FAA Airworthiness Directive for Boeing 777 aircraft

The Federal Aviation Administration (FAA) has issued a new Airworthiness Directive (AD) for Boeing Model 777 aircraft. This is not an emergency AD.

This new AD requires a one-time inspection to determine the part number of the left and right air supply and cabin pressure controllers (ASCPCs) and installation of new ASCPC software if necessary.

This AD results from a report of an ASCPC failure during flight. The FAA says that this unsafe condition is likely to exist or develop on other airplanes of the same type design.

The intention of this AD is to prevent an ASCPC failure that could stop airflow into the airplane, inhibit the cabin altitude warning message, and cause an incorrect display of cabin altitude. These failures could result in depressurization of the airplane without warning.

Here are some of the details from the document, designated AD 2007-07-05:

We have received a report indicating that the left air supply and cabin pressure controller (ASCPC) incorrectly shut off the right air conditioning pack and the left bleed, and erratically opened and closed the isolation valves, on a Model 777 airplane during flight.

This resulted in periods of loss of conditioned inflow to the cabin and flight deck. The flightcrew descended the airplane to 10,000 feet and returned to the airport.

Investigation into this event revealed that the actions of the ASCPC resulted from a solder defect in the Aeronautical Radio, Inc. (ARINC) 629 hardware that occurred during manufacturing.

The manufacturing error was determined to be an isolated event. However, subsequent analysis revealed a software deficiency within the ASCPC that would not detect this single point failure.

This defect caused an intermittent open to ARINC 629 built-in-test (BIT) 13 for all input words. This, in turn, caused the ASCPC to enter the auxiliary power unit-to-pack takeoff (APT) mode above 30,000 feet.

The ASCPC internal BIT did not detect the defect and allowed the ASCPC to continue to operate. This condition, if not corrected, could stop airflow into the airplane, inhibit the cabin altitude warning message, and cause an incorrect display of cabin altitude. These failures could result in depressurization of the airplane without warning.

Click here to read the entire FAA Emergency Airworthiness Directive, AD 2007-07-05.

Click here for a printable ('pdf') version of AD 2007-07-05.