What happened
During a flight, the crew operated an aircraft with a defect in the horizontal stabiliser de-icing system. Prior to departure, the flight crew evaluated the aircraft's status using the Quick Reference Handbook (QRH) and the Minimum Equipment List (MEL). At the time, certain cockpit warning lights were behaving abnormally, specifically a timer light that failed to illuminate. Because the MEL allowed for certain indication lights to be inoperative provided the aircraft was not flying in known or forecast icing conditions, and because the flight was planned to remain below the freezing level, the crew determined the aircraft was safe to depart.
During the flight, the crew did not experience any abnormal vibrations or changes in controllability. However, following the landing, a post-flight walk-around inspection by the First Officer revealed a torn de-icing boot that had not been identified during the pre-flight checks.
The investigation
Engineers examined the de-icing system and identified a torn de-icing boot as the sole mechanical fault. While the unlit timer light did not align with the specific symptoms of a torn boot described in the QRH, the investigation determined that the light's failure to illuminate was likely a separate system anomaly.
Investigators considered whether the tear existed prior to takeoff. If a small tear was present during the pre-flight inspection, it may have been too minor to be detected visually. It is believed that aerodynamic loading on the horizontal stabiliser during the flight likely caused the defect to expand to a size that was easily identifiable after the flight. Subsequent testing by the operator's engineering team, performed after the boot was repaired, confirmed that the timer light was actually serviceable.
Findings
- The flight crew's assessment of the risk was based on the incorrect assumption that the cockpit indications were merely due to a faulty sensor rather than a physical defect in the boot.
- The de-icing boot tear was likely small enough to evade detection during the initial walk-around.
- Aerodynamic forces during flight likely increased the size of the tear.
- The failure of the timer light to illuminate was an unexplained system anomaly that provided misleading information to the crew.