ENG24LA012

The use of a new long term fire retardant with a chemical composition incompatible with the retardant previously in use for the integration-operational field evaluation without adequate testing and qualification.

The United States Department of Agriculture Forest Service (USFS) had been using long term fire retardants (LTFRs) for many decades with no significant corrosion issues noted on their contract aircraft. Since at least 2006, the USFS had a single source supplier for LTFRs that provided retardants with monoammonium phosphate (MAP), diammonium phosphate (DAP), and/or ammonium polyphosphate (APP) used as the retardant salt. Beginning in 2020, the USFS allowed LTFRs to be submitted for qualification that contained retardant salts different from the historic phosphate-based retardants. The USFS granted conditional qualification to Fortress FR-100, a magnesium chloride-based (MgCl) dry powder concentrate, and Fortress FR-200, a MgCl-based liquid powder concentrate in May 2021 and August 2021, respectively, after they successfully completed uniform corrosion testing in accordance with USFS Specification 5100-304d, Long-Term Retardant, Wildland Firefighting at the USFS National Technology and Development Program (NTDP). The specified corrosion testing, which only examined the corrosive effects of a single retardant on one aluminum alloy, one steel alloy, one brass alloy, and one magnesium alloy, had been unchanged for many years. Beginning in June 2021, the first operational field evaluation (OFE) of FR-100 (MgCl-based) retardant began to show there was an incompatibility between MgCl-based and phosphate-based retardants when they were comingled in service. Increased corrosion was present on some aircraft components, and subsequent laboratory testing by NTDP confirmed the incompatibility between certain retardants. Test reports, USFS documents, and information received from operators and vendors suggested that the use of any new retardant should be delayed until further corrosion testing, more appropriate comingling studies, and updated operational procedures were completed. Additional testing by NTDP completed in July 2023 showed that alternating between phosphate-based and MgCl based retardants caused significant residue buildup in the level sensors used in the retardant tanks. Despite the evidence of incompatibilities between the retardants and without performing additional corrosion testing, the USFS decided to initiate an integration operational field evaluation (I-OFE) during the 2023 fire season with two large air tankers using Fortress FR-200 (MgCl-based) and Perimeter Solutions MVP-Fx (MAP and DAP-based) retardants. During the first part of the season both airtankers carried only FR-200 retardant then both airtankers alternated between FR-200 and MVP-Fx retardants for the second part of the season. At the conclusion of the 2023 fire season, post-season maintenance on the two airplanes involved found significant evidence of corrosion on the internal tank structure and components as well as the external airplane structure. Several items from each airplane were examined by the National Institute of Standards and Technology (NIST) lab under the direction of the investigation. For some of the items, NIST concluded that there was galvanic corrosion occurring where the steel and aluminum alloy components were coupled electrically in the presence of liquid retardants. For other items, NIST concluded that the corrosion was the result of a breakdown in material surface protection treatments in the presence of the liquid or dried retardants. NIST also examined the chemical interaction of the retardants and concluded that there is a likely interaction related to the nitrogen-based proprietary corrosion inhibitors present in the different retardants. Preliminary studies by NTDP based on the investigation findings validated the galvanic corrosion action between certain steel and aluminum alloys in the presence of a single liquid retardant, either FR-200 or MVP-Fx. NTDP also performed additional studies at the request of the investigation examining the galvanic corrosion action in comingled retardants that showed the comingled products resulted in more severe corrosion on certain alloys. The more severe galvanic corrosion in comingled retardants is likely due to the more acidic mixture that results from comingling. NTDP has initiated several corrosion studies to further examine corrosive mechanisms on additional alloys and built-up structures in the presence of LTFRs.

Tanker 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. On 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. Examination 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 t