Boeing says a simple wiring change will solve a persistent issue that operators of its 737 next-generation models have been experiencing with pitot probes—the external sensors that provide crucial airspeed readings to pilots and onboard systems.

The problem came to light when the crew of a Ryanair Boeing 737-800 on an instrument arrival to the Riga Airport in Latvia in snow and ice on Jan. 7, 2012, experienced divergent airspeed indications and other warnings, including the activation of the co-pilot's stickshaker throughout the final approach and landing. The Irish Air Accident Investigation Unit (AAIU) found that the co-pilot's pitot probe had experienced an internal short-circuit, allowing ice to corrupt the airspeed measurement. Due to the mechanics of the failure, a watchdog circuit designed to detect the problem and alert the pilots, did not activate.

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 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 notes. “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.”

AAIU's investigation puts doubt on the FAA and EASA's assessments because multiple pitot probes could fail without any notification, forcing pilots to use ground-speed estimates and power/pitch attitude tables to fly, a practice the AAIU says is infrequently practiced in training.

According to the AAIU's final report, the incident flight, with 140 passengers and six crewmembers onboard, was descending through 6,000 ft. toward Riga in “poor weather conditions with moderate snow” that day, after a flight from the U.K., when the airspeed indications on the pilot and first-officer's displays began to diverge, causing airspeed disagreement warnings as well as altitude and engine alerts.

The 737 has five identical and independent pitot probes—built by United Technologies—each of which has a thin heated wire element wound around the inside of the probe for anti-ice protection. A 2.5-amp circuit breaker protects the system from a short circuit, and a detection system monitors for a current of less than 30 milliamperes (mA), indicating an open circuit failure and sending an alert to the flight deck.

Three of the probes are on the nose for airspeed measurements—for the pilot, the co-pilot, and as a backup—and two are on the vertical stabilizer for the elevator feel-and-centering unit. If the airspeed difference between the pilot and co-pilot probes is greater than 5 kt. for 5 sec. consecutively, the pilots receive an “IAS (indicated airspeed) Disagree” alert.

The Ryanair pilots levelled at 4,000 ft. to assess the problem using Quick Reference Handbook (QRH) checklists. They determined 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 and the first officer's stickshaker activated for the duration of the flight. “The flight crew reported that the noise from the stickshaker was distracting and made communications difficult,” states the AAIU.

During the investigation, United Technologies examined 12 pitot probes of various ages that had been returned to the company due to un-annunciated heater failures signaled by “IAS Disagree” messages. The manufacturer concluded that corrosion-induced short circuits within the probes were providing only partial heating, which allowed ice to form but did not trip the circuit breaker or low-current monitor. Boeing says the probes, which are replaced on-condition, have a lifetime of 18,000-22,000 hr., depending on the operational environment. The Ryanair probe had accumulated 23,618 hr. at the time of the failure. An airline representative says they have now imposed life-limits on the parts.

Boeing, upon receiving the AAIU's draft recommendations in advance of the final report, began studying ways to improve the reliability of the pitot heat-monitoring system. “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),” states the AAIU. “This has been verified via analysis and testing of a shorted probe.”

Boeing says it has “standardized” the new polarity connection for all production 737s, and will issue a service bulletin in the second quarter of 2014 for the deployed fleet. Per the AAIU's recommendation, the airframer will also update QRH checklists for all aircraft models by the third-quarter to include the checklist for unreliable airspeed. The modified checklist for the 737 will include “various symptoms of possible unreliable airspeed including the activation of the stickshaker,” says Boeing.