Post-Season Maintenance On The Two Airplanes Involved Found Significant Evidence Of Corrosion On The Internal Tank Structure

Location: Missoula, Montana Incident Number: ENG24LA012
Date & Time: March 1, 2024, 09:00 Local Registration: N474NA (A1); N296EA (A2)
Aircraft: British Aerospace BAE 146 Series 200A (A1); McDonnell Douglas DC-9-87 (A2)
Aircraft Damage: Minor (A1); Minor (A2) Defining Event: Miscellaneous/other Injuries: N/A (A1); N/A (A2)
Flight Conducted Under: Public aircraft (A1); Public aircraft (A2)

Analysis: 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 phosphatebased 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 (IOFE) during the 2023 fire season with two large  air tankers using Fortress FR-200 (MgClbased) 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.

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this incident to be -- 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.

FMI: www.ntsb.gov