What happened
On 12 March 2009, an Airbus A320-233, registration HA-LPJ, was undergoing a pushback from Stand 40 at London Luton Airport. The operation, involving 136 passengers and 6 crew, began with the aircraft being pushed back towards the north apron. During the manoeuvre, the towbar's undercarriage began to lower unexpectedly, prompting the tug driver to halt the pushback and request that the undercarriage be raised.
In an attempt to correct the pushback track, the tug driver decided to pull the aircraft forward towards the stand. During this repositioning movement, the torque and axial pins on the towbar failed, causing the aircraft to detach from the tug. The aircraft continued to roll forward, resulting in a collision between the aircraft's right engine and the tug cabin. The impact caused damage to the aircraft's nosewheel tyre and the right engine inlet cowl and fan blades, while the tug sustained substantial damage to its cabin. There were no injuries to the crew or passengers.
The investigation
Investigators reviewed CCTV footage and examined the towbar components. The investigation established that the towbar was being used at a large acute angle. It was also noted that the towbar's undercarriage had been subject to a hydraulic leak in the 'up, down and hold' valve, which had caused the undercarriage to creep downwards.
Technical analysis of the towbar head revealed that the outer turn bush was sitting approximately 4 mm proud of the shear face due to previous damage. A manufacturer's representative suggested that this misaligned bush may have acted as a substitute for the shear pin, allowing the tug to maintain an acute angle without the pin breaking initially. However, the combination of the towing angle, the gradient of the taxiway, and the aircraft's thrust eventually caused the simultaneous failure of both pins.
Findings
- The simultaneous failure of both the torque and axial pins was the primary cause of the aircraft detaching from the tug.
- The incident was contributed to by the extreme towing angle and the gradient of the stand.
- A malfunctioning hydraulic valve caused the towbar undercarriage to lower, distracting the tug driver.
- The towbar's turn bush was improperly positioned, which likely prevented the shear pin from breaking at the appropriate load, allowing the angle to become critical.