Takeoff and fuel burn

[EngStudent]

I'm working on a project to understand the economics and fuel burn of large and small commercial jets over different distances.

I always thought that takeoff in big commercial jets was conducted at full throttle and then cruising is at a lesser burn rate, so basically once you have burned an initial large amount of fuel taking off and climbing to altitude for about 20 minutes you cruise and burn at a lesser rate for the duration of the flight, then land. (Whether you land at a cruise burn rate or something less because of descent, I don't know.)

I figured it takes a lot more fuel burn at takeoff for widebody (i.e. 2 aisle) jets, so the overall fuel consumption per passenger would be high unless you fly a long way - so on short routes narrow body jets are cheaper because takeoff and landing is better.

However I found a site on the net that says takeoff isn't conducted at full throttle at all...

Does anyone know where I could go to actually find some data about this stuff (like how many pounds per second get burned on takeoff vs while cruising?)

[BlueEagle]

Hi EngStudent,
It is highly probable that someone here will come along with a short answer for you, but, in the meantime, what you have heard about is the "Assumed Temperature Take-Off" and you will find some reading material here (http://www.google.com.au/search?hl=en&q=assumed+temperature+take+off&btnG=Google+Search&meta=) . Best of luck. (I think only the first four or so 'hits' are relevant out of the the 1.2million that came up!).;)

[Old Smokey]

EngStudent,

At the design stage, aircraft are Payload / Range optimised for their typical mission profile.

For example, an aircraft designed for Sydney - Los Angeles operations must be able to carry a profitable payload at the same time as carrying sufficient fuel for the flight. If Takeoff and Climb fuel efficiency was less than optimum, then it's still acceptable provided that cruise operations, which will take up about 95% of the flight time, are efficient.

On the other hand, if we could stuff enough fuel into a B737 to operate Sydney - Los Angeles direct, the payload would be nil or negligible. This aircraft has been designed for Short / Medium range operations, and Takeoff / Climb fuel efficiency (where the aircraft may spend up to 40% of the flight time) must be good, even if cruise performance is only mediocre.

There's absolutely no reason why a narrow body aircraft could not be built optimised for long range, or a wide body aircraft optimised for short range, the simple fact is that nobody builds such aircraft. (At the same time, wide body aircraft such as the A330 offer very good short range, as well as long range economics).

To respond to your one uncertainty (is fuel saved on descent), yes, absolutely it is, but fairly typically, the fuel saved on descent is then used for the approach, making little difference to calculating the flight at cruise to overhead the destination.

Regards,

Old Smokey

[Paul Wilson]

Saw a thread a little while ago called something like "Flex or Full power which uses less fuel?"

Seem to remember that the result was.

FLEX takeoff uses lower power, but uses it for longer to get to target altitude, total result is marginal INCREASE in fuel burn, but a decrease in maintenance bills, as engine core doen't get as hot.

Good search term to use on this site:-

FLEX (Airbus speak)
Derate
Assumed Temp(erature)
Also "Cost Index" which is a whole new topic in itself, basically your airline decides how oimportant 2 factors are to them, Fuel Price, and Hours in the air (Maintenance Price) you then dial a number between 1-100 (or is it 0-100) into the aircraft and it picks a speed to cruise at accordingly. Fuel price high - go slower, Fuel price dirt cheap - go faster.

[A4]

Hi ES,

On the A320 at fairly high weights, the fuel flow PER ENGINE is about 2500-3000 kgs/hr during take-off i.e. 5-6 tonnes per hour. Typical total burn to top of climb (usually FL350) is 2 tonnes. Cruise fuel flow per side is about 1200-1300 kgs/hr. Descent at idle is about 300kg/hr per side. Don't know about final approach - too busy - but it's high due to high drag configuaration.

The art of fuel management is complex. There are various ways of doing things but we are constrained by the requirements of ATC and arrival routings etc. But for example, descending at a slower speed is more economical than high speed. Why?

If you descend at a higher speed, you will start your descent later than if you were to descend at low speed. Therefore by descending earlier (slower) you spend less time in the cruise at cruise power and more time at idle and hence save fuel. Typical fuel burn approaching top of descent is about 40 kg/minute. So if you descend 1 minute earlier (say, 280 knots as opposed to 305 knots) you will save 40kgs in descent. (It works :) ).

Of course there are many other factors that can be taken into account to minimise overall costs - fuel just being one of them.

A4