Page:CAB Accident Report, American Airlines Flight 383.pdf/22

– 21 – rapidly reduced inflight visibility. In order to maintain VFR conditions and remain clear of these clouds it may have been necessary for the flight to initiate a descent from the downwind leg altitude. As the flight progressed along the base leg rain shower activity became increasingly heavy with inflight visibilities dropping to two miles or less. Towards the latter part of the base leg most probably the only airport lights visible to the crew would have been the sequenced "flashers" associated with the approach lighting system. In view of the rapidly deteriorating visibility as well as the rain being reported at the airport it is likely that the pilots were concerned with and perhaps principally occupied in maintaining visual contact with the airport.

Terrain Features

It is noted that the particular terrain (see Attachment #1) in the vicinity of the aircraft's flightpath may well have provide the crew with an illusion of proper altitude or adequate terrain clearance. The Ohio River basin directly to the left of the flightpath is approximately 400 feet lower than the terrain directly under or to the right of the aircraft's flightpath. Also the terrain south of the river in the direction of the airport is featured by a wooded unlighted hillside which rises steeply from the river to the approximate elevation of the airport. Except for the lights of residences located near the riverbank there are no lights which would provide terrain definition. At night under poor visibility conditions it is quite conceivable that the elevation of the lights in the river valley could be associated with the elevation of the terrain in the vicinity of the airport.

It should be remembered that at all times after turning onto the base leg, and throughout the descent, the airport remained well to the left side of the aircraft. In order to keep the airport in sight all observations by the pilots would have been to the left of the aircraft through the left side cockpit windshield or left side window. Then, the only lights visible to them would have been the ground lights along the riverbank which could have provided the pilots with a sensory illusion of adequate altitude over the immediate terrain.

It is realized that both pilots should have been familiar with all aspects of the terrain having flown into and out of the Greater Cincinnati Airport many times previously. However, at night in limited visibility conditions it is possible that the terrain, as viewed by the pilots, would not have triggered any immediate awareness of low altitude. Conversely, it is believed if the ground lights directly under or to the right of the aircraft's base leg flightpath had been observed an immediate low attitude situation would have become apparent to the pilots.

Misreading/Misinterpretation of Altimeter Presentations

Examination of the pilot's and copilot's altimeters revealed no evidence of an pre-impact failure or malfunction and both were set properly to indicate altitude above field elevation. The functional capabilities of the No. 3 altimeter could not be determined because of extensive impact damage to this instrument. However, based on the fact that the last ATC clearance altitude of 2,000 feet m.s.l. was maintained for approximately two minutes on the downwind leg prior to the initiation of the descent, it is believed that this altimeter was properly set and functioning normally throughout the approach.