Serious incident Pilatus PC-12/45 HB-FPC, Sunday 29 April 2018
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Date:Sunday 29 April 2018
Type:Silhouette image of generic PC12 model; specific model in this crash may look slightly different    
Pilatus PC-12/45
Registration: HB-FPC
MSN: 422
Year of manufacture:2001
Fatalities:Fatalities: 0 / Occupants: 3
Other fatalities:0
Aircraft damage: None
Category:Serious incident
Location:7.5 NM SSW of Zürich Airport (LSZH ) -   Switzerland
Phase: En route
Departure airport:Praha-Václav Havel Airport (PRG/LKPR)
Destination airport:Buochs Airport (BXO/LSZC)
Investigating agency: STSB
Confidence Rating: Accident investigation report completed and information captured
On April 29, 2018 at 14:06 UTC the flight crew took off on a private flight from Prague (LKPR) to Buochs (LSZC) with the Pilatus PC-12. There was one passenger on board. After an uneventful flight the aircraft was in descent at 15:27 UTC over the Dübendorf airfield at FL75. At 15:27:38 UTC the flight crew informed the air traffic controller of Zurich Departure that they were ready to end the flight according to instrument flight rules. At 15:27:43 UTC, the ATC issued the appropriate clearance.
Further on, at 15:28:29 UTC, the ATC gave the release to the flight crew of HB-FPC for a free choice of route and the change to the frequency of the Buochs airfield.
In this phase the aircraft suddenly moved around the vertical axis first to the left and then to the right. The flight crew assumed that this was an uncontrolled and automatic control input of the rudder trim runaway, whereupon the pilot, according to his statement, switched off the autopilot via the disconnect switch on the control column. The pilot pressed the trim interrupt rocker switch located on the central pedestal. This action corresponded to the first item on the corresponding checklist for emergency and abnormal cases (Quick Reference Handbook - QRH). At the same time, the pilot operated the adjacent flap interrupt rocker switch.
At 15:29:25 UTC the flight crew informed the air traffic controller of Zurich Departure that they had a problem with the aircraft trim. The air traffic controller then asked if they needed assistance. The flight crew answered yes and said that they could only fly right turns due to problems with the aircraft trim. When the ATCO asked if they could maintain the current altitude or if they wanted to continue descending, the flight crew answered "standby". According to the pilot, the altitude could be maintained well, but maintaining a heading was extremely difficult. The pilot further testified that he had seen Zurich Airport's runway 10 at the bottom right. He said he was almost on the opposite approach (downwind) to runway 10 and therefore decided to land on this runway.
At 15:30:05 UTC the flight crew therefore asked the air traffic controller if they could land on runway 10 in Zurich: "H-PC is it possible for 10? The air traffic controller replied if this meant a visual approach to runway 10. The flight crew replied that they needed an immediate approach to runway 10: "We need to go to runway 10, H-PC, right now". The air traffic controller immediately gave the go-ahead for a visual approach to runway 10, and at 15:30:24 UTC, the pilot instructed the pilot to make the following emergency call: "H-PC mayday, mayday, mayday, we need runway 10", which was confirmed by the air traffic controller. According to the statement of the two pilots, they concentrated on the upcoming landing and refrained from further processing the "trim runaway" checklist. At 15:31:23 UTC the controller informed the flight crew that HB-FPC was about 2 NM from runway threshold 10 and asked if it would reach the airport: "[...] can you make it? The flight crew said yes and at the same time requested landing clearance, which was given by the controller, indicating the prevailing wind. The landing was uneventful.

The yaw servo runaway, which caused a rudder deflection to the right, led to an uncoordinated flight condition around the vertical axis. As a consequence, the yaw servo gave a constant control signal to the rudder trim actuator to the right. Switching off the autopilot by means of the disconnect switch on the control wheel also released the clutch of the faulty yaw servo. This disconnected the yaw servo from the rudder and the corresponding trim signal to the rudder trim actuator, which controls the rudder trim tab, was lost. The pilot now manually controlled an aircraft which was trimmed to the right around the vertical axis and manually trimmed around all three aircraft axes. However, by pressing the trim interrupt switch, the pilot deactivated all trim systems and thus prevented manual trimming. By pressing the flap interrupt switch he also prevented the flaps from being extended.

Accident investigation:
Investigating agency: STSB
Report number: 
Status: Investigation completed
Duration: 2 years and 4 months
Download report: Final report



Revision history:

02-Sep-2020 19:40 harro Added

Corrections or additions? ... Edit this accident description

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