Aviation Accident Report: TWA Flight 891/Amendments
CIVIL AERONAUTICS BOARD
AIRCRAFT ACCIDENT REPORT
TRANS WORLD AIRLINES L-1649A, U.S. REGISTRY N—7313C
NEAR MILAN, ITALY, JUNE 26, 1959
English translation of the Report of Italian Republic Ministry of
Defense Board of Inquiry Appointed to Investigate the Accident.
AMENDMENTS TO THE REPORT OF THE BOARD or INQUIRY ON THE CRASH OF THE S. C. PLANE NO. 7313/0
Chapter XI
(1) Par. 11 4.18 These tests made in a specialized laboratory, disclosed that:
[1] On an L—1649 plane static discharges should occur at the vent outlets if the aircraft is struck anywhere by lightning and, if, it is not struck, they may occur when the aircraft flies through clouds that are charged with electricity.
[2] Static discharges, comparable in strength to those that can occur in flight, generated in still air in a container filled with flammable fuel vapors, cause those vapors to ignite.
[3] The tests mentioned in the preceding paragraph cannot, however, on the basis of the knowledge available at the present time, be taken as conclusive evidence that static discharges generated at the vent outlets of an aircraft in flight, from which (outlets) flammable fuel vapors are escaping, will necessarily cause those vapors to ignite, but they do indicate that such a danger is within the realm of possibility.
[4] The evidence and considerations referred to in the two preceding paragraphs indicate very definitely that adequate precautionary measures should be developed and adepted, the most important of which would be to insert anti—flame wire gauze onto the vent outlets and to have the said out—lets shaped and constructed in such a way as to render them less subject to the development of static discharges.
[5] Static discharges can, and generally should, develop without leaving on typical aircraft metals, and therefore on the vent outlets, any normally visible evidence.
Chapter XIV
(2) Par. 14.4.2.6 (b') (b') The possibility that static and nonstatic electrical discharges, under the testing conditions described in pars. 11.4.4 and 11.4.18, might ignite flammable gasoline vapors has already been tested in the United States, with positive results.
Amendments to the Report
[1] The tests mentioned in par. 11.4.18, made in a specialized laboratory, disclose that:
(a) On the strength of available knowledge, it cannot be stated positively that static electricity discharges generated at the vent outlets of an aircraft in flight, from which (outlets) flammable gasoline vapors are escaping, will necessarily cause those vapors to ignite, but the indication is that such a danger is within the realm of possibility.
(b) The atmospheric conditions existing at the time of the crash were extremely favorable for the development, at the vent outlets of Super Constellation aircraft No. 7313/C, of static—electricity discharges entirely capable of igniting flammable gasoline vapors under the test conditions described in par. 11.4.18.
[2] The tests referred to in par. 11.4.4, which were made in the tunnel:
(a) On one only of the original manifolds, located on the trailing edge of a wing contour, from whose four outlet pipes (compare par. 14.0) there was an escape of vapors containing a mixture compressed within the limits of flammability (tanks Nos. 6 and 7) and not included in said limits (tanks Nos. 3 and 4);
(b) At a pressure corresponding to the altitude of 1,700 feet;
(c) At an air flow speed of 170 knots I.A.S.;
(d) On the velocity of the vapors escaping from the individual pipes, corresponding to climbing speeds of 900, 600, and zero feet a minute, revealed that said vapors, as the result of suitable electric discharges of a nonstatic nature, become ignited only under conditions that would occur when the plane was climbing, and that the flame did not spread to the interior of the tanks.
In conclusion, although the evidence referred to in the aforesaid pars. 11.4.4 and 11.4.18 does not warrant the assertion that static electricity discharges generated at the vent outlets of the Super Constellation aircraft in flight can.ignite the flammable gasoline vapors issuing from those outlets, it nevertheless indicates that such a danger is within the realm of possibility and that the ignition of said vapors would actually take place if the electrical discharges were of sufficient intensity and of a non static nature. CONCLUSION
From the hypotheses listed in paragraph 14.4 and discussed in sub-paragraphs
14.4.1
14.4.2.1
14.4.2.2
14.4.2.3
14.4.2.4
14.4.2.5
14.4.2.6
The Board, having found in the facts under examination no significant and concrete indication or evidence that would warrant suggesting as the probable cause one or more of the hypotheses discussed in sub-paragraphs
14.4.1
14.4.2.1
14.4.2.2
14.4.2.3
14.4.2.4
14.4.2.5
points to the hypothesis dealt with in par. 14.4.2.6, namely
Explosion set off by static-electricity discharges (streamer corona)
as the probable cause of the accident.
(3) CHAPTER XVI
Remains as it now appears in the Report, except for the elimination of the last paragraph.
Elimination of the last paragraph is necessary, according to strict logic, on the basis of the following line of reasoning.
A hypothesis may be
All seven hypotheses discussed in the Report are possible and, as such, each one of them could have actually occurred.
Of these seven hypotheses, the first six are possible, but the inquiry has brought out no evidence or indication suggesting that any of them is probable.
On the other hand, with respect to the hypothesis mentioned in sub-paragraph 14.4 2.6, there are some evidence that would make it probable.
In other words, in the discussion of the seven hypotheses considered in the Report and insofar as their probability (likelihood) was concerned, a process of elimination was adopted whereby the first six are discussed and rejected, while the last one is discussed and recognized as probable.
Chief Inspector
For General Duilio Fanali, AF, Chairman of the Board
[s Col. P. Pernazza
This work was published before January 1, 1929, and is in the public domain worldwide because the author died at least 100 years ago.
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