COMMERCIAL AND BUSINESS AVIATION ADVISORY CIRCULAR
Notice to Pilots and Air Operators of Fairchild/Swearingen Metroliner SA226 & 227 Aircraft Brake Overheat and Wheel Well Fire Hazards
The purpose of this Commercial and Business Aviation Advisory Circular (CBAAC) is to alert flight crew and air operators to the potential hazards associated with brake overheat conditions in Metroliner aircraft.
This CBAAC replaces 0146 dated 98.09.16. This revision contains an addition to the procedures in ACTION and an amendment to CONCLUSION.
- Ongoing Transportation Safety Board (TSB) Accident Investigation - Mirabel Quebec, 1998.
- National Transportation Safety Board Accident Report #ANC88FA100 - Anchorage, Alaska, 1988.
- TSB Accident Report A#90C0024 - Winnipeg, Manitoba.
Research following a recent fatal accident at Mirabel has revealed a risk that overheated brakes in Metroliner aircraft can potentially result in a fire and/or explosion in the wheel well. The identification of this risk is supported by a number of reported wheel well fire occurrences. This research has also identified that information in the Aircraft Flight Manual (AFM) may be misleading in assisting flight crew to correctly diagnose the presence of a brake overheat or wheel well fire.
Indications that were common to the known Metroliner brake overheat/wheel well fires included:
- Hydraulic Pressure Warning Light and loss of pressure on affected side;
- Sluggish controls;
- Wheel Well and Wing Overheat Warning Light - steady then flashing;
- Abnormal aileron control forces; and
- Engine abnormalities
The AFM Emergency Procedure for "Wheel Well and Wing Overheat Warning Light ON"states that a steady warning light indicates "a wheel well or air conditioning duct overheat". A flashing warning light, on the other hand, indicates a "wing leading edge bleed air line failure or an overheated generator wire". No reference is made to the potential for other heat sources, such as a fire in the wheel well, to activate the warning light.
The Wheel Well and Wing Overheat Warning Light is linked to temperature sensors in the wheel well adjacent to bleed air lines and inside the leading edge of the wing root. The sensors are designed and located to detect a bleed air line failure. A steady warning light is triggered by sensors that detect a temperature of 350° F (177° C) in the wheel well or 450° F (232° C) in the air conditioning duct. A sensor in the wing root triggers a flashing light when temperature exceeds 250° F (121° C) in the wing leading edge. These temperature sensors will activate the applicable annunciator light whenever the threshold temperatures are exceeded, regardless of whether the heat source was related to a brake overheat/wheel well fire, air conditioning or bleed air malfunction, or an overheated generator wire heat.
The AFM does not address the possibility that another heat source can trigger the wing overheat annunciator light, nor does it alert crews as to symptoms that might be associated with a wheel well fire.
The following information is to passed to all Metroliner flight crews immediately upon receipt:
At the first indication of a flashing or steady wing overheat annunciator light, all checklist items should be carried out immediately, including the lowering of the landing gear. In the case of a steady light, if the light extinguishes within three minutes, press the annunciator PRESS TO TEST switch to confirm functioning of the anununciator light. Should the light fail to test, the sensor may have been damaged by fire, and there still may be a fire in the wheel well. The check list should be completed as if the light failed to extinguish. If additional symptoms, especially those found in the list contained in BACKGROUND above, or if engine instrument indications do not conclusively substantiate a bleed air malfunction, a wheel well fire should be considered and an emergency declared. Landing should be carried out as soon as possible.
Due to the susceptibility of the brakes to overheat, flight crews should immediately report any hard braking action, suspected brake overheat, directional control anomaly, brake/wheel drag and have the brakes inspected by a qualified technician if in doubt.
Transport Canada has submitted a recommendation for amendments to the AFM and checklists to the FAA manufacturer. In the meantime, flight crew are urged to consider the contents of this CBAAC when faced with a Wheel Well and Wing Overheat Warning Light.
Commercial & Business Aviation
Commercial & Business Aviation Advisory Circulars (CBAAC) are intended to provide information and guidance regarding operational matters. A CBAAC may describe an acceptable, but not the only, means of demonstrating compliance with existing regulations. CBAACs in and of themselves do not change, create any additional, authorize changes in, or permit deviations from regulatory requirements.