Correspondence Concerning the Fluid Hammer Scenario
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After reading the official NTSB Report, I was disturbed by the lack of fundamental explosion dynamics analysis. Consequently, I examined the available documentation from the point of view of a riveted pressure vessel failure vs. aircraft structural failure. The conclusions I came to were significantly different from the official version.
A fluid hammer event, which was not addressed either in Boeing's design or certification, nor in the Aloha or FEAT accident investigations, occurs when the "flap" becomes corked by a large object (the Flight Attendant in the Aloha case). Slamming the door on a 700 mph jet stream creates a localized, short duration, high pressure spike, up to several orders of magnitude times the allowable design pressure. This is a "fluid hammer".
I have had our technical staff review the documentation you supplied from a Mr. Matt Austin of Honolulu concerning the Aloha Airlines decompression accident in 1988. Based on their review, we believe that Mr. Austin provides a valid scenario in the abstract……We don't disagree with Mr. Austin's explanation about how an airplane can decompress at 24,000 feet after a 10" x 10" hole is blown open in the skin and about how devastating the "fluid hammer" effect can be at this altitude.
Thank you for the information you sent by fax regarding Aloha Flight 243. The scenario you pose is very interesting and a viewpoint that I had not heard or thought of previously…..If your concern is that the B-727 and B-737 are not fail-safe as designed because of the fluid hammer phenomenon you described, that is an issue best discussed with the Certification Office assigned to those models of aircraft. Since your question deals with the original design of the aircraft and its certification as a safe aircraft, that is within the purview of the Transport Directorate, Northwest Mountain Region.
This is in regard to your letter of June 30,1998, concerning fluid hammer effects on decompression loads. We do not concur that the Aloha airlines fuselage failure can be attributed to overpressure, caused by a flight attendant becoming lodged in the opening…. However, your theory is interesting and to our knowledge has not been considered in any decompression analysis……. Even so, we intend to perform some studies to obtain better estimates of pressures.
I requested (through our R&D coordinator, Paul Hawkins) that the FAA Technical Center perform a study to evaluate the effects of sudden stoppage of the airflow through an opening in the pressure cabin. Paul advises me that Congress reduced the R&D budget for this fiscal year, so the work will have to be postponed.
I have reviewed the information you sent me regarding the fluid hammer effect and have considered it's potential influence on Boeing's fail safe criteria concerning safe decompression.
Many of the tests we have performed in the past and some in-service events have in fact experienced the corking of the opening through which cabin air is flowing during decompression.
The fluid hammer effect does not invalidate our fail safe criteria.
To my knowledge, no tests have been conducted of the worst case scenario for a flapping event, a 10"x10" flap with a human type corking element. While validity of the "safe decompression" is legally the domain of Boeing and the FAA, from an engineering stand point, until the worst case realistic scenario (as described above) is tested, engineering validity is not proven.