On 26 May 2013, a Boeing 737-900 (PK-LHQ) being operated by Lion Air had just arrived at Singapore Changi Airport on a scheduled passenger flight. Passengers had disembarked and baggage for the next flight was being loaded during the approximately one hour turn round when the aircraft nose was suddenly moved 3.5 metres to the right by the prevailing wind during a tropical storm. Impact damage was caused to the aircraft, the attached air bridge and adjacent baggage loading equipment. None of those on board or adjacent to the aircraft were injured.
An Investigation was carried out by the Accident Investigation Bureau (AAIB) of the Singapore Ministry of Transport.
It was established that once the aircraft had arrived at its final parking position, wheel chocks were inserted both fore and aft of one of the nose wheels and to the aft only of the outboard wheels of each main gear. The Nose Landing Gear (NLG) steering by pass pin had also been inserted in preparation for pushback. The air bridge was positioned at door 1L for passenger, crew and ground staff access and it remained there as would normally be the case for a one hour turn round. When the unexpected movement of the aircraft occurred, crew members were on board and baggage loading equipment was positioned at the access to both the front and rear baggage compartments on the right hand side of the aircraft. The front compartment was empty and loading of baggage into the rear compartment had commenced. Refuelling of the aircraft was also in progress from a refuelling truck parked adjacent to the right engine and substantially complete - both wing tanks were full and the centre tank was estimated to contain around 3,000 kg.
Twenty three minutes after the aircraft had arrived and after heavy rain had started to fall a few minutes earlier, a gust of wind caused the aircraft nose to swing rapidly about 30° to the right pivoting about the main gear so that the nose gear was moved approximately 3.5 metres away from the parking stand centreline where it had previously been. When this happened, the aircraft parking brakes had already been released in accordance with the operator's SOP.
The movement caused impact damage to door 1L and to the lower fuselage and to both the attached air bridge and to the forward baggage loader.
A nearby wind velocity sensor recorded a gust of 26.4 knots at the time of the event. A Meteorological Service Warning of an imminent thunderstorm over the airport had been issued which forecast a wind speed of 10 knots gusting to 20 knots. It was noted that the Service provides a web-based weather alerting system but neither the airport operator nor the aircraft operator nor their ground handling agent (GHA) were subscribed to, and that they did not make use of any other sources to obtain wind information.
It was found that although Boeing recommended that the front baggage compartment should be loaded first so as to achieve a forward centre of gravity and thus reduce the risk of any pivoting about the main gear in the event of strong side winds and the aircraft operator's SOPs reflected this, "there was no evidence that this procedure had been conveyed to the GHA" by the Operator. It was also noted that "there was no evidence that the load control officer had emphasised the need to load the front baggage compartment first to the GHA staff concerned" and neither did the Loading Instruction Form specify which compartment should be loaded first.
The disposition of wheel chocks at the time of the event was found to have been the GHAs "default configuration" for narrow body aircraft, used in the absence of any specific instruction from an aircraft operator which appeared to be the case. Boeing documentation was found to specify a minimum wheel chock configuration of one chock fore and one chock aft of one of the two pairs of Main Landing Gear (MLG) wheels, either inner or outer, on the left and right hand sides. It was found that as a result of past events, Lion Air had issued a 'Quality Assurance Notice' dated 21 March 2013 which required that eight chocks should be inserted during short turn arounds when wind speeds would not exceed 35 knots, one pair fore and aft of both nose wheels and each set of outboard main wheels but "this requirement was apparently not conveyed to the GHA".
The Investigation concluded that as actually chocked and "based on the aircraft’s weight distribution at the time of the incident, the wind speed limit, beyond which the aircraft could be vulnerable to being moved by wind, is estimated to have been between 20 to 25 knots" - less than the probable gust which was recorded around the time of the movement. It was considered that if either the baggage had been loaded in the front baggage compartment first, or wheel chocks had been inserted in front of as well as behind the main wheels, the aircraft may not have moved. It was also noted that "Had the NLG steering bypass pin not been inserted, the nose wheels would have remained aligned with the aircraft longitudinal axis (i.e. facing forward) and the wheels’ resistance to sliding could have contributed towards countering any tendency of the aircraft nose to swing".
It was considered that when the GHA adopted its default chock configuration in the absence of any other instructions, "it should have made its default configuration known to the Operator and ascertained if the configuration was acceptable".
Safety Action taken by Lion Air as a result of the event was noted as informing their Ground Handling Agent at Singapore of the following aircraft turn round handling requirements:
- Eight wheel chocks should be inserted for transit aircraft when the wind strength is less than 35 knots with two at the front and rear of both nose wheels and both outer main wheels.
- Baggage should be loaded into the front baggage compartment first.
- The nose landing gear steering bypass pin should only be inserted when the tow bar is about to be connected with the aircraft.
Four Safety Recommendations were made as a result of the Investigation as follows:
- that the Aerodrome Operator monitor wind speed forecasts and disseminate any relevant wind speed warning to the airline operators or their handling agents. [R-2014-006]
- that the Ground Handling Agent review its apron procedures to ensure that wind speed information is monitored and timely action can be taken to stabilise or tie down its aircraft. [R-2014-007]
- that the Ground Handling Agent review its default wheel chock configuration taking into consideration the aircraft manufacturer’s recommended wheel chock configuration. [R-2014-008]
- that the Ground Handling Agent make its default configuration for the wheel chock positions known to its airline customers and ascertain if the configuration is acceptable to them. [R-2014-009]
The Final Report of the Investigation was published on 1 August 2014.