REX SF34 at Esperance on May 26th 2018, de-icing boot failure
Last Update: October 26, 2018 / 13:29:22 GMT/Zulu time
The ATSB released their final report concluding the probable causes of the incident were:
- It was likely that the crew conducted a flight with a small defect in the aircraft’s horizontal stabiliser de-icing boot, which increased to a readily detectable size during the flight.
- The timer light for the de-icing system did not illuminate, giving the flight crew indications that were inconsistent with a torn de-icing boot. The reason for the unlit timer light could not be determined.
- The flight crew managed the risk of airframe icing prior to continuing with the flight.
The ATSB described the sequence of events:
While taxiing, the first officer (FO) conducted an ice protection test and received a Master Caution. The crew observed that the STAB BOOT IND light was not illuminating and the Ice Protection Master Caution light on the Central Warning Panel (CWP) was on. The crew noted that the TIMER light did not illuminate during the test as expected. The crew conducted a lamp test and concluded that the TIMER light was functioning correctly.
Before entering the runway, the crew conducted their failure management procedure and went through the Quick Reference Handbook Checklist (QRH); ‘Boot remains inflated and boot indication light remains on or off’. In addition, the flight crew checked the Minimum Equipment List (MEL) to ensure there were no operational requirements that would ground the aircraft.
The Captain and the FO discussed the warnings and their observations that:
- no visible defects were detected during the daily walk-around check
- the TIMER light was working, but did not illuminate in conjunction with the STAB light
- the warnings received did not follow any of the scenarios in the QRH
- the MEL permitted dispatch outside of known or forecast icing conditions
- the forecast freezing level was 7,500 ft and if necessary, the flight to Perth could be conductedas low as 4,000 ft.
Following their discussion, the flight crew concluded that the warnings were most likely the result of a faulty sensor and there was no risk to the safety of the flight. Subsequently, they elected to proceed with the flight as scheduled and the flight was conducted without further incident.
During the post-flight walk-around, the FO observed that the right-hand stabiliser boot had a tear approximately six inches (150 mm) in length (Figure 1). The de-icing boot was replaced in accordance with the aircraft maintenance manual before the aircraft was returned to service. There were no other faults found with the de-icing system.
The ATSB analysed:
On examination by the engineering team, the only fault found within the de-icing system was a torn de-icing boot. It was therefore likely that the cockpit warning lights were indicative of that defect, despite the fact that the unlit TIMER light was not consistent with a torn boot as described in the QRH. The TIMER light not illuminating during testing was likely to have been a system anomaly and there was, in fact, a defect present in the de-icing boot that either went undetected during visual inspections or ruptured during the pre-flight test.
The TIMER light not illuminating during the de-icing system testing was not a known issue to either the operator or the manufacturer. The operator’s engineering team tested the TIMER light after repairing the de-icing boot and found it to be serviceable.
The FO did not observe any defects in the surface of the de-icing boots during the walk-around inspections, either the evening before or on the morning of the incident. During the post-flight walk-around however, the FO identified the tear without difficulty. It was therefore likely that if there was a pre-flight tear, it was small in nature, allowing it to go undetected. That being the case, aerodynamic loading on the horizontal stabiliser during the flight would have resulted in the defect increasing to the size that was readily identified post-flight.
Prior to departing, the flight crew assessed the criteria in QRH and the MEL (section 30-10-5, Boot indication lights), to ensure the aircraft was safe to depart. Under MEL 30-10-5, ‘all [indication lights] may be inoperative provided the aircraft is not operated in known or forecast icing conditions’. That MEL did not require any PIC maintenance. Based on the assumption the abnormal indications were due to a faulty sensor and knowing that the scheduled flight could be conducted below the forecast freezing level, the flight crew felt satisfied that there was no risk to the safety of the flight to continue as scheduled. That assessment, while understandable, included a likely incorrect assumption regarding the defect in the de-icing system that led to the flight being conducted with a torn de-icing boot.
The flight crew reported that they did not experience any in-flight controllability issues or abnormal vibrations during the flight to indicate that there was a torn de-icing boot. While a torn boot is not guaranteed to result in controllability issues, without any inflight indications there was no reason for the crew to suspect a torn boot and conduct a ‘Flaps 0’ landing as per the AD recommendation.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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