History of the flight
The aircraft, a Cessna 550 Citation (registration G-FCDB), taxied with two crew and one passenger for a flight to Copenhagen. ATC instructed the crew to taxi via taxiway Alpha for Runway 23 and cleared them for takeoff before reaching holding point Alpha One. The commander, acting as handling pilot, performed a rolling takeoff following the taxiway centreline onto the runway. Takeoff power was set and engine indications were normal. As the aircraft accelerated through an estimated 70 kt, the commander felt a bump and the aircraft yawed right. The co-pilot reported seeing a hare or rabbit run from the right, associating the bump with the animal. The commander applied opposite rudder, correcting the yaw. The heading deviation was later recorded as 10°; the co-pilot assessed it as 30° to 40°. The commander initially suspected a tyre failure but was able to bring the aircraft back to runway heading by the end of the concrete dispersal area, though the aircraft was on the edge of the marked runway. It partly left the paved surface, crossed a grass area between the dispersal concrete and the disused section of Runway 01/19, and then crossed that runway. The right wheel entered soft grass, causing further yaw. The commander applied differential braking and increased right engine power, and the aircraft re-entered the runway. The crew taxied back to the apron and shut down. The commander could not explain the bump or yaw and was unsure if it was caused by an animal. He noted the lead-in area was poorly lit with no runway centreline lights.
Aerodrome information
Teesside Airport's main runway 23/05 is 2,291 m long, 46 m wide, with a magnetic heading of 232°. It is crossed by Runway 01/19, of which only the southern part is usable. The airport, originally a military base, has two concrete dispersal areas at each runway end on the north side. Runway edge markings are unbroken white lines extending to each threshold. Runway 23 has high intensity bi-directional edge lights at 60 m spacing, set 2.5 m outside the edge markings, and high intensity centreline lights at 30 m spacing. Taxiway Alpha joins the runway with a yellow line and green centreline lights; its edge is marked by blue reflectors. The white runway edge line crossing the dispersal area is supplemented by red reflectors 1 m outside the line. There are no ground markings on the Runway 23 dispersal area other than the white edge line. The UK AIP entry for Teesside notes that at both ends of Runway 05/23, its width is twice that of the edge lights due to extra pavement on the northwest side, advising pilots to ensure correct lineup, especially at night, when the runway is contaminated or in low visibility.
Meteorological information
At 0620 hrs, Teesside reported surface wind from 200°(M) at 8 kt, visibility 6,000 m, temperature and dew point 8°C. Sunrise was at 0752 hrs.
Flight data recorder
The FDR indicated airspeed (IAS) commenced at 34.3 kt as the aircraft stabilised on runway heading. Eight seconds later, a heading deviation of 2° right occurred for about two seconds, followed by a larger deviation left to about 7°, then right again to 3°, then left to a maximum of 15°, coinciding with the aircraft re-entering the runway. Engine RPM initially reached 79.1% at the first right deviation, about one second before maximum IAS of 72.5 kt. During the second right deviation, number 2 engine RPM increased from idle to 81.7% then reduced. Accelerations were consistent with passage across an unprepared surface; no initiating event was indicated.
Examination of site
Airport operator examination revealed tyre marks entering grass at point B, 140 m from a notional point A (start of takeoff roll along taxiway centreline). The marks were 26.2 m from the runway centreline and continued 23.3 m parallel to the runway to point C on crossing runway. Another tyre mark started at point D, 28 m from C, continuing 37.2 m to the disused runway edge at point E. As the mark crossed the disused runway, it deviated slightly right. At point E, a second tyre mark appeared closer to the runway. Both tracks deviated left; the closer track re-entered the runway 10 m further. The main track paralleled the runway edge then deviated right, with the second track reappearing. Both tracks turned left and re-entered the runway at point F, 134 m from point E. An extensive search for the hare or rabbit found no remains.
Aircraft examination
Company engineering examined the aircraft the day after the incident. No fault was found that could account for the yaw. The aircraft was returned to service with no reoccurrence at the time of writing.
Analysis
Although both crew reported a significant yaw, the recorded data did not show a large deviation. The commander recalled correcting the yaw by the time the aircraft reached the first grass area. For a 10° heading change to cause the aircraft to reach grass at point B, it would have had to begin almost immediately after takeoff roll, but the aircraft would have been controlled long before the runway edge. Assuming the aircraft followed the taxiway lead-in line, it aligned at point A, 140 m from the grass. IAS first registered at 34.3 kt at this point. The maximum airspeed was recorded about 237 m from point A, around the time the aircraft entered grass a second time. The slight right yaw occurred between 167 and 237 m, coinciding with crossing the disused runway. The first left deviation occurred after 273 m, matching ground marks. The possibility that the initial tyre marks were not from this aircraft or that a higher momentary deviation occurred was considered, but the recorded data and lateral accelerations did not support a major yaw. The investigation concluded that the aircraft was lined up for takeoff on the runway edge line instead of the centreline; the commander's report that the runway lacked centreline lighting (though it had it) supported this. The initial heading deviation was likely caused by drag from the right main wheel encountering grass, exacerbated by contact with a drain cover or the disused runway edge, accounting for the bump felt by both pilots.