Ramon Lopez/WASHINGTON DC
All US Army Bell AH-1 Cobra attack helicopters must be inspected immediately for problems that could cause the engine's N2 spur gear to fracture and fail.
The safety-of-flight message stops short of grounding the 474 gunships, 34 in the active army and 440 in the Army National Guard. A similar safety concern grounded more than 900 Bell UH-1 utility helicopters in May. The AH-1 and UH-1 have common dynamic components and powerplant.
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-1s remain grounded pending development of a permanent modification.
The N2 spur gear passes turbine engine speed to the powerplant's governor and tachometer.
The US Army says there have been four N2 spur-gear-related failures in the Cobra fleet since 1992. Cobra aviation units must test their T53 engines within the next 10 flight hours or 15 days with upgraded aviation vibration analyser test sets that have already been fielded.
Cobras with potentially damaging vibrations will be grounded either until their powerplants are replaced or a long-term fix is applied.
An interim fix involves spray-coating the N2 spur gear, which is housed in a carrier drive assembly in the T53 powerplant. The UH-1 and AH-1 helicopters must be inspected after every 25 flight hours until a long-term solution is incorporated.
The excessive vibrations are created in the engine's nose reduction gearbox, which transfers engine power to the transmission.
The US Army says the vibrations will be eliminated through repair or replacement of the nose reduction gearbox, which includes installing new planetary and output gears.
Source: Flight International
