Philippine Air Lines Flight 741

Investigation

On the day of the crash, a test was made on a DC-3 with the rudder trim tab indicator set to 12° "nose left". Results showed the aircraft acting normally during the initial takeoff roll until it gained enough airspeed when the right rudder pedal appeared hard and the aircraft yawed to the left. Opposite rudder force and adequate pressure on the yoke counteracted the yaw.

Various technical examinations and inspections conducted on the airframe, power plants and their components and accessories did not reveal any evidence of failure prior to the accident.

Testimonies of both pilots showed that while taxiing from the ramp to the runway and during takeoff, there was free movement of the rudder pedals. When the aircraft became airborne, it started to veer slightly to the left. Pressure was applied to the rudder pedals to counteract the turn but was unsuccessful due to the restriction of travel of the rudder pedal. When the airspeed decreased, the pilots managed to move the right rudder pedal forward and the left turn and bank were corrected.

When stationary, with the rudder trim tab indicator set to 8°45" "nose left", the travel of the rudder pedal was unaffected, but as the airspeed increased, the deflected rudder trim tab generated a force which was directly proportional to the square of the airspeed, causing a corresponding deflection of the rudder which deflected the rudder pedals, as if human force was applied to the left rudder pedal, which caused the aircraft to veer to the left.

When the airspeed was around 85–90 mph, the rudder pedal force necessary to offset the asymmetric condition created by the rudder trim tab corresponding to the rudder trim tab indicator setting of 8°45" was less than the maximum force a pilot can exert. However, when the captain took over the controls, while the speed was at 105 – 110 mph, the rudder resistance might've given the impression that it has jammed, but as the airspeed decreased, the rudder pedal was able to be moved forward.

Although the pilots testified that they performed the normal pre-flight inspection and observed the pre-takeoff checklist, the Board believed that the rudder trim tab had been deflected prior to takeoff. During the investigation, the pilots stated that during the flight they did not think of the rudder trim tab, but the captain was concentrated on the control yoke and the rudder pedals to correct the left turn, and that the trim tab should've been checked if the yoke and pedals did not produce a reaction.

The fact that the aircraft was loaded slightly above its allowable gross takeoff weight, the testimony that the landing gear was retracted late, and the carburettor airscoop level locked between the hot and cold position, which could've reduced engine power, when considered separately may not have significantly affected the low altitude reached, but if these facts were considered altogether, including the bank of the aircraft which did not exceed 45°, they could've caused the low altitude during the flight.

It was determined that the duration of flight from takeoff to up to the initial impact was approximately 60 seconds. The time span from the moment the captain took over the controls up to the time when he was able to correct the banking for a short period was approximately 16 seconds. It felt that if the aircraft had gained a higher altitude, the pilots would've had more time to perform all the necessary corrections to avoid the accident.

Cause

As a result of the investigations, the Civil Aeronautics Administration determined that the probable cause of the accident was the failure of the pilots to set the rudder trim tab to the proper position before takeoff and during the flight. The continuous left turn and bank resulting in the crash was due to the undetected deflection of the rudder trim tab.

It was further determined that the slight exceeding of the maximum takeoff weight at Manila contributed to the accident.