What happened
During the 2023 fire season, the United States Department of Agriculture Forest Service (USFS) conducted an integration-operational field evaluation (I-OFE) to test the suitability of a new fire retardant, Fortress FR-200, alongside the existing Phos-Chek MVP-Fx. The evaluation involved two large airtankers: Tanker 02, a British Aerospace BAE-146-200A operated by Neptune Aviation Services, and Tanker 106, a McDonnell-Douglas DC-9-87 (MD-87) operated by Aero Air.
During the first part of the season, both aircraft carried only the FR-200 product. However, during the second part of the season, the aircraft alternated between the FR-200 and the MVP-Fx retardant. Following the conclusion of the season, maintenance activities identified significant corrosion on both aircraft. On Tanker 02, corrosion was found on the internal tank structure, specifically near fasteners and joints, including the aft vent door and spar assembly. Tanker 106 exhibited more extensive damage, with corrosion found on both the internal tank and the external airplane structure, including the right-wing forward spar lower cap doubler, brake cooling ducts, and landing gear doors. Additionally, corrosion was discovered on three Pratt & Whitney JT8D-219 engines installed on Tanker 106.
The investigation
The NTSB investigation included examinations by the National Institute of Standards and Technology (NIST) laboratory. NIST analyzed metal components and the retardants themselves using Nuclear Magnetic Resonance Spectroscopy. The investigation also reviewed previous laboratory testing and field evaluations conducted by the USFS National Technology and Development Program (NTDP).
Findings
- The investigation identified galvanic corrosion occurring where steel and aluminum alloy components were electrically coupled in the presence of the liquid retardants.
- Corrosion was also attributed to the breakdown of material surface protection treatments when exposed to the liquid or dried retardants.
- Chemical analysis suggested a likely interaction related to the nitrogen-based proprietary corrosion inhibitors present in the different retardant products.
- Testing of the combined retardants showed that the mixture had a lower pH, indicating higher acidity, which likely accelerated the corrosive processes.
- Previous studies had already indicated that mixing magnesium chloride-based and phosphate-based products could result in a thick, gel-like substance that interferes with aircraft systems.