Service Difficulty Advisory - Bell 206 Series - Generator Failure Warning Light
November 8 2001
Bell 206 Series
Generator Failure Warning Light
A recent crash of a Canadian registered Bell 206B has been attributed to a loss of electrical power.
Initially, the pilot was alerted to an inflight problem by a low rotor RPM warning horn and light with coincident gauge indications of 108 per cent rotor RPM. The pilot reduced the RPM to about 102 per cent and began a precautionary landing. During the descent, the boost pump warning light illuminated and the pilot then initiated a full autorotational landing. The engine out horn activated just prior to touchdown. The helicopter came to rest on its side with the engine still operating. The pilot subsequently shut down the engine and exited the aircraft.
A post-accident investigation revealed that the 24-volt sealed lead acid battery was in a low charge state, indicating that the generator was not charging the battery during flight. The cockpit instrumentation was fitted with an electrical load meter but had no generator failure warning light that would heighten the pilot’s awareness.
The dual main rotor and gas producer tach indicators on the Bell 206B are driven by tach generators, and the low rotor RPM and engine out warnings are powered by the 28-volt DC primary electrical system. The interface between the indicator systems and the warning systems are sensor boxes that monitor the tach generator output frequencies.
When the main rotor or engine N1 RPMs decrease to below the normal operating range, contacts within the sensor boxes close and complete a circuit-to-ground to activate the warning systems. The low rotor and engine out sensor boxes are designed to function with a system voltage of between 20 and 30 volts. However, component testing has determined that false warnings may occur if voltage drops below 20 volts.
This particular rotorcraft was fitted with an electrical fuel/fuel pressure gauge that was located in the lower left side of the instrument panel. The fuel normally indicates 10 to 20 per cent electrical load in flight, with the indicator needle positioned approximately one needle width above the zero position indication. The difference between a normal indication and a generator out indication is not easily detected during a cursory instrument scan. It was not determined if the generator had been left off prior to departure or if it had come off during flight.
This helicopter had previously experienced a loss of electrical power including an array of false warnings resulting in a precautionary landing. Histori8cal records also indicate other cases of multiple warnings on Bell 206 series as a result of electrical failure and/or low battery state, and that not all operators have installed such generator failure warning lights.
The circumstances of this accident demonstrate that there are risks associated with not having such a warning light installed. BHT Service instruction 206A-52 provides information on how to install a generator failure warning light on the Bell 206A. Bell helicopter Textron has added a generator failure warning light on 206 models, serial numbers 4311 and subsequent.
Further occurrences of this or any other defect, failure or malfunction should be reported by sending a Service Difficulty Report (SDR) to Transport Canada, Continuing Airworthiness, Ottawa.
For further information, contact a Transport Canada Centre, or Mr Barry Caldwell, Continuing Airworthiness, Ottawa, telephone (613) 952-4358 or facsimile (613) 996-9178.
For Director, Aircraft Certification
ORIGINAL SIGNED BY
Chief, Continuing Airworthiness
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