|Title:||Uncommanded yaw, Boeing 737-81Q, G-XLAD|
|Micro summary:||This Boeing 737-81Q experienced an uncommanded yaw in flight.|
|Event Time:||2002-12-23 at 0030 UTC|
|Publishing Agency:||Aircraft Accident Investigation Board (AAIB)|
|Publishing Country:||United Kingdom|
|Diversion Airport:||Gatwick Airport, London, England|
|Site of event:|
|Departure:||Tenerife South Airport (Reina Sofia Airport), Tenerife, Canary Islands, Spain|
|Airplane Type(s):||Boeing 737-81Q|
|Type of flight:||Revenue|
|Diverted to:||Gatwick Airport, London, England|
|Executive Summary:||The flight was uneventful until the burning smell previously encountered on the flight deck became apparent briefly whilst descending through about FL120. There were no other problems indicated and after landing it was agreed with the company that the aircraft should be flown to Gatwick, the company's main engineering base, for further investigation. The aircraft then departed Brussels at 2300 hrs with only the crew and the engineer on board. |
When established in the cruise at FL240 both pilots and the engineer became aware of the aircraft repeatedly yawing gently to one side and then returning quickly to a normal attitude. Descriptions differed between the crew as to the duration of each event, varying from 20 seconds to two minutes. Their description of how often the events occurred also varied, ranging from continuous to once every four minutes. The Commander recalls the indications during each disturbance in the cruise showing a slip to the right requiring about 2.5 units of right aileron to keep the aircraft straight. There was no asymmetric power and the yaw damper indicator showed a central position. The rudder trim was slightly displaced to the left, which was not considered unusual for this phase of flight. On attempting to apply right rudder the Commander found that his rudder pedals could not be moved, although between each disturbance the pedals could be moved again normally.
The pilots and engineer checked the flight deck for any abnormal switch selections or popped circuit breakers but could find none. The engineer then went to the rear of the passenger cabin where he turned to look forward towards the flight deck. He described the aircraft seeming to drift to the right before kicking back abruptly to the left. The kick was pronounced but was such that he was able to stand without support. These deviations, each of which lasted for about 20 seconds, continued throughout the few minutes that the engineer remained in the cabin and were consistent in their direction. He reported this to the Commander who, convinced of the seriousness of the situation, briefed the purser for a possible emergency landing.
The aircraft commenced its descent for Gatwick during which the divergences seemed to be more pronounced. As a result of this, and whilst on a closing heading for the localiser at FL080, the crew transmitted a PAN call to Gatwick approach advising they had problems with the rudder. The Commander described the divergences at this point showing the slip indicator displaced just over half way with the aircraft yawing to the left through about 20°. Gatwick approach acknowledged the PAN call and shortly afterwards asked the aircraft to slow to 230 kt. The commander reported that at this stage he disconnected both the autopilot and autothrust since the divergences were now more extreme, however, he decided to leave the YAW DAMPER engaged since it appeared to be behaving normally. The non-normal checklist for UNCOMMANDED YAW OR ROLL requires that the autopilot and autothrottle be disengaged (if engaged) and verification that symmetric thrust is applied; the YAW DAMPER should then have been selected to OFF.
ATC cleared the aircraft to descend to 3,000 feet where the Commander recalls at one point having to apply 5 units of left aileron to keep it tracking straight. The aircraft was then established on the ILS with the intention of completing a flap 25 landing, in VMC, with a surface wind reported as light.
When passing 300 feet AGL the Commander reported that the aircraft progressively yawed left, to the extent that the slip indictor was displaced three-quarters to the right and full right aileron was required to keep the aircraft straight. As on the other occasions his rudder pedals could not be moved despite him now applying full force to the right pedal. Simultaneously with the automated 30 foot radio altimeter call the uncontrolled divergences suddenly stopped. However, the extreme control inputs necessary to control the aircraft up to this point could not be corrected quickly enough to prevent the aircraft yawing and rolling to the right. The crew reported that the aircraft landed about 30° off the runway track touching down first on its right main gear.
The Commander attempted to regain the centre line by use of the rudder, but although he could now move the pedals they appeared to be ineffective. He therefore used the nosewheel steering to control the aircraft. The aircraft cleared the runway and was brought to a halt on the taxiway whilst the crew spoke to the fire service, on their discrete frequency of 121.6 MHz, to confirm their services would not be required.
The commander then taxied the aircraft to the allocated parking stand during which time he attempted to move his rudder pedals again, only to find that they would not move. When finally parked and with the engines still running he tried once more and this time found they had become free again. After shutdown an engineer from the maintenance organisation used by the Company was briefed on the control problems that had been encountered.
On previous flights there had been reports associated with airframe vibration, smells of burning, display irregularities and an unannounced autopilot disconnect. No faults had been identified to account for these reports. During the flight from Brussels to Gatwick the crew reported disturbances in yaw which concerned them sufficiently to warrant a distress message. These divergences occurred in both directions, for varying periods of time, at irregular intervals and were accompanied by an inability to move the rudder pedals. The flight crew did not complete the non-normal checklist for UNCOMMANDED YAW OR ROLL. Unusual but small rudder displacements were recorded by the FDR but the large amplitude excursions reported by the flight crew were not evident
The rudder flight control system and its components were subjected to a series of inspections agreed between the operator, its maintenance organisation and the aircraft manufacturer. No faults were found and the aircraft has since operated without further problems of this nature. A detailed study by the aircraft manufacturer considered the FDR data and analysed a number of mechanical scenarios in an attempt to identify a possible source of the input into the rudder flight control system. This study concluded that the FDR data could only be matched with the scenario for higher friction in that portion of the control circuit forward of the load transducer. However, in order to generate the observed aircraft behaviour some form of input at the pedals would still be required and this characteristic of the rudder flight control system would then be expected to persist for subsequent flights. There have been no further reports of problems with the rudder control system. This investigation has therefore been unable to explain the aircraft behaviour that was reported by the flight
|Learning Keywords:||Operations - Airspace - Air Traffic Control|
|Operations - Cabin or Cockpit Smoke|
|Operations - Upset - Uncommanded or excessive Yaw|
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