History of the Flight
The crew reported for duty at Belfast Aldergrove Airport at 1135 hrs for a series of six sectors between Belfast and Luton. After an uneventful first flight and turnaround, the crew departed the stand at Luton at 1412 hrs for the return sector, with the commander acting as handling pilot. Runway 08 was in use. The pilots completed flight control checks during taxi, which involved the commander displacing the rudder pedals fully while the co-pilot followed through, and the co-pilot checking control wheel and column movement. All checks were normal.
After an uneventful taxi, the aircraft was cleared to enter, backtrack, and line up on Runway 08, then cleared for takeoff. The takeoff run was normal until rotation, when the commander raised the nosewheel and the aircraft suddenly jolted to one side, although the crew could not recall which direction. The aircraft became airborne and handled normally. The pilots discussed the jolt and consulted the checklist, but found no relevant drill. The commander contacted the Senior Cabin Crew Member (SCCM), who reported that the jolt was severe enough to lead her to believe the aircraft was about to deviate off the runway.
Unable to find a cause, but suspecting an uncommanded rudder event, the commander contacted company operations, who advised return to Luton for inspection. The aircraft climbed to FL330 and received radar vectors back. The commander briefed for approach, reviewed immediate actions for uncommanded rudder input, and added 10 kt to approach speed. A normal landing was made, and the aircraft was shut down.
Aircraft Examination
Operator engineers conducted a thorough examination of aircraft systems and performed checks per Boeing Service Letter 737-SL-27-110 ('Unexpected roll and yaw event troubleshooting'). No faults were found. The aircraft returned to service the next day, and no similar events have occurred.
Flight Data Analysis
Analysis of flight data showed the yaw damper was engaged and functioning correctly during taxi and takeoff. During the takeoff ground roll, there were small heading deviations with corrective rudder inputs. At about 128 kt, a lateral acceleration to the right occurred, peaking at 0.21 g, with no pilot or rudder input initiating it. The aircraft's heading deviated right at about 1°/sec. The yaw damper applied corrective left rudder, and the pilot also applied left pedal input, preventing further deviation. The lateral acceleration peaked when the aircraft rotated at about 133 kt. After takeoff, a left bank of about 5° occurred but was corrected. Engine thrust was symmetric, and brake pressures were normal.
External Factors
No aircraft or control input cause was found, so the investigation focused on external influences.
Weather: Conditions were good with light wind (7 kt, varying through 40°), visibility above 10 km, and scattered fair-weather cumulus at 4,800 ft. No weather phenomena could explain the lateral loading.
Jet Blast: A runup bay exists to the left of Runway 08. However, no aircraft were using the bay at the time, so jet blast was discounted.
Wake Turbulence: The preceding departure was the same aircraft type (Boeing 737) of similar weight (164 kg lighter) three minutes earlier. Light winds may have caused vortices to persist and drift across the runway. ICAO guidance does not require separation time for two Boeing 737s taking off from the same runway, suggesting wake turbulence would not cause significant control problems.
Conclusion
The evidence provided no definitive cause for the sudden yaw divergence. Objective evidence suggested an external influence, but none was identified. It was concluded that the jolt was caused by an unspecified atmospheric disturbance acting on the aircraft at rotation.