Hey there!
I just got back from a trip and one question is going thru my mind: how do airplanes taxi?
I’ve seen some that are towed along, especially as they pull away from the gate, but what about when they’re out on the runway, lined up for takeoff?
I have two theories; 1) There’s a small electric motor with incredible torque that drives the wheels via chains or the like OR can the engines on said planes (and jets, too) be revved up to actually push the plane along the ground?
Airplanes taxi under their own power using the engine(s). Large aircraft (like airliners) have a little steering wheel to turn the nose gear. Smaller aircraft use the rudder pedals to steer the nose wheel (usually a bungee or spring arrangement, I think), or use differential braking.
You just apply a little throttle, and taxi away. All aircraft taxi that way, whether it has a propeller or jet engines. Putting motors for taxiing would be way too heavy for an airplane.
My airplane didn’t even have a steerable nosewheel. Just bring the prop rpm up to 1100 or so, and use the brakes to steer. The nosewheel was free castering, so it would just turn towards the wheel that had the brakes applied.
Jets with thrust reversers can back up, but they use pushback tractors because you really don’t want to rev up the engines around the terminal building because it’s dangerous and it blows crap everywhere.
Sam Stone: I remember the AA-5 being much easier to taxi with its differential braking than the Cessna with its steerable nosewheel. The Cessna always seemed heavier on the ground.
And as for why you see jets being towed in airports, a couple of reasons. Most aircraft can’t back up under their own power on the ground*, so they have to be pushed back, and sometimes you don’t want to rev the engines up because of where they are pointed (like towards windows and things).
*Never saw this, but I’ve heard that certain multi-engine prop planes can pull this off. Think WWII bombers, like B-29’s. They would move the prop on one outboard engine to reverse thrust and rev it up while locking up the brake on the opposite wheel, then switch sides and rev up the opposite engine and lock the other brake. This would let the plane sort of “walk” backwards.
Besides the dangers to the terminal building already mentioned about using the reverser to back up, there’s also the danger of foreign object damage to the engines with the reverser engaged at low airspeed. To be honest I don’t understand precisely why this is the case, but I think the risk of FOD is one of the reasons that reversers are usually disengaged as the aircraft is slowing through 80 knots when rolling out. Can someone more knowledgeable enlighten me on this?
Not more knowledgable, but I can confirm that I have heard the same thing. I believe it is standard procedure to not use thrust reversers below 60 to 80 knots because of FOD. IANAAP, I read it in the MS flight simulator manual.
Also not a pilot, but…
Thrust reversers, at least the ones I’ve seen, work by opening a pair of exhaust diverters to blow the jet exhaust forward instead of backward. (It’s pretty cool to watch them open.) In particular, they’re probably really good at lifting up any debris on the runway right in front of the engine intake. :eek:
How likely this is probably depends a lot on the specific engine. The ones like this (I’ve seen these on Boeing 727 and 737) redirect the exhaust upward and downward, so they probably blow ground debris around a lot. I think the ones on B767’s redirect horizontally instead of vertically and probably cause fewer FOD problems, but I can’t find pictures, so I might be wrong.
Yep. That’s exactly what happened in 1892 to Air Florida Flight 90, whose cowboy captain got impatient waiting for a tow during a snowstorm at DC’s airport. So he used the reverse thrusters to back away from the terminal, sucking snow into the engines (he also forgot to engage the wing deicers). A few seconds after takeoff, WHAM, the plan stalled and crashed directly on top of the 14th Street Bridge, which spans the Potomac River. Four cars were crushed, resulting in five deaths on the ground. The plane then slid off the bridge and sank to the bottom of the Potomac. 47 people died.
Source: http://www.airdisaster.com/special/special-af90.shtml
Several posts have talked about the “wash” being dangerous near the terminal.
Just to carry that on a bit further - don’t forget about Newton’s Third Law. If the engines are pushing the multi-ton beast forward, that means the jet velocity coming out the back of the engines must be tremendous. Even though the jet engines are not at full throttle, there is awesome power there.
I can absolutely confirm that American Airlines jets (I think they were MD80’s) at MCI airport will use thrust reversers to back away from Terminal A - I’ve watched them do it from the Admiral’s Club up above near Gate 12, and been sitting on the plane when they did it - this as late as this Summer.
Don’t know why they choose to do this, when there is a tug sitting right there though. 
For military aircraft, both the C-130 and the C-17 can back up under their own power. The C-130 (like most turboprops) has variable pitch propellers, and just adjusts the pitch so that they are providing reverse thrust. The C-17 uses its thrust reversers (but I understand it’s far from a routine operation, requiring the loadmaster to be looking out the cargo door spotting, and several additional checklists).
Thanks, everyone, for the informative responses!
Just to carry that on a bit further - don’t forget about Newton’s Third Law. If the engines are pushing the multi-ton beast forward, that means the jet velocity coming out the back of the engines must be tremendous. Even though the jet engines are not at full throttle, there is awesome power there.
That’s what I’ve always thought. The backwash must be pretty bad! Don’t stand behind those planes when they taxi!
I hadn’t thought of this, but it is likely much safer for MD-80s (or MD-90s or DC-9s) to use thrust reversers for backing up, since their engines are mounted high up near the tail rather than slung under the wings. Much less of a chance for FOD.
They’d have to call up the guy on the radio, then he’d have to hook them up, then push them back at two miles per hour, and they were probably already late as it was. Heck, it may even be totally normal procedure to use thrust reversers for push-backs in DC-9 type aircraft.
Actually, I thought that asked Latka to help them … 
Just a few clarifying remarks:
Any jet with reversers (not all have them) can back up under its own power. Yes, FOD is an issue, but mainly the restriction is from throwing pebbles and junk against glass windows on the terminal, or at other things/people that wouldn’t appreciate it. It’s also noisier. So, the practice is OK at some airports and some terminals, but not others.
Yes, tail-mounted engines have less of a FOD problem than wing-mounted ones, being further from the ground, but that’s true whether or not you’re using thrust backup.
It also cuts into engine life, which for most parts is measured in cycles (idle-max-idle throttle excursions). It also matters that the engines are reingesting some of their own exhaust gases, reheating them further, and making the metal parts hotter than they’re comfortable at. The moving parts in the reverser also get worn down faster. Some airlines ban the practice, even if it’s OK by the airport’s rules, because of higher maintenance costs.
Dr. Lao is right about the time used to hook up the tug sometimes being the deciding factor.
The C-17 can actually taxi backwards slightly uphill. To do that, it has reversers on the core as well as the fan. Part of the reason reverse thrust is not as high as forward thrust is that, on airliners, only the fan flow is reversed, and has to fight the forward core thrust.
Any newer prop aircraft with variable pitch (at least the ones I know of) has a reverse-thrust setting on the blades and can taxi in reverse that way.
Venkman may be thinking of the way to back up a B-17, which did not have a reverse setting on the props (I don’t think) but did have a taildragger gear layout, with the mains on the inboard nacelles. The trick was to lock the brake on one side, go to full thrust on the outboard engine on that side with the others at idle, pivoting the aircraft around that wheel. The other side would then actually move backward. After a bit, do the same on the other side, and the plane would actually “walk” backward, although at some cost to tire and brake wear and to the pilot’s relations with his crew chief.
You’re exactly right. My dad (who was a gunner on a B-17 in the war) told me this, but I garbled it badly in trying to remember what he had said. Thanks for the explanation.