What happened
On 03 April 2022, a Malaysia Airlines Boeing 737-800, registration 9M-MLS, was performing a scheduled flight from Kuala Lumpur to Tawau. While cruising at 25,000 feet and deviating from the standard departure route to avoid weather, the aircraft's autopilot disconnected, and the plane experienced an uncommanded pitch-up. The pilot in command noticed a rapid drop in indicated airspeed on his display, while the first officer's instruments showed increasing speed. This discrepancy triggered an "IAS DISAGREE" warning and an altitude disagreement alert.
In an attempt to recover, the pilot pushed the control column forward, which caused the aircraft to exceed its maximum operating speed and trigger the cockpit clacker. Although the crew managed to stabilize the aircraft and re-engage the autopilot, a second airspeed discrepancy occurred shortly after while the crew was performing non-normal checklists. During this second event, the first officer's airspeed indications also became unreliable. Following consultation with maintenance operations, the crew decided to abort the flight and return to Kuala Lumpur. The flight concluded with two minor injuries to cabin crew and two slight injuries to passengers during the abrupt maneuvers.
The investigation
AAIB Malaysia examined flight data recorder information, which revealed that the captain's pitot probe heat was in the OFF position throughout the entire flight. The investigation focused on the aircraft's flight path through clouds and the subsequent instrument discrepancies. Investigators also reviewed the cockpit voice recorder and maintenance records, noting that a circuit breaker had popped out, which the crew had failed to notice during their pre-flight inspection.
Findings
- The primary cause of the instrument discrepancies was likely ice build-up on the captain's pitot probe due to the probe heat being left in the OFF position while the aircraft flew through icing conditions.
- The flight crew's manual recovery maneuver was abrupt, involving significant force on the control column that disconnected the autopilot.
- The crew pairing, consisting of a highly experienced captain and a relatively junior first officer, may have impacted the effectiveness of Crew Resource Management (CRM) during the crisis.
- A pre-flight oversight occurred when the crew failed to notice a popped circuit breaker during their visual inspection.
Safety action
To prevent similar occurrences, the following recommendations were issued to the operator:
- Enhance UPRT (Upset Prevention and Recovery Training) modules to specifically include scenarios involving airspeed and altitude disagreements.
- Ensure more diligent visual inspections of the cockpit overhead panel prior to departure to identify popped circuit breakers.
- Improve crew awareness regarding Master Caution warnings and effective CRM during automation-induced upsets.