US investigators are still trying to understand why the UPS Boeing MD-11F involved in the fatal take-off crash at Louisville failed to climb away after shedding its left-hand engine.
After the powerplant separated from the wing, the inquiry says, flight-data recorder information reveals “minor perturbations” in the low-pressure and high-pressure spool speeds of the centre engine.
Its low-pressure rotor speed mainly varied from about 110-112% of N1, dipping momentarily to 100% a couple of times, while the high-pressure speed was around 105-110% of N2 during the aircraft’s brief flight.
According to the recorder data, the right-hand engine’s speeds were steadier, at around 110-112% of N1 and 107-110% of N2.
The National Transportation Safety Board, in an update on its inquiry into the 4 November accident, says the take-off roll was normal until 17:13:11 local time when the left-hand engine’s parameters were no longer reliable and a fire sensor for the engine was triggered.
Flight-recorder information shows the throttle resolver angle – which is a measure of the position of the thrust levers – increased for the centre and right-hand engines.
While the engines’ rotor speeds appear to have remained at typical take-off thrust levels, and the aircraft maintained a nose-up pitch of around 10°, the radio-altimeter data indicates the MD-11F only reached a height of some 25ft before striking buildings and crashing into an industrial zone, about 20s after the fire indication.
Certification criteria for commercial aircraft require the ability to climb away after failure of a single engine during take-off.
“[Flight-data recorder] and other engineering data will be incorporated into the [inquiry’s] airplane performance study to understand why the airplane’s altitude did not substantially increase after separation of the left pylon and engine,” says the safety board.
It adds that the performance study will explore the effects on aircraft-handling qualities resulting from the engine separation.
