British Aerospace BAE 146 SERIES 200A and Mcdonnell Douglas DC-9-87 incident near Missoula, Montana, March 1, 2024
On March 1, 2024 at about 4:00 pm local time, 2 aircraft, British Aerospace BAE 146 SERIES 200A (N474NA) and Mcdonnell Douglas DC-9-87 (N296EA), were involved in the same incident near Missoula, Montana (Missoula Montana airport). No one was hurt. The weather was conditions the NTSB did not record.
The NTSB's probable cause their words, unchanged
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 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.
Source: NTSB aviation accident database, copy made October 12, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- March 1, 2024 · about 4:00 pm local time · cases in 2024
- Place
- Missoula, Montana · Missoula Montana · cases near this airport · map
- Type
- Incident
- Injuries
- No one was hurt.
- Weather
- conditions the NTSB did not record
- Aircraft 1
- British Aerospace BAE 146 SERIES 200A, built 1987 · all BAE 146 SERIES 200As on the register
- Registration
- N474NA · registry record · serial E2084
- Damage
- Minor damage
- Flight
- Aerial application (crop spraying) flight · public-use (government) rules
- Aircraft 2
- Mcdonnell Douglas DC-9-87, built 1991 · all DC-9-87s on the register
- Registration
- N296EA · registry record · serial 53212
- Damage
- Minor damage
- Flight
- Firefighting flight · public-use (government) rules
The NTSB's narrative for the British Aerospace BAE 146 SERIES 200A final · quoted from the NTSB record
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.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The NTSB's narrative for the Mcdonnell Douglas DC-9-87 final · quoted from the NTSB record
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.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record for the British Aerospace BAE 146 SERIES 200A from the NTSB's investigation tables, in plain English
What happened, in order
- Miscellaneous/other during standing defining event
- Aircraft inspection event during standing
The NTSB's findings
- Organizational issues › Development › Selection/certification/testing › (general) › Other government
- Aircraft › Aircraft structures › Fuselage › Bulkheads (main fuselage) › Fatigue/wear/corrosion other cases naming the airframe structure
The aircraft
- Airframe total time: 34,458.1 hours
- Last inspection: approved inspection programme, February 22, 2024
- Maximum gross weight: 91,200 lb
- Seats: 2
- Landing gear: retractable
- Engine 1: Honeywell ALF-502R-5 (turbojet); 10,469 hours total
- Engine 2: Honeywell ALF-502R-5 (turbojet); 31,860 hours total
- Engine 3: Honeywell ALF-502R-5 (turbojet); 40,734 hours total
- Engine 4: Honeywell ALF-502R-5 (turbojet); 27,622 hours total
- Operator: Neptune Aviation Services INC
The flight
- A second pilot was aboard
Weather at the time
- Light: not reported
- Temperature: 0°F (-18°C), dew point 0°F (-18°C)
The factual record for the Mcdonnell Douglas DC-9-87 from the NTSB's investigation tables, in plain English
What happened, in order
- Aircraft inspection event during standing
- Miscellaneous/other during standing
The NTSB's findings
- Organizational issues › Development › Selection/certification/testing › (general) › Other government
- Aircraft › Aircraft structures › Fuselage › Plates/skins (aux fuselage) › Fatigue/wear/corrosion other cases naming the airframe structure
- Aircraft › Aircraft structures › Wing structure › Spar (on wing) › Fatigue/wear/corrosion other cases naming the airframe structure
The aircraft
- Airframe total time: 35,874.3 hours
- Last inspection: continuous airworthiness programme, December 16, 2022
- Maximum gross weight: 132,000 lb
- Seats: 3
- Landing gear: retractable
- Engine 1: P & W JT8D-219 (turbofan); 63,564 hours total
- Engine 2: P & W JT8D-219 (turbofan); 34,386 hours total
- Operator: Aero Air LLC
The flight
- A second pilot was aboard
Documents from the investigation the NTSB's docket: the evidence folder behind the report
41 documents, released by the NTSB on November 20, 2025. View them here, or download them; the NTSB redacts some personal details before release, and this site shows the NTSB's own titles rather than its file names.
The same docket at the NTSB · documents without a copy here are fetched from the NTSB when you open them.
Connections how this case links to the rest of the register
Aircraft
- N474NA on the register today · the tail number may have moved to another aircraft since
- British Aerospace BAE 146 15 registered today · the model family and its accident history
- British Aerospace 133 aircraft on the register
- N296EA on the register today · the tail number may have moved to another aircraft since
- Mcdonnell Douglas DC-9 93 registered today · the model family and its accident history
- Mcdonnell Douglas 254 aircraft on the register
- Engine: Honeywell ALF-502R-5 2 aircraft carry this series today · as the NTSB recorded it
- Engine: Honeywell ALF-502R-5 2 aircraft carry this series today · as the NTSB recorded it
Place
- Missoula Montana Airport · nearest airport
- Accidents and incidents in Montana 1,119 since 1982 · 164 in the last ten years
- Montana 6,044 aircraft registered
The whole map of how records connect · these connections as data: /graph/accident/ENG24LA012.json.
The bigger picture in Aviation Analytics: accidents over time since 1982, by model, by phase of flight and conditions, by place and on the map.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text does not name anyone involved, its policy at this time. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.