Graham Warwick/WASHINGTON DC

The US Army hopes to begin returning some Bell UH-1s to flight by the end of April, after grounding the helicopters on 28 March. All Army and National Guard UH-1s - 907 in all - have been grounded because of failures of a spur gear on the AlliedSignal T53 turboshaft which transmits turbine speed to the engine governor and tachometer.

Anew inspection has been devised to allow aircraft to be returned to flight while a long-term solution is developed.

Failure of the N2 spur gear causes the governor to think the engine is losing power, so that it increases fuel flow and overspeeds the engine. At the same time, the tachometer drops to zero, causing the crew to think the engine has failed. There have been 22 failures since August 1996, with three aircraft damaged, but no fatalities.

Failure of the gear, the vast majority of which have been produced by a third-party vendor, Keraton, has been linked to resonance. As a first step, grounded aircraft are to be checked for gear vibration. If none is detected, aircraft will be returned to flight with restrictions imposed in November 1997, and re-inspected every 25h.

Some of vibration-free aircraft will receive a new gear with a metallic spray coating to improve damping. This will allow the helicopters to be returned to flight without restrictions.

Aircraft that exhibit vibration will remain grounded until a new spur gear carriage assembly is installed. AlliedSignal has received a contract to supply the assembly, which will take six months. Retrofitting the entire UH-1 fleet will take 18-24 months and cost $15-20 million, says the US Army.

Failures have not occurred in civil UH-1 derivatives, which have a spur gear designed by AlliedSignal. Of more than 6,000 gears bought by the Army since competitive procurement was introduced in 1980, only 40 were produced by AlliedSignal, the service says.

Source: Flight International