Background
During a pre-flight de-icing system test, the flight crew observed abnormal cockpit warning lights, including an unlit TIMER light. The Quick Reference Handbook (QRH) indicated that an unlit TIMER light was not consistent with a torn de-icing boot. The crew assessed the aircraft's Minimum Equipment List (MEL) section 30-10-5, which permits all boot indication lights to be inoperative provided the aircraft is not operated in known or forecast icing conditions. The MEL did not require pilot-initiated maintenance. Based on the assumption that the abnormal indications were due to a faulty sensor, and knowing the flight could be conducted below the forecast freezing level, the crew determined the aircraft was safe to depart.
Findings
After the flight, an engineering team examined the de-icing system and identified a torn de-icing boot as the only fault. The TIMER light had not illuminated during the pre-flight test. This was not a known issue to the operator or manufacturer. However, following repair of the boot, the TIMER light was tested and found to be serviceable. The crew reported no in-flight controllability issues or abnormal vibrations that might have indicated a torn boot. The first officer (FO) had conducted walk-around inspections the evening before and on the morning of the incident without observing any boot defects. During the post-flight walk-around, the FO identified the tear without difficulty. It was therefore likely that if a pre-flight tear existed, it was small and went undetected, with aerodynamic loading during the flight causing it to enlarge.
Flight Crew Actions
The flight crew relied on the QRH and MEL criteria to decide to proceed. They believed the abnormal lights were due to a faulty sensor, not a physical defect. Although this assessment was understandable, it included a likely incorrect assumption about the de-icing system defect, leading to the flight being conducted with a torn de-icing boot. The crew had no reason to suspect a torn boot in the absence of in-flight indications and therefore did not conduct a 'Flaps 0' landing as per an Airworthiness Directive recommendation.
Post-Flight Inspection
The torn boot was readily identified after the flight. The engineering team found no other faults in the de-icing system. The TIMER light anomaly was not reproducible after the boot repair, suggesting it was a system anomaly that occurred only during the pre-flight test.