What happened
On February 9, 2001, a Fokker F27, Mk050, was performing a ferry flight from Saarbrücken to Cologne-Bonn with two pilots on board. During the takeoff run, the pilot flying experienced significant difficulty maintaining the centerline, as the aircraft pulled strongly to the right.
As the aircraft climbed and gained speed, the flight crew had to apply continuous, maximum left rudder and aileron inputs, along with maximum rudder trim, to maintain the departure route. The aircraft exhibited significant bank angles of up to 50 degrees and deviations of approximately 30 degrees from the intended heading. After the landing gear and flaps were retracted, the pilot in command took control and reduced engine power.
To regain control, the pilot in command set the flaps to 10 degrees, believing that a lower airspeed would improve controllability. The crew eventually stabilized the aircraft at approximately 130 knots using 60% to 70% left rudder trim and slight left aileron input. Following a visual inspection of the flaps and ailerons by the second pilot, which showed no defects, the pilot in command decided to abort the flight and return to Saarbrücken. The crew performed an ILS approach to runway 27 under conditions of 3 km visibility and a 400 ft cloud ceiling. The landing was completed with no injuries and no damage to the aircraft.
The investigation
The BFU examined the aircraft's maintenance history and the mechanical state of the flight controls. The investigation revealed that a 250-hour maintenance check had been performed prior to the flight. During that check, technicians had noted that the rudder trim was not centered and identified significant play in the rudder trim tab bearing. While the bearing was replaced and the tab was readjusted, the investigation focused on the subsequent mechanical behavior during the flight.
Technical personnel from the operator's maintenance base subsequently inspected the rudder control surfaces. During this inspection, they discovered that the rudder balance tab was operating outside of the specified tolerances.
Findings
- The primary cause of the directional instability was that the rudder balance tab deflections exceeded allowable tolerances.
- A contributing factor was that after the recent repair of the rudder trim tab bearing, the maintenance staff failed to verify that all rudder and trim settings complied with the required tolerances.