Page:CAB Aircraft Accident Report, American Airlines Flight 320.pdf/12

 Tests were accomplished on the effect of a possible static leak in the nose wheel well static system components; of rain and icing on the static ports; of a large spoiler ahead of the static ports; and of simulated careless washing and ground splash on the static system. The Electra static system was evaluated in flight by flying through freezing water dumped from an Air Force tanker. The result of these tests indicated ice buildup on the fuselage was confined to the forward section and did not approach the area of the static ports even when the test Electra was yawed drastically. Inflight vibration measurements were obtained from the instrument panel and outer case areas of drum altimeters. Seven drum altimeters were then subjected to these vibrations and exercised repeatedly in a manner approximating the profile of the fatal flight and over a period of more than 500 hours. No sticking, lagging, or other malfunction was noted. In addition, one altimeter was enclosed in a transparent case which permitted examination with a strobe light in a vibration environment similar to that of the Electra instrument panel. Tests were conducted to measure the effect of contamination in the inlet filter screen of drum altimeters by liquids, dry-wing insects, and other contaminants. The effects were measured for a wide range of rates of climb and descent. The greatest error produced was 285 feet too high during a descent at close to sea level. This was caused by completely coating the filter screen with turbo oil.

In addition to the tests arranged by the Board, tests were accomplished by Lockheed Aircraft Corporation in two groups of ten altimeters wherein they were mounted in a test panel and exercised from ground level to 10,000 feet m.s.l. at an average rate of climb and descent of 500 feet per minute. The second group was moved outdoors where it was subjected to the changes in ambient temperature occurring over 24-hour periods.

Similarly, an exhaustive review has been accomplished of all maintenance records including pilot complaints of all operators, civil and military, utilizing this type altimeter. Prior to and since this accident, several malfunctions of drum altimeters of the type installed on N 6101A have been reported. The more significant malfunctions which have occurred are described below:

On an American Airlines Electra training flight at Fort Worth, on February 7, 1959, one drum altimeter reportedly stuck on three occasions; once at 2,350 feet, and twice at 12,320 feet. Inspection of the internal mechanism at the National Bureau of Standards laboratory disclosed that lint was adhering to the pinion teeth, removal of which permitted the instrument to function satisfactorily. The factory seal had been broken thereby preventing determination of the time lint had been introduced into the instrument.

A drum altimeter installed as a "third" altimeter on American Airlines Electra N 6107A stuck at 1,310 feet on an ILS approach and landing at Detroit, Michigan, on April 3, 1959. During the removal of the instrument from the panel, the sticking ceased and the instrument indications became normal. Examination and TSO tests by Kollsman and the National Bureau of Standards disclosed this altimeter to be in satisfactory operating condition.

Of the twelve other drum-type altimeter malfunctioning reports prior to and since February 3, 1959, five reported sticking above 9,000 feet; one reported sticking (no altitude given); one read 600 feet at any altitude (with the condition reported as being remedied by removing a kink in the connecting hose); one reported lagging and sticking due to sticky stops (no altitude given); two reported barometric pressure knobs difficult to turn because of loss of lubricant, but with no sticking