Know how your aircraft’s avionics and autopilot will react

The ATSB is highlighting the consequences of pilots not fully understanding how their aircraft’s avionics and autopilot systems operate.

Accident site of Cessna 172S, VH-ZEW, near Millbrook, Victoria

The investigation into the fatal accident of a Cessna 172S has resulted in the Bureau issuing safety recommendations to the aircraft and autopilot manufacturers about providing limitations, cautions and warnings for autopilot systems and audible pitch trim movement.

The accident occurred on 8 September 2015, when a Cessna 172S was conducting a solo navigation training flight from Point Cook Airfield in Victoria. GPS data later showed the aircraft was on the third leg of the planned journey, cruising at about 3,000 ft above mean sea level, when it started to descend rapidly. The aircraft impacted rising terrain at about 2,200 ft and was destroyed. The pilot, who was the sole occupant, was fatally injured.

The investigation found that it was likely the pilot had 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’s low operating height above the ground, along with rising terrain in front of the aircraft, would have given the pilot limited time to diagnose, react, and recover before impact. The investigation indicates that the pilot may have been unaware of the exact properties of the autopilot.

ATSB Executive Director, Mr Nat Nagy, said that this accident highlighted a matter of serious concern.

“...if automation is not performing as expected, then the safest option under most circumstances is for them to disengage the system...”

“It is now common for general aviation aircraft to be equipped with advanced avionics and autopilot systems,” Mr Nagy said. “And while these systems can be very useful, it is vitally important that pilots understand how the systems will react in different circumstances.”

After the accident, the flight training organisation conducted flight testing and has emphasised to their students the hazard of manually manipulating the flight controls with the autopilot engaged. They have also updated their operations manual to include warnings about the operation and function of the autopilot system - warnings that are absent in the manufacturer’s documentation.

“This is knowledge that all pilots need to possess,” Mr Nagy said. “Pilots should also be aware that if automation is not performing as expected, then the safest option under most circumstances is for them to disengage the system and fly the aircraft manually.”

The ATSB issued recommendations to the aircraft and autopilot manufacturers, calling for them to provide limitation, cautions, and warnings for autopilots and audible pitch trim movement.

Read the final report AO-2015-105: Collision with terrain involving Cessna Aircraft Company 172S, VH-ZEW, at Millbrook, Victoria on 8 September 2015.

Last update 02 May 2018