Page:CAB Accident Report, American Airlines Flight 383.pdf/20

– 19 – The recorder determined ground track was in agreement with the ATC and witness determined tracks and the recorder derived flight profile correlated in all essential elements with the ATC and witness data. A brief review of the flight profile shows that N1996 entered a downwind approach leg approximately four miles east of runway 18 at approximately 2,000 feet m.s.l. (1,110 a.f.l.) and that an approximate 800 feet per minute rate of descent was commenced during the turn onto the base leg. This descent was continued to an altitude of 210 feet a.f.l. at which point a gradual turn toward the final approach course was initiated. Coincident with this turn (20 seconds before impact) the descent rate increased to about 2,100 feet per minute for 10 seconds, during which time the aircraft descended into the valley formed by the Ohio River, below the elevation of the airport. The final 10 seconds of flight showed a decrease in descent rate to approximate 625 per minute until impact.

Analytical studies of the flight recorder data relating to the aircraft's approach configuration were carefully reviewed and evaluated. As was previously stated, only during the descent from 5,000 feet m.s.l. to 2,000 feet m.s.l. could the aircraft's configuration be positively identified; i.e., spoilers extended and engines at idle thrust. However, based on the recorder data and AA operational procedures the other probable configuration sequences were determined relative to the flight profile. This analysis showed that traffic pattern entry was made at an airspeed of approximately 210 knots; that the spoilers were retracted and two degrees of flaps were extended at that time; that airspeed diminished as the aircraft turned toward the base leg; that just prior to the completion of this turn, at an airspeed of 170 knots, flaps where lowered to fix degrees; that near the midpoint of the base leg at an airspeed of 160 knots flaps were extended to 15 degrees; and that the final flap extension to 25 degrees was made during the turn toward the final approach course approximately 20 seconds before impact.

Although the increase in the rate of descent to approximately 2,100 feet per minute appears to correlate with the extension of flaps from 15 degrees to 25 degrees the Board find it extremely difficult to suggest a proper rationale for the pilot permitting such high descent rates to develop at this point in the approach. It would appear, however, that the increased drag and nosedown pitch mode accompanying the 25-degree flap extension was not initially checked either through the use of longitudinal control forces or increased thrust and that the high rate of descent was inadvertently developed. However, airspeed bleed-off from 160 knots to 145 knots coupled with a decrease in the rate of descent to approximately 625 feet per minute indicates that the pilot subsequently utilized the longitudinal control system to arrest the high descent rate. Because of the relatively high airspeed at this point in the approach it would appear that this action was consistent with an attempt to further reduce airspeed. Had added thrust alone been used to decrease the high rate of descent it would not have achieved the apparent desired results of slowing the aircraft.

The traffic pattern airspeeds and aircraft configuration sequences utilized by N1996 were compared to appropriate airspeeds and recommended configuration sequences outlined in the VFR training pattern. From this study it was indicated that the various flap extensions during the approach conformed with the AA normal operating extension speeds. However, because of the comparatively high airspeeds maintained by the flight throughout the approach, these flap extensions were concentrated or "bunched-up" on the base leg.