Aircraft Accident Report: Pan Am Flight 214/Attachment II

by Leon H. Tanguay

ATTACHMENT II

 

B-80-96

 

DEC 17 1963

Mr. George S. Moore
Director
Flight Standards Service
Federal Aviation Agency
Washington, D. C., 20553

Dear Mr. Moore:

The investigation to date of the Pan American World Airways, Inc., Boeing 707-121, N709PA, aircraft accident near Elkton, Maryland on December 8, 1963, has identified in a gross sense the causal areas. That an inflight explosion occurred in the No. 1 reserve fuel tank is readily evident. It is believed the explosion stemmed from ignition of combustible fumes at the fuel tanks vent outlet by a lightning strike. Positive evidence of lightning striking in the wing tip area, some near the vent system outlet, has been noted. The investigation, to attempt to more precisely define the mechanism of initial ignition and progress to explosion, is continuing.

Fuel aboard at the time of the accident was approximately a 68/32 Jet A/Jet B by volume mix. Based on data furnished by PAWA, it is estimated the temperature of the fuel was approximately 42 degrees F. Except for normally unuseable fuel, all was being carried in the four main tanks. Considering all factors, it is concluded the fuel vapors in all tanks were within the flammability limits.

This accident has focused attention on a serious problem; not new as evidence by the studies previously made and the great number of reports and papers dealing with it. There is no doubt in our mind that our concern regarding this matter is shared equally by yourself and staff. It is appreciated also that immediately attainable corrective measures to provide total protection are not readily evident. However, we do feel and urge that all available government and industry resources should be marshalled to implement protective measures. Even partial measures effecting direct improvement appear warranted before the ultimate protection can be provided.

The following recommendations for your consideration are submitted:

1. Install static discharge wicks on those turbine powered aircraft not so equipped.

2. Reevaluate problems associated with incorporation of flame arrestors in fuel tank vent outlets. We believe positive protection against fuel tank explosion from static discharge ignited fuel/air mixtures at fuel tank vent outlets can be provided by flame arrestors having sufficient depth.
3. A possible alternative to No. 2 that may be considered is to render the mixture emitting from the vent outlet non-ignitable by the introduction of air into the vent tube.
4. We believe the surge tanks located just outboard of the reserve tanks, by virtue of their location near the wing tip, are vulnerable with respect to lightning strikes. Burn marks on the skin in the tip area of N709PA substantiates this belief. This being the case, it is believed a measure of protection will be attained if the wing skin is not utilized as part of the surge tank walls. This could be accomplished by providing an inner wall with an air gap between it and the wing skin to form the surge tank. It is recommended that this concept be considered. Another alternative appears to provide sufficient thickness of the skin in this area to prevent burning through by lightning strikes.
5. Suggested for consideration is the requirement that only Jet A fuel be used commercially. Vapor flammability temperature charts provided by Esso show that much less of the operations would occur with the vapor in the flammability range while using Jet A fuel as compared with Jet B fuel.
6. Finally, it is recommended that every effort be expanded to arrive at a practical means by which flammable air/vapor mixtures are eliminated from the fuel tanks. There appears to be at least two approaches to accomplish this act. There is the possibility of inerting the space above the fuel by introduction of an inert gas. An alternate approach is to introduce sufficient air circulation into the tanks to maintain a fuel/air ratio too lean for combustion. There may well be other approaches to attain this goal; if so, they should be explored. Other problems of like complexity have been resolved and we feel the resolution of this problem is likewise attainable at a cost commensurate with the benefits. We recommend that FAA/CAB solicit the aid of the aviation and petroleum industry as well as government and defense agencies to provide a solution to this problem that is applicable to aircraft in service as well as new aircraft.

The above recommendations amplify our telephone conversation of December 11 and our meeting on December 12. Through your participation in the investigation of the accident as well as by direct contact from this office, you will be advised of current development as well as of any other thinking we may have regarding corrective measures. My staff will also be available for any assistance we can provide to resolve this safety problem.

Sincerely yours,

Original signed by
Leon H. Tanguay

Leon H. Tanguay
Director, Bureau of Safety.