NTSB probes trim settings in fatal Philadelphia Learjet 55 crash

US investigators are still trying to work out what role trim settings may have played in the crash of a medical Learjet 55 that went down shortly after leaving Northeast Philadelphia Airport in January last year. The jet hit the ground at high speed about a minute after takeoff, killing all six people aboard and one person on the ground.

The National Transportation Safety Board has not reached a conclusion on the cause. The lack of a flight-data recorder has complicated the inquiry, and the cockpit-voice recorder captured nothing useful about the accident sequence. Investigators had hoped to pull data from the ground-proximity warning system, but abandoned that plan after finding the unit too damaged to yield anything.

Weight and balance calculations, along with a checklist recovered from the wreckage, indicate the takeoff trim setting should have been 5.5 degrees. But examination of the recovered pitch-trim actuator showed a setting equivalent to 3.97 degrees instead. Inspectors found no sign of damage or runaway on the jackscrew mechanism.

Trim settings were not recorded during the flight, so investigators say they cannot determine whether the trim was set correctly before takeoff or whether it shifted at some point in the brief flight that followed. To help close that gap, manufacturer Bombardier used pitch, altitude and speed data from a trajectory analysis to model the control forces a pilot would have needed under different trim assumptions.

Under one scenario, using a constant 5.5-degree setting, the modeling suggests the pilot would have first pulled back on the controls, then shifted to a forward push later in the flight. The investigation into the cause remains open.