Irish investigators are recommending that Boeing reviews its pitot-heat indicator for modern 737 variants to ensure that crews are warned in the event of pitot heat failure.
It follows an incident on a Ryanair 737-800 during descent into Riga, in snowy conditions, during which the pilots’ airspeed indications started to diverge.
The cause was traced to a shorted pitot probe heater – on the side of the first officer, who was flying – but the warning system did not alert the crew.
Ireland’s Air Accident Investigation Unit, in its report into the 7 January 2012 event, says 20 other probes in the carrier’s 737-800 fleet had suffered unannunciated pitot-heater failures.
The Riga incident occurred at about 6,000ft, when the first officer’s airspeed indication began to decrease while the captain’s increased, and the crew levelled at 4,000ft to assess the problem.
Air traffic control assisted by relaying ground speed information to the pilots, who switched roles before continuing the descent.
As the aircraft decelerated for the approach, and the crew selected flaps, the stick-shaker activated on the first officer’s side and continued until the jet had landed. The crew also told investigators that the approach was flown manually because the autopilot had self-disconnected and the autothrottle kept tripping out.
Investigators have recommended that the FAA reviews effects of latent failure of pitot-heat indicator warnings, and that Boeing should provide guidance to crews on the systems which might be affected in such cases.