What happened
On 29 June 2000, a Fokker F27 Mark 500, registration G-JEAE, was conducting a cargo flight into London Stansted Airport. While established on the ILS approach to Runway 05, the commander attempted to correct an excess airspeed by retarding the power levers to flight idle. During this maneuver, the propellers unexpectedly entered the ground fine pitch (gfp) range. This caused a significant increase in propeller noise and a rapid deceleration of the aircraft.
As the propellers moved into the gfp position, the pilot attempted to advance the power levers to recover, but encountered a mechanical resistance that felt like a jam. In an effort to mitigate drag and resolve the issue, the commander selected the landing gear up. The aircraft's altitude dropped significantly, reaching a minimum of approximately 25 feet above ground level near the perimeter track. Following the incident, the crew performed a missed approach and eventually landed the aircraft without further complications.
The investigation
The AAIB examined the aircraft and analyzed recordings from both the cockpit voice recorder (CVR) and the flight data recorder (FDR). The investigation established that the propellers had indeed moved to the 0° stop. Analysis of the CVR confirmed that the movement of the power levers into the gfp position was accompanied by a sound signature consistent with the levers being lifted and moved rearward.
Engineers examined the throttle mechanism of G-JEAE and found that the forward movement of the RPM control levers could be restricted by the interaction between a baulking lug and a stationary segment within the throttle box. This interaction created the sensation of a mechanical jam. It was determined that releasing the grip on the levers allowed the internal springs to move the lugs below the flanges, restoring normal movement.
Findings
- The propellers entered the ground fine pitch range because the power levers were lifted and moved rearward far enough to activate the microswitches required for the gfp range.
- Significant turbulence encountered during the approach likely contributed to the accidental lifting of the levers over the mechanical gate.
- The pilot's perception of a jammed throttle was caused by the specific way the levers were gripped, causing the baulking lugs to interfere with the throttle box flange.
- The crew was under the mistaken impression that a 'weight-on-wheels' protection system prevented the selection of ground fine pitch while airborne.