Page:CAB Accident Report, Standard Airways Flight 388C.pdf/9

 excessive brake clearance of ·052 which rendered the brake incapable of propeller control.

It was evident from examination of the brake cage that it had not been tightened properly at the time it was last installed and had been subjected to repeated cycles of loosening and tightening over a prolonged period of time. If the cage is not properly tightened during installation it will tend to work loose even with the safety bolts in place due to the high torque of the pitch change motor and the drag by the brake. As the pitch change motor calls for a lower blade angle the cage will tend to unscrew and, conversely, a higher blade angle selection will produce a tightening effect.

The wear patterns on the cage safety bolt slots were indicative of repeated cycling predominantly in the loosening direction. These repeated cycles of loosening and tightening eventually caused failure of the safety lock bolts. This was evidenced by the battered condition of the splined disc duplex brake as well as the wear patterns on the top and lower surfaces of the inboard brake cage ring. After the failure of the safety lock bolts, the cage would tend to unscrew to whatever extent would be provided by the cycling action of the pitch change motor ln addition to normal vibration effect.

It was learned from the crew of N 189S that the No. 3 propeller r.p.m. surged when approach power was being established for the landing at Manhattan. This surging was due to the lack of adequate propeller brake action. As the airspeed and power were reduced for landing the synchronizer continued to maintain the selected 2400 r.p.m. However, with the brake inoperative on the No. 3 power unit, the blades of that propeller continued to move to a lower blade angle in order to maintain the selected r.p.m. as the airspeed was reduced. Eventually, the blades went beyond the low limit switch and into the reverse pitch range. The fact that this propeller had moved into the reverse range was confirmed by the observer/pilot who saw the No. 3 reverse light come on when the aircraft was approximately 75 feet above the ground.

With the No. 3 propeller in the reverse pitch range, the increased drag would tend to yaw the aircraft to the right, the right wing would drop due to loss of lift behind the propeller disc, and an increased rate of descent would result. Addition of engine power at the airspeed involved (115 knots or less) resulted in forward thrust from engines Nos. 1, 2, and 4, and negative thrust with high drag forces from engine No. 3. The combination of these forces at the airspeed and altitude at which N 189S was operating, resulted in an accelerated descent that could not be overcome prior to contact with the ground.

Descent rate calculations using known factors of weight, airspeed, engine power, and aircraft configuration were made by the Lockheed Aircraft Corporation. A descent rate of approximately 730 feet per minute prior to the propeller malfunction was calculated using the above factors. Since absolute values for loss of wing lift and drag, due to reversal of the No. 3 propeller, were not available, assumed values of 15 percent loss of lift over the right wing and 6,000 pounds