NTSB probes trim settings and spatial disorientation in fatal Learjet 55 crash near Philadelphia
Investigators strive to explain why ambulance jet came down just after departure from Philadelphia. US investigators probing the fatal medical Learjet 55 crash in Philadelphia are exploring the significance of trim settings and the effects of spatial orientation as part of an inquiry complicated by the absence of flight-recorder information. The aircraft came down at high speed just 1min after taking off from Northeast Philadelphia airport on 31 January last year. It was not carrying a flight-data recorder, and the cockpit-voice recorder did not capture any information about the accident. While investigators had previously stated that they might extract data from the ground-proximity warning system, this attempt was abandoned owing to damage and the limited nature of the data which might have been obtained. The National Transportation Safety Board has yet to draw any conclusions about the cause of the accident. None of the six occupants survived, and the crash also resulted in a fatality on the ground. From weight and balance figures, and the checklist located in the wreckage, investigators believe the take-off trim should have been set to 5.5°. Examination of the jet's pitch-trim actuator, retrieved from the debris, indicates that it equated to a trim of 3.97°. Inspection of the jackscrew did not reveal evidence of damage or runaway. Since the trim settings were not recorded, the inquiry has not determined whether the trim was correctly or incorrectly set at take-off, or whether it was adjusted during the flight. But using pitch, altitude and speed figures from trajectory analysis of the aircraft's flightpath, Bombardier has calculated separate pilot control-force profiles based on different assumed trim settings. If the 5.5° setting had been used throughout, for example, the calculated pitch at each point of the flight suggests the pilot would have initially applied back-pressure on the control column but transitioned to a push force, and that this force would have steadily increased. A setting of 3.97° would trim the aircraft more nose-down. Pitch trim can be adjusted through a switch on the control column. Trim setting is a particular point of interest given the aircraft's unexplained transition from a climb to a descent after it peaked at about 1,320ft above ground. This occurred about 30s after it entered an overcast cloud base at about 400ft. The cloud, and the fact that the flight took place at night, has highlighted the possibility of spatial disorientation. As it entered the cloud the aircraft was travelling at 191kt and climbing at 2,300ft/min, but the rate of climb began slowing about 15s later, to 400ft/min, and the aircraft started descending some 15s after that. Energy analysis indicates that there was no decrease in engine thrust during the flight. The aircraft's descent rate reached 10,000ft/min. Loss of visual references, particularly in cloud and darkness, can generate somatogravic illusions, because the vestibular system cannot distinguish between the effects of pitch and acceleration. The inquiry has not reached any conclusion as to whether the crew might have experienced such an illusion, and whether this could have influenced forces applied to the control column or possible changes in trim setting. Details of the various analysis efforts – including examination of medical equipment such as oxygen cylinders, interviews with training personnel, and operator risk assessments – have been newly published by the safety board as its investigation continues.