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Mr. B. R. Allen Director, Bureau of Safety Civil Aeronautics Board Washington, D. C. 20428

Dear Mr. Allen:

This is in answer to your letter of October 9, 1964, reference B-80-96, concerning the CO2 fire extinguishing system on the United Air Lines Vickers Viscount aircraft. It supplements our acknowledgment letter of October 19, 1964. Your letter of November 17, 1964, and our answer of December 15, 1964, also bear, in part (as will be explained), on this same subject.

We have evaluated the design of the Pyrene Duo Head Model DCD-10 fire extinguisher system. During this evaluation particular attention was paid to the two discrepancies mentioned in your October 9, 1964, letter. The first of these, spraying CO2 into the cockpit when extinguishers were discharged, was the subject of your letter of November 17, 1964, and our answer of December 15, 1964. The second discrepancy was in regard to the improper installation of the metal diaphragm, Pyrene P/N 49/90140, installed in the discharge head of the extinguisher.

As a result of our investigation, we have published a proposed airworthiness directive, applicable to the Vickers Viscount Model 744, 745D, and 810 series aircraft equipped with Pyrene fire extinguishers. The proposal appeared in the Federal Register of June 29, 1965, as Docket No. 6734.

During our investigation of this subject, we worked with representatives from UAL and the Air Registration Board, as well as your own technical personnel. Several independent flight tests were conducted, and also ground (using an installed system), and laboratory tests. Reports on these tests were submitted as exhibits at the public hearing at Knoxville, Tennessee, January 11–16, 1965.