History of Flight
On March 11, 2024, the United States Department of Agriculture Forest Service (USFS) informed the NTSB about corrosion discovered on two large airtankers during post-season maintenance. The aircraft, Tanker 02 (a British Aerospace BAE-146-200A) and Tanker 106 (a McDonnell-Douglas DC-9-87, or MD-87), were operated by Neptune Aviation Services and Aero Air, respectively, under contract for the 2023 fire season. Both were part of an integration-operational field examination (I-OFE) of a new fire retardant, Fortress FR-200, manufactured by Fortress North America. During the season, the aircraft alternated between Fortress FR-200 and another retardant, Phos-Chek MVP-Fx.
Aircraft Information
Tanker 02 had been in service as an airtanker since 2017. Before the 2023 season, it received a full inspection and cleaning; no corrosion was noted, though pink staining from previous operations was present. It delivered only FR-200 until August 4, 2023, then alternated between FR-200 and MVP-Fx for 22 loads from August 16 to November 21. Tanker 106 was converted to an airtanker in early 2023 and operated its first season during the I-OFE. Its retardant tank was new, and the aircraft was inspected and approved by the USFS. It delivered only FR-200 until August 9, 2023, then had alternating loads, followed by 12 loads of FR-200 alone, and finally six alternating loads.
Damage to Aircraft
Examination of Tanker 02 revealed significant corrosion to the internal tank structure, primarily where retardant was carried. Most internal surfaces were stained pink or purple, with corrosion more prevalent near fasteners and joints. The aft vent door and spar assembly were retained due to severe corrosion on the aluminum vent door and spar. Bubbled paint and corrosion were found around steel Hi-Lok fasteners with aluminum collars on the aft bulkhead.
Tanker 106 exhibited corrosion on both internal tank and external structure, more extensively than Tanker 02. External corrosion was limited to the lower aft area of the airframe. The right-wing forward spar lower cap doubler, a primary structural element, showed exfoliation corrosion over a 4-inch section near the inboard end. Other affected components included tank pod door arms, brake cooling ducts, cargo door skin, main landing gear doors and hinges, and tank door surfaces. Numerous crystalized deposits were stuck to the tank door interiors. Several items were retained for further examination.
Aero Air also discovered corrosion on three Pratt & Whitney JT8D-219 engines installed on Tanker 106. The right engine (132.5 hours) had extensive corrosion on cases, spacers, disks, blades, and shafts, leading to rejection of 61 parts due to excessive corrosion or blocked cooling holes. One left engine (116.1 hours) had 53 rejected parts, while another left engine (16.4 hours) showed only light corrosion after cleaning, with no parts rejected.
Tests and Research
Aerially applied long-term fire retardants (LTFRs) have been used for decades. Since 2007, the USFS specification allowed only phosphate-based salts, but a 2020 update permitted other salts. Fortress North America introduced magnesium chloride (MgCl)-based retardants. The qualification process includes uniform corrosion testing of metal coupons submerged for 90 days. During an early operational field evaluation (OFE) of Fortress FR-100 (MgCl-based), comingling with phosphate-based retardants produced a gel-like substance that interfered with equipment and caused corrosion and coating breakdown. The USFS report recommended avoiding contact between the two products.
Subsequent testing by the USFS National Technology and Development Program (NTDP) found significant incompatibility between MgCl-based Fortress products and APP-based Perimeter products, recommending no comingling. An OFE of FR-200 using fiberglass tanks showed no significant findings, and the product was fully qualified in December 2022. However, operators expressed concerns about precipitate reactions, corrosion, and inadequate testing. In 2022-2023, NTDP studied level sensor buildup; alternating between phosphate- and MgCl-based retardants caused significant residue. Galvanic corrosion studies found that aluminum coupons coupled with steel showed more pitting in FR-200 than in phosphate-based retardants. Additional testing was underway at the time of the report.