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Safetv Week > June 18. 2001 > Article> Print
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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.
Analysis:
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.)
Procedures:
For approach to
runways:
With known gusty
environments, especially if these conditions generate vertical
gusts due to the surrounding terrain,
Or,
When the reported
gust wind increment (max. wind minus average wind) is greater than
10 kt.
Or
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
COPYRIGHT 2001 pm
Media,
LLC COPYRIGHT 2001
Gale Group
Air
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