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Accident Spurs Action to Give Pilots More Control Authority of Computerized Aircraft

A March 7 landing accident of an A320 in Spain has prompted manufacturer Airbus Industrie to announce that modifications will be made to give pilots more authority over the airplane s computerized flight control system. The Airbus action deals with the so-called alpha protection designed into the airplane s computerized flight control system to prevent excessive angles of attack. The alpha protection, designed to guard against stalling the airplane, is one of the crown jewels of the Airbus flight control system. The pending relaxation of the iron law of alpha protection already has fueled the seemingly perennial debate between pilots and other experts over the degree of control that should be allocated between pilots and their flying machine. Since the advent of the fly-by-wire A320 in 1988, Airbus officials have hailed the added margin of safety provided by their alpha protection feature (see box B). In the approach to stall regime, alpha protection limits the amount of pitch-up that can be commanded, thereby preventing too high an angle of attack. Should the situation deteriorate further in the approach to stall regime, the alpha floor protection also will apply take-off go-around thrust (TOGA). The alpha floor feature is designed, according to a 1998 Airbus brochure on its fly-by-wire design philosophy, To protect against stall, even in high dynamic maneuvers or in gusts.

The A320 Flight Crew Operations Manual (FCOM) advises aircrews that the alpha protection feature "protects against stall and windshear (and) has priority over all other protections.

Accident details And yet the system designed to protect in gusty conditions and windshear is now being modified in the wake of a crash-landing in these conditions. During a nighttime flight from Barcelona to Bilbao, the pilots of Iberia (Iberia Líneas Aéreas de Espana) Flight 1456 were planning to land their A320 with 136 passengers and seven crew on Runway 30 at Bilbao s Sondica Airport. As it was a training flight for the first officer, there were three pilots in the cockpit. During their final ILS approach, the aircraft encountered heavy turbulence at about 200 feet above the ground (AGL). With gusts up to 65 mph., the winds were much more severe than the 9-10 mph winds at 240° with light turbulence initially reported to the crew. The aircraft encountered a 1.25G updraft, then a downdraft, followed by a tailwind gust as the aircraft was just 50 feet AGL. The dramatic and sudden shifts in wind direction and intensity are the classic symptoms of windshear. The airport is not equipped with windshear detection technology, although Spanish pilots reportedly have been calling for its installation. The Iberia crew had not been advised by local control that three aircraft had tried unsuccessfully to land at Bilbao and had diverted to their planned alternates. Sources advise that the airport s notorious wind conditions contributed to two other weather-related accidents during the preceding 15 days and to three other accidents in the previous five months. According to Capt. Michel Brandt, deputy director of flight operations support for Airbus, the pilots at the controls applied forward sidestick during the updraft (to keep the nose down), and then an aft input (to keep the nose up) as the sink rate increased. When the Ground Proximity Warning System (GPWS) alerted the crew with a Sink rate warning to the ground rapidly rising beneath them, the crew elected to abort the landing and applied TOGA power. Crew vs. computer The combination of dynamic winds and crew actions created a situation that triggered the airplane s alpha protection system. As the crew applied TOGA power for a go-around, the alpha protection ordered the elevators into a nose-down position. Seeing the runway coming up fast, both pilots pulled their sidesticks fully aft to avoid premature contact. Instead of a go-around, the aircraft struck the runway with a vertical speed of some 1,400 feet per minute (fpm). The airplane would normally descend in the last moments before flare at a rate of about 500-600 fpm. The 1,400 fpm descent rate translates to about 23 feet per second, slightly more than the certification requirement of 21.3 feet per second. In the landing at Bilbao, the main gear and the nose landing gear all struck at virtually the same time, and the nose gear collapsed under the force of impact. The airplane continued some 3,280 feet down the runway before coming to a stop. During the subsequent emergency evacuation, some of the crew and four passengers received minor injuries. One passenger, a 75-year old woman, was hospitalized. The aircraft, only six months old, received substantial damage to the engine nacelles and the wing structure. Sources say it is regarded as a total loss. The accident is being investigated by the Spanish CIAI (Comisión de Investigación de Accidents e Incidentes). More than a technical adjustment Airbus engineers replicated the Iberia accident on their A320 simulator and have developed a software modification to modify the alpha protection. Higher angles of attack will be permitted before the protection feature activates. As such, the modification will increase the flight crews control authority. One A320 captain with a major European carrier advised that the airplane is still flyable without stalling at a speed around 5-8 knots lower than the airspeed where alpha protection activates. The revised software is to be implemented rapidly after validation and certification by European regulatory authorities, which is expected to be completed later this month. It will affect the worldwide A319/A320 fleet of some 1,270 aircraft. The stretched A321 is not affected. Some 23 feet longer than the A320, allowing for a higher-nose-up command from the pilots during landing could possibly increase the likelihood of a tail scrape or tail strike. This is not a technical change, opines the A320 captain, but rather marks a shift in philosophy by Airbus towards granting pilots more control over the aircraft. Actually, the A320 alpha protection software was quietly modified two years ago, in response to operators  reports that heavy windshear could bring the aircraft too close to a stall warning. The earlier modification reduced somewhat the pilot authority, which it is now conceded was a step too far that is now being relaxed with the latest adjustment to the software. The Iberia accident marks the latest event where man-vs.-machine and man-vs.-man interactions have led to incidents and accidents. In the meantime, the French DGAC (Direction Générale de l Aviation Civile) issued an airworthiness directive (AD) in early April requiring A320/A319 operators to fly at least 10 knots faster and to use only a setting of CONFIG 3 (Flaps 3, as opposed to Flaps 4) during approach with gusts higher than 10 knots or when moderate to severe turbulence is expected on short final. The AD also mandates an immediate go-around if the GPWS Sink rate alert sounds below 200 feet. The U.S. Federal Aviation Administration (FAA) has not yet issued comparable guidance. Airbus issued guidance to its worldwide A319/A320 operators in an Operations Engineering Bulletin dated April 2001. One pilot explained that the new procedures avoids the pitch down effect during the last phase of the approach due to gusts causing alpha protection to kick in close to the runway.

Alpha Floor  The Airbus View 

"...(If) a rapid avoidance maneuver is required to avoid terrain, windshear or another aircraft, it is safe to rapidly pull the sidestick fully aft and/or bank and hold it there. The aircraft will pitch up to max Alpha (angle of attack), engage TOGA thrust and climb away. Such precise maneuvering around the low speed edge of the flight envelope is virtually not possible in any conventional aircraft." (Emphasis in original)

Source: Airbus, FBW at a Glance; A Pilot s First View

High Angle of Attack Protection 

Source: Airbus