I'm wondering how bad it is to fly a short distance, such as ~50km, with a big airplane. How much less fuel does it take to fly at high altitudes compared to low? And how much of the fuel is spent on start/landing? Flying between continents is never a problem, but if a politician flies a shorter distance all hell breaks loose.
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$\begingroup$ landing is with engines mostly at idle (you are descending and slowing down most of the way down). $\endgroup$– ratchet freakCommented Apr 16, 2015 at 10:12
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3$\begingroup$ The hell breaks loose not just because short flights are more expensive per mile than long ones but mainly because due to the time spent on the airport and need to drive to the airport and get cars at the destination airport to drive from there the time saving compared to just going by car is small while the cost is significantly higher. And it's not just fuel cost, but also some maintenance depends on pressurization cycles, i.e. flights, and there is all the more complicated organization that also costs something (especially if we talk about government plane, not regular airline). $\endgroup$– Jan HudecCommented Apr 16, 2015 at 11:20
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2$\begingroup$ In this case, a swedish politician flew 41km to another airport, where a few other politicians were picked up before they continued to new york. So it's a matter of two cars going 50km and back (200km total) vs the government airplane making the short flight to pick them up. $\endgroup$– DrathierCommented Apr 16, 2015 at 11:24
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2 Answers
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The fuel consumption or fuel burn rate is dependent on the aircraft type, size, engine configuration, cruise altitude, temperature, thrust setting and other factors.
In general: The larger the aircraft, the less economic do short flights become due to weight/size and fuel burn on takeoff/climb.
Here are some values I found for the CRJ2, the source may not be entire citable:
Approximate total burn pounds / hour
Taxi: 700 (Both engines and APU)
Takeoff: 6000-7000 (depending on temperature, elevation, reduced/full thrust, etc)
Climb: 6000-3500 (reducing as as altitude increases)
Cruise: 2500-3000 (depending on altitude/weight/etc... subtract 500 for long range, add 500 for high speed)
Holding: 1800-2400 (depending on altitude/speed)
Normal descent: 1500
Idle descent: 600
Approach: 2000 (fully configured)
(Source: www.airliners.net - Author: Flyf15)
You can also read the following related questions to understand how altitude affects fuel efficiency:
Why do jet engines get better fuel efficiency at high altitudes?
How does flight duration affect the hourly fuel consumption in heavy aircraft?
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The uproar was justified. The Swedish politicians should have driven to one airport and flown from there in one leg.
There are two effects which make short flights inefficient:
- The relative length of unproductive taxiing and initial acceleration becomes bigger the shorter the flight is, and
- Flying at low altitude is slower, so much more fuel is consumed per distance flown.
You also need to burn more fuel for climbing, but you will get that back on descent when the aircraft can reduce thrust, just like you can coast downhill in a car.
Jet aircraft reach their highest efficiency when flown near the tropopause. Here, air density is only a quarter of what it is at the ground, and the aircraft will fly twice as fast up there when flying at its optimum lift to drag ratio. In addition, the colder air at altitude will make jet engines more efficient. Therefore, flying those 41 km used more than twice as much fuel per km traveled than flying from Sweden to New York. Add to this that the aircraft needed to taxi for several km twice instead of only once, and also needed to spend the fuel for the take-off run twice. While the potential energy added during climb can be regained during descent, the initial acceleration energy is mostly wasted as brake heat, spoiler turbulence and reverse thrust. This adds up to several 100 kgs of fuel for a large airliner.
answered Apr 16, 2015 at 19:57