Pitot Tube problems receiving ongoing scrutiny
Pitot probe vulnerability is again receiving scrutiny from regulators following detailed reports on two inflight emergencies traced back to the systems.
The first, involving a United Airlines Boeing 757 descending into Dublin in October 2013, was chronicled in a recent issue of this magazine (AW&ST May 23, p. 32). The report by Irish investigators cited two probable causes: A temporary blockage of the right main pitot tube due to icing, leading to an inaccurate low-airspeed indication on the first officer’s display and the crew’s non-standard response to the low-airspeed reading. The Irish Air Accident Investigation Unit’s report included eight recommendations, including for the FAA to “study whether a safety deficiency exists in pitot probe icing protection” for aircraft certified before January 2015, when enhanced certification standards went into effect. Those changes were triggered in part by the investigation into the June 2009 Air France Flight 447 accident, which determined that inconsistent airspeed readings between the captain’s and first officer’s displays started the chain of events that led to an aerodynamic stall.
A month after the United incident, industrious mud-dauber wasps took less than 3 hr. to build a nest in the pilot’s-side pitot probe of an Etihad Airways Airbus A330 on the ground at Brisbane Airport in Australia, triggering a series of troubling events, an Australian investigation found. The undetected blockage of mud resulted in an aborted takeoff that was followed by an inconclusive troubleshooting effort by maintenance technicians and a second takeoff for Singapore. That departure was quickly followed by a Mayday call by the pilots, who promptly returned to Brisbane. Several organizations affected by the incident instituted changes based on the investigation, an Australian Transport Safety Bureau’s (ATSB) report explained. The airport instituted multiple operational changes, Airbus modified its maintenance troubleshooting manual, and Etihad began requiring ground crews to install pitot probe covers at Brisbane “irrespective of ground time.”
Investigators determined that the nest blocked the captain’s pitot tube, resulting in a red “speed flag” display on the avionics as the aircraft accelerated through 50 kt. on the first takeoff attempt. Per standard operating procedures (SOP), the captain rejected the takeoff. The A330 has three open-face pitot tubes-a captain’s probe, first officer’s probe and standby probe-on the underside of the fuselage near the nose, devices that measure ram air pressure that is converted to airspeed readings by the avionics.
Maintenance technicians relied on two procedures in the A330 troubleshooting manual (TSM), neither of which identified the pitot probe as a possible root cause for the airspeed indication problem. The ATSB noted that Airbus had sent out a service letter to operators prior to the incident, linking airspeed discrepancies to potential pitot probe problems. The airframer in October 2014 updated the TSM to include the additional information.
The A330 was cleared for departure after a few minor avionics configuration changes, but the captain’s airspeed indicator again failed during the takeoff run, this time at a speed where SOPs called for continuing the takeoff. The ATSB questioned the captain’s recollection that the airspeed failed after “V1” (151 kt.), the speed at which crews are advised to continue the takeoff, noting that the flight data recorder information showed that the failure flag should have appeared after reaching 50 kt.
Once airborne, the sensor issues caused the A330’s fly-by-wire flight control logic to revert to alternate law and various slat and flap warnings occurred. The pilots declared an emergency and landed at Brisbane at an aircraft weight of approximately 200 metric tons, 18 heavier than the A330s 182-metric-ton maximum landing weight.