Aviation safety investigations & reports

Unreliable Airspeed Indication involving Pilatus PC-12/47E, VH-OWI, near Albany, Western Australia, on 22 June 2020

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


Download Final Report
[Download  PDF: 1.19MB]

What happened

On the evening of 22 June 2020, a Royal Flying Doctor Service, Pilatus Aircraft Ltd. PC-12/47E registered VH‑OWI was conducting a positioning flight from Jandakot to Albany, Western Australia. Approximately four minutes into the descent to Albany Airport in instrument meteorological conditions the pilot observed an airspeed miscompare indication on the left primary flight display (PFD).

Having assessed that a blocked pitot tube was the likely cause of the issue, the pilot elected to climb the aircraft in an attempt to get clear of cloud. During this climb the pilot’s indicated airspeed increased and exceeded the aircraft’s maximum allowable speed. The pilot reported also receiving an overspeed alert at this time and consequently elected to discontinue the planned flight and return the aircraft to Jandakot.

When unable to obtain visual conditions, the pilot elected to descend the aircraft. During this descent the left indicated airspeed reduced to zero, however, no stall warning was activated. At 6,000 ft visual conditions were obtained, however, the turbulence at this level was severe. At this time the pilot observed a heading miscompare on both the left and right PFDs. Due to the severity of the turbulence at 6,000 ft the pilot climbed the aircraft first to 8,000 ft and then 10,000 ft on the return to Jandakot.

The pilot reported that on approach to Jandakot all indications had returned to normal and remained that way until short final when an altitude mismatch and low airspeed warning was identified on the PFDs.

What the ATSB found

The ATSB determined that during the flight, water entered the aircraft’s pitot tube either as rain or an accumulation of moisture from flying through cloud. Due to a blockage in the pitot tube drain the water had been unable to escape. This in turn obstructed the flow of air to the aircraft’s air data attitude heading reference system, resulting in an incorrect airspeed being displayed on the left PFD and triggering miscompare indications on both PFDs.

In addition, a heading miscompare was likely caused by the aircraft’s movement through an area of moderate to severe turbulence during the return to Jandakot.

Finally, the ATSB found that recent training that the pilot had undertaken helped them to identify the erroneous airspeed data.

Safety message

Spurious instrument readings can create a more complex scenario for flight crew than an instrument failure.

In this case the pilot’s recent training assisted in effectively assessing the situation, determining the likely failure mode and identifying the most accurate source of available data for a safe return to the departure airport.

Download Final Report
[Download  PDF: 1.19MB]

The investigation

General details
Date: 22 June 2020   Investigation status: Completed  
Time: 2155 AWST   Investigation level: Short - click for an explanation of investigation levels  
Location   (show map): near Albany   Investigation phase: Final report: Dissemination  
State: Western Australia   Occurrence type: Avionics/flight instruments  
Release date: 21 July 2021   Occurrence category: Incident  
Report status: Final   Highest injury level: None  
Anticipated completion: 3rd Quarter 2021    

Aircraft details

Aircraft details
Aircraft manufacturer Pilatus Aircraft Ltd  
Aircraft model PC-12/47E  
Aircraft registration VH-OWI  
Serial number 1232  
Operator Royal Flying Doctor Service of Australia (Western Operations)  
Type of operation Medical Transport  
Sector Turboprop  
Damage to aircraft Nil  
Departure point Jandakot, Western Australia  
Destination Albany, Western Australia  
Last update 22 July 2021