Britain's air safety watchdog is calling on both Airbus and Boeing to take action on failure modes in their respective narrow-body aircraft that can result in the loss of electronic cockpit instrumentation.

The recommendations from the Air Accident Investigation Branch (AAIB) result from incidents involving a British Airways' Airbus A319 in November 2005, and an EasyJet Boeing 737-300 in March of the same year. The AAIB is voicing concerns about six inflight failures of primary flight and navigation displays (PFDs and NDs), as well as the electronic centralized aircraft monitor (ECAM) upper display on the Airbus A320 family. Most of the flight deck lighting also was lost at the time. The safety officials have now devised remedial steps Airbus should take after examining the Nov. 25 incident--an A319 departing from Heathrow to Budapest--as well as five prior malfunctions.

IN THE A319 scenario, the pilot and co-pilot lost the three displays, leaving them with only the lower ECAM, which then was used to display warning messages usually shown on the upper screen. One concern of safety officials is that the Airbus manual states an aircraft can be dispatched with the lower display inoperable. But in the case of the past mishaps, that would have left pilots without a display and "there would not have been any information readily available to the crew as to how to manage the failure and restore affected systems," they warn.

In the case of the 737-300, the failure of a contact post in an electrical relay resulted in the loss of power from a Battery Busbar. The AAIB report notes "the flight crew experienced progressive abnormal annunciator indications. For some of these there were no procedures in the Quick Reference Handbook." The crew made a PAN (urgent) call--based on the "lack of an engine fire detection and indications systems . . . and that the aircraft systems were not operating normally." The aircraft landed at Lyons-Satolas airport without incident. During descent the standby attitude indicator began to topple, while color was lost from the pilot's and co-pilot's electronic attitude direction indicator displays.

The AAIB also raises doubt regarding the "assertion in Flight Operations Technical Bulletin that 'loss of only the battery bus is not considered a hazardous situation.'"

Since the incident, Boeing has issued a bulletin on a proposed modification to the electrical system. The AAIB notes this "should provide a means to preserve the main attitude displays following the loss of the Battery Bus, although it is not known . . . if it will address the loss of other significant systems, such as the engine fire detection and indication."

The damaged 737 R1 Relay Contacts

This failure mode applies not only to the 737-300, but also to the -400 and -500. The AAIB contends that had there been a "specific procedure for the problem," this would have made "diagnosis, crew actions and subsequent decisions significantly more straightforward." It recognizes, however, that, "the many different configurations of the electrical system for the 737-300/-400/-500 fleet have made it difficult for the manufacturer to produce a generic procedure for this failure, although Boeing has provided information to enable operators to write a procedure for their own aircraft." This resulted from the recommendations of the Danish Air Accident Investigation Board into a similar occurrence on a 737-500 in 1997.

The AAIB now wants the FAA to require that Boeing "examine the various electrical configurations of in-service 737s with the intention of providing operators with an Operations Manual Procedure that deals with the loss of power from the Battery Busbar."

In the case of the British Airways A319, the pilots were able to regain use of the screens following several trouble-shooting steps. However, the action to restore screen function was only ninth or 10th on a list of steps pilots were advised to take. In fact, it was determined that it took 90 sec. to get to that point. The AAIB asserts: "Loss of both the commander and co-pilot's PFDs and NDs, at a critical phase of flight instrument conditions, could affect safe operation of the aircraft and . . . is therefore undesirable."

Note the Relay's lever arm bent back

The sequence of actions recommended on the lower ECAM should be switched to more quickly restore the displays, the investigators say.

THE INCIDENT REVIEW highlighted another shortcoming. Airbus aircraft come with two configurations for the standby artificial horizon. One is a single power supply that can lead to the standby horizon having power only for 5 min. if AC Bus 1 fails. The other approach has redundancy. The AAIB questions whether the single-fault configuration should be acceptable, but, at the least, recommends that operators understand the pitfall. They also point out that the operation manual for the affected A319 indicated it featured the single-fault design, when it didn't.