Airspeed miscompare indication due to pitot tube blockage


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Key points:

  • Blockage in the left pitot tube prevented water escaping, obstructing air flow;
  • Incorrect airspeed indications were displayed on the left PFD, triggering miscompare alerts;
  • Spurious instrument readings can create a more complex scenario for pilots than a complete instrument failure

Differing airspeed indications on a PC-12 aircraft’s two primary flight displays due to a blocked pitot tube underscores the complexity of managing spurious instrument readings, an Australian Transport Safety Bureau (ATSB) investigation notes.

On the evening of 22 June 2020, a Royal Flying Doctor Service Pilatus PC-12/47E was conducting a positioning flight from Perth’s Jandakot Airport to Albany, on Western Australia’s southern coast, with a pilot, a doctor and a flight nurse on-board.

On descent to Albany while passing through flight level 180 and in icing conditions, the pilot observed an airspeed miscompare indication on the aircraft’s left primary flight display (PFD).

The pilot then compared the airspeeds displayed on the left and right PFDs with the airspeed indication on the aircraft’s standby instruments (the electronic secondary instrument system/ESIS), and determined that the left PFD was likely displaying incorrect information.

Assessing that a blocked pitot tube was likely causing erroneous data, the pilot requested clearance from air traffic control to climb to FL230 to get clear of cloud and avoid the worst of the icing conditions.

During the climb, the airspeed displayed on the left PFD exceeded the aircraft’s maximum operating speed, resulting in audible overspeed alerts, and the pilot elected to return to Jandakot rather than continue to Albany.

During the return to Jandakot, while descending the aircraft to exit icing conditions and cloud, the left PFD’s indicated airspeed reduced to zero, however, no stall warning activated. In addition, heading data on the left and right PFDs diverged, resulting in a heading miscompare indication on the left PFD, while the left PFD would also display an incorrect attitude.

Approaching Jandakot the pilot reported that all indications returned to normal until the aircraft was on short final to land when an altitude mismatch and low airspeed warning was identified on the PFDs

The aircraft landed without further incident.

Post flight, an initial aircraft examination revealed a small amount of foreign material was blocking the left pitot tube drain.

“During the flight, water entered the left pitot tube either as rain or an accumulation of moisture from flying through cloud,” said ATSB Director Transport Safety Stuart Macleod.

“The blockage meant the water was unable to escape, and 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.”

In addition, the heading miscompare was likely caused by the aircraft’s movement through moderate to severe turbulence during the return to Jandakot, Mr Macleod noted.

About a month prior to the incident the pilot had undertaken an operational proficiency check flight under the guidance of a check and training captain, which included observing the aircraft’s performance at various engine power settings and attitude combinations.

“The pilot advised the ATSB that this check flight had given them more confidence in the assessment that it was likely a pitot tube blockage and that the right PFD and ESIS were showing the correct information,” Mr Macleod said.

“Spurious instrument readings can create a more complex scenario for pilots than a complete instrument failure.

“Unlike in this incident, erroneous overspeed readings have had serious consequences when not properly diagnosed.”

You can find here the report: AO-2020-030 Unreliable airspeed indication involving Pilatus PC-12/47E, VH-OWI, on descent to Albany Airport, Western Australia, on 22 June 2020.

Last update 21 July 2021

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