Mr. George S. Moore (2)
A second discrepancy concerned improper installation of the metal seal diaphragm, Pyrene P/N 49/90140, which is installed in the discharge head of the extinguisher. This seal diaphragm covers the outlet port of the discharge valve and is installed prior to charging the extinguisher. Brass terminal blocks attached to each end of the seal are connected to electrical wiring in the discharge head for normal electrical discharge. When the discharge button is pressed in the cockpit current flows through the seal diaphragm which acts as a resistance and results in heating the seal. When the temperature rises sufficiently, the metal is softened and the 200 psi pressure in the extinguisher causes the seal to rupture and the CO2 flows into the underfloor compartment.
A number of these seal diaphragms were removed from the extinguisher discharge heads during the tests and examined. Several were found to have been installed off center resulting in improper or incomplete rupture. Photographs of representative seals are enclosed for your information. Comparison of the seal terminal blocks to their locating slots in the discharge heads indicates the reason for the off-center installation. The slots are three-fourths of an inch wide while the terminal blocks are five-eights of an inch wide. This one-eighth of an inch tolerance allows the seal to be mispositioned a critical one-sixteenth of an inch off center. The fact that mispositioning of the seal by such a small amount can result in questionable operation of the extinguisher system would indicate marginal, if not poor, design.
The Viscount Maintenance Manual contains explicit instructions for ensuring that the seal diaphragm is centered over the discharge valve opening. However, from the number of mispositioned seals noted during the tests, it is apparent that the manual instructions are not being followed.
Based on the results of the static and inflight tests as detailed in the preceding paragraphs, the Board believes that the Pyrene fire extinguishing system for the underfloor cargo compartment as installed on Viscount aircraft is not only inadequate for its intended purpose but also poses a danger to the flight crew.
Therefore, the Board recommends that the Federal Aviation Agency evaluate the design of the Pyrene Duo Head Model DCD-10 fire extinguisher system and take such corrective action as will increase its reliability and prevent release of CO2 into the habitable portions of the aircraft.
Flight Standards Service personnel assisting in the investigation of this accident are aware of these discrepancies and will continue to receive information relative to the problem. Personnel of our Engineering Division will also be available for further consultation on this matter.
Enclosure
