Why do airbridges move when boarding?

Elegant Chaos

I was boarding in LHR yesterday, and as often happens we were queuing to enter the aircraft. The airbridge "moved" slightly a couple of times as if it were re-adjusting, and it got me wondering - why does this happen? It's a pretty frequent occurrence; in other airports (BRU?) I've noticed more pronounced readjustments of the "is this going to collapse" type! Surely once the aircraft is stationary, and the airbridge is maneuvered into position, it is "locked" in place? Am I missing something obvious?

Irish Steve

Depending on how the loading is going, the nose wheel level can change, which then can trigger an adjustment by the airbridge to avoid damage to the doors, there are a number of sensors on the head of the bridge that detect the exact aircraft position. There is also the possibility that cargo pallets are being loaded at the rear, which can also cause a change in the relative position of the aircraft in relation to the airbridge, as that can be 2 or 3 tonnes of weight being put on a long way down towards the back, so the main gear can change, as can the nosewheel.

More rare would be s slight movement of the aircraft as a result of strong winds, though in theory, the chocks and brakes should minimise that.

Stovepipe

Steve has it; the airbridges will correct themselves so you sometimes get slight shunting movements and you'll hear the leg motors running. You will notice a black plastic "shoe" positioned under the open door and this is part of the level sensing equipment of the airbridge. On aircraft like the A330, there are marks painted on the hull near the main door of the aircraft, to which the bridge operator must place the airbridge. Airbridges will also default to stop so that they won't begin to "travel" without operator input. We had an airbridge try and jack up an A320 when it made an uncommanded movement, with no operator present and it nearly tore the pax door off.

lintdrummer

Think of an aircraft like a ship in a dock, as weight is added it will sink a little. As has been said if the airbridge were to be totally locked in position this could lead to damage or a pronounced step at the aircraft door.

jimbis

Wow I didn't know the self adjusted. Learn something new everyday!