Page:CAB Accident Report, Zantop Logair Flight 60-16.pdf/7

 temperature at the brush and commutator assembly. The operating capability of the feather pump motor was exceeded by repeated feather and unfeather cycles. This conclusion is further substantiated by the overspeeds observed by witnesses and the crew.

The feathering procedure for the C-46 published in the Zantop Operations Manual was not followed. The feathering switch was depressed, however, the mixture was found in the full rich position rather than idle cut off. The propeller control piston was found against the low pitch (high r.p.m.) stop indicating either that the propeller control in the cockpit was not selected to the high pitch (low r.p.m.) position as prescribed in the manual, or the feathering switch was still depressed and the feathering pump had driven the propeller to the low pitch stop. The observations of fire by the witnesses indicate that the firewall shutoff valve was not operated. From the above cited evidence it is apparent that the feathering procedure was not properly executed and this led to an intermittent overspeed condition as the propeller went through the feather-unfeather cycle. The feather-unfeather cycle could have been terminated at any time had the pilot manually released the feathering switch from its depressed position as he did when he performed the ground check of the feathering pump motor during his engine runup before takeoff.

The pilot's decision to land at Thun Field must be considered in the light of the circumstances and the information given to him by the RAPCON controller. Believing that he had a runway in excess of 5,000 feet available and closer to him than McChord AFB, he attempted to land under emergency conditions. RAPCON did not inform the pilot of physical facts essential to a proper evaluation of the airport's suitability. The omitted information included: partial runway lighting; tall trees in the approach zone; amount of useable hard-surfaced runway, and the fact that the runway was unusually narrow. His attempt to go around when he saw the actual condition of the field is understandable. In addition, there may have existed, in the pilot's mind, some doubt as to whether he was actually lined up with Thun Field. The brightly lighted drag strip to his left while on the final approach may have contributed to this doubt.

The estimated 120-knot airspeed on final approach is verified, in part, by the significant gain in altitude during the go-around in a relatively steep turn. At the gross weight of this airplane with a windmilling propeller on the left side, maximum lower on the right engine and operating in ground effect, the aircraft would have little, if any, climb capability at a normal single engine approach speed. It is therefore obvious that either the left engine was developing some power or the aircraft had a relatively high airspeed on the final approach. The copilot stated that he did not know whether he put the landing gear down; however, the sound of the gear warning horn can be heard in the background of the recorded radio transmissions during the final phase of the flight indicating that the landing gear was not down and locked. It is not possible to determine whether the left turn during the go-around was caused by excessive drag from a windmilling left propeller or was induced by the pilot. From all evidence available it appears that the turn was intentional and that some power was available from the left engine. This probability is substantiated by a witness statement that the pilot apparently "had it made" after levelling off over the trees 175 to 200 feet above the ground. Then the propeller oversped and apparently created an asymmetrical drag condition which pulled the airplane down and to the left. The pilot saw the open field in front of him and attempted a wheels-up forced landing.

In summation, the pilot's interpretation of an engine malfunction led to a decision to feather the engine. The improperly rigged pressure cutout switch caused the feathering button to stay depressed, held by the holding coil. This, in turn, allowed the feathering pump motor to run until the propeller was driven to the low pitch stop, causing the propeller to overspeed. This malfunction, combined with the pilot's actions, caused the propeller to go through a series of feather-unfeather cycles with