Bombardier has appealed to the US Federal Aviation Administration for an exemption for its latest Challenger 350 business jets from the need to meet existing regulations on protection against uncontrollable high engine thrust, on the grounds that such an event is “extremely improbable”.
An “uncontrollable high thrust” (UHT) event refers to an incident in which the full authority digital engine control system fails, allowing the thrust, rotational speed (rpm) and internal engine temperatures to increase above the normal upper limits, and then fails to respond to power lever commands to reduce thrust. The only protection is shutting off the fuel supply to the affected engine, and the risk is potential engine mechanical failure: contained or uncontained.
The rule Bombardier is seeking exemption from is 14 CFR 25.901(c), which says: "For each powerplant and auxiliary power unit installation, it must be established that no single failure or malfunction or probable combination of failures will jeopardise the safe operation of the aeroplane except that the failure of structural elements need not be considered if the probability of such failure is extremely remote."
The Bombardier request affects its deliveries of Challenger 350 business jets with Honeywell AS907-2-1A engines. Those 350s with AS907-1-1A engines already have exemption and, as Bombardier’s appeal to the FFA points out, so do certain Airbus, Boeing, Embraer and Gulfstream aircraft. The manufacturer has appealed for an exemption lasting four years – by which time, it promises, a UHT protection system will have been developed, certificated and installed in the entire fleet.
Bombardier argues: "The overall level of safety of the thrust control system of the [latest] Challenger 350 will not be less than that of the current fleet. This fleet has never had an UHT event since its entry into service back in 2003. However, full compliance with 14 CFR 25.901(c) would require introduction of complicated and novel design changes" to the 350. That, says the company, is why it needs time to develop a fix.
The manufacturer argues that UHT is rare – in the world fleet it has only ever caused one accident, and that was not fatal. In September 1997 a Saudi Arabian Airlines Boeing 737-200 with Pratt & Whitney JT8D-15 engines taking off from Nejran on a domestic flight suffered an uncommanded No.2 engine thrust increase and exhaust gas temperature warning. The crew attempted to reduce thrust but there was no response. Take-off was aborted, but the aircraft overran the runway into sand, its gear failed, and the No 2 engine separated.