The US Marine Corps has delayed the operational test phase of its Bell Helicopter UH-1Y/AH-1Z upgrade by at least four months, after discovering "unforeseen" tail boom weakening caused by exhaust heat.

Flight tests were suspended in early February to allow a second panel of aluminium sheeting to be riveted to the tail boom, which lies in the path of the engine exhaust. A permanent fix may involve replacing the design with an off-the-shelf kit which turns the exhaust outlet outboard.

A routine cooling and temperature survey on the airframe last year revealed the fault. Higher exhaust temperatures generated by the AH-1Z's upgraded General Electric T700 engine caused an unexpected level of metal annealing along the tail boom. Within months, more tests caused flight operations to be shut down for several days.

"AH-1Ws that have flown in excess of 3,000h don't have the annealing that we have with these aircraft after 300h," says USMC assistant programme manager for systems engineering, Lt Col Dave Anderson.

About 36 W-model Cobras were due to receive an upgrade turning kit for the exhaust system later this year. Tail boom heating caused by the aircraft's original exhaust duct orientation - nearly vertical, pointed aft - has been a known problem, but concerns for earlier models focused on aircraft survivability. The modification had been intended to cut the heat signature, but it is now hoped the same solution can remedy the UH-1Y/AH-1Z tail boom weakening problem.

An expected increase in flat drag rate caused by the turn kits should be balanced by a 1-2% jump in power, says Anderson. The new exhaust system will also slightly lower overall aircraft weight.

A clear budget estimate of the delay and modification was unavailable last week, but programme officials speculate this could amount to tens of millions of dollars.

The new problem comes about two years after the UH-1Y/AH-1Z programme was nearly cancelled after costs soared by about $2 billion to $6.2 billion. The programme to remanufacture 100 AH-1Zs and 180 UH-1N Hueys had recently been praised by the Pentagon's acquisition leadership for fixing earlier technical and budgetary miscues.

Bell Helicopter and FlightSafety International have, meanwhile, received a $45.5 million contract to produce training devices for the upgraded H-1


Source: Flight International