The Pratt & Whitney F135 engine of a U.S. Air Force F-35A caught on fire during start, severely damaging the aircraft, due to a tailwind, says the Air Force Incident Investigation Board’s (AIB) report released this week. The pilot of the aircraft received some burns during egress and damage to the aircraft is expected to exceed $17 million. According to the report, “The mishap was caused by a tailwind blowing hot air from either the mishap aircraft’s Integrated Power Pack (IPP) exhaust or the mishap aircraft’s engine exhaust into the IPP inlet. The hot air entering the IPP inlet started a sequence of events ultimately ending in an uncontained engine fire.”
The IPP on the F-35 functions as a combined auxiliary and emergency power unit. Power from the IPP is used to start the main engine. When hot exhaust gases entered the IPP’s inlet, an automatic shutdown of the IPP was triggered. Shutdown of the IPP cut power to the F135’s engine starter, just prior to the engine reaching a self-sustaining speed. The AIB concluded that “since engine combustion had already began, an increasing amount of fuel was delivered to the engine in an effort to increase combustion and overcome the slowing acceleration.” However, since the engine was turning below a self-sustaining speed, the additional fuel, rather than increasing engine speed, resulted in a fire that spread beyond the combustion section of the turbine. The tailwind, in addition to causing in the original IPP fault, then spread the flames across the aircraft.
Based on the local weather at the time of the incident, the tailwind component where the mishap aircraft was parked is estimated to have been 30 knots. There was a note in the pilot checklist stating that “issues could occur” starting with a tailwind, but no maximum tailwind limit during start had been established.
