Aircraft Weight Question

[pcoltas]

This is really confusing me.

If an Aircraft has an empty weight of 12 tonnes and a zero fuel weight of 17 tonnes, what is the max payload it can carry?

I would say 5 tonnes, zero fuel weight - empty weight. Is this right?

Then it says the maximum take off weight is 21 tonnes. So does this mean that the maximum fuel weight is 21-17 = 4 tonnes?

Because the question says calculate the lift generate by the aircraft in kN after the aircraft has used 22,000lbs of fuel.

But 22,000lbs = 10 (metric) tonnes? How can it use this if the max fuel weight is 4 tonnes?

Its probably just me misinterpreting something but any help would be greatly appreciated

Thanks in advance

[BOAC]

5T is correct, assuming that by 'zero fuel weight' you mean MAX ZFW?

If a/c was EMPTY, then max fuel = MTOW-BW (Basic weight) = 21-12T = 9T.
If a/c was actually at 17T ZFW, max fuel =4T.

I think you may have misread the Q or it is wrong?

[Alex Whittingham]

Ah, BOAC got in first!

It's a bit of an odd question with some very inexact wording.

It would be better if they used a term like Dry Operating Mass (DOM) instead of 'empty weight'. 'Empty Weight' tends to mean 'the weight of the aircraft with only unusable fuel and fluids in sealed systems on board, no crew and no catering equipment etc.', items sometimes called the 'variable load'. The variable load would not count as 'freight'. DOM = 'empty weight' plus variable load.

Assuming that the 'ZFM' in the question actually means MZFM, it would be true to say that the maximum freight limited by ZFM would be MZFM - DOM, in this case 17t - 12t = 5t.

If we continue with that assumption then there is no need to assume that all the freight is carried, the actual ZFM could be less than 17t and more than 4t of fuel could be carried. If 'ZFM' means 'actual ZFM' you are right, you can only carry 4t of fuel.

What is really odd is that a DOM of 12t and a MTOM of 21t only allows a fuel load of 9t, and that assumes no freight at all. It's difficult to see how a plane could burn 10t if it could only carry 9 on take-off.

Where did this come from?

[pcoltas]

Its off a past university exam paper, I have an exam 2moro.

I think it must be a misprint it should say 2200lbs instead of 22000lbs, that would be 1 tonne right?

I just thought it was weird that calculating the payload is worth 5 marks, when its just 17-12.

This is my understanding of the terms

Empty weight - All equipment necessary for flight, including unuseable fuel but excluding pax/cargo

Zero fuel weight - All equipment necessary for flighht, including unuseable fuel and any pax/cargo. Any additionl weight after zero fuel weight is fuel only.

Max take-off weight - All equipment necessary for flight, including unuseable fuel, pax/cargo and fuel.

Is this correct?

Would the lift generated after 2,200lbs of fuel has been burnt simply be the weight of the aircraft times 9.81 to give Newtons?

ie. 21 tonnes Max take-off weight - 1 tonne fuel burnt = 20 tonnes

20 tonnes = 44000lbs

44000 x 9.81 = 431640N = 431.640kN ?

Thanks again for all the help

[BOAC]

As Alex has said, the industry has gone over to 'xxx Masses' rather than 'xxx Weights', and his definitions are more correct than yours, but, you have to play the examining board's game.:D

As for calculating Newtons - I'm FAR too old for that sort of nonsense. :D :D

Good luck.

[pcoltas]

Just realised my calculations were wrong, I converted lbs into N and it should have been kg

So,

Max Take off Weight = 21tonnes
Fuel Used = 2,200lbs = 1tonne

Therefore lift generated after 1 tonne of fuel used =

20-1 = 20tonnes = 20 x 1000 = 20,000kg

20,000 x 9.81 = 196200N = 196.2kN

Thats the only way I can think of doing it?

Cheers

[BOAC]

I'd love to say I noticed.............................................:D

[OverRun]

Huumph. It wasn't one of my exams. Aviation is an exact science, and I'd chop someone for this mistake.

The discussion about converted lbs into N and it should have been kg becomes overly loud in the silence when the engines have gone quiet because the fuel is finished.

[ft]

If an Aircraft has an empty weight of 12 tonnes and a zero fuel weight of 17 tonnes, what is the max payload it can carry?

Assuming that you mean "a maximum zero fuel weight of 17 tonnes" you don’t know, as the maximum P/L is the difference between the operational empty weight (OEW) and the MZFW (I’m industry W&B and stuck with MZFW, even though I appreciate the idea behind using the more correct MZFM). On top of the BEW you have the operational items (pilots, galley supplies, various fluids, ELTs and a number of other items) which are not payload.

As worded, you know that the payload is 17 tonnes minus the BEW and the weight of the operational items, or 17 tonnes minus the OEW, and nothing about the max P/L.

I would say 5 tonnes, zero fuel weight - empty weight. Is this right?

Sorry, no.

Then it says the maximum take off weight is 21 tonnes. So does this mean that the maximum fuel weight is 21-17 = 4 tonnes?

No. It means that if the aircraft is loaded up to the MZFW you can only fill it up with 4 tonnes of fuel. Most aircraft can be fuelled with quite a bit more than this – but you will not be able to use MZFW.

Because the question says calculate the lift generate by the aircraft in kN after the aircraft has used 22,000lbs of fuel.

(MTOW in kg – 22,000 lbs/2,205)*9,82

But 22,000lbs = 10 (metric) tonnes? How can it use this if the max fuel weight is 4 tonnes?

See above.

One solution to the problem of the BEW (which should have been OEW, making matters even worse) plus the fuel used exceeding the MTOW is that the maximum ramp weight can exceed the MTOW. In other words, you can weight more sitting on the ramp than taking off. This enables aircraft to take off at MTOW even after a long taxi. You also have a max taxi weight, which can be lower than the MRW, meaning you will have to burn fuel at the stand from MRW before being able to taxi.

Good luck!
Fred

Repeating parts of previous posts, but I figure a complete reply to the original question is often better than connecting patches of other replies.