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.
Safety Recommendation 2005-65
recommended that the FAA require that the Boeing Airplane
Company examine the various electrical configurations of
in-service Boeing 737 aircraft with the intention of
providing operators with an Operations Manual Procedure that
deals with loss of power from the Battery Busbar.
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.'"
Boeing published a Flight Operations Technical Bulletin,
Boeing has no technical objection to an airline
incorporating a loss of Battery Bus procedure in their
Operations Manual. However, since there are so many
different electrical configurations throughout the 737
fleet, Boeing is unable to publish a generic procedure in
the Boeing Operations Manual which will work for all
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
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
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.