Boeing 757 Air-data Computer Issue May Cause Canceled United Flights

Steps skipped in FAA required modification temporarily grounds all 96 United 757s

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Boeing 757 Air-data Computer Issue May Cause Canceled United Flights

United Continental Airlines, the world's largest airline which was formed last October, has announced that it has temporarily grounded all 96 of its "original" United Boeing 757 aircraft. None of the Boeing 757s flown by Continental are affected, says this story at Bloomberg News.

According to this article in the Wall Street Journal, United discovered that some of the steps required in modifying the aircrafts' air-data computers as required by a 2004 US Federal Aviation Administration (FAA) airworthiness directive (AD) were either not done or done out of sequence.

The WSJ says that the 96 grounded 757's make up more than 25% of United's mainline fleet.

United says that the required checks will take up to 90 minutes to complete, and that some flights may need to be canceled as a result. The airline hopes to complete all the checks by later today. This story appearing at the Chicago Tribune says that seven United flights were canceled yesterday as a result on the voluntary grounding order.

United also stated that none of the air-data computers have malfunctioned because of the maintenance oversight.

The WSJ, in explaining the problem, said that:

"The airworthiness directive in question dates from 2004, when the FAA required modifications to the air-data recorder system, including installing new circuit breakers, relays and related components and making various wiring changes. For certain aircraft types, this amendment also required actions necessary to ensure that the pilots are able to silence certain erroneous warnings of overspeed and stall. Otherwise the false warnings could lead the flight crew to act on misleading information."

This blog post at the Airline Industry Review says the airworthiness directive was FAA AD 2004-10-05 which

"... warns against malfunction of the aircraft’s Air Data Computer ( ADC ) which could feed the Flight Control and Flight Management Computer with incorrect data, such as incorrect airspeed readings thus providing erroneous overspeed or stall warnings... The AD affects 747, 757 and 767 series aircraft."

The Tribune story says that United mechanics performing routine checks on Tuesday discovered the problem. It further states that United installed the required software modifications to the air-data computers in 2004. 

Why this issue was not discovered years ago by United during other "routine maintenance checks" has not been explained. I expect the FAA to ask United for an explanation why.

A United spokesperson said that:

"We apologize for any inconvenience and ask customers to check their flights status on before going to the airport."

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