N571UP, the aircraft involved, seen taking off from Dubai in November 2008
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A UPS Boeing 747-400 flying between Dubai International Airport and Cologne Bonn Airport crashed close to Dubai airport, killing the two crew members, on September 3, 2010.
The aircraft had departed Dubai International earlier, but returned after the pilots reported smoke in the cockpit.
The General Civil Aviation Authority (GCAA) of the United Arab Emirates released a comprehensive, 322-page accident report, on July 24, 2013. The GCA found “with reasonable certainty” that the fire which caused the crash originated in a cargo container which held thousands of lithium batteries.
The GCAA’s final report states that even now its investigators have been unable to determine the initiating action that resulted in the cargo fire.
“One line of testing outside the scope of this investigation was the investigation of the possible effect of structural-acoustic coupling to determine the acoustic principle sources and transmission paths for airframe junction vibration transmission, the effect of multi-frequency phased vibration of the fuselage structure caused by tonal disturbances, either engine derived or by airbourne structural excitations, affecting the modal characteristics and fuselage dynamic responses,” states the report.
Structural-acoustic coupling phenomenon in an aircraft fuselage is a known characteristic of aeroelastic structures and yet the vibration and acoustic signatures of large aircraft cargo areas are not well understood.
The GCAA said the FAA working with European colleagues at EASA and Boeing should evaluate the Boeing 747 Freighter/Combi aircraft Class E cargo compartment for a structural acoustic coupling phenomena in the aircraft fuselage.
Further, the GCAA urges the US air accident investigation bureau at the NTSB, the FAA and EASA fire test divisions to perform tests on lithium batteries to determine their ignition properties when subjected to external sources of mechanical energy, including acoustic energy in flight range modes and acoustic harmonic modes.
“The purpose of this testing is to determine the safe limits for the air carriage of lithium type batteries in dynamic aeroelastic, vibrating structures where the battery electrolyte composed of an organic solvent and dissolved lithium salt could become unstable when exposed to these forms of mechanical energy.”
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A U.S. Department of Transportation (DOT) document notes:
Rechargeable lithium batteries (also called lithium ion (Li-ion), or secondary lithium batteries), and nonrechargeable lithium batteries (also called lithium metal, or primary lithium batteries), provide more energy and a longer operating life than other battery chemistries. They have the potential to generate a significant amount of heat or catch fire if damaged or improperly packaged, cared for, or constructed than do other batteries.
Reference: Battery Guide
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The U.S. DOT 173.159a vibration test level for batteries is:
(1) Vibration test. The battery must be rigidly clamped to the platform of a vibration machine, and a simple harmonic motion having an amplitude of 0.8 mm (0.03 inches) with a 1.6 mm (0.063 inches) maximum total excursion must be applied. The frequency must be varied at the rate of 1 Hz/min between the limits of 10 Hz to 55 Hz. The entire range of frequencies and return must be traversed in 95 ±5 minutes for each mounting position (direction of vibrator) of the battery. The battery must be tested in three mutually perpendicular positions (to include testing with fill openings and vents, if any, in an inverted position) for equal time periods.
The corresponding acceleration levels are: (10 Hz, 0.31 G) & (55 Hz, 9.3 G)
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One lithium battery vendor has noted that microscopic metal particles may come into contact with other parts of the battery cell on rare occasions, leading to a short circuit within the cell.
Although battery manufacturers strive to minimize the presence of metallic particles, complex assembly techniques make the elimination of all metallic dust nearly impossible.
A mild short will only cause an elevated self-discharge. Little heat is generated because the discharging energy is very low. If, however, enough microscopic metal particles converge on one spot, a major electrical short can develop and a sizable current will flow between the positive and negative plates. This causes the temperature to rise, leading to a thermal runaway, also referred to ‘venting with flame.’
Perhaps the aircraft vibration stirred up these hypothetical metal particles in one or more cells, ultimately resulting in the fire. This possibility must be considered in any future test program.
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– Tom Irvine