Hundreds of US Black Hawks were grounded except for mission-essential flights from May 1 after a potential transmission problem was discovered.

About 960 Black Hawks in the active Army and National Guard have been restricted, officials said, as the US Army conducts an assessment to see if the problem is isolated or if other Black Hawks have a similar problem. Under the restriction, only mission-essential flights such as medical evacuation and combat are allowed.

The restriction involves the UH-60A utility helicopter, EH-60A electronic warfare version and the UH-60Q medivac chopper. The UH-60L helicopters were not affected because they have a different transmission.

The transmission problem was detected when an US Army aviation unit, 57 Medical Coy, at Fort Bragg, North Carolina, experienced persistent low oil pressure problems with the transmission in a UH-60A Black Hawk.

The transmission was sent to Corpus Christi Army Depot, Texas, for repair. The part was disassembled and a cracked planetary carrier was determined as the reason for the low oil pressure. The planetary carrier, manufactured by RAF Industries, is a large plate-shaped component within the transmission that has no finite replacement life.

US Army Aviation and Missile Command spokesman Bob Hunt said this was the first cracked planetary carrier found in 3.5 million flight hours.

"The [US] Army Aviation and Missile Command, Corpus Christi Army Depot and Sikorsky are working vigorously to solve this issue. A lot of people are putting in a lot of hours to get to the bottom of this.," he said.

US officials said if a planetary carrier cracked, stress was placed on the transmission gears and could cause internal shim damage. If pieces of the shim fell into the transmission oil sump, the transmission filter could become blocked and cause low oil pressure.

When the part doesn't function, it can't transfer engine power to the main rotor system. The rotor system could continue to operate without power allowing an autorotation landing or the failed carrier could cause a jam causing the rotor system not to work.

Australian Army S70-A9 Black Hawk are fitted with a a different production (later model) main rotor gearbox (transmission) which is not affected by the US Army's Safety of Flight message issued five weeks ago.

Logistic managers of the Black Hawk in AASPO are being kept informed by Sikorsky and the US Army.

Lt-Col Andrew Hastie, SO1 AVN, said if there was any concern about Australian aircraft appropriate inspection action would be immediately instigated but none was expected or planned.

By Tonya Johnson
US Army News Service