BOEING 717-200

Full Narrative

HISTORY OF FLIGHT On August 27, 2009, about 1603 eastern daylight time (EDT), a Boeing 717-200, N994AT, operated by Airtran Airways as flight 163, encountered convective turbulence while climbing through flight level 250 near Inverness, Florida. One flight attendant sustained serious injuries. The other two flight attendants, the two airline transport pilots, and the 81 passengers were not injured, and the airplane was not damaged. Instrument meteorological conditions prevailed, and an instrument flight rules flight plan had been filed for the flight that departed Tampa International Airport (TPA), Tampa, Florida, destined for Hartsfield-Jackson Atlanta International Airport (ATL), Atlanta, Georgia. The scheduled passenger flight was conducted under Title 14 Code of Federal Regulations (CFR) Part 121.

According to the flight crew, after departure, the airplane was assigned a heading of 350 degrees, and was cleared to climb to its cruise altitude. The flight then entered instrument meteorological conditions (IMC), and encountered "very mild light chop." There were "no reports of turbulence from PIREPS or ATC and no cells were displayed on the radar." The seat belt sign was illuminated. After the airplane passed through 10,000 feet, the flight attendants began their cabin service. As the airplane passed through flight level 250, still in IMC, it encountered turbulence for approximately 15 seconds. The non-flying pilot checked the airborne radar, and changed the range settings to see if the airplane had "hit a cell." He saw "nothing" on the radar. The turbulence encounter occurred approximately 60 miles north of TPA.

According to the lead flight attendant, the cabin crew consisted of three flight attendants, designated as Lead, R1, and L2. The lead attendant was working business class at the front of the cabin, and the R1 and L2 attendants were working the service cart in the aisle at the rear of the airplane. The R1 attendant was forward of the cart, and the L2 attendant was aft of the cart. At the time of the turbulence encounter, the lead attendant was in the galley, and he "came off the floor about a foot." He reported that the turbulence lasted about 15 to 20 seconds, that coffee was "splashing out of the pots," and that beverage cans were turning over. According to the operator's incident report, during the turbulence encounter the L2 flight attendant and the service cart "came off the floor," and the cart struck the flight attendant's leg.

The turbulence subsided, and the L2 flight attendant was observed to be lying on the floor, and complaining of "intense pain" in her right leg. The flight attendant's leg was immobilized and a passenger who was a nurse helped her to an unused row of seats. The flight crew was notified of the injury, and informed that the injured attendant could not perform her duties. The flight crew and the dispatch department arranged for medical personnel to meet the flight in ATL. Cabin service was discontinued, and the R1 flight attendant occupied the L2 station at the tailcone door for the landing. The injuries were subsequently diagnosed as a fractured tibia and a crushed ankle.

PERSONNEL INFORMATION

Captain

According to records provided by the operator, the captain held an airline transport pilot certificate with multiple ratings including airplane multi-engine land, and a type rating for the Boeing 717. His most recent Federal Aviation Administration (FAA) first-class medical certificate was issued May 2009. The captain reported 18,700 total hours of flight experience, including 5,000 hours in the accident airplane make and model. The captain's length of employment with the operator was not determined.

First Officer

The first officer held an airline transport pilot certificate with ratings for airplane multi-engine land, and multiple type ratings. His most recent FAA first-class medical certificate was issued in February, 2009. The first officer reported 10,000 total hours of flight experience, including 3,000 hours in the accident airplane make and model. The first officer's length of employment with the operator was not determined.

Cabin Personnel

The lead flight attendant was employed by the operator as a flight attendant for nearly 5 years. The L2 (injured) flight attendant was employed by the operator as a flight attendant for 2 1/2 years. The R1 flight attendant's length of employment with the operator was not determined.

AIRCRAFT INFORMATION

The airplane was a variant of the Douglas Aircraft DC-9, and was manufactured in 2002. It was powered by two Rolls-Royce BR 715 series turbofan engines, and at the time of the turbulence encounter, the airplane gross weight was approximately 99,000 pounds. According to the operator, the airplane was configured for 2 flight crew, 4 cabin crew, and 117 passengers.

METEOROLOGICAL INFORMATION

General Forecast

Detailed meteorological information regarding the flight conditions was documented in a separate report by a National Transportation Safety Board (NTSB) meteorologist. That and other information is summarized briefly in this section.

The National Weather Service (NWS) data issued at 0800 depicted conditions which supported widespread upward vertical motion over the region of the flight track. The 1230 NWS Convective Outlook indicated that only general airmass-type convective activity was expected, and did not forecast any organized severe weather for the region.

The 1345 area forecast for northern Florida called for scattered clouds at 1,500 and 4,500 feet, broken cloud layer at 12,000 feet, with tops to 17,000 feet, broken cirrus clouds above, with scattered thunderstorms and light rain. Cumulonimbus cloud tops were forecast to reach 45,000 feet. The same forecast stated that after 2000, scattered clouds at 2,000 feet, a broken cloud layer at 12,000 feet, with widely scattered thunderstorms and light rain, were expected.

NWS information issued at 1400 depicted conditions in the immediate vicinity of the turbulence encounter that provided additional support for the generation of convective activity, including moderate to strong multicellular-type thunderstorms in lines and clusters.

AIRMETs and SIGMETs

The NWS website stated that "AIRMETs (AIRman's METeorological information) are issued by the Aviation Weather Center to advise of weather potentially hazardous to all aircraft but that does not meet SIGMET criteria." The NWS stated that "moderate turbulence" was one of the conditions that would result in the issuance of an AIRMET, and that "AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancellation of a SIGMET." AIRMETs are routinely issued for 6-hour periods beginning at 2245 EDT.

The NWS website stated that a "SIGMET (SIGnificant METeorological information) advises of weather potentially hazardous to all aircraft other than convective activity," and are issued for several reasons, including "severe or extreme turbulence." SIGMETs are typically issued for 4-hour periods. According to the NWS, "if conditions persist beyond the forecast period, the SIGMET is updated and reissued."

Convective SIGMETs are issued for "severe surface weather," including "embedded thunderstorms, lines of thunderstorms, and thunderstorms greater than or equal to video integrator and processor (VIP) intensity level 4 affecting 40% or more of an area at least 3000 square miles." The NWS stated that "any convective SIGMET implies severe or greater turbulence," that they are issued hourly at 55 minutes past each hour, and are valid for up to 2 hours.

Between 1255 and 1555, the NWS issued three convective SIGMETs that overlaid the planned flight track in the vicinity of the turbulence encounter, and which forecast embedded thunderstorms, with tops above flight level 450, moving approximately south to north. At 1255, the NWS issued convective SIGMET 37E. That convective SIGMET was valid until 1455, which was about 1 hour prior to the planned departure time. At 1455, the NWS issued convective SIGMET 43E, which overlaid a portion of the area encompassed by 37E, and was valid until 1655. Convective SIGMET 43E forecast a diminishing area of embedded thunderstorms. At 1555, about 6 minutes after the flight took off, convective SIGMET 46E was issued. Convective SIGMET 46E was valid until 1755, forecast embedded thunderstorms, and included areas common to 37E and 43E.

Meteorological Conditions in Vicinity of Turbulence Encounter

The 1600 NWS radar composite reflectivity mosaic image depicted several defined echoes associated with thunderstorms north of the departure airport, including a short but intense line at the latitude and longitude of the turbulence encounter, but below the airplane's altitude. The 1620 NWS radar summary chart depicted an extensive area of thunderstorms in the vicinity of the flight track, and also depicted the solid line of thunderstorms in the vicinity of the turbulence encounter. Weather observations from several airports in the vicinity of the encounter included the presence of thunderstorms prior to, during, and after the encounter.

Correlation of the ground-based WSR-88D with the flight path information revealed that the airplane was penetrating a line of radar echoes when the turbulence encounter occurred. The data depicted radar echoes of magnitude 20-30 dBZ (decibels) at the airplane's altitude, and echo magnitudes of 50 to 55 dBZ below the airplane. The radar data also depicted echoes that overlaid the airplane's flight path, with tops above the airplane's altitude.

Infrared satellite imagery from 1602 depicted the presence of cumulus clouds, with tops near 36,500 feet, at and around the location of the turbulence encounter. The radar and satellite imagery indicated conditions that were consistent with the airplane flying through clouds at the time of the turbulence encounter.

"Tail-end Charlie" Phenomenon

The operator's submission to the NTSB suggested that the airplane encountered a "Tail-end Charlie," which was a common-use term for the trailing ("tail-end") cell in a line of build-ups or thunderstorms. Review of the available ground-based WSR-88D radar data revealed that a continuous line of echoes was oriented along a southwest to northeast axis. The line was translating to the north-northwest, and was expanding in length to the southwest. The line was depicted on the radar summary chart with reflectivity Levels 5 or 6, which were defined as "intense to extreme." The airplane penetrated the southwestern portion of the line, which was its advancing segment.

AIDS TO NAVIGATION

The initial segment of the planned route of flight was from TPA 101 miles north to the Cross City (CTY) very high frequency omni-range navigation facility (VOR), and then northwest to WYATT intersection. Although not cited in the flight plan as waypoints, two VORs which were equipped to broadcast hazardous inflight weather advisory service (HIWAS) were located within reception range of the airplane's flight track. The St. Petersburg (PIE) VOR was located about 10 miles west of TPA, and the Gators (GVN) VOR was located about 35 miles east of CTY VOR. Review of aeronautical charts indicated that at the time of the turbulence encounter, the airplane was about 60 miles north of the PIE VOR, and about 60 miles southwest of the GNV VOR.

COMMUNICATIONS

According to air traffic control (ATC) radio communications documentation provided by the FAA, the flight was issued taxi clearance at 1541, and takeoff clearance at 1547. Two minutes later, the flight was handed off to the Tampa TRACON (traffic control). About 1553, the flight made initial radio contact with Jacksonville Air Route Traffic Control Center (ARTCC, designated as "ZJX") sector R88. The turbulence encounter occurred at 1603, and the flight was handed off to ZJX sector R17 about 1 minute after that.

According to Chapter 2, section 6 of FAA Order 7110.65R (Air Traffic Control), "Controllers shall advise pilots of hazardous weather that may impact operations within 150 NM of their sector or area of jurisdiction. Hazardous weather information contained in HIWAS (hazardous inflight weather advisory service) broadcasts includes ... Significant Meteorological Information (SIGMET), Convective SIGMET (WST)...and Center Weather Advisories (CWA)." The section also stated that "Controllers within commissioned HIWAS areas shall broadcast a HIWAS alert on all frequencies, except emergency frequency, upon receipt of hazardous weather information. Controllers are required to disseminate data based on the operational impact on the sector or area of control jurisdiction."

Review of the communication transcripts revealed that none of the four ATC controllers who communicated with the flight prior to the turbulence encounter mentioned the existence of either SIGMET 43E or 46E, or broadcast a HIWAS alert, while the flight was on their frequency.

FLIGHT RECORDERS

The flight data recorder information was downloaded and provided to the NTSB Recorders Laboratory. The data indicated that when the airplane was climbing through an altitude of 25,300 feet, and at an airspeed of 307 knots, it experienced a rapid vertical acceleration increase that reached a peak value of approximately 2.5 g (acceleration due to gravity), and decreased to approximately 0.5 g within 1 second. The vertical acceleration values then fluctuated between 0.6 g and 1.2 g for the next 11 seconds, before they stabilized near the normal value of 1 g. During that same period, the data indicated pitch and roll excursions of approximately 5 and 20 degrees, respectively, and an airspeed decrease of approximately 15 knots. The altitude at the end of the encounter was approximately 25,800 feet.

The acceleration sensors were installed near the airplane's center of gravity (CG). However, since an airborne airplane has freedom of motion in six degrees, the accelerations measured at the CG may not be representative of those experienced elsewhere in the airplane. Accelerations in the cockpit or the aft cabin can be significantly less than or greater than those at the CG, and can differ significantly from each other as well.

ADDITIONAL INFORMATION

CAST, JSAT and JSIT

The Commercial Aviation Safety Team (CAST) was founded in 1998 as a joint industry -government effort to develop and apply an integrated, data-driven strategy to reduce the commercial accident fatality rate, and to improve commercial aviation safety in the United States. The CAST processes consisted of two sequential phases; analysis and implementation. The Joint Safety Analysis Team (JSAT) identified the top safety areas by the analysis of accident and incident data, produced "problem statements" which identified specific safety deficiencies, and identified and prioritized intervention strategies for use by the follow-on efforts of the Joint Safety Implementation Team (JSIT). The JSITs developed the means to implement the specified safety improvements through various equipment, process, operational and regulatory changes.

Several JSAT/JSIT teams, each assigned to a specific subject matter area, were formed; the Turbulence JSAT/JSIT focused on cabin safety issues related to turbulence. The Turbulence JSAT produced a total of 21 problem statements, and identified 18 safety enhancements for implementation.

The Turbulence JSAT developed the following three flight-crew-related problem statements and definitions:
No. 601: Flight Crew - Failure to process and interpret available, relevant data. Definition: Flight crew failure to process and interpret available, relevant data, including decisions arising from Collaborative Decision-Making.

No. 602: Failure to communicate with cabin crew/Passengers. Definition: Failure of flight crew to communicate to cabin crew and/or passengers information about impending turbulence or directions to be seated/restrained.

No. 604: Not adequately prepared for the task. Definition: Flight crew not adequately prepared with briefings, in assessment of weather factors or other information received, or not mentally prepared.

The Turbulence JSIT 's "Detailed Implementation Plan For Cabin Injury Reduction During Turbulence" recommended several air carrier actions, including implementing "a policy and related SOPs for FAs to remain seated during periods of significant turbulence risk including climb and descent' and "a policy and related SOPs permitting FAs to prioritize their duties and cabin service schedules, including the option to be seated ... prior to and during turbulence encounters to minimize unnecessary FA exposure."

FAA Advisory Circular

In 2005, in response to the Turbulence JSIT, the FAA issued Advisory Circular (AC) 120-88 ("Preventing Injuries Caused By Turbulence"), and issued a revision to the AC in 2006. The stated purpose of the AC was to provide "information and practices that can be used to prevent injuries caused by turbulence," and it was "intended for implementation in the operations and training of air carrier flight crews, flight attendants, aircraft dispatchers, and managers." The AC highlighted "the data-driven methods of the FAA and its government and industry partners in identifying practices known to be effective against injuries caused by turbulence."

Paragraph 11 of AC 120-88 provided recommended dispatch procedures, including:

"a. Keep communication channels open full-time.
Dispatchers can communicate with flightcrews, and flightcrews can communicate with dispatchers, before, during, and after a flight, and can be encouraged to do so whenever necessary...Communication should be encouraged by an air carrier's management to improve the flow of real-time information regarding turbulence.

b. Weather Briefings.
Preflight weather briefings... must include forecasts of turbulence and pilot reports of turbulence ....

c. Real-time Information Sharing.
During a flight, the pilot and dispatcher must communicate any changes in the forecast or actual turbulence conditions ... in order to pass real-time turbulence information along to other flights."

Appendix 1 of the AC presented standard terminology for turbulence, excerpted from the FAA Aeronautical Information Manual. The terminology and definitions were very similar to those in the operator's FAM and FOM, and included:

"Light Chop - Slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude.

Moderate Turbulence - Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed. Occupants feel definite strain against seatbelts. Unsecured objects are dislodged. Food service and walking are difficult.

Severe Turbulence - Large, abrupt changes in altitude and/or attitude. Usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Occupants are forced violently against seatbelts. Unsecured objects are tossed about. Food service and walking are impossible."

Operator Guidance Regarding Turbulence

The operator provided crew guidance in two separate documents, one each for the cabin crew (Flight Attendant Manual; FAM) and for the flight crew (Flight Operations Manual; FOM). The guidance was consistent across the two documents.

Section 10.0 (page EMS-37) of the then-current FAM was entitled "Turbulence," and stated that "crew communication and coordination is essential during turbulent and bumpy" conditions, and that "if turbulence is anticipated, the Captain will turn on the 'Fasten Seatbelt' sign and may request that Flight Attendants sit in jumpseats and fasten their seatbelts." The FAM stated that during light turbulence, "normal cabin duties may be accomplished." It also stated that "if moderate or severe turbulence is anticipated...Flight Attendants are to secure the cabin and galleys (stow carts) and check customer compliance," and that "If instructed to do so, Flight Attendants are to take their seats and fasten their seat belts." Section 10.3 (Levels of Turbulence), listed three separate turbulence levels (Light, Moderate, and Severe) and provided descriptive text to enable categorization of the turbulence levels based upon their effects in the cabin. The FAM defined severe turbulence as that in which "unsecured objects are tossed about."

The FOM required that the flight crew provide a preflight briefing to the cabin crew prior to each flight. The stated purpose of the briefing was to "pass pertinent information from the flight deck crew to the cabin crew in a timely manner." If turbulence was anticipated or expected, the flight crew was required to provide that information, as well as its "recommendations or instructions on conduct of cabin service relative to weather conditions," to the cabin crew. The FOM provided for the option of a "standard crew briefing," which was to be used when only limited time for the briefing was available. If turbulence was not expected, or not expected to adversely affect cabin service, the flight crew could use the phrase "standard briefing," and forego a specific mention of turbulence. The FOM also stated that the flight crew was to communicate with the cabin crew during flight, if the expected or encountered turbulence warranted that the cabin crew should be or remain seated. The FOM did not require the flight crew to provide the cabin crew with an estimate of the time available before the onset of any anticipated turbulence.

The JSAT substantiating data revealed that turbulence was a principal cause of flight attendant injuries, and the JSAT/JSIT demonstrated that emphasis on cabin service can adversely affect cabin safety. Comparison of the FAM and FOM contents with the information in the Turbulence JSIT and JSAT reports, and AC 120-88, revealed that the operator's guidance incorporated some, but not all, of the recent industry and FAA material regarding turbulence hazard mitigation. The operator's procedures placed the bulk of the communication and decision-making responsibility with the flight crew, while the recent mitigation strategies emphasized a more mutual approach, with a strong emphasis on inter-crew communication. In its accident submission to the NTSB, the Association of Flight Attendants (AFA, which represented the cabin crew) recommended that the FAM be modified to provide greater autonomy for the flight attendants when it came to decisions about when to provide service, when to remain in or re-take their seats, and task prioritization in the event of turbulence. The decreased emphasis on cabin service, as advocated by the AFA, is congruent with the JSIT recommended actions.

No records or statements that indicated that a preflight briefing was conducted, or that any specific preflight information regarding turbulence was provided by the flight crew to the cabin crew, were provided to the investigation. During the flight, the flight crew did not advise or instruct the cabin crew to remain seated, nor did they advise the cabin crew of any anticipated turbulence.

Cabin and Flight Crew Statements

The lead flight attendant filed a "Cabin Service Report" with the operator, which included a narrative of the turbulence encounter. The narrative stated that "there was only light turbulence" prior to the event, and that "about 5 minutes" after the attendants had moved the cabin service cart to the aft end of the airplane, the airplane "hit severe clear-air turbulence" which lasted for "15-20" seconds. An incident report signed by the injured flight attendant stated that "during service jet hit turbulence," that the "service cart and [flight attendant] came off the ground," and that the cart struck the flight attendant's right leg in the "medial shin area."

The captain's written account of the event stated that the airplane was in instrument meteorological conditions when it "encountered unexpected and unforeseen moderate turbulence for a few seconds." The first officer provided a more detailed account. He stated that the airplane was "in the clouds... since takeoff," and that neither air traffic control (ATC) nor any aircraft preceding the event airplane reported any "adverse weather." He also noted that "no adverse weather [was] depicted on the onboard weather radar," but he did not specify the range or tilt settings in use at the time. He stated that captain was the pilot flying, and that the seatbelt sign was on, but that it "likely" would have been off if the airplane was in cruise flight at the time. The first officer stated that he "did not know what caused" the turbulence, but offered that the airplane might "have flown over a dissipating cell below our altitude that was not detectable" by the ATC or airplane radar systems.

None of the cabin or flight crew statements indicated whether there was a pre-flight briefing, or whether the potential for in-flight turbulence was discussed.

Operator's Dispatch Department Information

The flight dispatch department was responsible for providing the flight crew with appropriate flight planning information, and for monitoring the progress of the flight. Per CFR 121.601, before and during the flight, the department "shall provide the pilot in command any additional available information of meteorological conditions...that may affect the safety of the flight."

The hardcopy document of weather information that was provided to the flight crew by the operator's dispatch department was issued at 1356, about two hours before the turbulence encounter. The document included surface observations, forecasts, and notices to airmen for the departure, enroute, and destination airports. It also included pilot reports and wind and temperature aloft forecast data for the route, as well as AIRMET and convective SIGMET information. The weather section included convective SIGMET 37E, which was only valid until 1 hour before the planned departure time. The document did not include convective SIGMET 43E (issued 1 hour prior to the planned departure time, and which overlaid the flight track and turbulence encounter location) or SIGMET 46E (issued 6 minutes after the actual departure time, and which also overlaid the turbulence encounter location).

There was no indication that convective SIGMET 43E, or the pertinent information it contained, was issued to the flight crew. No weather-related documents, products, or advisories, other than the original dispatch document, were provided to the crew by the operator. In addition, there were no indications that the crew received updated weather-related advisories from other sources.

Post Event Modifications by the Operator

In late March 2010, the operator's Manager of In-Flight Standards issued a "Must Read" interoffice memo addressed to all the operator's flight attendants. The stated subject was "In-Flight Turbulence." The first sentence of the memo stated that "Although turbulence...is a routine occurrence, Flight Attendants must always be mindful of their safety..." The remainder of the memo discussed the general cabin crew procedures for expected or encountered turbulence, and the contents were consistent with the guidance provided in the FAM.

According to the manager of the dispatch training program, as a result of the lessons learned from the turbulence event, the operator initiated implementation of two modifications to the information provided to flight crews by dispatch. Final implementation of those changes was still pending as of February 2011.

One planned modification was to provide flight crews with quantitative forecast turbulence levels associated with specific navigational fixes. The operator purchased a subscription for forecast turbulence data from a commercial weather services vendor. The intensity of the forecast turbulence was to be divided into seven levels, with each level assigned a numerical value from "1" to "7." The flight planning system will be modified to incorporate the forecast turbulence data (numeric intensity value, geographic location and valid time period), and output it on the flight crew dispatch documents, with a forecast turbulence level for each navigation fix in the flight plan.

The other planned modification was the incorporation of graphical weather information into the dispatch paperwork. AIRMET and SIGMET data were output in graphical format and overlaid on the charts that depicted the planned flight route, to enable crewmembers to readily discern the intersection or relative proximity of the two. Flight crew procedures will be changed, and crews will be advised to contact dispatch for an expanded briefing if the route of flight intersects any AIRMET or SIGMET volume.