Tim van Beveren analyse



http://www.iasa.com.au/folders/Safety - Issues/others/Bilbao.html


Airbus Industrie initiates a move to increase pilots’ authority over their automated safety system


by Tim van Beveren, Miami 2001-06-01


The landing accident of an Iberia Airbus A 320 on March 7 in Bilbao, Spain, shows vital consequences for Airbus Industrie. The European aircraft manufacturer recently surprised the aviation co mm unit y by announcing that it is going to revise the software programming of its automated Angle-of-Attack (AoA) protection, also known as "alpha­protection". This highly sophisticated safe-guard system has been installed on aIl of Airbus' fly-by-wire aircraft, dating back to the first delivery of an A320 in 1988. The alpha protection has always been regarded as one of the prestigious, outstanding and truly safety-enhancing features of the modern Airbus products. The computer guided system prevents the aircraft of entering a staIl, resulting from a excessive angle of attack. If, for example, the pilot inadvertently commands too high a nose-up attitude in a climb, which would lead to a subsequent loss of lift, the system will automatically lower the no se by acting on the elevators and thereby prevent the aircraft from entering into a stall. The Airbus A 320 Flight Crew Operation Manual (FCOM) defines it as a system "which provides protection against stall and windshear, (and) has priority over all other protections." (Emphasis added)

However, according to first released findings of the Bilbao accident investigation, the 'activity' of this safety feature was a contributing factor in the event: the alpha­-protection contradicted the desired pilots action. During the final approach to runway 30, the Ground Proximity Warning System "sink rate" warning was triggered and the crew applied TOGA-power (Take-Off/Go-Around power) to abort the landing.

Yet, special attention should be given to the specifics of the scenario: At around 23:10 local time Iberia flight 1456 was approaching Bilbao's Sondica airport. The flight originated in Barcelona. On board were 136 passengers and a crew of seven. As the flight was a training flight for the First Officer, there were three pilots in the cockpit. At the time of their ILS approach the crew encountered a thunderstorm and was advised of light turbulence and surface wind speeds from 240 degrees of only 8 to 9kts, but no windshear. The airport is dreaded by pilots for its critical conditions, - especially in the winter, and is not equipped with improved weather measuring equipment or modem windshear detectors. The airport was the scene of two other weather related accidents that occurred during the preceding 15 days and another three in the previous five months.

Bilbao's Air Traffic ControI did not mention to the Iberia crew that, just shortly before the A320'S approach, three other aircraft had tried unsuccessfully to land at Sondica and had finally decided to divert. According to statements of airport personnel to local media after the event, other flights also diverted directly to their alternate, without even trying to land in Bilbao.

During the final approach the A320 encountered heavy turbulence at about 200 ft AGL, with gusts up to 65mph, an 1.25g- updraft, followed by a downdraft and tailwind gusts at an altitude of 70-50ft. The associated change of wind direction during the event clearly points to a windshear encounter.

According to information released by Airbus's deputy director of flight operations support, Cpt. Michel Brandt, the flight crew applied a forward sidestick input during the updraft, then an aft input to reduce the subsequent increasing sink-rate. When the GPWS alerted the crew about their unusual increased sink rate, the pilots decided to perform a go-around and applied TOGA-power. But the crew's desired and commanded action was not performed by the aircraft. As the alpha-protection was triggered during this event, the system commanded a nose down signal, which was performed, even though bath pilots had their sticks full backward, commanding a "climb". Nevertheless the airplane touched down with all three gears struts almost simultaneously and with an estimated vertical speed of 1,400ft./min. The nose gear subsequently collapsed and the plane slid along the runway for about 3,280 ft. before coming to a stop. During the emergency evacuation, four passengers and some of the crew received minor injuries, among those, one passenger, a 75 year old female, was hospitalised.

The barely six month old aircraft received substantial damage, including the wing structure and the engine nacelles. It is beyond economical repair and therefore should be regarded as a total loss. The accident has been under investigation by the Spanish CIAl (Comision de Investigacion de Accidentes e Incidentes). For Spain's national carrier Iberia this accident represents the first loss of an Airbus A 320. The company operates 85 modern Airbus fly-by-wire aircraft (A319, 320, 321 and 340) and last year transported over 30 million passengers. The Spanish company CASA is part of the European Airbus consortium.

Back in early April, the French Civil Aviation Authorities (DGAC) had issued an airworthiness directive (AD) for the A319/320 aircraft. It ordered the crews to fly at least 10 kts. faster and use only "CONFIG 3" (flaps 3) setting on approach in conditions with gusts greater than 10kts reported wind increment (max. wind minus average wind), or when moderate or severe turbulence on short finals has to be expected. In such events, the crew must select no more than flaps 3 and maintain a minimum approach speed of VLS ("lowest selectable speed") + 10kts. If the GPWS "sink rate" warning occurs below 200ft, an immediate go-around is required. Operators incorporated this AD-note into special bulletins for their pilots, but no additional information about the nature of this special procedure was given so far.

In the light of the accident occurring only four months ago, and the sometimes lengthy 'normal' timeframe for implementation of safety revisions after an accident, it appears to be of "amazing speed" how Airbus Industrie, - even in the absence of a final report, has already decided and performed a modification on the alpha- protection control laws. This was done in an approach "to increase the flight crew's authority", - as Cpt. Brandt was quoted saying by media. - A step applauded even by staunch Airbus critics among the international pilot community. A revised software version is expected to be validated this month and has already received certification by the French Civil Aviation Authority (DGAC) and the European Joint Aviation Authority (JAA). Airbus plans to implement a "rapid retrofit program" for its entire A319/320 fleet.