The ATSB is advising Pilatus PC-12 operators to review their pitch trim system training and checking processes to ensure pilots are adequately prepared to manage a trim runaway emergency.
The advice forms part of an ATSB investigation report into a pitch trim runaway and partial loss of control incident experienced by a Royal Flying Doctor Service (RFDS) Pilatus PC-12 aircraft near Merredin, in Western Australia, in April 2019.
The investigation describes how shortly after midnight and approximately 90 seconds after departing Merredin, the PC-12/47E, with a pilot, flight nurse, doctor and patient on board, was climbing about 1,400 ft above ground level when the Pitch Trim Runaway warnings activated and the aircraft started to nose over.
The pilot initiated the Pitch Trim Runaway emergency procedure from memory but the investigation found that the pilot inadvertently moved the Flap Interrupt switch instead of selecting the Trim Interrupt switch. Both switches are located in close proximity to each other on the PC-12’s centre console, which the investigation found to be a safety issue.
Consequently, the pitch trim runaway continued uninterrupted with full nose-down reached 16 seconds after the warning message activations, with the aircraft pitch attitude moving from +9.5 degrees nose-up down to -7.5 degrees.
As a result of the abnormal trim setting and aircraft airspeed, the nose-down control forces were excessively high. Despite exerting maximum effort with both hands on the control column, the pilot was unable to fully counteract the nose-down force and there were times when the aircraft developed high rates of descent, descending to a minimum height of 400 ft above ground level.
The ATSB advises PC-12 operators to review their training and checking processes related to the pitch trim system to ensure that pilots are adequately prepared to manage a trim runaway emergency.
The report describes how the pilot continued with the emergency procedure from memory but was unable to resolve the full out-of-trim condition and serious control difficulties. With the assistance of the doctor who pulled on the other control column, the pilot was able to return to Merredin for a flap-less landing. The aircraft was undamaged and the occupants uninjured.
ATSB Executive Director Transport Safety Director Nat Nagy said the cause of the pitch trim runaway was a malfunctioning relay in the manual stabiliser trim system. (At the time of the incident Pilatus was progressing the development of a replacement relay for the manual and autopilot stabiliser trim systems.)
“The ATSB advises PC-12 operators to review their training and checking processes for the aircraft’s pitch trim system to ensure that pilots are adequately prepared to manage a trim runaway emergency,” Mr Nagy said.
“The similarities between the Trim Interrupt and Flap Interrupt switches and their co-location unnecessarily continues to increase the risk of mis-selection and contributed to the excessive out-of-trim condition in this incident”, he noted.
In this incident, after selecting the wrong switch, the pilot pulled the Pitch Trim Circuit Breaker, in line with the checklist, but only after the trim had reached the full nose down position so there was no noticeable change in the controllability of the aircraft.
“More broadly this incident also reminds pilots and operators of all aircraft types to understand the expected system behaviour from switch and other control selections.”
Mr Nagy also reinforced that for flight control emergencies such as out-of-trim conditions, maintaining control of the aircraft while resolving technical issues is imperative.
“A critical consideration for pilots during flight control emergencies is the control of airspeed and engine power,” he said.
“In this incident, as the pitch trim runaway progressed, the reinforcing cycle of increasing control loads, forced descent, and increasing airspeed was initially exacerbated by high engine torque. The airspeed reached 210 kt with an increasing risk of descent into terrain before the pilot reduced engine torque and airspeed to partially alleviate control loads and to arrest the descent.”
The ATSB found that the emergency procedures and systems information in the PC-12 Pilot Operating Handbook/Airplane Flight Manual and Quick Reference Handbook did not provide effective guidance or sufficient information for pilots contending with a pitch trim runaway.
The investigation also notes that incomplete systems knowledge and unrealistic practice exercises undermined the effectiveness of RFDS training and checking processes for a pitch trim runaway.
You can find here the investigation report AO-2019-019: Pitch trim runaway and partial loss of control involving Pilatus PC-12/47E, VH-OWJ, near Merredin, Western Australia, on 14 April 2019