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FindArticles > Air Safetv Week > June 18. 2001 > Article> Print friendly


Software Changes Being Made to Help Prevent Landing Mishaps

A February 7 landing accident of an Iberia A320 in Spain has prompted manufacturer Airbus Industrie to develop a modification to its flight control software. It will prevent the airplane's built-­in protection against stall from being activated by a high rate of change in angle of attack.

The software change is intended to help pilots safely land their airplanes in gusty wind conditions, as was the case in the accident.

It has been widely reported in other media that Airbus was expanding the allowable angle of attack (AOA) the pilots could apply in its computer- controlled fly-by-wire A320/ A319 aircraft. This is not the case. Rather, the modification returns to the threshold AOA criterion for stall protection that was certified originally in 1988.

Until the software change is distributed to the fleet, Airbus has advised A319 and A320 operators to maintain a higher speed when landing in gusty wind conditions and to limit landing slats and trailing edge flaps to Configuration 3. At this setting, one step short of full slat/flap deployment, the slats are at 22°  and the flaps are at 20°.  The guidance applies to a worldwide A319/A320 fleet of some 1,270 aircraft.

In related action, 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" 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) issued comparable guidance, contained in AD-200l- 08-26, which has an effective date of May 11. This is only part of the interim story. Both the French and the D.S. ADs are being modified to permit autoland operations without flap restrictions under the same environmental conditions.

The Airbus software change 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.

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. 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 (AOA). Should the situation deteriorate further in the approach to stall regime, the alpha floor protection also will apply take-off go-around thrust (TOGA).

In the Iberia case, two aspects of the alpha protection feature apply. One is the angle of attack (AOA). The flight control laws programmed into the computers will not allow the aircraft to exceed a predetermined AOA, based on the aircraft's weight and configuration.

The other aspect governing alpha protection is the rate at which AOA is allowed to change before reaching the protection limit. The alpha protection is triggered by two combined conditions: a threshold AOA and the rate of AOA change. To change the outcome in dynamic wind conditions near the ground, Airbus plans to modify the software to eliminate pitch rate as a controlling factor in alpha protection. In plain language, with the rate of change in the value of AOA being removed, the modification basically reverts the software to an earlier standard where pitch rate was not part of alpha protection (the pitch-rate limitation was installed as a result of post-1988 flight tests). The software change, contrary to some reports, does not alter the allowable AOA. However, Airbus officials said the change stems directly from the Bilbao accident. By implication, pilots will have a greater ability to control the rate of pitch change, which should help them to better cope with dynamic wind conditions during landing.

Accident details

During a night time flight from Barcelona to Bilbao, the pilots of Iberia (Iberia Lineas Aereas 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, 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 below 150 ft. the airplane encountered a potent downdraft. The first officer as the pilot flying (PF) pulled back his sidestick to arrest the rate of des cent. The downdraft was followed by a tailwind gust as the aircraft was just 70 feet AGL.

The dramatic and sudden shifts in wind direction and intensity are the c1assic 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 previously 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 conditions contributed to two other weather-related accidents during the preceding 15 days and to three other accidents in the previous five months.

When the Ground Proximity Warning System (GPWS) alerted the crew with a "Sink rate" warning, the captain called for a go-around while pulling on the sidestick - reportedly without pressing his priority control button.

Protection triggered

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, with both pilots pulling back on their sidesticks, the alpha protection law reduced the elevator nose-up commando. Instead of a go- around, the aircraft struck the runway with a vertical speed Airbus officials relate was some 1,200 feet per minute (fpm). The airplane would norma11y descend at a rate of about 500-600 fpm and, depending on the pilots skills and his discretion in the last moment during flare, it would descend at a rate of 50-120 fpm. The 1,200 fpm descent rate translates to about 20 feet per second, slightly less than the certification requirement of 21.3 feet per second.

In the landing at Bilbao, the main gear and the nose landing gear ail struck at virtually the same time, and the nose gear collapsed under the force of impact (one report, unconfirmed, has the airplane bouncing once and the nose gear collapsing on the second 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 may be a total loss.

The accident is being investigated by the Spanish CIAI (Comision de Investigacion de Accidents e Incidentes). This body may have its work cut out for it. No two of the publicly available descriptions of the final sequence of events agree. As one veteran A320 pilot remarked, "There is a lot going on here." Consider, he said, there's (1) windshear. (2) The aircraft is below 50 feet. (3) The aircraft is in the flare mode. (4) Take off go-around (TOGA) has been commanded. (5) Both sidesticks are being pulled back without use of the priority button. As he put the matter colorfully, using a "rope" metaphor to describe the aircraft's computerized flight control system, in the Bilbao accident "there are five guys pulling on the rope."

'Strictly Adhere' To These Procedures

Extracts of Airbus Industrie Operations Engineering Bulletin No. 146/1

Reason for issue: An A320 operator encountered a case of unexpected activation of high AOA protection during flare.


The AOA protection law can be triggered by AOAs lower than the stated threshold due to the advance phase term introduced in ELAC (elevator/aileron computer) L80. This advance term is only activated by sidestick input...

The combination of specific wind gradient/updraft and pilot inputs...caused the aircraft to enter the high AOA protection, which prevented the normal flare.

Note: During extensive simulator sessions, including simulation of the encountered wind gradient/updraft, it was difficult to reproduce the event, unless specific sidestick inputs were performed in a specific sequence and timeframe. (ASW note: What this is all saying is that under specific gusty conditions, the protection logic could restrict nose-up elevator orders.)


For approach to runways:

With known gusty environments, especially if these conditions generate vertical gusts due to the surrounding terrain,


When the reported gust wind increment (max. wind minus average wind) is greater than 10 kt.


Where moderate to severe turbulence is expected on short final,

The flight crew should strictly adhere to the following procedure:

- Use CONF 3 for approach and landing.

- Minimum V APP (approach speed) is VLS (lowest selectable speed at CONF 3) + 10 kt. The recommendation to use managed speed remains valid.

- Correct the landing distance for the speed increment (ASW note: With a 20% adjustment in landing distance, this guidance can reduce the choice of airports with limited runway conditions.)

- If the "SINK RATE" GPWS warning occurs below 200 ft., immediately initiate a go-around.

Source: Airbus


The Changes Explained

Airbus Industrie responds to the pertinent questions:

ASW: What is being changed?

Airbus: As presently implemented, the logic that triggers the Alpha Protection law is based on two values: Angle of Attack (Alpha), and the rate at which Alpha is changing. This latter term was added to the logic recently (two years ago) as part of an enhancement of the aircraft behaviour following some flight test work showing that very aggressive pitch inputs could result in transient exceedance of alpha max. However, as the experience at Bilbao demonstrated, this additional rate, or anticipatory term, could prematurely trigger alpha protection law under a very specific set of circumstances, viz., a combination of severe vertical and horizontal gusts and aggressive flight control inputs, that could result in a hard landing. To minimize the probability of this, the decision was made to revert to a definition close to the previous one; i.e., alpha protection will be triggered only by alpha, with the rate of change term deleted.

ASW: Is the change being made as an outgrowth of the accident at Bilbao?

Airbus: Yes, the decision to delete the rate term was made directly as a result of the experience at Bilbao.

ASW: If it does not affect AOA, why is the stretch A321 not affected?

Airbus: The A321 is not affected because the logic change for alpha protection was never made on the A321; due to the dynamic response characteristics of the longer fuselage on that aircraft, the issue noted during the flight tests mentioned above did not apply. (ASW note: the A320 is 123 ft. long (37.57m), and the A321 is 146 ft. long (44.51m). The angle between the ground and the tailcone on the A320 is 13.3°, and the angle from the ground to the tailcone on the longer A321 is 11.13°)

ASW: If the change is a reversion to previous software, why does it need approval of the certifying authorities?

Airbus: Certification is of the entire Elevator Aileron Computer (ELAC) software package. In the new standard the only change is the alpha protection law, however it is still necessary to re-certify the entire Elevator and Aileron Computer (ELAC) software package.

ASW: How would this change have helped to prevent the accident at Bilbao?

Airbus: Analysis of the Bilbao data has shown that alpha protection law was triggered by a very specific combination of environmental factors including vertical gusts and wind shear, and pilot inputs. As noted previously, this unique combination of circumstances triggered alpha protection because one of the two logic conditions that could trigger alpha protection was met: a very high rate of change of alpha along with large amplitude side stick inputs from the pilots. In the absence of this rate of change term, alpha protection law would not have been triggered. Very precise timing was necessary to reproduce the Bilbao event on a simulator. Intensive simulator and flight tests proved that the new software would have worked as planned in Bilbao.

ASW: From various accounts, the pilots were thwarted in their attempt to go-around. Would not a TOGA command override? Even if their airplane has contacted the runway, as a general proposition aren't the systems designed to allow the pilots to execute a go-around?

Airbus: Because of the premature triggering of alpha protection law caused by the combination of aggressive maneuvering and severe vertical gusts, 'side stick input was commanding angle of attack, not load factor, as is normally the case. During the Bilbao event, the crew selected TOGA (take off and go- around) thrust, and the engines spooled up to TOGA rating. However, the selection of TOGA thrust has no relation to the fly-by-wire control laws and, therefore, could not result in any "override" of alpha protection. The solution to the problem noted in Bilbao is to avoid the early triggering of alpha protection law in the first place, which is what the new ELAC standard accomplishes.

ASW: When is revised software to be distributed? Will the temporary recommended restrictions in gusty conditions be lifted at that time?

Airbus: Distribution of the revised ELAC standard will begin immediately upon certification. However, due to production and installation time requirements, we estimate it will be as long as one year before the entire fleet of aircraft has been modified. In the meantime, the temporary operating limitations will remain in place for all unmodified aircraft. These restrictions no longer will apply to an airplane once the new ELAC standard has been installed. It should be noted that operating restrictions also do not apply to autoland approaches, and under the terms of the Airbus OEB (Operations Engineering Bulletin), if autoland is otherwise approved, autoland approaches may be made without operating restrictions.

ASW: From what is known, did the Iberia airplane hit alpha protection or alpha max?

Airbus: Due to the early triggering of the alpha protection law in combination with a wind shear encountered after alpha protection was triggered, neither alpha protection nor alpha max was reached. The max value of alpha reached was less than alpha prot or alpha max.

ASW: Somewhere below 150 ft., but it's not clear where, both pilots reportedly pulled full aft side stick (FO was PF). If, say, the captain pulled aft side stick, while overlooking the need to press his priority button, the sum of the two pitch inputs might be greater than alpha max. What would the airplane do: 1) Not respond, 2) go to alpha max, or 3) go to alpha protection and maintain there?

Airbus: Simultaneous side stick inputs are summed only to the point where the command equals the equivalent of a single, full-stick commando In the event of dual side stick inputs that exceed the value of maximum deflection on a single side stick, the aircraft would respond as if there was a single, maximum input on one stick. These inputs were done below 50 ft, two seconds before impact.

ASW: At around 50 ft. AGL, during the transition to landing flare law, the airplane is programmed to nose over about 2° over an 8 second period, using as a reference point the last side stick position.

If TOGA has been commanded, would it override, or would this lowering of the nose in the flare somehow conflict with the need for the nose to pitch up to execute a go-around?

Airbus: Phasing in a nose down input during the final stages of a landing is done to give the pilots the "feel" of a normal flare. In order to hold the desired pitch attitude during the flare, the pilot must make increasing aft stick inputs, thus making the aircraft behave conventionally from a pilot's point of view. When TOGA thrust is commanded, the engines spool up to TOGA thrust and the Flight Director provides pitch guidance to the pilot. TOGA thrust selection has no relation with the flare law of the fly-by-wire system and therefore does not affect the 2° nose-over feature. >TK


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