The US Army is expected to decide by 19 June whether its 400 Bell Helicopter AH-1S Cobra gunships must be inspected for a safety problem that grounded over 900 UH-1 light utility helicopters in May, according to Brig Gen Robert Armbruster, the US Army Aviation and Missile Command's deputy for systems acquisition.
The UH-1 flight restrictions were ordered earlier this year after it was discovered that excessive engine vibration was causing failure of the N2 spur gear on the AlliedSignal T53 engine. About 200 UH-ls remain grounded pending a permanent solution. Both helicopters use the T53 engine.
The engine manufacturer has created an interim remedy that is being incorporated initially on the 700 UH-ls cleared to resume flying. It was determined that a spray coating can be applied to dampen the vibrations in the N2 spur gear, which is housed in the carrier drive assembly of the T53 powerplant. The utility helicopters must be re-inspected after 25 flight hours until the long-term solution is incorporated, however.
Shipments to the US Army of the specially coated gears, which began in May, are being accelerated, but it could take up to two years to install the new equipment in all 907 UH-1s flying in active units.
It was discovered that the excessive vibrations are created in the engine's noise reduction gearbox, which transfers reduced engine power to the transmission. Although they have pinpointed the source of the vibrations, US Army and AlliedSignal officials have yet to determine their cause. Issues such as gear tolerance and metal fatigue are under investigation. The permanent cure will include development and incorporation of a redesigned N2 spur gear and reduction or elimination of the vibration source.
Source: Flight International
