The World From Your Airplane Window
A science writer's guide for the inquisitive air traveler.
- By Brian Clegg
- AirSpaceMag.com, February 07, 2012
Ruud de Vries
After what can seem like a long wait, it’s time for pushback. The aircraft reverses away from the stand and taxies off to the end of the runway. Unlike a car, there’s no power to the wheels in a plane; most of the maneuvering on the ground is driven by the aircraft engines. This is not a very efficient way to travel when the plane isn’t in the air, particularly in reverse, so to get away from the terminal an aircraft tug (sometimes called a pushback tractor) is usually brought in.
The tugs used on 747s are typically 200-300 horsepower—less than a high-performance car. In principle, an airliner can back away from the terminal using reverse thrust. This involves the crude technique of placing a deflector behind the jet engines, so the blast of air is pushed towards the front of the plane. Reverse thrust is usually deployed on landing to slow the aircraft down—this is what is being engaged when you hear the engines suddenly surge as you touch the ground. But it isn’t practical to use reverse thrust when close to a terminal (‘on stand’ in airline parlance). The blast from the engines is liable to send any debris on the ground hurtling towards the glass of the building, which is why tugs are used instead.
You may wonder, given the inefficiency of taxiing on jet engines, why the tug doesn’t take the plane all the way to the runway. Virgin Atlantic did come up with the idea of doing just this in 2006. The idea was to pull the plane to a ‘starting grid’ at the end of the runway. This would have produced significant fuel savings—Virgin reckoned that they could save two tonnes of CO2 per flight, as well as reducing noise and cleaning up the air near the terminal.
Unfortunately, despite its green credentials, the technique soon had to be shelved. This was partly because airports were not willing to provide the starting grid locations, which would have produced delays while tugs were decoupled and moved clear of the jet blast. But more significantly, the aircraft manufacturers warned that increasing the amount of towing would put too much strain on the undercarriage, meaning the struts that hold the wheels would have to be replaced more frequently.
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Comments (9)
Thank you for this very interesting and informative article. As a regular flyer myself, and a former student pilot, I would like to suggest adding some more information about what a passenger can see from the vantage point of sitting just behind the wing. I have had lots of fun looking at the control surfaces on the wing -- ailerons, spoilers, flaps -- and using what I see in the deflections of those surfaces, particularly the ailerons to deduce what the pilot is doing.
For example, on a landing approach, from my view out the left window just behind the wing, I see left ailerons deflected downward, indicating that there is a crosswind component from the right, causing the pilot to slip the airplane to the right to correct for the crosswind.
I have also seen pilots using spoilers to slow the airplane to safe turbulence penetration airspeed during straight and level flight, and also once, when there was a steep descent required due to traffic conditions on approach to a very busy international airport. That was a thrilling adventure as the pilot on that flight was also using thrust reversal together with spoilers in order to maintain safe airspeed.
Flaps of course, are quite obvious when landing or taking off.
I know, I have been verbose here, but if I could use some pictures, everything would be much clearer.
Thank you for considering this suggestion.
Posted by Gerard Pereira on February 8,2012 | 04:44 PM
Thrust reversers used in flight? No way, they are interlocked that they can not be used unless the plane is on the ground.
Posted by John Bell on February 12,2012 | 12:31 PM
@John Bell: Yes, many aircraft are certified for in-flight use of thrust reversers. The DC-8 is so certified. However, there are conditions of use which vary aircraft to aircraft...it is unusual!
Posted by Michael Bludworth on February 22,2012 | 05:38 PM
Maybe he meant speed brakes?
Posted by Robb Cole on February 22,2012 | 08:05 PM
When seated in a window seat near the wing leading edge or slightly aft, I have noticed several times the air density change, in certain atmospheric conditions, that allowed a normal shock wave to be visible in the vicinity of the engine nacelle intersection with the wing leading edge on a jet aircraft. This shock wave would be quite stable and proved that the gas dynamic textbook illustrations were accurate. The aircraft itself was not travelling supersonically but the air was being locally accelerated enough to approach or exceed sonic conditions.
Posted by Tom Lisec on February 23,2012 | 09:05 PM
Thanks for the snippet of information. Keep up the good work. My personal joy in flying is land use and geography / geology. Saw my first wind farm a year ago (Minnesota) and it took me a while to recognise the site and its minimal impact.
Posted by ErnestPayne on February 24,2012 | 08:26 AM
Was that steep approach into Washington National by chance? They tend to name it after the basic tool we know as the crowbar. I rode it in a USAIR Dc-9 once. Quite a ride, but I'm pretty sure he didn't use any reverse.
Posted by Markus on February 27,2012 | 11:53 AM
Would a runway facing due North be numbered 36 or 01?
Posted by Dave R. on February 28,2012 | 08:50 PM
A runway facing north is numbered 36. Approach it from the opposite end and it's numbered 18.
Posted by Auntie Analogue on March 23,2013 | 12:14 AM