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"Aircraft Tire Explosions Can Discharge Energy Equivalent to Dynamite"


 
Tuesday, August 8, 2000

Tire Explosions Can Discharge Energy Equivalent to Dynamite
French authorities focusing on failure of landing gear water deflector in
Concorde crash
Air Safety Week


Bursting airplane tires are like "rubber bombs." Under extreme conditions of
pressure and heat buildup, an exploding tire can release the energy
equivalent of 4-5 sticks of dynamite. The potential for cascading, possibly
catastrophic damage to nearby fuel tanks and engines is a well-recognized
hazard.

The fiery July 25 crash of an Air France Concorde has cast the issue of
bursting tires into chilling focus. French officials have said that tire
debris from the accident airplane was found on the runway at Paris' Charles
de Gaulle International Airport. The debris was strewn along the runway past
the V1 point on the airplane's takeoff run - in other words after the
imaginary line where the airplane had gained so much speed there was not
enough runway remaining to bring it to an emergency stop. In these cases,
the standard practice is to get airborne and prepare the airplane for an
immediate emergency landing. According to radio transmissions, this course
appears to have been uppermost in the mind of Capt. Christian Marty as he
acknowledged to the tower controllers the trail of fire streaming from his
aircraft during its takeoff run.

Meanwhile, Air France officials have been at pains to explain why they had
not followed a British Airways modification to the Concorde landing gear. A
piece or pieces of the water deflector were found on the runway after the
Concorde crash. According to an Air France statement, the British Airways
modification was not incorporated into the Air France Concorde fleet because
it was not designed to prevent the deflector from separating from the
landing gear in the event of an impact. "It was designed simply to ensure
that the deflector would remain in one piece should it come loose from the
landing gear," the Air France statement declared, apparently attempting to
downplay the importance of the British Airways modification. The Air France
statement added somewhat defensively that the modification was never
required by French Civil Aviation Authorities.

Tires as "armed bombs"

Tires can be dangerous even during routine ground maintenance, as evidenced
by this grim example, contained in an article titled "Like Rubber Bombs"
that appeared about 20 years ago in a U.S. Navy publication called Mech:

"The tire/wheel assembly exploded, tearing the hub into two pieces. One
piece bounced off a railing, hitting one helper in the head, killing him
instantly. His body was found 10 feet from the spot where he had been
standing. The other portion of the hub struck the crew chief with so much
force that he was thrown some 30 feet. His head and right arm were severed
from his body. All of the wheel bolts were found bent...and the threads on
five bolts were stripped. Only four of the wheel nuts were found."

This article about the extreme care needed in tire maintenance was triggered
by a spate of incidents. The author, Navy materials engineer Marcelo
Fontanoz, cautioned that an inflated aircraft tire/wheel assembly needs the
cautious handling of "an armed bomb."

While subsonic airliners have experienced considerable damage on occasion
from bursting tires (to include tire debris penetrating fuel tanks),
Concorde represents perhaps the extreme example for airline service. For sea
level conditions, a B747-400 at maximum weight rotates and begins climbing
into the air at a speed of about 163 knots (187 mph). The tires on the jumbo
are inflated to 205 psig, or pounds per square inch gage (estimates from
various pilots placed the jumbo's tire pressure in a range of 180-210 psig).

At the point where the B747 lifts into the sky, the Concorde is still
accelerating to its fully loaded rotation speed (VR) of 199 knots (228 mph).
It must fly 40 mph faster to get airborne. Its tires are inflated to 225
psig, or about 10 percent higher than the pressure in the tires of a
subsonic jumbo jet. One pilot vividly described the Concorde's takeoff
thusly: "Concorde rotates at a higher speed than subsonics because the wings
do not provide lift so much as...stability to what is really a rocket."

Catastrophic Deflation

In his "rubber bombs" article, Fontanoz calculated the energy released by
exploding tires in equivalent sticks of dynamite. For comparison purposes,
he also showed the velocity a 3,000-lb. automobile would have to travel in
order to possess the kinetic energy equivalent to the energy released by an
exploding tire. Calculated from the various ideal gas laws and thermodynamic
equations, Fontana's figures show the energy released by exploding tires on
various Navy aircraft. The 200 psig inflation pressure for main gear tires
of the P-3, a 4- engine turboprop patrol aircraft, seems closest to the
B747. The 225 psig for Concorde tires approximate the 220 psig for the A-6,
an attack bomber. As shown, if the tire should burst due to some mechanical
failure (e.g., tread letting go), the energy released is equivalent to about
half a stick of dynamite.

Under conditions of pressure increasing to the burst point, as could occur
during heat buildup during taxi and takeoff, a tire deflating
catastrophically would release the energy equivalent of four sticks of
dynamite (see P-3 Main, burst pressure, second row from bottom in table).
The table vividly captures the tremendous energy released by an exploding
tire. The physics of the hazard have not changed one whit since the
publication two decades ago of Fontanoz' cautionary article. Among his
recommendations:

Initial inflation: Always place the tire/wheel assembly in the safety cage
for initial inflation. In the event of a tire explosion, the cage will
absorb the destructive energy.

Inflation on the aircraft: Always approach the wheel assembly in a fore or
aft direction - never in line with the axle.

Notes From the Past

Prudence and precaution respecting the condition of the tires on the
Concorde may be even more important than for subsonic jets. Not only are the
tires under higher pressure, rolling at high speeds, the main landing gear
is located close to the engine inlets and the wing fuel tanks. A 1997 report
by the Federal Aviation Administration's (FAA) Office of Aviation Research
referenced the particular hazard regarding the Concorde. The report, titled
"The Potential for Fuel Tank Fire and Hydrodynamic Ram From Uncontained
Engine Debris," contains a wealth of obviously related information regarding
the hazard to vital aircraft systems posed by debris from exploding tires
(hydrodynamic ram is the overpressure produced by the motion of a fragment
inside a fuel tank).

With respect to Concorde, the FAA report said, "The risk of fire and
explosion in aircraft fuel tanks in the Concorde SST (supersonic transport)
due to uncontained failure of the Olympus 593 engine was initially
recognized in a British Aerospace report by Wallin (1976). The probability
of ignition for Jet-A was estimated...to vary from 0.05 to 0.8 depending on
fragment trajectory and fuel temperature."

The fragments for this analysis were hot blades from an uncontained failure
in the turbine section and "consequently, the potential for ignition is
high". Nevertheless, the point is relevant to exploding tires. In 1979 an
Air France Concorde on takeoff from Washington-Dulles experienced two tire
blowouts on the left main landing gear; tire debris and wheel shrapnel
punctured three fuel tanks, tearing a hole in the top of the wing and
damaging the No. 2 engine (see ASW, July 31). The case, with a luckier
outcome, has obvious parallels to the recent crash. The recommendations sent
November, 1981 by the National Transportation Safety Board (NTSB) to the
French Bureau Enquetes Accidents (BEA), primarily to inspect wheels/tires
before each takeoff, and not to retract the landing gear in the event of a
blown tire, unfortunately did not prevent a raft of future occurrences. For
one thing, visual inspection would cover perhaps 85 percent of the tires'
surface, but not that portion in contact with the ground. Unless the
airplane were moved slightly to make the rest of the tire surface available
for inspection, the potential would remain for a deep cut or a bald spot
from a hard touchdown to go undetected. Unfortunately, such a procedure may
not be operationally convenient when the airplane is parked at a jetway
preparing for departure.

Nevertheless, the documentation of the damage to that Air France Concorde in
1979 can serve as a "control sample" for the present investigation,
particularly as it pertains to the trajectory of liberated debris.
Meanwhile, as of an announcement late last week by French Transport Minister
Jean Claude Gaysot, the five remaining airplane's in the Air France Concorde
fleet will remain grounded until investigators turn up more about the cause
of the crash.

A Partial Listing of Concorde Tire/Landing Gear Incidents

* October 1993: British Airways Concorde taxiing at Heathrow suffers brake
lock,
causing a main gear tire to burst. Fragments of the water deflector punch
holes in wing fuel tank.

* July 1993: BA Concorde landing at Heathrow experiences brake lock and
subsequent blown main gear tire. Flying debris damages wing, hydraulics and
No. 3 engine.

* Feb. 1981: Air France Concorde experiences blown tire on left main gear
during
takeoff at Washington-Dulles. Engine problems resulting from damage force
the crew to land at New York's JFK with one engine shut down due to
vibration.

* Sept. 1980: BA Concorde experiences blown tire on takeoff at
Washington-Dulles. Upon landing, pieces of tire damage the engine and
airframe.

* Oct. 1979: Air France Concorde experiences two blown tires during takeoff
at
New York's JFK. Landing gear is retracted and the flight continues to Paris.

* July 1979: Air France Concorde experiences blown tire on takeoff from
Washington-Dulles. A compressor stall occurs while climbing through 27,000
feet, possibly from foreign object damage.

* June 1979: Air France Concorde experiences two blown tires on left main
gear
during takeoff from Washington-Dulles. The plane's magnesium wheels strike
the runway, break apart, hurling shards into fuel tanks, engines and
fuselage. Unable to retract gear, the crew returned to Washington.

Source: Compiled from multiple sources

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