National Transportation Safety Board Report 215

speeds above that at which the wing would stall. However, the stall speed for the left wing increased to 159 KIAS [knots indicated air

speed].

The evidence was conclusive that the aircraft was being flown in accordance with the carrier's prescribed engine failure procedures. The consistent 14' pitch attitude indicated that the flight director command bars were being used for pitch attitude guidance and, since the captain's flight director was inoperative, confirmed the fact that the first officer was flying the aircraft. Since the wing and engine cannot be seen from the cockpit and the slat position indicating system was inoperative, there would have been no indication to the flight crew of the slat retraction and its subsequent performance penalty. Therefore, the first officer continued to comply with carrier procedures and maintained the commanded pitch attitude; the flight director command bars dictated pitch attitudes which decelerated the aircraft toward V,, and at V, + 6, 159 KIAS, the roll to the left began.

The aircraft configuration was such that there was little or no warning of the stall onset. The inboard slats were extended, and therefore, the flow separation from the stall would be limited to the outboard segment of the left wing and would not be felt by the left horizontal stabilizer. There would be little or no buffet. The flight data recorder also indicated that there was some turbulence, which could have masked any aerodynamic buffeting. Since the roll to the left began at V. + 6 and since the pilots were aware that V, was well above the aircraft's stall speed, they probably did not suspect that the roll to the left indicated a stall. In fact, the roll probably confused them, especially since the stickshaker had not activated.

The roll to the left was followed by a rapid change of heading, indicating that the aircraft had begun to yaw to the left. The left yaw-which began at a 4' left wing down roll and at 159 KIAS-continued until impact. The abruptness of the roll and yaw indicated that lateral and directional control was lost almost simultaneous with the onset of the stall on the outboard section of the left wing.

The simulator tests showed that the aircraft could have been flown successfully at speeds above 159 KIAS, or if the roll onset was recognized as a stall, the nose could have been lowered, and the aircraft accelerated out of the stall regime. However, the stall warning system, which provided a warning based on the 159 KIAS stall speed, was functioning on the successful simulator flights. Although several pilots were able to recover control of the aircraft after the roll began, these pilots were all aware of the circumstances of the acci-