Sunday, August 07, 2005

Backing Up

Someone who has been watching carefully as jets leave the gates e-mailed me with two questions about airplanes leaving the gate.

Why is it that the nosewheel steering is disconnected prior to push-back?

Nosewheel steering is typically a mixture of hydraulic and mechanical connections allowing the captain to steer the nosewheel through thirty degrees or so on either side. When a towing bar is connected to the noseweel, and the aircraft is pushed backwards, or pulledto the side, the nosewheel can be turned a lot further, perhaps through ninety degrees. Damage could result to the steering if it were left connected. Obviously it is equally, if not more important to ensure that steering is properly reconnected after the ground handling.

Is it because of the lack of a rear-view camera that aircraft can't use reverse thrust to push themselves backwards instead of having to wait for tow-tractors?

Not really. The guy driving the tractor has better visibility than the guys in the cockpit, but it's still pretty hard to tell what's behind a large aircraft being pushed back. Accidents happen that way, even with wing walkers and walkie-talkie-equipped crews.

Remember, an airplane taxiing isn't like a truck driving. The motive force comes from the movement of air. In crowded gate areas, aircraft are sometimes damaged or even overturned by the jet blast of other airplanes manoeuvring. In order for an airplane to back out of the gate, the thrust of the engines would have to be directed forward, through the plate glass window where you're standing watching the airplane. The ground idle danger zone for jet blast extends 450 feet behind a medium jet, and it takes more than idle to start taxiing from a standstill.

Some airplanes simply have no reverse. They go forward or they are pushed backwards. Some airplanes are designed to develop reverse thrust on the ground, but with limits, for cooling. I flew one that was restricted to one minute in low temperatures, and only as absolutely required up to 30 seconds in high temperatures. Reverse thrust is generally intended to be used to slow down the airplane on the runway, after landing not for ground manoeuvering.

"I'm okay with women flying airplanes, just so long as they don't have to reverse them into parking spaces." -- old timer captain

Right after I wrote this page, I discovered that Boeing has developed an electrically powered nosewheel to allow ground movement with no tug, with all engines shut down. Air Canada successfully tested it in June.


Lord Hutton said...

Some people just dont do reversing. It's bad enough letting them loose in a car, let alone a plane.

ScottJ said...

Great answer! There's another good reason why most aircraft don't "power back". Applying reverse thrust from a standstill has a tendency to blow FOD up off the tarmac and forward, where it's sucked into the fan.

dibabear said...

There was a time, very short lived, when jets used reverse to pull away from the gate. Seriously. I was on a couple of flights (two to be exact) where this was done.

They throttled up, backed just far enough to clear the gate and then taxied forward into a tight turn.

I have no idea what prompted it and I haven't seen it done before or since.

Oh...and I had a good laugh on a 777 flight recently. The passengers were slow getting settled and the FA announced that everyone has to be seated so the Captain can see down the aisle to back up. The stragglers were in their seats quickly.