Aviation safety investigations & reports

Pitch trim runaway and partial loss of control involving Pilatus PC-12/47E, VH-OWJ, near Merredin, Western Australia, on 14 April 2019

Investigation number:
AO-2019-019
Status: Completed
Investigation completed
Phase: Final report: Dissemination Read more information on this investigation phase

Final Report

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What happened

On 14 April 2019, the pilot of a Pilatus PC-12/47E aircraft, registered VH-OWJ and operated by Royal Flying Doctor Service - Western Operations (RFDS), was conducting a medical transport flight under instrument flight rules from Merredin to Jandakot within Western Australia. A RFDS aeromedical crew consisting of a flight nurse and doctor were on board with a non-critical patient who was being transferred to a hospital in Perth. For the midnight departure, there were almost clear skies with minimal ambient and celestial lighting.

About 1.5 minutes after take-off, ‘Pitch Trim Runaway’ warnings activated and the pitch trim continued to move nose-down without any pilot or autopilot inputs. The pilot initiated the applicable emergency procedure but inadvertently selected the Flap Interrupt switch rather than the Trim Interrupt switch. Consequently (before the next checklist item was actioned), the pitch trim continued to runaway until it reached full nose-down with associated serious control difficulties.

The pilot did not identify the mis-selection and continued to address the emergency procedure without resolving the full out-of-trim condition. With the assistance of the doctor seated in row 2, the pilot managed to return to Merredin for a flapless landing. The aircraft was undamaged and the occupants uninjured.

What the ATSB found

The ATSB found that the pitch trim runaway occurred because of a malfunctioning relay in the manual (main pilot-engaged) stabiliser trim system.

As the (uninterrupted) 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 kts with increased risk of descent into terrain before the pilot reduced engine torque and airspeed to partially alleviate the control loads and arrest the descent.

After the pilot addressed items 2 and 3 of the emergency procedure, the malfunction was neutralised and the alternate stabiliser trim system was available to adjust the trim. However, the pilot did not identify those positive conditions and continued with items 4 to 8 of the procedure, which disabled the alternate stabiliser trim system, prevented pitch trim adjustment and prolonged the serious control difficulties.

The similarities between the Trim Interrupt and Flap Interrupt switches and the proximal location of the two switches, unnecessarily increased the risk of mis-selection and contributed to the excessive out-of-trim condition.

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. If the pilot selects the Trim Interrupt switch early in the sequence and does not need to adjust the pitch trim, the risk is not significant. In this incident, the lack of effective guidance and systems information probably had an adverse influence on the pilot’s capability to resolve the uninterrupted trim runaway condition and was a critical factor.

As a factor that increased risk, the effectiveness of RFDS training and checking processes for pitch trim runaway was undermined by incomplete systems knowledge and unrealistic practice exercises associated with training/checking in the aircraft (non-simulator).

What's been done as a result

Pilatus advised that a design change, to reduce the likelihood of a trim runaway, was developed before the occurrence to replace the mechanical pitch trim relays with solid-state relays but was not fully implemented due to limited parts availability. Both applicable service bulletins have now been published.

Pilatus also advised that the probability of erroneous activation of the Flap Interrupt switch instead of the Trim Interrupt switch has been reduced by the publication and active distribution of a Safety Information Letter (SIL-003) to all customers, operators and service centres. This includes a reminder of procedures when encountering a trim runaway condition.

The ATSB acknowledge these positive safety actions but notes that the Trim interrupt and Flap Interrupt switches on the PC-12 do remain identical and co-located, and there is potential for engineering controls to eliminate the mis-selection of the interrupt switches.

RFDS investigated the occurrence and implemented safety action such as increasing pilot awareness about the pitch trim systems and enhancements to their related training and checking processes.

Safety message

The ATSB advises operators of PC-12 aircraft to review their training/checking processes related to the pitch trim system to ensure that pilots are adequately prepared to manage a runaway emergency. More generally, operators and pilots are advised to enhance awareness of expected system behaviour from switch and other control selections.

For flight control emergencies such as out-of-trim conditions, there is an imperative to maintain control while resolving the technical problem. A critical factor for pilots to consider is control of airspeed and associated engine power. 

Operators are encouraged to submit reports of PC-12 pitch trim defects to the Defect Reporting Service to facilitate the Civil Aviation Safety Authority’s monitoring of continuing airworthiness data.

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The occurrence

Context

Safety analysis

Findings

Safety issues and actions

Pilot details

Sources and submissions

Appendices

Safety Issue

Go to AO-2019-019-SI-01 -

Pilatus PC-12 trim and flap interrupt switches

The similarities between the Trim Interrupt and Flap Interrupt switches and the proximal location of the two switches unnecessarily increased the risk of mis-selection and contributed to the excessive out-of-trim condition.

Safety issue details
Issue number: AO-2019-019-SI-01
Who it affects: Operators and pilots of PC-12 aircraft
Status: Partially addressed
General details
Date: 14 April 2019   Investigation status: Completed  
Time: 0010 AWST   Investigation level: Defined - click for an explanation of investigation levels  
Location   (show map): 4 km west of Merredin   Investigation phase: Final report: Dissemination  
State: Western Australia   Occurrence type: Loss of control  
Release date: 13 May 2020   Occurrence category: Serious Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer Pilatus Aircraft Ltd  
Aircraft model PC-12/47E  
Aircraft registration VH-OWJ  
Serial number 1411  
Operator Royal Flying Doctor Service of Australia (Western Operations)  
Type of operation Medical Transport  
Sector Turboprop  
Damage to aircraft Nil  
Departure point Merredin, Western Australia  
Destination Jandakot, Western Australia  
Last update 13 May 2020