Singapore B744 at Singapore on Dec 2nd 2011, runway excursion
Last Update: September 4, 2013 / 14:01:09 GMT/Zulu time
2.1 Deactivation of auto-brakes
2.1.1 According to the operatorâ€™s Operations Manual Volume A
Gen/Basic (OMV A), auto-brakes should normally be deactivated when the aircraft has decelerated to between 60 and 30 knots (the latter being the typical taxi speed). The operatorâ€™s Flight Crew Training Manual (FCTM) also stated that the speed at which to deactivate auto-brakes would depend on such factors as deceleration rate, runway conditions, etc. This implies that flight crews could deactivate auto-brakes above 60 knots if they judged that the aircraft would be able to slow down and vacate the runway safely.
2.1.2 The FO was aware of the guidance stipulated in the OMV A, and FCTM. According to him, he deactivated the auto-brakes at 99 knots because he wanted to vacate the runway as expeditiously as possible. He believed that the aircraft would
occupy less time on the runway by having it rolling down the runway at a higher speed and moderating the brakes to control the aircraftâ€™s speed down to the safe level for exit via W3. He believed that retaining the auto-brakes until the aircraft had decelerated to 60 knots before deactivating them would result in the aircraft staying longer on the runway.
2.2 Attempt to vacate via W3
2.2.1 The FO intended to exit via W3. He did not check his speed just before he attempted to vacate the runway via W3. Moments earlier when the aircraft was abeam W4, the FO noted that his speed was about 60 knots and decelerating.
He then focused his attention outside the cockpit for his intended exit W3 and did not check his speed again before attempting to vacate the runway via W3. Based on visual cues, he perceived that the speed had decelerated to a safe level to turn off at W3.
2.2.2 The FO had deactivated the auto-brakes by applying manual brakes but he did not maintain the manual brakes. He only applied manual brakes again about five seconds later. Had he maintained manual brakes, the aircraft would have had further decelerated through a distance of 280m and would have reached the turning point for W3 at a speed lower than 59 knots. It was possible that the aircraft could have safely vacated the runway via W3, despite early deactivation of the auto-brakes.
2.3 Nose wheel steering tiller operation
2.3.1 Although the FO said that he applied right rudder pedal input, the FDR data indicated that only very negligible right rudder pedal input was applied. As a result, the aircraft continued on a relatively straight path along the runway instead of turning right towards W3.
2.3.2 As the FO felt that the aircraft was not turning right, he used the nose wheel steering tiller to try to turn the aircraft.
2.3.3 Tiller for nose wheel may be used when the aircraft is at or below typical taxi speed. The FO was looking outside the cockpit as he attempted to vacate the runway. Although he felt (based on the visual cues) that the aircraft was at a speed which was safe to vacate via W3 and for tiller operation, the aircraftâ€™s speed was actually about 59 knots, which was higher than the typical taxi speed.
2.3.4 Thus, while the use of the tiller managed to get the aircraft to start turning to the right, the inertia of the aircraft also caused the aircraft to skid, resulting in the aircraftâ€™s excursion off the runway.
The MoT reported that the first officer (34, ATPL, 3,440 hours total, 3,173 hours on type) was pilot flying, the captain (40, ATPL, 9,095 hours total, 3,625 hours on type) was pilot monitoring. The aircraft was approaching runway 02L and had been configured to touch down with flaps at 25 degrees and auto brakes set to 2 (medium).
The aircraft touched down about 120 meters past the aiming marker, the thrust reversers were activated. While slowing through 99 KIAS the first officer deactivated the automatic brakes by briefly pressing the brakes pedals, but released the brakes pedals again. The first officer again applied manual braking about 5 seconds later after a travel of 280 meters, the thrust reversers were stowed at about that time while slowing through 68 knots. The first officer looked out for rapid turn off W3, felt from the visual cues available the speed had sufficiently reduced to take exit W3 and attempted to turn right off the runway providing a right hand rudder pedal input - the flight data recorder showed a marginal rudder input only at a speed of 59 knots. The aircraft didn't turn, hence the first officer began to apply steering tiller input. The aircraft started to turn right, however also began to skid prompting the first officer to try to steer the aircraft back onto the runway center line, the captain took control of the aircraft and steered the aircraft back onto the runway center line. In the meantime tower control had already seen the aircraft veering off the runway onto grass surface and had alerted the emergency services, the right main gear, right body gear and nose gear had temporarily left paved surface. The aircraft subsequently vacated the runway via taxiway W2 and stopped on the taxiway where emergency services responding to the tower alert checked the aircraft. The crew did not observe any abnormal indications, arriving maintenance engineers however reported engine #3 (right hand inboard engine, PW4056) had ingested mud. That engine was shut down and the aircraft taxied to the apron.
The MoT reported that all wheels except #4 had received damage and needed to be replaced, the brakes assemblies of the right hand main and right hand body gear were damaged and needed to be replaced. One fan blade of engine #3 was damaged by mud that had been ingested.
This article is published under license from Avherald.com. © of text by Avherald.com.
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