Hot rocket exhaust ingested into engines curtails exercise after power surge
The US Navy has halted the final phase of weapons testing with the Bell AH-1Z Super Cobra while it studies whether to introduce a firing envelope restriction following a serious problem that occurred during a 22 July live fire test.
An AH-1Z was firing single 2.75in (70mm) rockets simultaneously from pods on both sides of the aircraft when the glitch occurred. Twelve pairs of rockets were supposed to have been fired in a salvo, but a pocket of hot rocket exhaust not dispersed by the Super Cobra's rotor downwash was ingested into the helicopter's engines after only three or four rockets had been launched.
The heated air caused the engine power to surge, over-stressing the helicopter's drive train. The test team ordered the pilot to land the aircraft.
Programme engineers are now studying a range of options to avoid the problem from arising in the future, including possibly restricting the envelope for weapons firing. The current pause on weapons testing is not expected to delay the USMarine Corps' overall schedule, which covers the remanufacture of 100 AH-1Ws into AH-1Zs and 180 UH-1N Huey utility helicopters into UH-1Ys for the US Marine Corps.
The over-torque issue is the second major glitch reported by navy officials as the AH-1Z programme advances toward an operational evaluation phase scheduled to start next February. Programme development stopped for three weeks from early February 2004 to address a tailboom annealing problem caused by hot exhaust from the engines.
The problem has been solved by installing a "turn kit" that re-directs the exhaust outboard, rather than directly on to the boom. Bell is being awarded a $7.2 million contract to supply the first 24 turned exhaust kits by December 2005.
The design change also is expected to reduce infrared signature, improving survivability from heat-seeking missiles.
STEPHEN TRIMBLE / WASHINGTON DC
Source: Flight International