Don’t be scared.
They’re a common sight at airports around the world: Passenger Boarding Bridges (PBBs), also known as “jet bridges” or “Jetways” (which is a brand name), are some of the more distinctive architectural features in aviation. In the United States, they’re used by most airports handling passenger jets after being developed in the 1950s to funnel passengers directly from the terminal interior to the aircraft interior, sparing them from the outdoor elements.
Anybody who has spent a fair amount of time on a PBB (or is really observant) is familiar with the rather jarring sensation of the bridge seeming to have a seizure—jerking and wobbling for no apparent reason as the floor jars and jolts. It almost never fails to get everybody’s attention and elicit a flurry of remarks.
But what is the mysterious rumble?
Ever felt an automobile settle onto its tires when you get in? It’s the same concept. During turnarounds, aircraft slowly rise off their landing gear as passengers, baggage, and cargo are offloaded, and they slowly sink again as new passengers, baggage, and cargo are loaded. This means that without adjustment, the level of the PBB can differ by several inches from the floor of the aircraft at various points during the process—that’s a trip hazard for people entering or exiting the aircraft, and too much movement can put pressure on the aircraft exits, a situation that often requires a lengthy damage assessment from an aircraft maintenance team before the aircraft can be cleared to safely depart.
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The obvious solution is to put down ramps, but there’s another wrinkle: aircraft skins are uniquely sensitive to wear-and-tear. The seal on an aircraft door is vital to maintaining a pressurized cabin while inflight, so it’s also a safety risk to scrape away a micrometer of metal each time a ramp is placed, so all the servicing equipment is designed to come up close to the aircraft skin, but not remain in contact with it.
Baggage loaders, PBBs, and catering trucks all inch close to the aircraft, but generally don’t make contact. There are certain exceptions, like the cloth canopies on PBBs, which are soft enough not to be an issue, or the metal ramps temporarily used for catering carts and passenger boarding on some regional jets are similarly designed to bridge the gap without constant contact with the aircraft’s skin.
The floor level of a PBB already adjusts up and down so that it can service different aircraft types at differing heights above the ground: way down low for regional jets and occasionally turboprop aircraft, and way up high for the big widebody jumbo jets. When the PBBs are parked against the aircraft, there are sensors that can determine how much the aircraft has moved relative to where the floor level was originally set, and make adjustments to keep the floor level with the aircraft floor.
While the newer auto-level systems use video and lasers to detect movement, the older ones are pretty low-tech. When the PBB is parked, an arm comes out with a rubber wheel that sits against the aircraft skin to measure how much the level has changed. When it reached a certain threshold, it suddenly adjusts on the wheel back to where it was. It’s not always a gentle movement, and it can rub the canopy against the aircraft skin. This isn’t unsafe, but over time it leaves a rather unsightly square of gray smudge around the door, which is particularly on aircraft painted a light color like white, cream, or grey (as so many are).
The important thing to remember is that this is completely normal. Nobody messed up, the systems aren’t malfunctioning, and the passengers aren’t about to spill out onto the ramp (although sometimes they might need to grab onto a railing). It’s just a byproduct of a handy invention designed to keep travelers safe boarding and deplaning from their flights.
