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they approached the localizer and the
Airbus turned to intercept it. As they tracked
the localizer, all appeared normal.
As they closed to 22 miles from the runway,
the airplane pitched up abruptly. The crew
realized that the airspeed had deteriorated below
135 knots. Quickly, the Captain disengaged
the autopilot and autothrust systems. As
expected, the A319 responded normally to the
manual inputs made by the Captain. They descended
back to 3,600 feet and accelerated
back to 160 knots. What just happened? Why
did it occur? Would it happen again?
After reengaging the autopilot and autothrust,
the airplane flew a normal coupled approach
and auto-landing. Arriving at the gate the Captain
entered the pitch event into the maintenance
logbook. As one would expect, the
outbound flight canceled.
First indications were that an angle of attack
sensor provided incorrect information, and that
caused the pitch up. Later, upon further analysis,
this did not prove to be correct. The crew
contacted the Company and the ALPA Central
Air Safety Committee.
After some initial conversations with the
Fleet Manager, Captain Bob Skinner, we realized
that there were several unanswered questions
and very similar levels of concern. This
was a significant inflight event and there was
not a good explanation of the cause.
We decided that this required much more
investigation. A call to our Airbus avionicsengineering
experts in Tampa verified the de10
US AIRWAVES – April 2000
Central Air Safety
tails of the event. As we studied the flight data
recorder, several anomalies appeared. We began
to ask specific questions of Airbus to help
us understand what happened.
A conference call followed with the Airbus
engineers at the factory in Toulouse. They explained
what they saw in the flight data recorder
data. At the conclusion of the call, there were
still many unanswered questions. We discussed
using the engineering simulator to recreate the
event. Airbus concurred, and they set up the
time and the needed experts for the tests. After
a short debate, we decided that it would be
prudent for a couple of us to attend the tests.
Just before leaving for Toulouse, we were
able to contact ALPA Safety representatives at
other airlines that operate the Airbus and ask if
they were aware of any similar events. Surprisingly,
the answer came back that some of the
maintenance engineers were aware, but the pilots
were not.
Two days later, we arrived in Toulouse and
met with the Airbus engineers in person. Over
the next two days, we agreed on the conditions
that we would use in the simulator. Additionally,
we agreed that the participants in a postsimulator
meeting would decide on future action.
The stage was set to not only understand
the event but to take newly learned information
and distribute it to other operators.
Early in the morning, we met at the engineering
simulator. Along with the engineers was a
pilot from the Flight Testing Department. The
technicians loaded the data into the simulator,
and we began our tests. As expected, the simulator
followed the flight path of the event airplane.
It began to pitch up 22 miles from the
runway threshold just as the event airplane had.
The flight data recorder had recorded a glide
slope signal prior to the event. As the A319
tracked the localizer, all the conditions were
armed for a glide slope intercept. This would
have been normal except for the large scallop in
the glide slope signal that occurred causing the
flight management guidance computer to believe
that the aircraft was approaching the glide
slope. It then switched into glide slope ‘*’
(capture) mode. The scallop was very quick so
that the computer believed it was below a valid
glide slope and began to climb. This explained
the pitch-up and the quick rate of the pitch-up
because glide slope * can command up to three
times the normal pitch rate to capture the glide
slope. In addition, we then understood the
airspeed decay. It is normal for the Airbus to
“ The event was
the result of the
airplane performing
exactly as
designed. Pilot
intervention was
appropriate and
necessary. ”
trade airspeed for altitude in an effort to capture
the glide slope. We also learned that the
airplane remains within its protected flight envelope.
There is no danger of a stall. (You
cannot stall an Airbus when it is in normal law.)
The event was the result of the airplane performing
　
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