Summary
On 30 June 1998 at 1212 UTC, a Jetstream 31 (registration G-LOVA) operated a non-scheduled passenger flight from London (Stansted) Airport to Le Bourget, France. The aircraft was crewed by two pilots and carried eight passengers. The runway in use was 23, with a dry asphalt surface and an available take-off distance of 3,048 metres. The commander, aged 61 and holding an Airline Transport Pilot's Licence with 18,000 total flying hours (425 on type), was the handling pilot.
Pre-takeoff
The aircraft was parked at the Business Aviation Terminal. Engine start was normal except that the left engine was slightly slow to accelerate, a known characteristic within limits. After receiving ATC clearance, the aircraft taxied to the holding point with normal engine and brake parameters. After a brief hold, it entered the runway for a short backtrack and lined up on Runway 23 abeam the Hotel/Juliet rapid exit turnoff. Takeoff clearance was received at 12:12:30 hrs, with surface wind reported as 330°/12 kt.
Takeoff and Runway Excursion
A rolling takeoff was initiated with full power. All cockpit indications were normal: both torques and RPMs at 100%, EGTs approximately 610°C. At 70 kt (actual ground speed 73 kt as derived from runway centreline lighting sounds), the commander relinquished nosewheel steering and placed his left hand on the control yoke, calling "my stick." At about 80 kt, both pilots noted a slight low-frequency vibration, but it was considered insignificant. The aircraft then began a rapid left turn, described by the pilots as similar to a simulated engine failure at low speed, but no engine asymmetry was perceived and no cockpit warnings activated. The commander applied full right rudder and right brake but could not stop the heading deviation. As the aircraft was about to leave the runway, he closed both throttles and, once on grass, selected full reverse thrust symmetrically. The first officer notified ATC of the aborted takeoff. The aircraft came to rest on a heading roughly parallel to the runway, about 50 yards from the edge. After parking brake application, the commander started emergency shutdown checks. The first officer observed smoke from the right engine intake; another aircraft reported smoke from the left side. The commander shut down both engines using feather levers and saw flames in the right engine intake, then fired both right engine fire extinguishers. All passengers evacuated through the main left rear door without injury. Airfield fire services arrived promptly.
Runway Marks and Examination
Runway tyre marks showed a progressive uncommanded left nosewheel steering offset. Nose tyre marks diverged left from the centreline, followed by left and right main wheel marks. Darker marks from nose and right main wheels indicated pilot's corrective efforts (right brake and rudder) had some initial effect, but the aircraft continued left, departing the runway at about 15° to the left. The aircraft travelled about 145 m on grass before stopping. The nose tyre tracks remained separated, showing the steering angle never became extreme.
Examination of the aircraft found no sooting or heat damage near engine intakes; brief fire indications were likely due to wind-induced back-flow of gases after shutdown. Brake system function checks were normal. The investigation focused on the nosewheel steering system.
Nosewheel Steering System Description
The nose gear steering system is hydraulically actuated via a linear actuator. Pilot inputs are made using a tiller wheel on the left cockpit panel, transmitted via a chain and sprocket closed-loop system to the input shaft of a steering control valve assembly mounted above the nose gear strut. The assembly includes a hydraulic spool valve and a mechanical summing linkage that compares pilot input and steering-angle negative feedback. The summing linkage comprises an input shaft with a yoke and rollers, a rocker (differential) interposed between input and feedback shafts, and a centering mechanism with spring plungers. The rocker is pivoted on the feedback shaft; its outer cut-outs engage the yoke rollers, with one cut-out providing clearance. Tilting of the rocker positions the control valve spool via a lever. Centering spring cartridges return the rocker to neutral; their screw stops must be accurately adjusted.
Probable Cause
The runway excursion was caused by a progressive uncommanded nosewheel steering offset to the left.