A Boeing 737-476 aircraft was in the final stages of preparation for an 0810 EST scheduled departure from Melbourne. It was the aircraft's first flight of the day. The ground engineer had completed the pre-flight and other assigned pre-departure tasks. There was little else for him to do prior to his duties for the pushback which included assisting the ramp towmotor driver to connect the towbar to the towmotor and walk out with the aircraft. The flight crew had started the Auxiliary Power Unit (APU) and were completing the Before-Start Checklist in preparation for pushback.
The towbar was delivered by ramp personnel at 0803 and was connected to the aircraft nose landing gear by the engineer. At 0805, cargo loading was still in progress in both the forward and rear cargo bays. The engineer was aware that the towmotor should only be driven by the rostered ramp towmotor driver who was still involved with the loading of the aircraft. Concerned that the departure of the aircraft was about to fall behind schedule, he decided to connect the towmotor to the towbar himself.
The engineer moved the towmotor close to the towbar, but stopped when he noticed that the towbar was at the wrong height for connection. He then left the cab, with the towmotor's engine running, to correct the towbar height. While re-adjusting the towbar, he heard the towmotor engine shut down. He reported that he then climbed back into the towmotor, restarted the engine, and attempted to drive forward the remaining 10 cm required for connection.
At that time the towmotor appeared to him to move forward "...relentlessly..." and collided with the aircraft. He indicated that he had applied the brakes and selected reverse gear, without success. He then vacated the cabin of the towmotor across the deroofed front of the vehicle.
The flight crew indicated that they heard and felt a "...big thump..." which continued and shook the aircraft for about 4-5 seconds. The pilot in command advised the cabin crew to "...brace..." and noticed the towmotor wedged under the aircraft nose. He assessed that there was no danger to the aircraft and ordered that the passengers be disembarked normally through the forward door via the aerobridge, which was still in place. He shut down the flight deck systems, set the park brake, but left the APU running to power the hydraulics and disembarked to view the extent of the damage.
The aircraft had sustained substantial damage to the radome, forward pressure bulkhead, forward fuselage skin and nose landing gear doors.
The engineer had been employed by the operator for more than 26 years and was highly regarded as a Licensed Aircraft Maintenance Engineer (LAME) and company employee. Supervisors indicated that the engineer was very cooperative and willing to assist ramp staff at all times to despatch aircraft on schedule.
At the time of the accident, the engineer was on the first day of a shift rotation, having been off-duty for the preceding three weeks.
The engineer held a current ramp driver's licence and had considerable recent experience towing aircraft using another towmotor type. He had last driven the accident towmotor about 6 months previously.
Towmotor and towbar information
The towmotor was designed specifically for B737 use. It had last undergone maintenance action on 5 March 2001. According to the operator's maintenance staff, the accident towmotor was used only on the ramp as it was unsuited for maintenance purposes. It was the only one of its type used by the operator.
The design of the towmotor was such that the towbar and connection point at the front of the vehicle could not be seen from the driver's seated position. Towbar connection was a two-person operation. Any attempt by a single person to connect the vehicle and the towbar, involved the driver having to stand up and lean over the steering wheel to view the operation. From a standing position, it was possible for the driver to place a foot over both the accelerator pedal and the brake pedal simultaneously.
The cabin forward structure had been torn upwards and rearwards during the initial contact with the aircraft. The rear left corner of the cabin structure was forced down through the vehicle's upper body panel by the weight of the aircraft's nose. Inspection of the towmotor found no mechanical fault that could have contributed to the accident.
The towbar for the Boeing 737 aircraft underwent a complete service on 28 March 2001 and had been correctly connected to the nose landing gear strut of the aircraft. Impact from the towmotor sheared the towbar's rear shear pin, which allowed the bar to swivel out of the way, pivoting on the remaining shear pin, as the towmotor drove forward into contact with the aircraft. The towbar was inspected and found to have been serviceable at the time of the accident.
Aircraft pushback procedures
Aircraft pushback procedures at the time of the accident were contained in the operator's Ramp Standard Operating Procedures (SOP). The SOP stated that the towbar was to be delivered to the aircraft by ramp personnel, then connected to the nose landing gear by the engineer. Connection to the towmotor was to be accomplished by the rostered ramp towmotor driver, with the assistance of the engineer. Line engineers normally towed the aircraft for maintenance purposes only. In addition, the SOP stated:
"Use a marshal (usually the engineer) when manoeuvring for hook up to the tow bar when it is attached to the aircraft. Do not stand up to see the hitch point."
The operator's Engineering and Maintenance Procedures Manual stated that:
"...Prior to push-out operation, the engineer will...(2) Connect the towbar and towmotor by front or rear connection as appropriate..."
A ramp driver's licence was valid for a 12 month period. Renewal was devolved to airline operators by the airport owner and consisted of a written examination covering rules, procedures, airfield speed limits and ATC communications and light signals. No driver refresher training was conducted. Company towmotor drivers who were qualified to tow aircraft on the ramp were to conduct towing operations in accordance with their respective company SOP.
Concerned that the departure of the aircraft was about to fall behind schedule, the engineer decided to connect the towmotor to the towbar as a single person operation. That decision, combined with the design of the towmotor, led the engineer to operate the towmotor from a standing position. The subsequent movement of the towmotor was consistent with the driver inadvertently placing his foot on both control pedals simultaneously.
Local safety action
As a result of the accident, the operator implemented the following safety actions:
1. The Engineering and Maintenance Procedures Manual has been amended to include an instruction requiring drivers to have a marshaller present during towbar hook ups and for the marshaller to stand to one side of the vehicle during this operation.
2. The Engineering and Maintenance Procedures Manual has been amended to include a requirement for drivers to operate tugs from the seated position in those tugs designed for operation from this position.
3. An evaluation of the handling characteristics of the tug has been conducted with a view to an assessment as to the suitability of the tug for further operations.
4. Anti-slip paint has been applied to the brake pedal and has been incorporated into the system of maintenance.
5. An external overhead mirror has been fitted to enable drivers to view the towbar hitch point from the seated position in the control cabin.
6. An assessment has been made as to whether other tugs used by the operator require modification with anti-slip paint and external overhead mirrors.
7. A lockout switch has been fitted so that the transmission selector cannot be moved from the neutral position unless pressure is applied to the brake pedal.
8. A recurrent proficiency program has been implemented for drivers who have not driven a particular type of tug for 6 months.
In addition, the operator is assessing the feasibility of fitting transmission selector lockout switches to other tugs used by the operator, which do not have these switches fitted as standard equipment.
|Date:||01 April 2001||Investigation status:||Completed|
|Time:||0810 hours EST|
|Release date:||04 December 2001|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Substantial|
|Departure point||Melbourne, VIC|
|Role||Class of licence||Hours on type||Hours total|