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- andyc
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Hi, I'm confused with this one. My calculations are different!

I did a practice exam, question was "maximum distance aircraft can plan to fly".

NORMAL TAS.......................190 kt

ASYMMETRIC TAS...................110 kt

NORMAL FUEL FLOW.................120 lt/hr

ASYMMETRIC FUEL FLOW.............85 lt/hr

FUEL TANK CAPACITY...............680 lt

FIXED RESERVE 45 min CALCULATED AT CRUISE RATE.

VARIABLE RESERVE OF 10 percent OF THE TRIP FUEL.

START UP AND TAXI ALLOWANCE......15 lt

I got it wrong due to where I put the fixed reserve in the below calculations.

Your workings show we include the fixed reserve in the first bit and take off 10% as if a variable reserve had been added to the fixed:

Flight fuel available = (680 [full] - 15 [taxi] - 90 [fixed res]) รท 1.1 [var res] = 523 litres.

My workings assumed the "trip fuel" (on which the variable reserve is calculated) does*not* include the fixed reserve, meaning we haven't added 10% to the fixed reserve so we don't need to take it off - so remove fixed reserve *after* the variable reserve has been removed:

Flight fuel available = (680 [full] - 15 [taxi]) / 1.1 [var res] = 604 litres - 90 [fixed reserve] = 514 litres

This made a difference to the answer I chose

I guess assuming you're right, i'm wondering why we add 10% to the fixed reserve? Does that make the fixed reserve "trip fuel"?

I did a practice exam, question was "maximum distance aircraft can plan to fly".

NORMAL TAS.......................190 kt

ASYMMETRIC TAS...................110 kt

NORMAL FUEL FLOW.................120 lt/hr

ASYMMETRIC FUEL FLOW.............85 lt/hr

FUEL TANK CAPACITY...............680 lt

FIXED RESERVE 45 min CALCULATED AT CRUISE RATE.

VARIABLE RESERVE OF 10 percent OF THE TRIP FUEL.

START UP AND TAXI ALLOWANCE......15 lt

I got it wrong due to where I put the fixed reserve in the below calculations.

Your workings show we include the fixed reserve in the first bit and take off 10% as if a variable reserve had been added to the fixed:

Flight fuel available = (680 [full] - 15 [taxi] - 90 [fixed res]) รท 1.1 [var res] = 523 litres.

My workings assumed the "trip fuel" (on which the variable reserve is calculated) does

Flight fuel available = (680 [full] - 15 [taxi]) / 1.1 [var res] = 604 litres - 90 [fixed reserve] = 514 litres

This made a difference to the answer I chose

I guess assuming you're right, i'm wondering why we add 10% to the fixed reserve? Does that make the fixed reserve "trip fuel"?

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- John.Heddles
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- ATPL/consulting aero engineer

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I think you've headed off up a blind valley. In effect, you have applied the variable reserve to your fixed reserve .. admirably conservative but not what the question is looking to see. Perhaps you are missing the point because the quoted calculation is done all at once rather than bit by bit ?

The first calculation is correct. The question asks for the planning situation ie before flight.

Rationale is

(a) you know what you start with in the tanks (680)

(b) you need to quarantine start taxi (15) and fixed reserve (90) - they have naught to do with the variable reserve factor. (If your operation included a final manoeuvring and landing allowance that, also, would be part of the initial quarantine calculation).

(c) Once you have done this you have 680 - 15 - 90 = 575 available for flight fuel plus variable reserve. This amounts to 110% of flight fuel so, to reduce that to flight fuel you need to prorate it by 100/110 which is done simply by dividing by 1.1. When you do this you get 575 / 1.1 = 522.7, call it 522. On this point I differ from the quoted calculation .. all round offs should be to the conservative side. However, be guided by Bob et al as to what the examiner wants to see in this respect. If you end up in commercial operations, though, expect to have your wrist slapped if you round off non-conservatively.

What you have done, in effect, with your calculation sequence, is quarantine the start/taxi allowance. only, which then puts the fixed reserve into the variable reserve factor calculation and you end up double dipping unnecessarily.. Treat this calculation requirement strictly as the sequence in the quoted solution and things will work out just fine.

** i'm wondering why we add 10% to the fixed reserve?**

In the solution calculation, you haven't added the factor to fixed reserve, while in your calculation, you have. As I suggested above, I think that you have been confused by the single calculation in the solution. Try doing it step by step until it becomes clearer and then move to the single calculation.

Just to recap - start with the fuel in the tanks. Then quarantine (remove) those bits which don't attract the 1.1 variable reserve factor viz., start/taxi and fixed reserve. What you have left after that calculation is 110% flight fuel. Reduce this to 100% flight fuel and Bob's your uncle, as they say ....

The first calculation is correct. The question asks for the planning situation ie before flight.

Rationale is

(a) you know what you start with in the tanks (680)

(b) you need to quarantine start taxi (15) and fixed reserve (90) - they have naught to do with the variable reserve factor. (If your operation included a final manoeuvring and landing allowance that, also, would be part of the initial quarantine calculation).

(c) Once you have done this you have 680 - 15 - 90 = 575 available for flight fuel plus variable reserve. This amounts to 110% of flight fuel so, to reduce that to flight fuel you need to prorate it by 100/110 which is done simply by dividing by 1.1. When you do this you get 575 / 1.1 = 522.7, call it 522. On this point I differ from the quoted calculation .. all round offs should be to the conservative side. However, be guided by Bob et al as to what the examiner wants to see in this respect. If you end up in commercial operations, though, expect to have your wrist slapped if you round off non-conservatively.

What you have done, in effect, with your calculation sequence, is quarantine the start/taxi allowance. only, which then puts the fixed reserve into the variable reserve factor calculation and you end up double dipping unnecessarily.. Treat this calculation requirement strictly as the sequence in the quoted solution and things will work out just fine.

In the solution calculation, you haven't added the factor to fixed reserve, while in your calculation, you have. As I suggested above, I think that you have been confused by the single calculation in the solution. Try doing it step by step until it becomes clearer and then move to the single calculation.

Just to recap - start with the fuel in the tanks. Then quarantine (remove) those bits which don't attract the 1.1 variable reserve factor viz., start/taxi and fixed reserve. What you have left after that calculation is 110% flight fuel. Reduce this to 100% flight fuel and Bob's your uncle, as they say ....

Engineering specialist in aircraft performance and weight control.

Last Edit: 7 months 1 week ago by John.Heddles.

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- andyc
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Thanks John.

This part made it click:*"you need to quarantine start taxi (15) and fixed reserve (90) - they have naught to do with the variable reserve factor."*

This part made it click:

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