A misunderstanding of how an autopilot would respond to manual input may have a contributed to the fatal crash of a Cessna in 2015, according to an ATSB investigation report released today.
The pilot of Cessna 172 VH-ZEW was on a training flight from RMIT Flight Training at Point Cook on 8 September 2015 when it crashed into high ground east of Ballarat, Victoria. The pilot was on her first navigation exercise and did not survive the impact.
"The site and wreckage inspection identified that the aircraft impacted terrain in a level, slight right-wing low attitude," the report states. "That indicated that the pilot likely stopped the aircraft’s descent and started to initiate a manoeuvre to avoid the terrain. It is likely that the pilot manually manipulated the controls while the autopilot was on and engaged in a vertical mode.
"As a consequence, the autopilot re-trimmed the aircraft against pilot inputs, inducing a nose-down mistrim situation, which led to a rapid descent."
The aircraft was approaching the 2200-foot summit of Black Mount at the time, which reduced the time the pilot had to switch off the autopilot and recover from the nose-down pitch.
Data recovered from the G1000 systems fitted to the aircraft showed that the pilot had engaged several modes during the accident flight, and that the autopilot mode had been changed six times in the last 30 seconds of recorded data. It also showed that at one stage the pitch angle of the aircraft was higher than the pitch command given to the autopilot.
The ATSB also found that there was nothing in the aircraft manuals that specifically warned that the trim would operate against any manual inputs.
"There was no advice, limitation, or warning in the aircraft pilot operating handbook or avionics manual to indicate that if a force is applied to control column while the autopilot is engaged, that the aircraft’s autopilot system will trim against the control column force, and possibly lead to a significant out of trim situation," the ATSB report states.
"Training requirements for autopilot systems was rudimentary at the recreational pilot licence (RPL) level due to stipulated operational limitations for its use. At the time of the accident there was no regulatory requirement for pilots to demonstrate autopilot competency at the RPL level."
Garmin responded to the ATSB by saying it didn't consider a warning necessary because it was "common knowledge" that pilots shouldn't try to apply manual control inputs whilst the autopilot is on. Regardless, the ATSB has recommended that all cautions and warnings should be documented because of the inexperience of pilots being confronted with autopilots.
The full report is on the ATSB website.