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Electra N 6101A had approximately 302 hours of flight time since manufactured during all of which the drum type altimeters installed had operated satisfactorily. Of the 302 hours of flight time, approximately 150 hours had been flown prior to delivery to American Airlines.
Prior to departure of Flight 320 from Chicago, the altimeters were checked by the crew and were reported operating satisfactorily; also, during the flight they appeared to be operating satisfactorily. During letdown on approach to runway 22, both altimeters were reportedly cross-checked at 900 feet and again at 600 feet.
The drum altimeters in N 6101A were calibrated prior to delivery by the manufacturer, Kollsman Instrument Corporation, according to accepted government-industry standards. Examination and testing of these altimeters subsequent to the accident did not reveal any mechanical failures other than those attributable to impact, shock, and immersion in salt water. When the wreckage was recovered, the captain's altimeter read minus one thousand five hundred feet, with a pressure altitude setting of plus 85 feet (barometric setting 29.83). The first officer's altimeter read minus one thousand six hundred and fifty feet, with a barometric setting of 29.79.
Chemical analysis of the contents of the casings of the captain's and the first officer's altimeters disclosed the presence of hydrated aluminum oxide and sea water, with traces of iron and other metallic elements ordinarily found in sea water. No marks were found on the altimeter faces that could have been caused by impact of the pointers. The glass coverings of the faces of the two altimeters were in place and not broken.
Before opening the casings of the altimeters for examination of their internal mechanisms, it was found that the manufacturer's lead seal, which entirely covers the head of one of the screens that secures the casing on the altimeter, was missing from the captain's altimeter. The purpose of this seal is to determine, when an instrument has been returned to the manufacturer with a complaint, for overhauling, or adjustment, whether the altimeter had been opened since its delivery to the purchaser.
Internal inspection of the altimeters revealed considerable corrosion within each of the instruments. A broken link pin was found in the captain's altimeter and a rocking shaft pivot was found broken in the first officer's altimeter. The corrosion was removed to permit a detailed inspection of the moving parts. The broken components were replaced, after which both instruments were checked for mechanical freedom and were found to operate without significant restriction over a range of minus 1,000 feet to 12,000 feet.
Since the diaphragms of both altimeters were overstressed due to submersion, it was impossible to establish a calibration curve which would be representative of the calibration of the altimeter before the crash.
Static Pressure System
Two separate static systems supply barometric pressure to the captain's and first officer's flight instruments, respectively. A third (alternate) static system is also provided as an emergency source of pressure to which either or both sets of flight instruments may be connected by means of static system selectors.
