IAS DIASAGREE / ALT DISAGREE
Speeds Disagree
Flight crew received no warning that a pitot probe heater had failed.
Stick Shaker Activated
Boeing 737-800. No damage. No injuries.
The 737 was en route with 140 passengers and six crewmembers the evening of Jan. 7, 2012, from East Midlands Airport in England, to Riga Airport in Latvia, where weather conditions were deteriorating. As the aircraft neared the airport, Riga was reporting a 500-ft overcast and a runway visual range of 1,800 ft (550 m) in rain and snow.
Shortly after entering a cloud layer at 6,000 ft during descent, the flight crew noticed a discrepancy in their primary flight display (PFD) indications. The first officer, the pilot flying, saw indications that airspeed was decreasing and that the descent rate was increasing, while the commander saw an indication that airspeed had increased to about 280 kt.
The first officer disengaged the autopilot and autothrottle system, and leveled the aircraft at 4,000 ft. “When the flight crew cross-checked, they noticed ‘IAS DISAGREE’ [and ‘ALT DISAGREE’] warnings on the PFDs and that the first officer’s altimeter read 250 ft higher than the commander’s,” said the report by the Air Accident Investigation Unit of Ireland (AAIU).
While conducting the quick reference handbook (QRH) “IAS Disagree” checklist, the pilots confirmed that pitot heat was selected on and that there were no indications of a pitot heat failure. A cross-check of the standby flight instruments and the inertial reference system showed that the commander’s instruments were providing correct indications.
The commander then took control, and the crew resumed the approach. “Air traffic control volunteered and helpfully gave groundspeed readouts,” the report said.
Shortly after the flaps were extended, the first officer’s stick shaker (stall-warning system) activated. “As the stick shaker continued until the aircraft had landed, the crew commented that they found this very distracting, and the noise made communications difficult,” the report said. “Although … the circuit breaker for the stick shaker could have been pulled, they felt that it was unwise to look for [it] in a dark cockpit and in the prevailing circumstances.”
The crew re-engaged the autopilot and autothrottles, but both systems soon disengaged automatically. The commander continued the approach manually, and after sighting the runway at 300 ft, the crew landed the 737 without further incident.
The report said that a short circuit had developed in the heating element in the pitot probe on the first officer’s side, but the circuit had remained open. Because electrical power continued to be supplied to the heater, no warning of its failure was provided. The consequent ice build-up on the probe caused the air-data discrepancies.
The report noted that 20 similar incidents had been reported by the aircraft operator. “In the majority of the cases reported, the airspeed divergences returned to normal as the probe ice melted during the descent into the warmer temperatures at lower altitudes,” the report said.
Based on these findings, the AAIU recommended that the U.S. Federal Aviation Administration and Boeing review the pitot heating systems in 737NG series aircraft and provide guidance in the QRH about other systems that can be affected and about warnings that can be generated as a consequence of pitot system failure.