Frequently Asked Questions
What is the status of the Australian investigation, concerning the failure of critical flight data presentations to the pilots, during a B-777 flight from Perth?
The investigation of that incident is still ongoing. The Australian TSB has not as yet, issued a final report on this incident. The following is my summary of that Preliminary report.
On August 1, 2005, a Boeing 777-200, which had departed from Perth, received an EICAS (Engine Indication and Crew Alerting System) warning of low airspeed, as the plane was climbing through FL (flight level) 380. Simultaneously, the aircraft’s slip/skid indication moved full right, on the PFD (Primary Flight Display). The PFD speed tape also displayed contradictory information: that the plane was approaching both the high speed limit and the low speed (stall) limit. The aircraft, still connected to the autopilot, pitched up and climbed to approximately FL410 as the airspeed decreased from 270 kts to 158 kts. The stall warning devices also activated.
The PIC (pilot in command) "disconnected the autopilot and lowered the nose of the aircraft. The autothrottle commanded an increase in thrust which the PIC countered by manually moving the thrust levers to the idle position. The aircraft pitched up again and climbed 2,000 ft." The PIC advised ATC "that they could not maintain altitude and requested a descent and radar assistance. The crew was able to verify with ATC the aircraft speed and altitude."
The PFD indications became accurate again as they were descending through FL200. The PIC attempted to use both the left and right autopilots, but had to turn them off after each one produced undesired command responses. "There were no control difficulties experienced when the aircraft was flown manually, but the autothrottle `arm’ switches remained in the `armed’ position."
ATC radar vectors put the plane in position to conduct an ILS to R 03 at Perth. When they reached 3,000 ft, the PFD again began indicating erroneous low airspeed information. The autothrottle again responded by advancing the thrust levers. Since the pilot can override that command, simply by manually adjusting those thrust levers, the plane was able to land safely at Perth.
The FDR (flight data recorder), the CVR (cockpit voice recorder) and the ADIRU (air data inertial reference unit) were removed from the plane, for a detailed examination. Under the supervision of the American NTSB, the ADIRU was shipped to its manufacturer for detailed analysis.
The FDR data confirmed the erroneous acceleration values had been displayed on the PFDs, as the pilots reported. The ADIRU produced those erroneous acceleration values and they were used by the PFC (primary flight computer). As it was designed to do, the PFC compared the information from the ADIRU, to the information coming from the SAARU (Standby Air Data and Attitude Reference Unit). That comparison ability enabled the PFC to reduce the severity of the initial pitching motion of the aircraft.
The last comment in that Preliminary Report:
That comment perplexes me a bit. Since the SAARU is an essential redundancy backup component for critical flight instrument display data, all operators of the B-777 know it is a required dispatch item for all flights. Why then was it necessary to remind them that they should not fly the plane without that unit operating properly?
Additional Q and A:
This is the first time I have ever heard of this kind of incident on a B-777, which has been in commercial operation since about 1995. It should be noted that although the pilots had a real serious problem on their hands, the remaining standby systems, and the ability of the pilots to fly the plane without autopilots engaged, proved to be the ultimate backup systems which enabled a safe outcome.
The key is pilot simulator training. They learn how to fly their planes, even when most of the usual flight data information is no longer available and, most importantly, how to quickly identify when the flight data information being presented, is no longer to be trusted.
So long as they have some form of attitude information available, they can successfully fly the airplane to a safe landing just with a knowledge of what amount of engine power will produce what kind of speed, in any given configuration. Obviously, the pilots of that particular B-777, were very well trained.
I will be happy to update this FAQ when more information from the investigation is made available from the Australian TSB.
September 29, 2005
Robert J. Boser
The Editor of this Web Page, now retired, was an airline pilot for 33 years and holds 6 specific Captain's type-ratings on Boeing Jet Airliners.