Page:CAB Accident Report, American Airlines Flight 383.pdf/21

– 20 – Based on an evaluation of airspeed bleed-off and descent rates, as well as the metal impingement pattern found in the engine turbines, the Board concludes that, except for the brief level period at 2,000 feet m.s.l., the entire descent was probably conducted at or near idle thrust. In this case, the only logical explanation for conducting the approach at such low engine power settings would have been an attempt by the pilot to expedite airspeed reduction to the appropriate approach/flap extension speeds. However, it can be seen that if the downwind leg altitude had been maintained or if a considerably lesser descent rate had been used while extending flaps in conformance with normal flap extension speeds, airspeed bleed-off would have been more rapid and the appropriate flap extensions could have been achieved further back on the base leg. Thus, with the aircraft slowed down and approach flaps extended, utilization of higher more desirable thrust settings would have been possible earlier in the approach. As it was, a number of aircraft configuration changes and landing checklist items remained to be completed as N1996 was turned onto the final approach course.

The Board recognizes that the VFR landing approach profile can and will vary depending upon the specific operational situation encountered. Since a rather wide downwind leg was flown by N1996 (4-1/2 miles from the runway) as compared to the training pattern downwind leg of 1-1/2 miles, proper technique would have dictated either: (1) remaining at or near the downwind leg altitude (1,100 feet a.f.l.) until much further along the base leg before starting the final descent, or (2) utilizing a considerably lower descent rate throughout the base leg, or (3) leveling off at and maintaining the circling minimum altitude of 600 feet a.f.l. until the aircraft had been turned onto final and a visual landing was assured, or (4) some combination of the single alternatives that would have assured proper terrain clearance. Because none of the above was accomplished and a descent was continued below field elevation it can only be concluded that the crew, possibly distracted by or preoccupied with the exigencies associated with continuing the visual approach into deteriorating weather conditions, did not give proper or sufficient attention to the primary altitude reference instruments during the approach.

It is difficult to reconcile how two experienced captains could spend almost two minutes descending below 1,200 feet a.f.l., at night, under adverse weather conditions and not properly monitor altitude. Even if both pilots were primarily concerned with maintaining visual contact with the airport it would be logical to assume that an occasional cross-check of the flight instruments would be conducted by one or both pilots.

It is therefore not reasonable to believe that the failure to properly monitor the altimeters and continued unchecked descent can be attributed to one or two factors, but is more properly enveloped within a number of significant, complex, and highly inter-related conditions. The following are considered to be the most prominent, consequential areas and although discussed individually must be evaluated within the frame work of their total effect upon the crew's conduct of the approach:

Deterioration Visibility Conditions

Prior to commencing the turn onto the base leg better than VFR conditions existed along the aircraft's flightpath. After the turn onto the base leg the flight began to encounter light rain showers and scud clouds which