Page:CAB Aircraft Accident Report, American Airlines Flight 320.pdf/13

 in altitude readings; one read 1,000 feet off because of being out of calibration; and one was reported as malfunctioning, but except for serial number and date, no information was given. In many instances, information supplied concerning these malfunctions was insufficient to make complete evaluations.

Of the drum-type altimeter malfunctions mentioned above, none of these malfunctions involved more than one of the drum-type altimeters aboard the aircraft. In addition, examination and testing of the altimeters involved in this accident failed to reveal any mechanical failure not attributable to impact shock and immersion in salt water. It should be noted that the failures causing known erroneous indications discussed earlier in this analysis could not have been detected in laboratory examination and test if the altimeters involved had been subjected to impact and submersion as were the altimeters installed on N 6101A.

The Board believes that the destruction of the two instantaneous vertical speed indicator internal mechanisms was caused by impact and submersion of the instrument panels in approximately 20 feet of water.

Following the erroneous altitude indication of a drum altimeter at Detroit, Michigan, April 3, 1959, mentioned above, use of the drum-type instrument as a "third" altimeter was discontinued by all air carriers operating Electras. Since that time, the Board has had no opportunity to observe or make further critical examination of drum altimeters in Electra aircraft even as a "third" or standby instrument. However, the service results of the use of this type of altimeter in other aircraft are being monitored.

On the basis of the available evidence, several possible equipment malfunctions and operational errors have been examined critically with a view to determining the most probable cause of this accident.

As far as could be determined, the two drum-type altimeters were installed in N 6101A at the Lockheed factory. Electra N 6101A had approximately 302 hours of flight time during all of which the altimeters had operated satisfactorily.

According to the testimony of First Officer Hlavacek and Flight Engineer Cook, the altimeter indications appeared normal throughout the approach to approximately 500 feet when the aircraft struck the water. From their testimony it is apparent that any error in the two drum altimeters must have been in the nature of a lag rather than pointer sticking. Both testified to a gradual reduction in altimeter indication during the approach from New Rochelle to the point of impact consistent with descent contemplated in the instrument approach procedure. At no time were they conscious of a prolonged indication of a fixed altitude. To have indicated 500 feet at the moment of impact, the lag in the altimeters must have been approximately 500 feet. None of the altimeter malfunctions known to the Board here have involved similar manifestations.

As required by the Civil Air Regulations, the altimeters installed on the pilot's and copilot's flight instrument panels have separate static sources. The static lines leading to the instruments from normal sources are completely independent. Since there exists no common element of the instruments or any of their related static system and sources, an identical and simultaneous malfunction of these instruments and associated systems of the magnitude suggested by the crew testimony would involve such an extreme mathematical improbability that the Board is compelled to reject it.