BRITISH AEROSPACE BAE 146 SERIES 200A

Full Narrative

HISTORY OF FLIGHTOn March 11, 2024, the United States Department of Agriculture Forest Service (USFS) informed the NTSB about corrosion found on two large airtankers during post-season maintenance activities. The first airplane, Tanker 02, a British Aerospace BAE-146-200A, was owned, operated, and maintained by Neptune Aviation Services, Inc. of Missoula, Montana. The second airplane, Tanker 106, a McDonnell-Douglas DC-9-87 (MD-87), was owned, operated, and maintained by Aero Air of Hillsboro, Oregon. Both airplanes were operated as public aircraft firefighting flights when dropping retardant.
The two airplanes were under contract to the USFS for the 2023 fire season and part of integration-operational field examination (I-OFE) of a new fire retardant, Fortress FR-200, manufactured by Fortress North America, LLC. The I-OFE was developed to examine the effects of integrating two retardants with different retardant salts into the airtanker systems and determine the product suitability during field use. The airplanes used the Fortress product for the first part of the season but alternated between the Fortress product and another retardant, Phos-Chek MVP-Fx, manufactured by Perimeter Solutions, for the second part of the season.
The USFS requested assistance from the NTSB to examine the airplanes. Following the examination, the USFS requested that the NTSB investigate the incident due to the potential for more serious corrosive effects on airtankers that could lead to an accident. AIRCRAFT INFORMATIONTanker 02, the BAE 146-200A, had been used as a large airtanker under contract to the USFS since 2017. Before the 2023 fire season, the airplane had undergone a full inspection and cleaning by the operator and a full inspection and approval by the USFS. There was pink staining of some of the internal tank structure from previous operations, but no corrosion was noted. Tanker 02 delivered only FR-200 during the first part of the season through August 4, 2023. Between August 16 and November 21, Tanker 02 delivered 22 loads of retardant that started with FR-200, ended with MVP-Fx, and alternated between the two each time.
Tanker 106, the MD-87, was converted to a large airtanker in early 2023 and was operating in its first season as an airtanker during the I-OFE. The retardant tank on the airplane was new, and the airplane had been inspected by the operator in accordance with a heavy check and undergone a full inspection and approval by the USFS before the season. After installation of the tank, the fuselage and vertical stabilizer were completely stripped and repainted, and the wings and horizontal stabilizers were sanded and repainted. Tanker 106 delivered only FR-200 during the first part of the season, through August 9, 2023. Between August 15 and August 17, Tanker 106 delivered 4 loads of FR-200 and 3 loads of MVP-Fx that started and ended with FR-200 and alternated between the two each time. Between August 25 and October 11, Tanker 106 delivered 12 loads of FR-200 with no comingling. Between October 30 and November 14, Tanker 106 delivered 6 loads of FR-200 and 5 loads of MVP-Fx that started with FR-200, ended with MVP-Fx, and alternated between the two each time. AIRPORT INFORMATIONTanker 02, the BAE 146-200A, had been used as a large airtanker under contract to the USFS since 2017. Before the 2023 fire season, the airplane had undergone a full inspection and cleaning by the operator and a full inspection and approval by the USFS. There was pink staining of some of the internal tank structure from previous operations, but no corrosion was noted. Tanker 02 delivered only FR-200 during the first part of the season through August 4, 2023. Between August 16 and November 21, Tanker 02 delivered 22 loads of retardant that started with FR-200, ended with MVP-Fx, and alternated between the two each time.
Tanker 106, the MD-87, was converted to a large airtanker in early 2023 and was operating in its first season as an airtanker during the I-OFE. The retardant tank on the airplane was new, and the airplane had been inspected by the operator in accordance with a heavy check and undergone a full inspection and approval by the USFS before the season. After installation of the tank, the fuselage and vertical stabilizer were completely stripped and repainted, and the wings and horizontal stabilizers were sanded and repainted. Tanker 106 delivered only FR-200 during the first part of the season, through August 9, 2023. Between August 15 and August 17, Tanker 106 delivered 4 loads of FR-200 and 3 loads of MVP-Fx that started and ended with FR-200 and alternated between the two each time. Between August 25 and October 11, Tanker 106 delivered 12 loads of FR-200 with no comingling. Between October 30 and November 14, Tanker 106 delivered 6 loads of FR-200 and 5 loads of MVP-Fx that started with FR-200, ended with MVP-Fx, and alternated between the two each time. DAMAGE TO AIRCRAFTExamination of Tanker 02 revealed significant signs of corrosion to the internal tank structure where the retardant was carried. Most of the internal tank structure was stained pink or purple, and the corrosion was more prevalent in areas adjacent to fasteners or where there were joints. The aft vent door and spar assembly were retained for further examination due to the presence of significant corrosion on the 6061-T651 aluminum alloy vent door and on the 7075-T6 aluminum alloy spar where the stainless-steel hinge was attached. There was bubbled paint and evidence of corrosion around many of the steel Hi-Lok fasteners with aluminum collars that fastened the aft bulkhead of the tank. The corrosion products and several fasteners were retained for examination.
Examination of Tanker 106 revealed significant signs of corrosion to the internal tank and external airplane structure. There was much more evidence of corrosion noted to the airplane structure exterior to the tank than on Tanker 02. The identified exterior corrosion was limited to the lower part of the airplane aft of the tank exit, with many individual locations identified. The most significant structural item that had corrosion was the right-wing forward spar lower cap doubler. The lower cap doubler spans from the side of body to the wing tip and is considered primary structure by the manufacturer. An area near the inboard end of the doubler, about 4 inches long, coincident with the inboard end of the slat, had evidence of exfoliation corrosion that affected most of the thickness of the doubler. A section of the 2024-T-351 aluminum alloy doubler was retained for further examination. Many of the tank pod door arms had staining, dried retardant, blistered paint, and areas of corrosion adjacent to the location where bushings were normally installed. One of the 2024-T3 pod door arms was retained for further examination. Several of the brake cooling ducts were removed due to the presence of a white powdery material adhered to the inner surfaces. A section of one of the 6061-T4 brake cooling ducts was retained for further examination. A small section of the 2014-T351 aluminum alloy lower right aft cargo door skin that had evidence of corrosion was retained for further examination. There was retardant staining and corrosion noted to the main landing gear (MLG) doors on both sides. The operator reported that the hinge pins on both MLG main doors were very difficult to remove, unlike their normal experience with the doors. After removal of the MLG main doors there was corrosion noted to the hinge halves and the pins. The forward section of the 17-4 PH stainless steel right MLG hinge and a section of the 17-4 PH stainless steel hinge pin were retained for further examination. The forward end of the right tank door had corrosion at the corners of the seal grooves and some areas of pitting on the inner surface that is normally in contact with the retardant when the tank is full. There were numerous small, hard, crystalized deposits stuck to the inner surface of the tank doors. A section of the 7075-T6 aluminum alloy right tank door was cut from the door and retained for further examination.
The retained items were examined by specialists at the NIST laboratory in Boulder, Colorado, under the direction of the investigation team. NIST also examined each of the retardants, FR-200 and MVP-Fx, alone and comingled using Nuclear Magnetic Resonance Spectroscopy to establish the chemical nature of each retardant and any interactions between them.
Aero Air discovered corrosion on three of the Pratt & Whitney JT8D-219 engines that were installed on Tanker 106 during the I-OFE. The most severe corrosion was found on the right (#2) engine that was installed for the duration of the I-OFE. Lesser amounts of corrosion were found on the two different left (#1) engines that were installed during the I-OFE. The three engines were shipped to Aviation Turbine Repair in Miami, FL, for teardown inspection. The staff of the teardown facility indicated that they typically do not see the level of corrosion, and in some cases see no corrosion, on the parts examined.
The inspection of the right engine that had accrued 132.5 hours revealed corrosion that affected the cases, spacers, disks, blades, and shafts. A total of 61 parts and modules were rejected by the facility due to excessive corrosion or contamination that had blocked cooling holes in the gas path, restricting airflow.
The inspection of the left engine that had accrued 116.1 hours revealed corrosion that affected the turbine cases, disks, and shafts. A total of 53 parts and modules were rejected by the facility due to excessive corrosion or contamination that had blocked cooling holes in the gas path, restricting airflow.
The inspection of the left engine that had accrued 16.4 hours revealed chemical residue and light corrosion through the stages of the low-pressure compressor and high- and low-pressure turbines. After cleaning, no parts or modules were rejected by the facility and the cooling holes in the gas path were free from contamination. TESTS AND RESEARCHAerially applied LTFRs have been in use by the USFS for many decades. Beginning in 2007, the USFS specification for LTFRs stated that only one or a combination of monoammonium phosphate (MAP), diammonium phosphate (DAP), or ammonium polyphosphate (APP) could be used as the retardant salt. Since 2006, Perimeter Solutions has been the sole source supplier of LTFRs to the USFS using MAP, DAP, and/or APP as the retardant salt. In January 2020, the USFS updated Specification 5100-304d, Long-Term Retardant, Wildland Firefighting, to allow for LTFRs using retardant salts other than phosphate-based products. The first new retardants, submitted for qualification by Fortress North America, used magnesium chloride (MgCl) as the retardant salt.
The USFS specification includes the physical properties, performance, testing, environmental, and delivery requirements for any product to be included on the USFS Qualified Products List (QPL) as a LTFR. All testing and qualification of retardants is performed by the USFS National Technology and Development Program (NTDP) in Missoula, Montana. Only uniform corrosion testing is required under the specification. For the fixed-wing airtankers, test coupons of 2024-T3 Aluminum alloy, 4130 Chromoly Steel alloy, UNS C26000 yellow brass, and Az31B Magnesium are fully and/or partially submerged in the retardant for 90 days at 70°F and 120°F and left undisturbed for uniform corrosion testing. The test coupons are measured and weighed before the test and then weighed again after the test to calculate a corrosion rate that must fall within the limits of the specification. The test coupons are examined for intergranular corrosion, and no intergranular corrosion is allowed under the specification.
As part of the qualification process for LTFRs, “the Government shall determine the need for an operational field evaluation.” The purpose of the operational field evaluation is to determine product suitability during field use after completion of laboratory testing.
Fortress FR-100, a MgCl-based dry powder concentrate, was tested by NTDP per the specification and received conditional qualification in May 2021. The USFS initiated an OFE in June 2021 to ensure the product was acceptable under actual firefighting conditions. During the OFE, airplanes were allowed to load and deliver Fortress North America FR-100 (MgCl-based) and Perimeter Solutions Phos-Chek LC95 (APP-based). Operators indicated that mixing of the two retardants resulted in a thick, congealed, gel-like substance that adhered to the tank structure when wet and a thick, clay-like substance when dry. The substance interfered with the operation of float sensors, valves, flow meters, and seals and required mechanical means for removal. Corrosion of actuators and a breakdown of surface coatings were also reported. The USFS test report suggested that third-party testing of all aircraft metals to an aeronautical standard and continued monitoring of corrosive properties over time was needed to better understand the corrosion. Additionally, more research was needed to understand the congealing when comingling retardants; wash procedures also needed updating. The report stated “it is imperative that these two products do not come in contact with one another.”
The NTDP initiated a study examining the compatibility of retardants on the Qualified Products List (QPL) after the FR-100 OFE. The study examined Fortress FR-100 and FR-200 (both MgCl-based) and Perimeter MVP-Fx, LCE20-Fx, and LC95 (DAP, MAP, and APP-based, respectively). Uniform corrosion testing of 2024-T3 aluminum coupons per the USFS specification showed that corrosion rates for all combinations was within the tolerances allowed. There was some intergranular corrosion identified with some individual combinations, which necessitated further study. The study concluded that there was a “significant incompatibility” between the two Fortress products and the Perimeter LC95-series APP-based products and recommended that these products not be comingled in any capacity. Further study of the retardants by agency partners and stakeholders was recommended in the report.
The USFS initiated an OFE at the end of the 2021 fire season to evaluate any operational limitations with the Fortress FR-200 retardant after it had passed the laboratory phase of testing under the specification. The OFE was performed using only single engine air tanker (SEAT) airplanes that were equipped with a fiberglass retardant tank. The OFE continued into the 2022 fire season and was completed in August 2022. The product met all requirements to receive full qualification and was added to the QPL in December 2022. Aircraft inspections during and after the OFE had no significant findings. The USFS report noted that ongoing work could necessitate operational changes for safe operation with existing equipment and systems. The report stated, “Product isolation to certain tanks and systems is currently the safest option for operation due to the list of unknowns at the time of this reports’ findings.”
In August 2022, the USFS met with the large airtanker operators to discuss their concerns with the Fortress retardants after the FR-100 and FR-200 OFEs. The operators were concerned with the precipitate reaction that occurred when mixing retardants, the possibility of increased corrosion from the precipitate, the magnesium chloride-based retardants and their corrosive effects on aircraft structure, and that the existing testing performed by NTDP did not include the various alloys found on air tankers. They suggested that the testing specifications should be updated to be more representative of actual conditions. The report of the meeting identified the following recommendations from the operators:

· Delay any further field evaluations until there are proper tests to ensure the safety of flight crews.
· Solicit the help of air tanker contractors, other entities or agencies to assist in developing appropriate tests and running the products through them prior to putting a new product inside an air tanker.
· Require comingled product tests prior to conditional qualification.
· Consider how new products will interface with air tankers in real life scenarios and the impact it could have on the safety of flight.
· Additional testing for impacts of MgCl and struvite on aircraft metals.
· Follow-up with ATBs (airtanker boards) on airports that restrict MgCl use.
The NTDP performed a study of the effects of comingling on the operation of the level sensors used in some of the retardant tanks between December 2022 and July 2023. The study concluded there was significant residue buildup in the level sensors for the tests that alternated between a phosphate-based retardant and a MgCl-based retardant. There were differences in the two tests that alternated between MVP-Fx and FR-200 that indicated the presence of primer coating on the level sensor made it less susceptible to residue buildup and made it easier to clean. The comingled LCE20-Fx and FR-200 resulted in significant residue buildup and required more aggressive cleaning procedures. The comingled LC95A-R and FR-200 resulted in severe residue buildup and adverse effects to the operation of the sensor that required very aggressive cleaning procedures.
The USFS requested information from suppliers, operators, and subject matter experts on potential risks associated with the handling and use of LTFRs in operation, the effects of comingling LTFRs, and risk mitigation strategies for the identified risks through the issuance of a Special Notice in December 2022. Many of the respondents identified the comingling hazard with mitigation strategies ranging from more thorough and complete washing of aircraft between products, to avoiding any comingling, and to eliminating the use of magnesium chloride-based retardants. The corrosion hazard was also identified by some respondents with mitigation strategies that included more complete testing involving all materials used in aircraft construction, testing of joints, built-up structure, and components common to aircraft, testing representative of real-world conditions, and more frequent washing of aircraft.
NTDP initiated some broad galvanic corrosion studies based on the preliminary investigation findings. The first study examined the effect on aluminum and steel test coupons galvanically coupled and submersed in retardant and seawater under the guidance of an existing American Society of Testing and Materials (ASTM) procedure. The study used test coupons of 2024-T3 and 6061-T6 aluminum alloys electrically coupled to 4130 steel alloy test coupons. The uniform corrosion rates for all coupons were within the limits in the specification. However, the pitting corrosion was more prevalent on all 2024 and 6061 aluminum coupons that were coupled with 4130 steel coupons and immersed in FR-200 or seawater as compared to the coupons immersed in MVP-Fx and LC95A-F. NTDP concluded that additional testing with more replicates was needed to establish statistical significance and solution cycling testing was needed to better match real world conditions. The second test was similar to the first but involved additional replicates of each galvanic couple that were only tested with MVP-Fx and FR-200. The results were substantially like those observed in the first test, with all uniform corrosion rates within the specification limits. In general, it was noted that the aluminum coupons exhibited higher uniform corrosion rates in FR-200 and the steel coupons exhibited higher corrosion rates in the MVP-Fx validating that a galvanic reaction was occurring. A third test was performed with galvanically coupled test coupons partially submerged in retardants comingled at a 97%/3% volume ratio. The results were mostly consistent with the first two studies and the uniform corrosion rates were all within the specification.
The USFS contracted with outside companies to perform additional corrosion testing based on the preliminary investigation findings. At the time of this report, the testing began but no results were available. The immersion test plan called for evaluating 20 different materials typically used in aircraft construction that would be immersed in retardant A for 48 hours, removed and allowed to air dry for 24 hours, and then immersed in retardant B for 48 hours, removed and allowed to dry for 24 hours. The process would be repeated 10 more times before the coupons were examined. The wicking test plan called for evaluating eight different materials typically used in aircraft construction fastened together with eight different common aircraft fasteners. The built-up test coupons would be immersed in a single retardant for six days, removed and allowed to air dry for one day, immersed in the same retardant for six days, removed and allowed to air dry for one day, and photographed. This process would be repeated fourteen more times before the test coupons were examined. The modified salt fog test plan called for evaluating five different aluminum alloys that would be exposed to an alternating spray/dry of one retardant (or the control salt) for a period of seven days before being examined.
Finally, NTDP performed additional galvanic corrosion testing at the request of the investigation team using identical coupon couples as before and retardants comingled in a 50%/50% volume ratio. Measurement of the pH of the individual and comingled retardants showed the comingled retardants had the lowest pH (highest acidity) of all which could accelerate the corrosive processes. The results for the galvanically coupled coupons immersed in the comingled mixture of FR-200 and MVP-Fx showed all the 2024 and 6061 aluminum test coupons exhibited moderate to severe pitting corrosion that significantly exceeded the specification.
HISTORY OF FLIGHTOn March 11, 2024, the United States Department of Agriculture Forest Service (USFS) informed the NTSB about corrosion found on two large airtankers during post-season maintenance activities. The first airplane, Tanker 02, a British Aerospace BAE-146-200A, was owned, operated, and maintained by Neptune Aviation Services, Inc. of Missoula, Montana. The second airplane, Tanker 106, a McDonnell-Douglas DC-9-87 (MD-87), was owned, operated, and maintained by Aero Air of Hillsboro, Oregon. Both airplanes were operated as public aircraft firefighting flights when dropping retardant.
The two airplanes were under contract to the USFS for the 2023 fire season and part of integration-operational field examination (I-OFE) of a new fire retardant, Fortress FR-200, manufactured by Fortress North America, LLC. The I-OFE was developed to examine the effects of integrating two retardants with different retardant salts into the airtanker systems and determine the product suitability during field use. The airplanes used the Fortress product for the first part of the season but alternated between the Fortress product and another retardant, Phos-Chek MVP-Fx, manufactured by Perimeter Solutions, for the second part of the season.
The USFS requested assistance from the NTSB to examine the airplanes. Following the examination, the USFS requested that the NTSB investigate the incident due to the potential for more serious corrosive effects on airtankers that could lead to an accident. AIRCRAFT INFORMATIONTanker 02, the BAE 146-200A, had been used as a large airtanker under contract to the USFS since 2017. Before the 2023 fire season, the airplane had undergone a full inspection and cleaning by the operator and a full inspection and approval by the USFS. There was pink staining of some of the internal tank structure from previous operations, but no corrosion was noted. Tanker 02 delivered only FR-200 during the first part of the season through August 4, 2023. Between August 16 and November 21, Tanker 02 delivered 22 loads of retardant that started with FR-200, ended with MVP-Fx, and alternated between the two each time.
Tanker 106, the MD-87, was converted to a large airtanker in early 2023 and was operating in its first season as an airtanker during the I-OFE. The retardant tank on the airplane was new, and the airplane had been inspected by the operator in accordance with a heavy check and undergone a full inspection and approval by the USFS before the season. After installation of the tank, the fuselage and vertical stabilizer were completely stripped and repainted, and the wings and horizontal stabilizers were sanded and repainted. Tanker 106 delivered only FR-200 during the first part of the season, through August 9, 2023. Between August 15 and August 17, Tanker 106 delivered 4 loads of FR-200 and 3 loads of MVP-Fx that started and ended with FR-200 and alternated between the two each time. Between August 25 and October 11, Tanker 106 delivered 12 loads of FR-200 with no comingling. Between October 30 and November 14, Tanker 106 delivered 6 loads of FR-200 and 5 loads of MVP-Fx that started with FR-200, ended with MVP-Fx, and alternated between the two each time. AIRPORT INFORMATIONTanker 02, the BAE 146-200A, had been used as a large airtanker under contract to the USFS since 2017. Before the 2023 fire season, the airplane had undergone a full inspection and cleaning by the operator and a full inspection and approval by the USFS. There was pink staining of some of the internal tank structure from previous operations, but no corrosion was noted. Tanker 02 delivered only FR-200 during the first part of the season through August 4, 2023. Between August 16 and November 21, Tanker 02 delivered 22 loads of retardant that started with FR-200, ended with MVP-Fx, and alternated between the two each time.
Tanker 106, the MD-87, was converted to a large airtanker in early 2023 and was operating in its first season as an airtanker during the I-OFE. The retardant tank on the airplane was new, and the airplane had been inspected by the operator in accordance with a heavy check and undergone a full inspection and approval by the USFS before the season. After installation of the tank, the fuselage and vertical stabilizer were completely stripped and repainted, and the wings and horizontal stabilizers were sanded and repainted. Tanker 106 delivered only FR-200 during the first part of the season, through August 9, 2023. Between August 15 and August 17, Tanker 106 delivered 4 loads of FR-200 and 3 loads of MVP-Fx that started and ended with FR-200 and alternated between the two each time. Between August 25 and October 11, Tanker 106 delivered 12 loads of FR-200 with no comingling. Between October 30 and November 14, Tanker 106 delivered 6 loads of FR-200 and 5 loads of MVP-Fx that started with FR-200, ended with MVP-Fx, and alternated between the two each time. DAMAGE TO AIRCRAFTExamination of Tanker 02 revealed significant signs of corrosion to the internal tank structure where the retardant was carried. Most of the internal tank structure was stained pink or purple, and the corrosion was more prevalent in areas adjacent to fasteners or where there were joints. The aft vent door and spar assembly were retained for further examination due to the presence of significant corrosion on the 6061-T651 aluminum alloy vent door and on the 7075-T6 aluminum alloy spar where the stainless-steel hinge was attached. There was bubbled paint and evidence of corrosion around many of the steel Hi-Lok fasteners with aluminum collars that fastened the aft bulkhead of the tank. The corrosion products and several fasteners were retained for examination.
Examination of Tanker 106 revealed significant signs of corrosion to the internal tank and external airplane structure. There was much more evidence of corrosion noted to the airplane structure exterior to the tank than on Tanker 02. The identified exterior corrosion was limited to the lower part of the airplane aft of the tank exit, with many individual locations identified. The most significant structural item that had corrosion was the right-wing forward spar lower cap doubler. The lower cap doubler spans from the side of body to the wing tip and is considered primary structure by the manufacturer. An area near the inboard end of the doubler, about 4 inches long, coincident with the inboard end of the slat, had evidence of exfoliation corrosion that affected most of the thickness of the doubler. A section of the 2024-T-351 aluminum alloy doubler was retained for further examination. Many of the tank pod door arms had staining, dried retardant, blistered paint, and areas of corrosion adjacent to the location where bushings were normally installed. One of the 2024-T3 pod door arms was retained for further examination. Several of the brake cooling ducts were removed due to the presence of a white powdery material adhered to the inner surfaces. A section of one of the 6061-T4 brake cooling ducts was retained for further examination. A small section of the 2014-T351 aluminum alloy lower right aft cargo door skin that had evidence of corrosion was retained for further examination. There was retardant staining and corrosion noted to the main landing gear (MLG) doors on both sides. The operator reported that the hinge pins on both MLG main doors were very difficult to remove, unlike their normal experience with the doors. After removal of the MLG main doors there was corrosion noted to the hinge halves and the pins. The forward section of the 17-4 PH stainless steel right MLG hinge and a section of the 17-4 PH stainless steel hinge pin were retained for further examination. The forward end of the right tank door had corrosion at the corners of the seal grooves and some areas of pitting on the inner surface that is normally in contact with the retardant when the tank is full. There were numerous small, hard, crystalized deposits stuck to the inner surface of the tank doors. A section of the 7075-T6 aluminum alloy right tank door was cut from the door and retained for further examination.
The retained items were examined by specialists at the NIST laboratory in Boulder, Colorado, under the direction of the investigation team. NIST also examined each of the retardants, FR-200 and MVP-Fx, alone and comingled using Nuclear Magnetic Resonance Spectroscopy to establish the chemical nature of each retardant and any interactions between them.
Aero Air discovered corrosion on three of the Pratt & Whitney JT8D-219 engines that were installed on Tanker 106 during the I-OFE. The most severe corrosion was found on the right (#2) engine that was installed for the duration of the I-OFE. Lesser amounts of corrosion were found on the two different left (#1) engines that were installed during the I-OFE. The three engines were shipped to Aviation Turbine Repair in Miami, FL, for teardown inspection. The staff of the teardown facility indicated that they typically do not see the level of corrosion, and in some cases see no corrosion, on the parts examined.
The inspection of the right engine that had accrued 132.5 hours revealed corrosion that affected the cases, spacers, disks, blades, and shafts. A total of 61 parts and modules were rejected by the facility due to excessive corrosion or contamination that had blocked cooling holes in the gas path, restricting airflow.
The inspection of the left engine that had accrued 116.1 hours revealed corrosion that affected the turbine cases, disks, and shafts. A total of 53 parts and modules were rejected by the facility due to excessive corrosion or contamination that had blocked cooling holes in the gas path, restricting airflow.
The inspection of the left engine that had accrued 16.4 hours revealed chemical residue and light corrosion through the stages of the low-pressure compressor and high- and low-pressure turbines. After cleaning, no parts or modules were rejected by the facility and the cooling holes in the gas path were free from contamination. TESTS AND RESEARCHAerially applied LTFRs have been in use by the USFS for many decades. Beginning in 2007, the USFS specification for LTFRs stated that only one or a combination of monoammonium phosphate (MAP), diammonium phosphate (DAP), or ammonium polyphosphate (APP) could be used as the retardant salt. Since 2006, Perimeter Solutions has been the sole source supplier of LTFRs to the USFS using MAP, DAP, and/or APP as the retardant salt. In January 2020, the USFS updated Specification 5100-304d, Long-Term Retardant, Wildland Firefighting, to allow for LTFRs using retardant salts other than phosphate-based products. The first new retardants, submitted for qualification by Fortress North America, used magnesium chloride (MgCl) as the retardant salt.
The USFS specification includes the physical properties, performance, testing, environmental, and delivery requirements for any product to be included on the USFS Qualified Products List (QPL) as a LTFR. All testing and qualification of retardants is performed by the USFS National Technology and Development Program (NTDP) in Missoula, Montana. Only uniform corrosion testing is required under the specification. For the fixed-wing airtankers, test coupons of 2024-T3 Aluminum alloy, 4130 Chromoly Steel alloy, UNS C26000 yellow brass, and Az31B Magnesium are fully and/or partially submerged in the retardant for 90 days at 70°F and 120°F and left undisturbed for uniform corrosion testing. The test coupons are measured and weighed before the test and then weighed again after the test to calculate a corrosion rate that must fall within the limits of the specification. The test coupons are examined for intergranular corrosion, and no intergranular corrosion is allowed under the specification.
As part of the qualification process for LTFRs, “the Government shall determine the need for an operational field evaluation.” The purpose of the operational field evaluation is to determine product suitability during field use after completion of laboratory testing.
Fortress FR-100, a MgCl-based dry powder concentrate, was tested by NTDP per the specification and received conditional qualification in May 2021. The USFS initiated an OFE in June 2021 to ensure the product was acceptable under actual firefighting conditions. During the OFE, airplanes were allowed to load and deliver Fortress North America FR-100 (MgCl-based) and Perimeter Solutions Phos-Chek LC95 (APP-based). Operators indicated that mixing of the two retardants resulted in a thick, congealed, gel-like substance that adhered to the tank structure when wet and a thick, clay-like substance when dry. The substance interfered with the operation of float sensors, valves, flow meters, and seals and required mechanical means for removal. Corrosion of actuators and a breakdown of surface coatings were also reported. The USFS test report suggested that third-party testing of all aircraft metals to an aeronautical standard and continued monitoring of corrosive properties over time was needed to better understand the corrosion. Additionally, more research was needed to understand the congealing when comingling retardants; wash procedures also needed updating. The report stated “it is imperative that these two products do not come in contact with one another.”
The NTDP initiated a study examining the compatibility of retardants on the Qualified Products List (QPL) after the FR-100 OFE. The study examined Fortress FR-100 and FR-200 (both MgCl-based) and Perimeter MVP-Fx, LCE20-Fx, and LC95 (DAP, MAP, and APP-based, respectively). Uniform corrosion testing of 2024-T3 aluminum coupons per the USFS specification showed that corrosion rates for all combinations was within the tolerances allowed. There was some intergranular corrosion identified with some individual combinations, which necessitated further study. The study concluded that there was a “significant incompatibility” between the two Fortress products and the Perimeter LC95-series APP-based products and recommended that these products not be comingled in any capacity. Further study of the retardants by agency partners and stakeholders was recommended in the report.
The USFS initiated an OFE at the end of the 2021 fire season to evaluate any operational limitations with the Fortress FR-200 retardant after it had passed the laboratory phase of testing under the specification. The OFE was performed using only single engine air tanker (SEAT) airplanes that were equipped with a fiberglass retardant tank. The OFE continued into the 2022 fire season and was completed in August 2022. The product met all requirements to receive full qualification and was added to the QPL in December 2022. Aircraft inspections during and after the OFE had no significant findings. The USFS report noted that ongoing work could necessitate operational changes for safe operation with existing equipment and systems. The report stated, “Product isolation to certain tanks and systems is currently the safest option for operation due to the list of unknowns at the time of this reports’ findings.”
In August 2022, the USFS met with the large airtanker operators to discuss their concerns with the Fortress retardants after the FR-100 and FR-200 OFEs. The operators were concerned with the precipitate reaction that occurred when mixing retardants, the possibility of increased corrosion from the precipitate, the magnesium chloride-based retardants and their corrosive effects on aircraft structure, and that the existing testing performed by NTDP did not include the various alloys found on air tankers. They suggested that the testing specifications should be updated to be more representative of actual conditions. The report of the meeting identified the following recommendations from the operators:

· Delay any further field evaluations until there are proper tests to ensure the safety of flight crews.
· Solicit the help of air tanker contractors, other entities or agencies to assist in developing appropriate tests and running the products through them prior to putting a new product inside an air tanker.
· Require comingled product tests prior to conditional qualification.
· Consider how new products will interface with air tankers in real life scenarios and the impact it could have on the safety of flight.
· Additional testing for impacts of MgCl and struvite on aircraft metals.
· Follow-up with ATBs (airtanker boards) on airports that restrict MgCl use.
The NTDP performed a study of the effects of comingling on the operation of the level sensors used in some of the retardant tanks between December 2022 and July 2023. The study concluded there was significant residue buildup in the level sensors for the tests that alternated between a phosphate-based retardant and a MgCl-based retardant. There were differences in the two tests that alternated between MVP-Fx and FR-200 that indicated the presence of primer coating on the level sensor made it less susceptible to residue buildup and made it easier to clean. The comingled LCE20-Fx and FR-200 resulted in significant residue buildup and required more aggressive cleaning procedures. The comingled LC95A-R and FR-200 resulted in severe residue buildup and adverse effects to the operation of the sensor that required very aggressive cleaning procedures.
The USFS requested information from suppliers, operators, and subject matter experts on potential risks associated with the handling and use of LTFRs in operation, the effects of comingling LTFRs, and risk mitigation strategies for the identified risks through the issuance of a Special Notice in December 2022. Many of the respondents identified the comingling hazard with mitigation strategies ranging from more thorough and complete washing of aircraft between products, to avoiding any comingling, and to eliminating the use of magnesium chloride-based retardants. The corrosion hazard was also identified by some respondents with mitigation strategies that included more complete testing involving all materials used in aircraft construction, testing of joints, built-up structure, and components common to aircraft, testing representative of real-world conditions, and more frequent washing of aircraft.
NTDP initiated some broad galvanic corrosion studies based on the preliminary investigation findings. The first study examined the effect on aluminum and steel test coupons galvanically coupled and submersed in retardant and seawater under the guidance of an existing American Society of Testing and Materials (ASTM) procedure. The study used test coupons of 2024-T3 and 6061-T6 aluminum alloys electrically coupled to 4130 steel alloy test coupons. The uniform corrosion rates for all coupons were within the limits in the specification. However, the pitting corrosion was more prevalent on all 2024 and 6061 aluminum coupons that were coupled with 4130 steel coupons and immersed in FR-200 or seawater as compared to the coupons immersed in MVP-Fx and LC95A-F. NTDP concluded that additional testing with more replicates was needed to establish statistical significance and solution cycling testing was needed to better match real world conditions. The second test was similar to the first but involved additional replicates of each galvanic couple that were only tested with MVP-Fx and FR-200. The results were substantially like those observed in the first test, with all uniform corrosion rates within the specification limits. In general, it was noted that the aluminum coupons exhibited higher uniform corrosion rates in FR-200 and the steel coupons exhibited higher corrosion rates in the MVP-Fx validating that a galvanic reaction was occurring. A third test was performed with galvanically coupled test coupons partially submerged in retardants comingled at a 97%/3% volume ratio. The results were mostly consistent with the first two studies and the uniform corrosion rates were all within the specification.
The USFS contracted with outside companies to perform additional corrosion testing based on the preliminary investigation findings. At the time of this report, the testing began but no results were available. The immersion test plan called for evaluating 20 different materials typically used in aircraft construction that would be immersed in retardant A for 48 hours, removed and allowed to air dry for 24 hours, and then immersed in retardant B for 48 hours, removed and allowed to dry for 24 hours. The process would be repeated 10 more times before the coupons were examined. The wicking test plan called for evaluating eight different materials typically used in aircraft construction fastened together with eight different common aircraft fasteners. The built-up test coupons would be immersed in a single retardant for six days, removed and allowed to air dry for one day, immersed in the same retardant for six days, removed and allowed to air dry for one day, and photographed. This process would be repeated fourteen more times before the test coupons were examined. The modified salt fog test plan called for evaluating five different aluminum alloys that would be exposed to an alternating spray/dry of one retardant (or the control salt) for a period of seven days before being examined.
Finally, NTDP performed additional galvanic corrosion testing at the request of the investigation team using identical coupon couples as before and retardants comingled in a 50%/50% volume ratio. Measurement of the pH of the individual and comingled retardants showed the comingled retardants had the lowest pH (highest acidity) of all which could accelerate the corrosive processes. The results for the galvanically coupled coupons immersed in the comingled mixture of FR-200 and MVP-Fx showed all the 2024 and 6061 aluminum test coupons exhibited moderate to severe pitting corrosion that significantly exceeded the specification.