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United Airlines has progressively grounded its Boeing 757 fleet as a voluntary measurement, for checks, after discovering that it is incompatible with an FAA Airworthiness Directive ( AD). The original FAA AD, 2004-10-05 , of 2004 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. United, after discovering the incompatibilities, as a voluntary measure, had ordered checks of it’s 757 fleet and is now releasing the inspected aircraft into service. Continental fleet is not affected. It is not clear whether United’s 747 and 767 fleets are also affected.

AD Summary

RIN 2120-AA64
Airworthiness Directives; Boeing Model 747-400, 747-400D, 747-400F, 757-200, 757-200PF,
757-200CB, 767-200, 767-300, and 767-300F Series Airplanes
AGENCY: Federal Aviation Administration, DOT.
ACTION: Final rule.
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SUMMARY: This amendment adopts a new airworthiness directive (AD), applicable to certain
Boeing transport category airplane models, as listed above. This amendment requires a modification
of the air data computer (ADC) system, which involves installing certain new circuit breakers, relays,
and related components, and making various wiring changes in and between the flight deck and main
equipment center. For certain airplanes, this amendment also requires accomplishment of various
other actions prior to or concurrently with the modification of the ADC system. For certain airplanes,
this amendment also contains an option that will extend the compliance time to accomplish the
modification of the ADC system. This action is necessary to ensure that the flightcrew is able to
silence an erroneous overspeed or stall aural warning. A persistent erroneous warning could confuse
and distract the flightcrew and lead to an increase in the flightcrew’s workload. Such a situation could
lead the flightcrew to act on hazardously misleading information, which could result in loss of control
of the airplane. This action is intended to address the identified unsafe condition.
DATES: Effective June 22, 2004.