On 17 May 2015, the pilot of a Raytheon B200, registered VH-ZCO, was conducting an aeromedical retrieval flight from Darwin, Northern Territory, to Jabiru, also in the Northern Territory. During the take-off, the stall warning horn sounded and the pilot noticed that the aircraft was not performing as well as normally expected.

As the initial climb continued, the pilot discovered that the pneumatic boots fitted to the wing leading edges were inflated. The boots are normally held against the contour of the leading edge by a vacuum, and only inflated as required to shed an accumulation of ice. The pilot deduced that the stall warning and relatively poor performance were attributable to the inflated condition of the boots. The pilot also noticed that the aileron control forces were abnormally light.

The pilot tried to deflate the boots by cycling the surface de-ice control switch, but to no avail. The pilot elected to return to Darwin and used a higher approach reference speed than normal to provide increased confidence with respect to aircraft controllability and performance. The circuit and landing were uneventful, but the pilot noticed that substantially more power than normal was required to hold the desired speed. The wing de-ice boots remained inflated until the aircraft was parked and the engines were shut down.

Subsequent inspection by engineering staff found that the wing de-ice boots inflated during an engine ground run, without having been selected. The boots returned to normal operation when the surface de-ice system control switch was cycled.

The pilot had checked the operation of the surface de-ice system prior to the flight, as required by the operator’s pre-flight checklist procedures. Although the system appeared to operate normally, the pilot could not specifically recall confirming that the boots had deflated at the completion of the check, as was normal practise. The pilot believed that the boots probably remained inflated following the check on this occasion, and that the inflated condition went unnoticed.

Pilots of Raytheon B200 aircraft, and other aircraft with similar surface de-ice systems, are cautioned that the status of leading edge de-ice boots may not be immediately obvious under some conditions. This incident also serves as a reminder that any aerofoil leading edge contamination, damage or distortion has the potential to significantly adversely affect aircraft performance and handling qualities, and aerodynamic stall characteristics.


Aviation Short Investigations Bulletin - Issue 43

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