This bulletin provides information informing flight crews of temporary recommended procedures for coupled approach anomalies.
During a test flight at Boeing, all three autopilot channels disconnected during a coupled approach. Analysis revealed that this condition is caused by an error introduced between the Flight Control Computers (FCC) during a power transfer. The error occurs if one source of electrical power fails after the LAND 3 annunciation appears during approach. During the power transfer, air data from the Air Data computers is interrupted. The time required for the electrical system to revert to normal, then re-isolate may exceed the limit allowed for interruption of air data, and result in loss of air data to one FCC for the duration of the approach. As a result, an autopilot disconnect may occur during flare or during goaround. Service Bulletin 767-22-0038 corrects this anomaly.
Disconnecting the autopilots causes the electrical system to revert to normal operations (non-isolated), and resets the autoland system and ASA to allow normal LAND 2 or LAND 3 operation on subsequent approaches.
If, during an autopilot (coupled) approach, following the LAND 3 annunciation, the Autoland Status Annunciator (ASA) is observed to change to LAND 2, execute a manual landing on that approach (weather permitting), or execute a manual go-around.
If the approach is continued, the autopilot must be disconnected prior to landing.
If a go-around is accomplished, the autopilot must be disconnected prior to executing the go-around.
A subsequent autopilot (coupled) approach and landing may be conducted provided LAND 2 or LAND 3 remains annunciated on that approach.
This Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes in the operators fleet have been modified by Boeing Service Bulletin 767-22-0038.