Boeing has made changes to its Boeing 737 pitot tube heating systems and checklists following an incident report by the Irish Air Accident Investigation Unit (AAIU).

That report, published Dec. 19, details a serious incident in which the crew of a Ryanair Boeing 737-800 on an instrument arrival to the Riga Airport in Latvia in snow and ice conditions on Jan. 7, 2012, experienced divergent airspeed indications and other warnings, including an extended stick shaker activation, with no annunciation of the pitot heater short circuit that was the root cause of the problem.

During the investigation, Ryanair told the AAIU that it had experienced 20 events of unannounced pitot heat failures on its 737-800s in 2012 alone, and that the issue was a “fleet-wide problem” that has occurred on all makes of 737 next-generation aircraft.

“[Ryanair] informed the Investigation that it and other operators had brought the failure of the pitot probe heater warning system to the attention of [Boeing],” the report says. “It stated that [Boeing] had consulted with the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) who agreed in 2011 that the current rate of unannounced pitot probe heater failures met the FAR 25 requirements and that it was not therefore a safety of flight issue.”

The AAIU’s investigation puts doubt on that assessment. According to the final report, the incident flight, with 140 passengers and six crew members, was descending through 6,000 ft. toward Riga in “poor weather conditions with moderate snow” when the airspeed indications on the pilot and first officer displays began to diverge, causing airspeed disagreement, altitude, engine warnings to activate. Pitot tubes, which measure airspeed, include heating elements to prevent ice from corrupting the measurement. The devices are replaced “on-condition,” and in this incident, the failed unit had been in service for 23,618 hr.

The pilots levelled at 4,000 ft. and delayed the approach to assess the problem using checklists. They later decided that the first-officer’s airspeed was incorrect, and set up for an instrument approach to Runway 18 based on the pilot’s airspeed.

During the approach, the 737’s autopilot and autothrottle disconnected, after which the pilots hand-flew the approach despite the first officer’s stick shaker activating from the later stages of the approach all the way through landing. “The flight crew reported that the noise from the stick shaker was distracting and that it made communications difficult,” says the AAIU. The crew had considered pulling the circuit breaker for the stick shaker to disable the system, but decided to “it was unwise to look for the particular circuit breaker in a dark cockpit and in the prevailing circumstances,” according to the report.

The airspeed divergence was later attributed to a pitot tube heater failure on the first-officer’s side due to a short circuit, a problem that should have tripped a pitot heater failure warning in the cockpit. “The pitot heater failure warning had not activated because the design of the warning system may not detect failures of this nature,” says the AAIU.

As such, the AAIU says the 737 does not meet its Part 25 certification requirements that “heated air data sensors” be monitored and failures annunciated. In this case there were situations where the pitot could malfunction and not provide anti-icing, but still continue to draw enough power so as to not trip the alerting system.

The agency recommends that the FAA and Boeing review the design and operation of the pitot heat indicator system for the Boeing 737 with respect to latent failures, as well as make changes to the aircraft’s quick reference handbook (QRH) to consider other systems that might be affected by a pitot failure, including spurious stick shaker activations.

The AAIU says that Boeing, after being given a draft of the final report, said it had analyzed ways to improve the reliability of the indicating system for the probe heaters. “It found that by reversing the connection polarity of the probe heat wiring, a partially shorted probe continues to provide adequate heat until the short burns through the wire element thus causing an open circuit (and associated indication). This has been verified via analysis and testing of a shorted probe.”

Boeing says the wiring change is now included in new production aircraft, and that it plans to release a service bulletin in the first quarter of 2014 for making the changes on existing 737 next-generation models. Coming in June 2014 will be an amended checklist that will include “various symptoms of possible unreliable airspeed including the activation of stick shaker,” says the AAIU.