BOMBARDIER CL600 2B19

Full Narrative

HISTORY OF FLIGHTOn September 28, 2010, about 1710 central daylight time, N498CA, a Bombardier CL600-2B19, operated as SkyWest Airlines flight 3074, landed on runway 7 right (07R) at the General Mitchell Airport (MKE), Milwaukee, Wisconsin with its left main landing gear (MLG) retracted. The incident flight originated from Eppley Airfield (OMA), Omaha, Nebraska, at 1538 CDT, with a planned destination of General Mitchell Airport (MKE), Milwaukee, Wisconsin. While on approach to MKE the crew attempted to extend the landing gear and observed an abnormal gear indication. The crew conducted a go-around and attempted to manually extend the gear, but was not able to extend and lock the left main landing gear (MLG), and decided to land before fuel ran low. The flight landed on runway 07R, coming to rest on the right MLG, nose wheel, and the left wing. The captain and first officer, 1 flight attendant, and 39 passengers were not injured.

According to interviews with the NTSB, the first officer indicated that he was the flying pilot and the captain was the monitoring pilot during their flight from OMA to MKE. The incident flight departed OMA about 1538 bound for MKE. According to the interviews, after selecting the landing gear down during approach to runway 07R, a GEAR DISAGREE message posted on the Engine Indicating Crew Alert System (EICAS) along with an associated aural alert and a triple chime. The flight crew performed a go around and received radar vectors from air traffic control (ATC) while trouble shooting the problem.

After executing the go around, the captain selected the gear back to the up position and initiated the "GEAR DISAGREE" procedure located in the Quick Reference Handbook (QRH). According to the interviews, fuel was critical so they asked for radar vectors back to final approach for runway 07R. During final approach and after pulling the landing gear manual release T-handle (the gear selector lever remained up), the captain took control of the airplane and asked the first officer to verify a note in the checklist regarding the landing gear manual release T-handle.. After discussing their interpretation of the note, the flight crew decided to leave the landing gear manual release handle in the extended position and proceeded to land. The captain stated that the notes in the checklist implied to him that the landing gear could come back up if he stowed the manual extension handle. He said about that time, the tower asked them if they wanted to do a fly by. He said they had already talked to ATC and declared an emergency so they did the fly by over runway 07R and the tower controllers confirmed that the left main gear appeared to be up.

The captain used right aileron to keep the left wing up as long as possible. The left wing eventually dropped to the runway and the airplane came to a stop at the runway's left edge. PERSONNEL INFORMATIONThe captain was a certificated airline transport pilot with a multi-engine rating. His most recent first class Federal Aviation Administration (FAA) medical certificate, dated May 11, 2010, bore no limitations. He reported 6,313 hours total flight time with 4,048 hours in the Bombardier CL600-2B19.

The first officer was a certificated airline transport pilot with a multi-engine rating. He held a first class medical certificate that was issued on September 20, 2010. He reported a total of 5,042 flight hours with 2,966 hours in the Bombardier CL600-2B19. AIRCRAFT INFORMATIONThis Bombardier Canadair Regional Jet CL-600-2B19 was manufactured in 2003 and was identified with serial number 7792. It was equipped with two General Electric (GE) CF-34-3B1 engines and was configured with 53 seats; 50 of these seats were passenger seats located in the main cabin. SkyWest Airlines leased the airplane from GE Capital Aviation Services (GECAS) on May-17-2007. At the time of the incident, the airplane had accumulated about 17,101 flight hours and 15,754 flight cycles.

The airplane was equipped with a tricycle landing gear system that comprises two main landing gear (MLG) assemblies mounted on the inboard part of each wing, and a nose gear assembly mounted directly below the flight compartment. Both MLG retract inward into recesses in the wing and center fuselage, and the NLG retracts forward. The landing gear system, operated by a selector lever, is electrically controlled by a proximity sensor electronic unit (PSEU) and hydraulically operated by the Number 3 hydraulic system. The MLG system comprises a priority valve, selector valve, run-around and bypass valve, a left and right MLG sidestay actuator and a left and right uplock assembly. The NLG system comprises a selector valve, extension/retraction actuator, uplock assembly, downlock, nose selector valve, and priority valve, bypass valves, restrictors, and check valves. Both MLG are secured in their up and locked position by their respective uplock assembly, which is attached to structure in the main wheel well. Each uplock assembly consists of a latch, an uplock lever, an actuator, and a manual release lever. When up and locked, the MLG uplock roller (which is attached to the gear) is positioned and held in place by the uplock latch.

Two forward NLG doors are operated hydraulically and are sequenced to operate independently of the NLG position, while the other (single aft NLG door) is mechanically linked to the nose gear position. During extension, the forward doors open before the NLG is released from the uplock. The nose gear assembly will then extend, simultaneously opening the rear door. Upon reaching full extension (when the NLG is down and locked), the forward doors close and remain in that configuration until a retraction command is selected.

The cockpit was equipped with a landing gear control panel, which contains a landing gear selector lever. When the selector lever is manipulated, an electrical command is sent to the PSEU to extend or retract the landing gear. Both MLG are extended in the outboard direction by their respective MLG sidestay actuators and are hydro-mechanically locked in place for landing. Each gear is retracted by the MLG side stay actuator in the inboard direction and mechanically locked in the MLG wheel wells during flight by their respective uplock mechanism.

During normal landing gear extension, when the landing gear selector lever is placed in the gear down position, the selector lever module sends an electrical extension command to the proximity sensor system and provides electrical signals to command the MLG and the NLG selector valves to their gear down position. When the MLG selector valve transitions to its gear down position, the valve is designed to allow no. 3 hydraulic system fluid, from the priority valve, to be ported, via the runaround and bypass valve, simultaneously to the uplock assembly and the extend side of the sidestay actuator for each MLG. The hydraulic pressure causes each uplock assembly to unlatch and release the MLG assembly. When unlatched, an uplock sensor (on the uplock mechanism) provides an input to the PSEU, which in turn signals the data concentrator units (DCUs) to generate an amber 'IN TRANSIT' gear indication on the EICAS display for each of the gear. When hydraulic pressure is supplied to the extend side of the gear actuators (sidestay), the actuator extends causing each MLG to extend to its full down and locked position; the extension rate is controlled by a restrictor in the actuator up line.

The landing gear manual release system provides the flight crew with another means to extend the landing gear in the event that an electrical or hydraulic failure within the landing gear system prevents the landing gear from being extended normally. The manual extension system is controlled by the vertical movement of a T-shaped manual release handle. To extend the landing gear manually, a flight crew member must pull up on the manual release handle. Movement of the T-handle is transmitted by a cable circuit to the NLG release mechanism and to the MLG release mechanism.

For the NLG system, the mechanisms activates the NLG door bypass valve and the NLG bypass valve and releases the NLG uplock and nose door lock. For the MLG system, rotation of the interconnect lever results in three actions: 1) the displacement is transmitted by two cables to the release levers on the left and the right uplock mechanism to unlock the uplock mechanism permitting the gear to extend by gravity and 2) re-positions the runaround and bypass valve into bypass mode and 3) positions the assist valve to pressurize the assist actuator.

In bypass mode, the runaround and bypass valve connects the extend pressure from the selector valve and both extend and retract pressure from the sidestay actuators and the extend pressure of the uplock assembly to an independent return line. The extend pressure of the uplock assembly is sent to the return line via the selector valve and check valves. The removal of all hydraulic pressure from the uplock mechanisms and sidestay actuators is designed to allow the gear to free-fall regardless of the position of the MLG selector valve. The activation of the downlock assist selector valve results in the valve porting no. 2 hydraulic system pressure to the MLG downlock assist actuators to assure down locking of the main gears after free-falling.

The landing gear indication system provides the status of each landing gear position on the landing gear display area on the EICAS primary page. The primary page contains three rectangles that will change color depending on the position of the landing gear. When a gear assembly is "up and locked", its respective rectangle will be colored white and display "UP". When the indication system detects that a gear assembly is not "up and locked" or "down and locked", its respective rectangle will transition to amber and when a gear assembly is "down and locked", its respective rectangle will be colored green and display "DN". If any landing gear remains in transit for longer than 28 seconds, the amber intransit indication of that affected gear will change to red (gear unsafe). Simultaneously a red 'GEAR DISAGREE' message will be displayed, accompanied by a 'GEAR DISAGREE' aural warning message. When any landing gear assembly remains in its up and locked position for longer than 6 seconds after the landing gear has been commanded down, a landing gear disagree aural warning will be annunciated along with an EICAS red gear disagree warning message. This warning will also be annunciated when any landing gear assembly remains in its downlock position for longer than six seconds when the landing gear has been commanded up.

If the landing gear selector lever remains "UP" during the manual extension, the EICAS immediately displays a "GEAR DISAGREE" message and the master warning illuminates and the corresponding cancellable voice message sounds. The landing gear indication and warning system comprises a PSEU and multiple proximity sensors and switches located within the control system. The PSEU logic analyzes inputs from these various proximity sensors and switches to determine the status of the landing gear and doors. Its output is displayed on the EICAS system (primary page) and master caution/warning panel on the glare shield.

The No. 3 hydraulic system is an independent hydraulic system that supplies the landing gear system, braking system, and certain flight control systems with hydraulic pressure. This hydraulic system comprises two alternating current motor pumps, identified as ACMP 3A and ACMP 3B, to generate hydraulic power (3000 psi), a pressure manifold, and a return manifold. Pressure generation comes primarily from ACMP 3A. However, because the system No. 3 accumulator had been removed from the airplane in accordance with the requirements of FAA Airworthiness Directive 2010-22-012, ACMP 3B was also ON at all times during the flight.

According to the MLG uplock component maintenance manual, the published wear limits for the uplock assembly's latch was:

1. The maximum permitted wear limit at overhaul is 0.003 in. (0.07 mm) on either or both upper and lower wear surfaces. Between overhauls, the maximum permitted wear limit can be a further 0.003 in. (0,07 mm), for a total of maximum 0.006 in. (0.15 mm) per surface.

2. If the wear on either upper or lower surface is greater than 0.006 in. (0.15 mm), replace the latch.

The hydraulic pumps are replaced on condition of failure to deliver 3000 ± 200 psi hydraulic pressure to the respective hydraulic system. AIRPORT INFORMATIONThis Bombardier Canadair Regional Jet CL-600-2B19 was manufactured in 2003 and was identified with serial number 7792. It was equipped with two General Electric (GE) CF-34-3B1 engines and was configured with 53 seats; 50 of these seats were passenger seats located in the main cabin. SkyWest Airlines leased the airplane from GE Capital Aviation Services (GECAS) on May-17-2007. At the time of the incident, the airplane had accumulated about 17,101 flight hours and 15,754 flight cycles.

The airplane was equipped with a tricycle landing gear system that comprises two main landing gear (MLG) assemblies mounted on the inboard part of each wing, and a nose gear assembly mounted directly below the flight compartment. Both MLG retract inward into recesses in the wing and center fuselage, and the NLG retracts forward. The landing gear system, operated by a selector lever, is electrically controlled by a proximity sensor electronic unit (PSEU) and hydraulically operated by the Number 3 hydraulic system. The MLG system comprises a priority valve, selector valve, run-around and bypass valve, a left and right MLG sidestay actuator and a left and right uplock assembly. The NLG system comprises a selector valve, extension/retraction actuator, uplock assembly, downlock, nose selector valve, and priority valve, bypass valves, restrictors, and check valves. Both MLG are secured in their up and locked position by their respective uplock assembly, which is attached to structure in the main wheel well. Each uplock assembly consists of a latch, an uplock lever, an actuator, and a manual release lever. When up and locked, the MLG uplock roller (which is attached to the gear) is positioned and held in place by the uplock latch.

Two forward NLG doors are operated hydraulically and are sequenced to operate independently of the NLG position, while the other (single aft NLG door) is mechanically linked to the nose gear position. During extension, the forward doors open before the NLG is released from the uplock. The nose gear assembly will then extend, simultaneously opening the rear door. Upon reaching full extension (when the NLG is down and locked), the forward doors close and remain in that configuration until a retraction command is selected.

The cockpit was equipped with a landing gear control panel, which contains a landing gear selector lever. When the selector lever is manipulated, an electrical command is sent to the PSEU to extend or retract the landing gear. Both MLG are extended in the outboard direction by their respective MLG sidestay actuators and are hydro-mechanically locked in place for landing. Each gear is retracted by the MLG side stay actuator in the inboard direction and mechanically locked in the MLG wheel wells during flight by their respective uplock mechanism.

During normal landing gear extension, when the landing gear selector lever is placed in the gear down position, the selector lever module sends an electrical extension command to the proximity sensor system and provides electrical signals to command the MLG and the NLG selector valves to their gear down position. When the MLG selector valve transitions to its gear down position, the valve is designed to allow no. 3 hydraulic system fluid, from the priority valve, to be ported, via the runaround and bypass valve, simultaneously to the uplock assembly and the extend side of the sidestay actuator for each MLG. The hydraulic pressure causes each uplock assembly to unlatch and release the MLG assembly. When unlatched, an uplock sensor (on the uplock mechanism) provides an input to the PSEU, which in turn signals the data concentrator units (DCUs) to generate an amber 'IN TRANSIT' gear indication on the EICAS display for each of the gear. When hydraulic pressure is supplied to the extend side of the gear actuators (sidestay), the actuator extends causing each MLG to extend to its full down and locked position; the extension rate is controlled by a restrictor in the actuator up line.

The landing gear manual release system provides the flight crew with another means to extend the landing gear in the event that an electrical or hydraulic failure within the landing gear system prevents the landing gear from being extended normally. The manual extension system is controlled by the vertical movement of a T-shaped manual release handle. To extend the landing gear manually, a flight crew member must pull up on the manual release handle. Movement of the T-handle is transmitted by a cable circuit to the NLG release mechanism and to the MLG release mechanism.

For the NLG system, the mechanisms activates the NLG door bypass valve and the NLG bypass valve and releases the NLG uplock and nose door lock. For the MLG system, rotation of the interconnect lever results in three actions: 1) the displacement is transmitted by two cables to the release levers on the left and the right uplock mechanism to unlock the uplock mechanism permitting the gear to extend by gravity and 2) re-positions the runaround and bypass valve into bypass mode and 3) positions the assist valve to pressurize the assist actuator.

In bypass mode, the runaround and bypass valve connects the extend pressure from the selector valve and both extend and retract pressure from the sidestay actuators and the extend pressure of the uplock assembly to an independent return line. The extend pressure of the uplock assembly is sent to the return line via the selector valve and check valves. The removal of all hydraulic pressure from the uplock mechanisms and sidestay actuators is designed to allow the gear to free-fall regardless of the position of the MLG selector valve. The activation of the downlock assist selector valve results in the valve porting no. 2 hydraulic system pressure to the MLG downlock assist actuators to assure down locking of the main gears after free-falling.

The landing gear indication system provides the status of each landing gear position on the landing gear display area on the EICAS primary page. The primary page contains three rectangles that will change color depending on the position of the landing gear. When a gear assembly is "up and locked", its respective rectangle will be colored white and display "UP". When the indication system detects that a gear assembly is not "up and locked" or "down and locked", its respective rectangle will transition to amber and when a gear assembly is "down and locked", its respective rectangle will be colored green and display "DN". If any landing gear remains in transit for longer than 28 seconds, the amber intransit indication of that affected gear will change to red (gear unsafe). Simultaneously a red 'GEAR DISAGREE' message will be displayed, accompanied by a 'GEAR DISAGREE' aural warning message. When any landing gear assembly remains in its up and locked position for longer than 6 seconds after the landing gear has been commanded down, a landing gear disagree aural warning will be annunciated along with an EICAS red gear disagree warning message. This warning will also be annunciated when any landing gear assembly remains in its downlock position for longer than six seconds when the landing gear has been commanded up.

If the landing gear selector lever remains "UP" during the manual extension, the EICAS immediately displays a "GEAR DISAGREE" message and the master warning illuminates and the corresponding cancellable voice message sounds. The landing gear indication and warning system comprises a PSEU and multiple proximity sensors and switches located within the control system. The PSEU logic analyzes inputs from these various proximity sensors and switches to determine the status of the landing gear and doors. Its output is displayed on the EICAS system (primary page) and master caution/warning panel on the glare shield.

The No. 3 hydraulic system is an independent hydraulic system that supplies the landing gear system, braking system, and certain flight control systems with hydraulic pressure. This hydraulic system comprises two alternating current motor pumps, identified as ACMP 3A and ACMP 3B, to generate hydraulic power (3000 psi), a pressure manifold, and a return manifold. Pressure generation comes primarily from ACMP 3A. However, because the system No. 3 accumulator had been removed from the airplane in accordance with the requirements of FAA Airworthiness Directive 2010-22-012, ACMP 3B was also ON at all times during the flight.

According to the MLG uplock component maintenance manual, the published wear limits for the uplock assembly's latch was:

1. The maximum permitted wear limit at overhaul is 0.003 in. (0.07 mm) on either or both upper and lower wear surfaces. Between overhauls, the maximum permitted wear limit can be a further 0.003 in. (0,07 mm), for a total of maximum 0.006 in. (0.15 mm) per surface.

2. If the wear on either upper or lower surface is greater than 0.006 in. (0.15 mm), replace the latch.

The hydraulic pumps are replaced on condition of failure to deliver 3000 ± 200 psi hydraulic pressure to the respective hydraulic system. WRECKAGE AND IMPACT INFORMATIONOn September 28, 2010, the state of Wisconsin's 128th Fire/Crash Rescue assisted SkyWest Airlines in removing the airplane from the runway. When the airplane was lifted with air bags during its removal, the left MLG remained up and locked within its wheel well. SkyWest Airlines maintenance personnel attempted, but failed, to extend the left MLG by cycling the landing gear selector lever and by pulling the manual release handle multiple times. The manual release handle was pulled one additional time, which resulted in the left MLG extending. The left MLG was pinned in place for safety and the airplane was towed to a SkyWest Airlines maintenance hangar for further evaluation.

During the period of September 29 through 30, 2010, and October 1, 2010, investigators from the NTSB, FAA and Bombardier examined the incident aircraft's landing gear system at the SkyWest Airlines maintenance hangar at MKE, Milwaukee, Wisconsin. ADDITIONAL INFORMATIONFlight Data Recorder (FDR) information from the airplane's previous landing shows that upon gear down selection, both the left and the right MLG "up and locked" discrete's transitioned to "not up and locked". Within approximately 12 seconds, both MLG "downlock" discrete's indicated "down and locked". The nose landing gear (NLG) "uplock" discrete transitioned from "up and locked" to "not up and locked" about 3 seconds after gear down selection and required approximately 9 seconds to reach down and locked.

Because small pieces of aluminum about 1mm by 3mm in size were found in the left sidestay actuator restrictor manifold, radiographic studies were done on July 21-26, 2011 in Chicago, Illinois to examine and document the internal configuration of the pressure and return hydraulic filters. The filters were documented using a combination of computed tomography (CT) scans and digital radiography. The filters were imaged using a total of 6 digital radiographs (DR) (3 DR's for the pressure filter and 3 DR's for the return filter) and 8,985 CT slices (4,841 slices for the pressure filter and 4,144 slices for the return filter).

Review of the images indicated that there were a substantial number of particles (ranging from 0.28 mm to 1.59 mm long) in the pressure filter. The debris inside the pressure filter was composed of both high density (bright) particles and lower density material. Some of the particles were found in clusters, while others were individual particles.

There were no particles found in the return filter, however there were several cracks noted in the epoxy material in the end caps of the filter, and there were some high density areas noted within the filter material itself.

A review of the CRJ Series Regional Jet Quick Reference Handbook Volume 2, model CL 600-2B19, revision 58 dated October 31, 2005 found that the GEAR DISAGREE checklist included two parts, a "Gear UP Disagree", followed by a "Gear DN Disagree" procedure. Interviews with the flight crew indicated that the procedure to be used was determined by the position of the landing gear selector lever. If the "GEAR DISAGREE" EICAS message was displayed when the selector lever was in the up position, the crew was to use the "Gear UP Disagree" procedure and if the message was displayed when the selector lever was in the down position, the "Gear DN Disagree" procedure was to be used.

The captain indicated that he may have initially used the "Gear UP Disagree" procedure but eventually used the "Gear DN Disagree" procedure for trouble shooting prior to landing.

According to interviews with the flight crew, upon reaching step 8 "landing gear manual release – stow" of the "Gear DN Disagree" procedure, the captain took over pilot flying duties, handed the QRH to the first officer, and asked him to interpret a note contained within the procedure that states: "In some cases, stowing the landing gear manual release handle may cause the main or nose gear to retract. If this occurs, the landing gear manual release handle must again be pulled to full extension and left in this position until the gear pins have been installed." After discussing their interpretation of the note, the flight crew decided to leave the landing gear manual release handle in the extended position and proceeded to land. The captain stated that the note in the checklist implied to him that the landing gear would come back up if he stowed the manual extension handle.

Following this note, was a procedure (step 9) to perform the "Landing Gear Up / Unsafe" landing procedure. The crew did not recall if they had completed this procedure and the captain stated that he was not aware that step 9 referred to another checklist until he reviewed the procedure after the event.

An NTSB review of the "GEAR DISAGREE" QRH procedure indicated that it had been revised a number of times since it was originally issued. An earlier version of the checklist included a step to leave the landing gear manual release handle in its extended position after use. The procedure was later revised and included a step to stow the handle after deployment of the landing gear. Revision RJ/98 dated, April 12, 2007, was then issued which maintained the step to stow the handle, and added notes indicating that stowing the handle could cause the landing gear to retract. After the incident, Bombardier Regional Aircraft Division's Flight Operations and Engineering groups initiated a review and evaluation of the GEAR DISAGREE procedure.

Based on their review, several editorial changes were made to the GEAR DISAGREE procedure in the AFM and QRH to clarify the procedure. An additional note was included to inform flight crew that they may have to reposition the seat and stand up prior to manual release handle deployment. The actual force to operate the landing gear handle of greater than 40lbs was also included in the note. The procedure has been unified in a single procedure. There are no longer two separate procedures for up and down based on the handle. The current version in the QRH is dated 30 Nov 2012 (Rev. 60).

After the incident, SkyWest Airlines published an informational article in an internal company flight operations newsletter available to flight crews. The article provided guidance on the use of the gear disagree checklist procedure and emphasized the identification of the proper procedure, the possibility that the procedure may need to be repeated multiple times to achieve success, and the direction to stow the manual extension handle after the gear has been extended.

SkyWest Airlines also modified their CRJ training program to include a discussion of gear disagree events during recurrent ground school, and a gear disagree scenario during flight crew recurrent simulator training.

Subsequent to the incident with N498CA, Bombardier revised the contamination limits in reference to hydraulic fluid in the aircraft maintenance manual. The manual was updated on June 9, 2011.The class 9 contamination level per NAS 1638 was previously identified as a "permitted" contamination level. The AMM Task 29-10-00-750-801, Hydraulic Fluid Sample Check was updated to restrict contamination level of the fluid as follows:

Class 8: Perform a repeat sample check within 3000 flight hours (FH)

Class 9: Within 30 calendar days or 200 FH W.C.F flush system as per AMM

Class 10 and above: Flush system as per AMM

The AMM task for the ACMP replacement after a pump failure was also updated. It now includes requirements to examine associated pump filter elements (pressure, case drain and return) for contamination and replace if required. In addition to that it requires to take a sample of the hydraulic fluid from the affected system and to do analysis for contamination.

Prior to SkyWest's September 2010 and June 2011 (Reference NTSB CEN11IA379) gear up landing events at the General Mitchell Airport (MKE), Milwaukee, Wisconsin, Bombardier required freedom of movement inspections at the 'C' checks (18 mos.) on each MLG uplock roller. After the landing gear events, SkyWest developed a task independent of Bombardier due to a Continuing Analysis and Surveillance System (CASS) board mandate which required the checking of the MLG uplock roller for freedom of movement at each service check (every 5 days). This task is still in effect.

During the period of December 5th – 17th, Bombardier conducted a landing gear system study on an exemplar CL600-2B19 airplane in Tucson, Arizona. Present during this testing were representatives from the National Transportation Safety Board, Bombardier Aerospace, Messier-Dowty, Tactair and SkyWest Airlines. The study was comprised of a series of simulation tests that induced failures into the landing gear and hydraulic system to generate data for use in a safety analysis of the landing gear system.

To obtain an initial understanding of the operation of a normally operating landing gear system, the landing gear system was cycled three times using the landing gear selector lever with both the Number 3A and 3B hydraulic pumps running. Testing found that when the landing gear selector lever was positioned to down, both MLG would simultaneously be released from the wheel well and extend to the down and locked position in approximately 13 to 14 seconds. When the landing gear was retracted, it took between 14 and 15 seconds for both MLG gear to retract from their down and locked position to their up and locked position.

Because information from the September 28th incident airplanes FDR indicated that the left MLG unlocked, but did not extend to its down and locked position when commanded, tests were performed to evaluate the effects of a hydraulic restriction at a sidestay actuator; one test simulated a restriction just prior to the extend port and another test simulated a hydraulic restriction at its return port. To simulate the hydraulic restriction at the extend port of the actuator, a valve was inserted into the hydraulic line between the actuator and its respective extend hydraulic line. With the NLG and MLG in their up and locked position and both the Number 3A and 3B hydraulic pumps running, the valve was closed (simulating a full restriction) and the landing gear was selected down using the selector lever. Upon gear down selection, both MLG simultaneously released from their uplock assemblies. The right MLG extended to its down and locked position in approximately 9 to 11seconds, and the left MLG extended approximately 2.0 inches out of its wheel well and then remained in this position for the duration of the test. This scenario was repeated two addition times with similar results. To simulate a hydraulic restriction at the return port of the sidestay actuator, a valve was inserted into the hydraulic line between the actuator and its respective return hydraulic line. With the NLG and MLG in the up and locked position and both the Number 3A and 3B hydraulic pumps running, the valve was closed (simulating a full restriction) and the landing gear was selected down using the selector lever. Upon gear down selection, both MLG simultaneously released from their uplock assemblies. The right MLG extended to its down and locked position in approximately 8 to 9 seconds and the left MLG extended approximately 4.0 inches out of its wheel well and then remained in this position for the duration of the test. This scenario was repeated two addition times with similar results.

A potential landing gear system failure scenario considered that during the landing gear extension, the left MLG sidestay actuator received hydraulic pressure prior to its respective uplock actuator creating a force fight between the sidestay and its respective uplock assembly. This force fight would hypothetically prevent the uplock latch from rotating and releasing the gear. To validate this potential scenario, a test was performed in an attempt to simulate the force fight condition. For this test, the Number 3A and 3B hydraulic pumps were running and providing 2,900 psi to the landing gear system (except to the left MLG uplock assembly). To control and apply hydraulic pressure to the uplock assembly, a hydraulic hand pump was connected to uplock actuator. When the landing gear selector lever was selected down, the nose and left MLG were released from their respective uplock mechanisms and extended to their down and locked positions; the left MLG remained in its up and locked position with its sidestay pressurized to extend. Using the hand pump, the pressure to the left uplock actuator was slowly increased until the left uplock assembly released the gear. At 1,000 psi, the uplock assembly released the left MLG allowing the sidestay actuator to extend the gear to its down and locked position. This test was repeated two additional times with the uplock mechanism releasing the gear between 900 and 950 psi. This test was performed again, but instead of manually releasing the gear with the uplock mechanism, the landing gear manual system was used. When the landing gear selector lever was selected down, the nose and right MLG extended to their down and locked positions. The manual release handle was pulled and the left MLG released and extended at a handle position of about 6.0 inches and a handle force of about 60 pounds. This test was repeated with similar results.

Testing was performed to simulate the effect of an improperly rigged left MLG uplock roller on the operation of the landing gear. For this test, the Number 3A and 3B hydraulic pumps were running and providing 2,900 psi to the landing gear system (except the left MLG uplock assembly). To control the hydraulic pressure to the uplock assembly, its actuator was connected to a hydraulic hand pump. When the landing gear selector lever was selected down, the nose and right MLG were released from their respective uplock mechanisms and extended to their down and locked positions; the left MLG remained in its up and locked position. Using the hand pump, the pressure to the left uplock actuator was slowly increased until the left uplock assembly unlocked. For this condition, the MLG uplock roller released between 950 and 1,000 psi of pressure and the gear extended to its full down and locked position.

Testing was performed to simulate the effect of a jammed (non-rotating) left MLG uplock roller on the operation of the landing gear. For this test, the Number 3A and 3B hydraulic pumps were operating and providing 2,900 psi to the landing gear extension/retraction system (except the left MLG uplock assembly). To control the hydraulic pressure to the uplock assembly, its actuator was connected to a hydraulic hand pump. When the landing gear selector lever was selected down, the nose and right MLG were released from their respective uplock mechanisms and extended to their down and locked positions; the left MLG remained in its up and locked position. Using the hand pump, the pressure to the left uplock actuator was slowly increased until the left uplock assembly unlocked. For this condition, the MLG uplock roller released between 950 and 1,000 psi of pressure and the gear extended to its full down and locked position.

Testing was performed to simulate a combination of improper rigging of the left MLG uplock roller, a jam of the uplock roller (prevented from rolling), and a worn uplock latch (groove of 0.01 machined on lower surface). For this test, the Number 3A and 3B hydraulic pumps were operating and providing 2,900 psi to the landing gear extension/retraction system (except the left MLG uplock assembly). To control the hydraulic pressure to the uplock assembly, its actuator was connected to a hydraulic hand pump. When the landing gear selector lever was selected down, the nose and right MLG were released from their respective uplock mechanisms and extended to their down and locked positions; the left MLG remained in its up and locked position. Using the hand pump, the pressure to the left uplock actuator was slowly increased until the left uplock assembly unlocked. For this condition, the MLG uplock latch was released between 500and 600 psi of pressure, but the MLG uplock latch did not rotate enough to release the MLG uplock roller, therefore the gear remained in the uplock. The pressure at the uplock actuator was increased to 3,000 psi and there was still no movement of the uplock latch. The manual release handle was pulled to its full extension (10 inches) and there was no additional rotation of the uplock latch. Turning off hydraulic system number 3 also did not cause the latch to rotate and release the gear.

Several tests were performed by varying the hydraulic system pressures and flow rates to observe their effect on the landing gear system operation. The hydraulic pressures were varied from 3,000 psi to 2,100 psi and the flow rates were varied from 6.0 gallons per minute (GPM) to 3.5 GPM. For each pressure and flow rate condition, the landing gear was extended and retracted. Testing revealed that for all hydraulic flow rates that were less than 5.0 gallons per minute, the NLG required more than 30 seconds to transition from its up and locked position to its down and locked position, which would have triggered a "gear disagree" message. Testing also revealed that for all of the hydraulic pressure and flow rate scenarios, both MLG uplock assemblies unlocked and released the landing gear. However, at hydraulic pressures less than 2,600 psi one MLG assembly would typically release and begin to extend before the other MLG assembly. Additionally, for all of the hydraulic pressure and flow rate scenarios except for one scenario (3,000 psi at 6 GPM), both MLG required more than 16 seconds to extend to their full down and locked position.

A test of the landing gear manual release system was performed to document the relationship between the position of the manual release handle and the position of the manual release levers on the MLG uplock assemblies. As previously stated, the manual extension system is controlled by the vertical movement of a T-shaped manual release handle. The handle is designed to move upward a maximum of 10 inches before it contacts it up-stop. To extend the landing gear manually, a flight crew member must pull up on the manual release handle with an upward vertical force. The force required to release the landing gear must be between 25 and 46 pounds and the actual travel dimension of the manual release handle must be a maximum of 7.75 inches. With the airplanes MLG positioned to down and locked, the left MLG uplock mechanism's latch was manually rotated to its up and locked position. To determine the relationship, the manual release handle was slowly pulled and the left uplock assembly was visually monitored. When the handle was pulled, the manual release levers on the left uplock assembly moved upward contacting and applying a force to the uplock lever in a direction to move the lever up and away from the uplock latch. As the manual release handle extended to approximately 6.5 inches the uplock manual release levers had displaced enough to move the uplock lever up and allow the uplock latch rotate in the gear release direction. However, when the handle was pulled beyond 7.25 inches, the manual release levers contacted their upstop pin and could not be moved any further. Any additional handle extension resulted in system compliance only. DAMAGE TO AIRCRAFTThe airplane sustained minor damage to its left wing tip, left flap assemblies, flap fairings, and flap actuation arms. The left MLG remained within its wheel well. The NLG and right MLG had extended and were in their down and locked position. FLIGHT RECORDERSThe airplane was equipped with a Fairchild FA-2100-1020 solid-state cockpit voice recorder (CVR). The CVR was sent to the NTSB's laboratory for readout evaluation. The CVR was in good condition, and the audio information was extracted without difficulty. The recording consisted of six channels encompassing two hours of audio information. Two of the six channels contained two hours of audio information. The remaining four channels each contained 30-minutes of audio information duplicating the last 30-minutes of the two hour recordings. In conjunction with the IIC, a brief summary was prepared covering the entire two hour recording.

The airplane was equipped with an L-3 Communications Fairchild model F1000 solid-state flight data recorder (FDR), which was sent to the NTSB Vehicle Recorder Division for download. The FDR was in good condition, and the data were extracted normally.

The FDR incident data indicated that at about 1643:35, during descent through a pressure altitude of approximately 5,200 feet, the left and the right MLG "up and locked" discrete's transitioned to "not up and locked". Within approximately 8 seconds, the right MLG "downlock" discrete indicated "down and locked" and the left MLG "downlock" discrete remained indicating "not up and locked". The nose landing gear (NLG) "uplock" discrete transitioned to "not up and locked" about 3 seconds after gear down selection and required approximately 9 seconds to reach down and locked.

At 1644:05 the CVR recorded a triple-chime warning tone followed by an electronic voice announcing a "gear disagree" warning. According to the FDR data, a "Gear Disagree Warning" was triggered approximately thirty seconds after the landing gear was selected to down. FDR incident data indicated that at 1644:26, all gear transitioned to "up and locked" and the CVR recorded decreasing wind noise sounds, consistent with gear retraction. At 1646:03, the CVR recorded sounds consistent with landing gear extension and the FDR data shows that at this time, the landing gear was selected down a second time with similar results as the first attempt to lower the gear. Approximately 28-seconds later, the CVR recorded an electronic voice announcing a "GEAR DISAGREE" warning.

The FDR incident data indicated that at about 1657:07, while at a pressure altitude of approximately 4,150 feet, the NLG and both the left and the right MLG "up and locked" discrete's transitioned to "not up and locked. Immediately after this, the FDR indicated that a gear disagree warning occurred and the CVR recorded an electronic voice announcing a "gear disagree" warning and a "nose door" warning indicating that the landing gear manual release T-handle was pulled while the landing gear selector lever remained in the "gear up" position. According to the QRH, the landing gear selector lever should have been positioned to down before pulling the manual release handle. Within approximately 5 seconds, the NLG "downlock" discrete indicated "down and locked" and then after an additional 8 seconds (13 seconds after unlock), the right MLG "downlock" discrete indicated "down and locked" and the left MLG "downlock" discrete remained indicating "not up and locked".

At 1659:48, the FDR data indicated that hydraulic system 3 pressure decreased from about 2,800 PSI to less than 200 psi. After 23 seconds, the hydraulic pressure returned to about 2,800 psi and remained at this pressure for the rest of the flight.

The FDR recording ended at 22:10:45 UTC time. TESTS AND RESEARCHA review of SkyWest Airlines maintenance records revealed that the incident airplane had been having hydraulic system number 3 discrepancies during the period of April 8, 2010 through May 10, 2010, along with twelve landing gear disagreement events during the period of May 10, 2010, through August 16, 2010. The landing gear system events were not isolated to only the left MLG. On eight occasions, the flight crew received a "Gear Disagree" messages associated with the right MLG during landing gear extension. In an attempt to address the landing gear discrepancies, SkyWest Airlines replaced the left and right MLG side stay actuators, the MLG selector valve, right MLG up-lock mechanism and the run around and bypass valve.

According to SkyWest Airlines maintenance records, the number 3A hydraulic system pump was replaced on April 8, 2010 to address a flight crew write-up regarding a number 3A hydraulic caution message. Inspection of the pump by an overhaul facility revealed that the pump had an internal failure and multiple components were found damaged. The number 3A hydraulic system pump was replaced again on April 16, 2010 to address a maintenance write-up regarding a hydraulic pump caution message, which was received while the airplane was on the ground. Similar to the previous pump, an inspection of the pump by an overhaul facility revealed that the pump had an internal failure and multiple components were found damaged beyond serviceable limits. On May 10, 2010, the number 3A hydraulic case drain filter was replaced to address a maintenance write-up indicating that the case drain filter differential pressure indicator (red button) was extended indicating that the case drain filter element was clogged or contaminated.

This indicator will extend about 0.19 inches when it is actuated by a pressure differential of more than 15.5 to 21.5 psi across the filter element. A review of SkyWest Airlines aircraft maintenance manual (AMM) 29-10-00, showed that there is maintenance procedure titled "Main Hydraulic Power – Inspection/Check" dated May 10, 2010. This maintenance procedure contains a filter check, numbered Task 29-10-00-780-801, which contains the maintenance procedures to visually inspect differential pressure indicators. The procedure states that if any pressure indicator is found extended, a hydraulic fluid particle contamination check must be performed according to Task 29-10-00-750-801 and also to replace the filter pertaining to the system. No maintenance records were found indicating that SkyWest Airlines had performed this hydraulic fluid particle contamination check or replaced the filter.

On July 4, 2010, SkyWest Airlines obtained a hydraulic fluid sample from the number 3 hydraulic system and sent it to Aviation Laboratories for analysis. The results of the analysis determined that the fluid appeared normal and was classified as Class 4.

On September 29, 2010, and in a hangar at the General Mitchell Airport (MKE), Milwaukee, Wisconsin, the systems group conducted inspections and performed function testing on the MLG landing gear system. After completing the landing gear system inspections and testing, SkyWest Airlines removed the left MLG sidestay actuator, the left MLG uplock mechanism, the MLG selector valve, and the runaround and bypass valve for examination. SkyWest Airlines also removed the number 3 hydraulic system pressure and return filter elements (15 micron disposable filter) from the airplane and sent them directly to the NTSB Materials Laboratory for metallurgical examination. SkyWest Airlines personnel also obtained hydraulic fluid samples from several landing gear system components and sent them to Exxcare Oil Analysis for a fluid analysis. The fluid analysis results indicated that all of the samples were in satisfactory condition. However, because an insufficient amount of fluid was removed from each of the landing gear system components, a particle count could not be performed on the samples.

In an attempt to duplicate the event condition (left MLG remains up when commanded down), the landing gear was cycled multiple times using the landing gear selector lever with both the Number 3A and 3B hydraulic pumps operating. Testing was unable to duplicate the event condition; both MLG consistently extended and retracted normally without hesitation or binding. Testing found that it took about 12.5 seconds for the gear to retract from its down and locked position to its up and locked position. During extension, it took about 12.0 seconds for the gear to extend from its up and locked position to its down and locked position.

A test of the manual landing gear release system found that at approximately 6.5 inches of handle extension and a force of about 55.0 pounds, both MLG released from their respective up-lock mechanisms and extended to their down and locked position. According to SkyWest Airlines AMM 32-34-00, titled "manual release system, landing gear – adjustment/test" dated May 10, 2009, the force required to release the landing gear must be between 25 and 46 pounds.

NTSB examination of the pressure filter found that it contained significant metallic particles noted of different color and texture; in addition there was discoloration of the hydraulic fluid (black).

A visual inspection of the left MLG uplock roller revealed that it exhibited slight wear and flat spotting; its bracket assembly remained properly connected to the landing gear. Using hand pressure, the uplock roller was found to rotate smoothly (as designed). The diameter of the roller was measured and found to be 0.555 inches, which was within the acceptable limits of 0.552 to 0.562 inches as specified in the CMM. A visual inspection of the left MLG uplock mechanism, while it remained on the airplane, revealed that its latch contained contact wear marks and foreign material on its upper and lower surfaces. A dimensional check of the rigging gap between the root of the hook and the edge of the MLG uplock roller was found to be 0.775 inches, which was within the acceptable limits of 0.730 to 0.910 inches. On November 10, 2010, Messier-Dowty conducted an examination/disassembly of the left MLG uplock mechanism at their facility located in Ajax, Ontario, Canada. Visual inspection confirmed the presence of contact wear marks on the upper and lower surfaces of the hook. The upper contact wear mark was measured and found to have a depth of about 0.001 inches and the lower contact wear mark was measured and found to have a depth of about 0.003 inches. Functional performance testing showed that the uplock mechanism locked and unlocked within all test specifications.

Each MLG assembly was equipped with a sidestay actuator that hydraulically extends or retracts its respective gear assembly. The actuator assembly includes a piston, cylinder, restrictor, and an internal locking mechanism. The cylinder body contains two hydraulic ports (port A and port B): one for fluid to extend the actuator piston, and the other used to port fluid from the retract port (on the opposite side of the cylinder). When the actuator is installed in the airplane, it is connected to hydraulic lines at two locations (port A and port B). When commanded to extend, the actuator receives hydraulic pressure (3,000 psi) at its extend port (port A). This pressure reacts against the piston head resulting in the piston rod extending. The rate of extension is controlled primarily by the retract line restrictor (in Port B).

A visual inspection of the left MLG sidestay actuator, while it remained on the airplane, found evidence that the actuator was leaking hydraulic fluid (black in color) from its end seal; no other anomalies were noted.

On November 10, 2010, Messier-Dowty conducted an examination/disassembly of the sidestay actuator at their facility located in Ajax, Ontario, Canada. Prior to conducting functional testing on the actuator, Messier-Dowty personnel removed hydraulic fluid samples (black in color) from the actuator's extension port, retraction port, and cylinder. Evaluation of these samples, by Messier Dowty M&P Laboratory, revealed that each of them contained metallic and organic contamination ranging in size from 10 to 100 microns. The fluid from the cylinder of the sidestay actuator failed the particle count test and the water content test. According to AMM 29-10-00, dated May 10, 2010, the in-service particle count limits should be class 9 or better. The fluid within the cylinder was determined to be class 10. The in-service limits for water content should be 0.10% to 0.30% fluid analysis found that the water content was 0.44%.

Functional performance testing showed that the sidestay actuator operated within all test specifications. The sidestay actuator was disassembled and no abnormalities could be found. The actuators restrictor assembly, (comprising a manifold, two screens, and an orifice) was disassembled exposing its two screens. Two small pieces of aluminum about 1mm by 3mm (1000 – 3000 microns) in size were found in the manifold. The two screens and the manifold were sent to Messier Dowty M&P Laboratory for inspection. Microscopic examination found no evidence of retained debris on the screens, but the examination did find several particles of debris, about 50 microns in size within the manifold housing.

On November 4, 2010, the MLG selector valve was examined at the Tactair Fluid Controls facility, Liverpool, New York. Tactair personnel performed functional tests on the selector valve and found no discrepancies that would have prevented it from operating.

On the same day, Tactair personnel examined and tested the by-pass and runaround valve. The runaround and bypass valve was subjected to acceptance test procedure (ATP) 3ATP750004000, minus the proof pressure test, external leakage and the pressure drop test. All tests, except the "operating force and bypass valve operation" test were found to be within acceptable limits. With a hydraulic pressure of 3,000 psi applied to the unit and a flow rate of 4.0 gallons per minute, the force to pull the handle of the bypass valve was measured and found to be 60 pounds which exceeded the ATP specified force requirement of 40 pounds. Additionally, the bypass spool valve exhibited binding and "sticking" during extension and retraction during the operational test.

The bypass spool and sleeve assembly were removed from the runaround and bypass valve and submitted to Tactair's customer support engineering group for a detailed inspection of its internal condition and diametrical clearance of the spool and its respective sleeve assembly. Results of the examination indicate that the diameter of one of the spools five lands exceeded the specified drawing limits, three of the sleeves five port walls exhibited burrs around the lands and contamination (RTV) was found throughout the valve.

A review of Tactair's records indicates that the subject runaround and bypass valve was repaired by Tactair in May 2010. The records show that during initial testing of the valve, it had similar test responses for the operating force test. The valve was overhauled with new seals and packing and re-tested per ATP 3ATP750004000, which found the valve to be within normal functional limits.