'Safety interlock' on pushback trucks?

[knobbygb]

I flew from LHR to MAN the other day (Sunday morning) on an A320, and as we were pushing back there was a 'jerk' and a noise from the front. Sounded/felt like the pushback bar had either come lose or banged into somthing.. .We were pulled back on stand and delayed for about 40 mins while they inspected it. In the end the reason given was that 'the safety interlock on the pushback truck was not working properly'. If this is true, why were we pulled back onto the stand when we were ready to go? Is there really a sefety interlock? - never heard of it before.. .I think it was more likely that they thought there might be some damage to the forward gear leg. They had to run both the engines up on stand to 'diagnose the problem' - could they have suspected damage to the hydraulics/brakes etc?If so, I can understand the pilot not wanting to alarm the pax by saying there might be some damage. Was anyone there and know what really happened?

[tinyrice]

Sounds a bit like the shear pin went on the tow bar if you were already under way. Dont know much about the A320, but most jets require either a steering bypass pin, or to disconnect the nose gear torque links so that when hydraulic pressure becomes available, the tug and two bar aren't trying to "backdrive" the nose steering gear. If you leave the steering bypass pin out and then start engines on pushback, there will be a large and embarassing bang as the hydraulics come on line and the shear pin breaks. This is usually closely followed by an anxious voice on the headset asking pointedly if everythings OK down there. This is usually followed by towing the aircraft back to the gate, having a look at the nose gear attach and the steering system for any damage, getting a new tow bar and trying again.

[TR4A]

The shear pin can also break if ther tug driver turns the nose too far. The tug driver the other day pushed me too close to a bag cart which made him turn the tug to sharp and we felt and heard a snap. The shear pin on the tow bar broke.

[knobbygb]

Yep, it happened as they started No 1, part way through pushback so sounds like the pin sheared. No 2 was already running, though. We didn't appear to be turning too tightly, but you can't really tell from in the back. Thanks - I'll be able to sleep at night now.

[Irish Steve]

From the description, a broken Shear pin sounds like the most likely scenario, but there's also another pin that can let go if there's a "snatch", which can happen if the tug driver changes throttle setting too fast, or if the tug or aircraft goes into a drainage channel at just the wrong speed, and the combination reacts.. .. .Could even be that the lock on the towbar head at the nosewheel end was not properly interlocked, or failed, and the aircraft and bar parted company. Given the forces involved, that for sure would generate an "incident", requiring inspection, as the bar would almost certainly hit something as it fell off.