Page:CAB Accident Report, Continental Airlines Flight 290.pdf/11

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unpa‘ertly had an adequate amount of control and power necessary to establish a ccnstant altitude from the Bluff Fan Marker to the runway. During this time neither crew member made any radio transmiSSion.

The flaps were found to have been at the 32-degree pOSition at impact and they had reached that pOSition by being retracted rather than by being extended. The nose landing gear had been knocked off the aircraft but the damage to the surround* ing structure indicated that it was not locked down at the moment of impact. The left main landing gear was down with the locks withdrawn 9/16 of an inch and the right main gear was down and locked.

The testmnony at the hearing indicates that it would not have been pQSSible to cause a noseover rotation of the aircraft, of the magnitude required for this accident, through the use of the trim tab or the flying tab with the elevator in an immovable peeition. Further, the autopilot was not capable of creating such a nose- over rotation within the altitude that is known to have existed at the time of this anoident. The only ways the aircraft could pitch down to an attitude of more than 22 degrees below the horizontal, in the altitude available, was by a loss of down loading on the horizontal tail surfaces, or by pilot induced maneuver. There is no ev1dence to indicate that such a pilot induced maneuver did occur. The testimony at the hearing indicated that horn shaped or concave ice formations on the leading edge of the horizontal stabilizer can produce a strong nosedown pitching moment when flaps are lowered This type of ice was reproduced in tunnel tests by the manufac~ turer. It is known that a negative angle of attack of the horizontal tailplane increases with a given airspeed with an increase in flap angle; a given flap setting with an increase in airSpeed, at a given airspeed and flap setting with a reduction in weight. Each of the above conditions requires more negative lift on the tailplane to maintain longitudinal trim. A stability test performed by the manufacturer showed concluSively that horn shaped ice formations can produce drastic reduction in the marimum negative lift obtained by the horizontal tailplane and the tailplane approach— es a stall condition. Because of the criticality of the shape of the ice, the results of the test were necessarily qualitative. Little could no gained quantitatively. It " was found, however, that for a particular shape of ice which could be trimmed against at a zero—degree flap setting at a constant Speed, a serious nosedown pitching moment could occur at 20 degrees of flaps. Similarly, a shape which could be trimmed against at any flap angle could produce a lessening of tailplane lift at any higher flap angle. Also, a shape which could be trimmed at a given flap angle and speed could produce a nosedown pitching moment at some higher airspeed.

In addition, the tests revealed that with the reduction of negative lift there is a change in lift distribution such that elevator hinge moments (and therefore wheel force) are increased more than normal as up elevator is applied to counteract the pitch. Additionally, up elevator could increase the flow separation to the pOint of reduCing tail load still further, resulting inra sharper nosedown pitching moment.

The fact that the aircraft was accalerating and climbing in a normal attitude negates the pOSSibility of a Wlng stall

Furthermore, the eVidence indicates that the powerplants were operating in a 15,000 r.p.m. range at impact and that the propellers were in the flight range The flight tests conducted by the manufacturer indicated that even if the low pitch stops had been removed the aircraft was still capable of erecuting the gc-around maneuver without any loss of control and that the propellers would not go into flat pitch cauSing an abrupt nosedown.