What happened
On September 19, 2006, at Prague Ruzyně Airport (LKPR), a Boeing 737-800, registration OK-TVC, was being towed from stand 15 to a new parking position on runway 04/22. The towing operation was performed by a Steward & Stevenson GT-110 tug using a standard tow bar.
As the aircraft transitioned from taxiway L to taxiway P, the aircraft began to oscillate, swinging left and right. While the tug driver attempted to correct the movement by steering right and using audible signals, the aircraft gained speed relative to the tug. During this maneuver, the tow bar reached a critical angle, causing the tow bar eye to strike the rear of the tug. This impact resulted in the structural failure of the tow bar's bolted connection.
Following the separation, the remaining portion of the tow bar remained attached to the aircraft. The sudden change in forces caused the nose gear to swivel sharply to the right. The aircraft then collided with the rear left corner of the tug, causing damage to the forward right fuselage of the Boeing 737-800. The tug sustained minor damage to its upper left bodywork, and the tow bar was torn into two pieces. No injuries were reported.
The investigation
The ÚZPLN investigation focused on the mechanical integrity of the towing assembly and the sequence of forces during the turn. Investigators examined the broken surfaces of the shear pin and the connecting bolts. Technical analysis by the VZLÚ institute confirmed that the shear pin had failed due to uneven loading and a localized increase in pressure near the upper shear plane. Additionally, a fragment of the lower part of the shear pin was recovered approximately 200 meters from the accident site on taxiway L.
Findings
- The primary cause of the collision was the destruction of the shear pin within the towing equipment.
- The failure of the shear pin was caused by an uneven distribution of loads across the pin connection during the maneuver.
- The change in loading characteristics—from a pulling force to a pushing force—combined with the turning moment, created the stress that led to the pin's failure.
- The aircraft's nose gear was forced to rotate significantly after the disconnected tow bar caught on the pavement.
- The crew's ability to react was hindered by limited visibility from the cockpit and the rapid nature of the mechanical failure.