Jet's skid marks raise puzzling questions

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Jet's skid marks raise puzzling questions
By PAUL KORING
Saturday, August 6, 2005 Updated at 2:03 AM EDT
From Saturday's Globe and Mail

The long skid marks raise troubling questions about whether the crew
should have aborted the landing, whether they made and then abandoned
an attempt to take off again, and whether the Airbus A340's
sophisticated anti-skid system was working properly in the wild and
wet conditions.

Investigators have confirmed that the aircraft “landed long” — that
is, touched down much farther down the runway than it should have —
and was still travelling at about 150 kilometres an hour when it ran
off the end of the 2.4-km runway.

Investigators also confirmed yesterday that a treasure trove of data
from the flight recorders and cockpit conversations survived the fiery
aftermath of the accident and has been successfully extracted from the
so-called black boxes in France.

“I'm very happy to report we have good, solid data,” said Réal
Levasseur, lead investigator for the Transportation Safety Board
probe. He said that will “allow us to . . . start removing wreckage
from the site much earlier than we would have anticipated.”

Investigators are using more than 650 data parameters; most were
measured several times a second, including the positions of individual
switches, airspeed, altitude and the control inputs from the crew.

The team will be able to minutely reconstruct the flight of the
aircraft, which arrived in the middle of a thunderstorm after a
transatlantic journey from Paris.

Investigators also completed their interrogation yesterday of the
unidentified Air France co-pilot who was flying the landing.

The captain remains in hospital with unspecified back injuries and has
yet to be interviewed.

“The co-pilot was very frank in his discussions with us,” Mr.
Levasseur said.

Although it may be months before the investigation is complete, the
flight and cockpit recorders should quickly provide answers to crucial
questions about whether the Air France pilots were considering
aborting the landing and briefly advanced the throttles to initiate a
“go-round” after landing halfway down the runway.

A go-round, usually initiated before touchdown, is the prescribed
procedure for aborting an unstable landing or one that would put the
aircraft down in the wrong place on the runway.

Pilots are trained to apply power, climb away, contact air-traffic
control and seek a return to the landing pattern rather than continue
with an unstable approach and landing.

If the Air France crew even momentarily began a go-round by advancing
the throttles after landing, it would have cancelled out the
preprogrammed automatic braking that pilots usually dial into the
Airbus's sophisticated flight management computers before landing.

And if the go-round decision itself was then reversed or countermanded
by the Air France captain, the flight crew would then be faced with
braking manually using the foot pedals.

According to Mr. Levasseur, an initial examination of the ill-fated
aircraft's tires showed no evidence of aquaplaning. That occurs when
the tires slide on a thin film of water.

With no evidence of aquaplaning, and of anything so far that indicates
something amiss with the aircraft, investigators will focus on the
flight crew's decisions, performance, training and whether they
followed the best procedures.

The crash, from which all 309 people on board scrambled to safety
before fire engulfed the fuselage, was the first serious accident
involving the long-range, four-engined, wide-body A340.

The long skid marks on runway 24L puzzled several aviation experts.

Three experts, all with extensive experience with the systems on the
A340, offered conflicting theories as to whether the skid marks could
have been made with the aircraft's sophisticated anti-skid braking
system, assuming that system was operating properly.

Normally, anti-skid systems (just like those on modern cars) prevent
wheels from locking up on hard braking, releasing just enough pressure
to keep the wheels turning, which provides the maximum possible
deceleration. Locked-up wheels that skid actually lengthen braking
distances.

One expert suggested the skid marks would on closer examination turn
out to be a staccato series of marks, left by a functional anti-skid
system.

Another suggested that the TSB investigators may have mistaken the
marks, believing a skid that long would have shredded the tires, even
in the wet conditions. At least one set of main landing wheels, with
tires apparently intact, is visible in photographs of the burned-out
wreck.

Finally, another expert suggested the crew may have disengaged the
anti-lock system — although he offered no explanation as to why they
would — either by flipping a switch in front of the co-pilot or
engaging the parking brake that applies maximum force to all the
aircraft's brakes.

All three experts agreed that unless there was some sort of
catastrophic series of multiple failures that rendered the main
braking computer and at least two layers of backups inoperable, then
the automatic anti-skid braking system would be available to the
pilots unless they disengaged it.