On recent commercial flights I have noticed unusual tugs pushing the aircraft back from the gates. I later learned these are “towbarless tugs,” and instead of connecting to the aircraft’s front landing gear with a ten-foot-long towbar, the tug simply picks the landing gear up off of the ground altogether:
Here’s one picking up an A-380:
According to the Wikipedia article on aircraft tugs, they can maneuver the aircraft at higher speeds, and without anyone in the cockpit.
First Question: when using conventional towbar-equipped tugs, why is somone required in the cockpit? The towbar is understood to be a large lever by which the tug can steer the aircraft’s front landing gear, so why is anyone’s input needed from up there?
Further reading led me to the Taxibot, a new system in which a towbarless tug is actually controlled by the pilot in the cockpit, and is used to transport the plane from the gate all the way to the runway for takeoff (or back to the gate after landing) without using the plane’s engines; the tug’s motor, either electric or diesel, provides the necessary motive power. The advantage is claimed to be substantial fuel savings and reduced airport pollution, because aircraft engines burn a lot of fuel under idle/low-thrust conditions, something around 50% of the rate they burn fuel at cruise. This of course means that the plane’s engines don’t even get started until it’s time to take off - which means they may not be anywhere close to “normal” operating temperature when they’re commanded to a high power setting. This is generally understood to be a bad idea for piston engines: you’re supposed to let those warm up at light loads and moderate RPM’s before you put your foot to the floor, but I wonder whether that’s also true of gas turbines.
Second Question: how much warm-up time do jet engines need before you can apply takeoff power without excessive wear and tear?
Can’t say about jets but I flew Hueys in the Army years ago.IIRC our EGT(exhaust gas temp.) was at operating temp just as soon as the engine was started. Don’t koow if the rest of the engine was at operating temp. because it wasn’t anything we worried about so I imagine didn’t matter.
Not an aviation mechanic but a few educated guesses:
[ul]
[li]Piston & turbine engines are very different. Turbines get much, ***much **hotter, they get hot much faster, and perhaps most importantly their combustion heat is more evenly and completely distributed around the entire engine compared to a piston engine. Piston engines have to heat the pistons so they expand and fit tighter in the cylinders, they have to heat all the oil that circulates thru the entire engine, and they have to generate enough heat to recirculate some back thru the intake manifold & fuel system to keep it from freezing even in warm weather.[/li][li]For a large airliner the nose gear steering will require hydraulic assistance (i.e. similar to power steering in a car), therefore in order for it to be turned by a towbar it would need to be set to a ‘neutral’ or ‘passive’ mode, which requires power from the plane itself (which necessitates having a flight crew on board).[/li][]By lifting the nose gear off the ground the tug turns ‘around’ it, like modern tow trucks that lift the car by the front tires.[/ul]
[quote=“Hail_Ants, post:3, topic:647757”]
Not an aviation mechanic but a few educated guesses:
[ul]
[li]For a large airliner the nose gear steering will require hydraulic assistance (i.e. similar to power steering in a car), therefore in order for it to be turned by a towbar it would need to be set to a ‘neutral’ or ‘passive’ mode, which requires power from the plane itself (which necessitates having a flight crew on board).[/ul][/li][/QUOTE]
Actually the nose wheel steering is unpowered unless there is hydraulic power turned on. So as long as the hydraulics are off you can tow with no problems. In other words, the nose wheel steering is able to be towed with no power at all to the aeroplane. On the other hand you need someone in the cockpit to guard the brakes so that if something goes wrong with the tug, or if it gets disconnected somehow, the person in the cockpit can stop the aeroplane with the wheel brakes. For that you need brake pressure which may mean you need some hydraulic systems on line. In my type, the Avro RJ, there is a dedicated DC pump that supplies hydraulic pressure to the brakes but doesn’t pressurise the other systems. This can be used for an unpowered tow.
For the RJ also, the warm up time is three minutes or until the oil temperature is over 30ºC, whichever is later. A normal taxi time with no delays at a small airport is around 10 minutes so you could still save at least 7 minutes of fuel by delaying the start.
First I’ve heard of this, but here’s a thought: If it’s true that aircraft are/can be towed all the way to the runway, it seems more efficient to lower the nose wheel to the ground and drive away so the aircraft can depart, than to disconnect the tow bar and tow it back for the next plane.
I was an aircraft mechanic for over 20 years. When we towed an aircraft to and from places such as a hanger, we had someone in the cockpit for 3 reasons.
(1) to use the aircraft’s brakes in an emergency such as a tow bar breaking. Never saw that happen.
(2) to release and set the parking brakes as needed.
(3) to maintain radio contact with the control tower. However if the tow tug had a radio with the proper frequencies, this wasn’t needed.
The largest aircraft that I worked on was a DC-9-30 series. Pretty small in comparison to an A-380.
Second Question: Very little time at all. By time they taxi out to the runway, the engines are warmed up enough. Actually the only thing to worry about is the engine oil being cold and that’s not much of a worry. Some military jets on alert duty can from engine start to take-off in less than 5 minutes.
Last time I towed an aircraft with a tow bar, my back was sore for a week afterward. And that was only a six-seater! And I only towed it about 10 feet, too.
On the other hand the Antanov 124’s engines are so rubbish that they have to sit on the runway for about 10 minutes before take off, slowly spooling them up, as just applying full throttle abruptly runs an extremely high risk of breaking them. Similarly it is unable to go around at anything like the same altitudes that most aircraft are, it’s commited to landing because of said spool up issues.
How often are problems discovered during engine startup? It seems to me there will be cases where the plane is towed to the runway, the flight crew start the engines, discover a problem, and have to be towed back. You’ll be in for a worse delay than if you’d stayed at the gate, and the returning aircraft will have to be sorted out by ground controllers. Those two factors will negate some of the advantage of using these things.
