Re: Thunderstorms

From:         Robert Dorsett <rdd@netcom.com>
Date:         26 Jun 94 16:18:26 
References:   1
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>In article <airliners.1994.1349@ohare.Chicago.COM>, you write:
>
>[sub: lightning strikes on airplanes]
>
>>
>>Usually it's a tiny hole in the fuselage.  Rarely, the airplane blows
>>up.  In one case, it triggered a bizarre chemical reaction, and the wing
>>of an Iranian Imperial Air Force 747 fell off over Spain.  Sometimes, you
>>can even get ball lightning in the cabin!
>
>Can you point me to a reference to the Iranian 747 wing problem? 
>I am interested in finding out more.  

See NTSB-AAR-78-12, "Special Investigation Report--Wing Failure of Boeing
747-131, Near Madrid, Spain, May 9, 1976," dated October 6, 1978.

>What happened to the
>airplane? 

It crashed.

The "bizarre chemical reaction" was an early theory.  

The following may be of interest.  Note that the NTSB could not render a
probable cause statement, since it was out of jurisdiction.

"Hypotheses of wing failure causes" summarized as one of (pp. 17-18):

"a.  Internal overpressures.  Loads resulting from an explosion or ignition
of fuel vapors within the wing.

"b.  Aerodynamic forced.  Loads developed by the wing's aerodynamic 
surface as the aircraft exceeds limit speeds through the air mass; loads
developed as a result of an encounter with high velocity horizontal wind
gusts, updrafts, or downdrafts; loads developed as a result of maneuvering
the aircraft through pilot or autopilot control system inputs; or 
any combination of the above.

"c.  Dynamic forces--loads developed when a structural body is excited to
an undamped vibration at or near its natural frequency."

On the specific subject of lightning protection:

"Lightning currents flowing in the skin and and structural elements of
an aircraft can induce some level of voltage in all of the electrical
wiring enclosed within the aircraft.  The metallic structure of an aircraft
acts as a conductor of the lightning stroke currents and prevents direct
attachment to units enclosed by the metallic shield. The currents of the
stroke generate magnetic fields while passing along this shell which, in
turn, penetrate the metal shell and induce voltages and currents in
elements within the shell.  Experiments in the past have led to estimates
that the highest level of induced voltage can be 1,000 to 2,000 volts.

"Systems installed within a wing, such as fuel quantity measuring devices,
fuel lines, and electrical wiring, will be subjected to induced voltages
and currents.  Discontinuities of various electrical paths within the wing
can produce arcing of the induced voltages which could become ignition
sources for a flammable fuel.  as long as these induced currents are
conducted along a continuous path through the wing and wing elements and
eventually off the aircraft, no arcing should occur. Discontinuities are
prevented at points where mechanical joints exist by electrical bonding or
"jumpers" to assure conductivity.  Fuel system components are located in
zones in which the magnetic coupling effect is greatly reduced by the mass
of the shielding structure and the geometry of the component's location.

"Fiberglas components of the aircraft's structure include metallic strips
which permit current to pass through the component and thus prevent
discontinuitis of the direct lightning strike current.  Lightning strike
tests on scale models pointed out the areas of the aircraft's geometry
that were more susceptible to strike attachment.  Such informatin helped
to determine the location of components that were considered critical. 
Service experience also contributes to the design of lightning
protection."




--              
Robert Dorsett                                                       
rdd@netcom.com