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## Safe endurance calculations

• Posts: 11

### GeeEss created the topic: Safe endurance calculations

Hi all,

I'm trying to get my head around the calculations for safe endurance (SE).

In the CPL Performance textbook and on other forum posts I've read through (like this one: www.bobtait.com.au/forum/performance/5868-pnr-endurance ). The post is more than 2 years old however.

It seems that calculating SE is done as follows:

SE = (Total fuel - Fixed reserve - Startup/Taxi - Holding) / 1.1 / Fuel Flow

The point that I don't understand is, the way the SE formula is set out, would this not be taking the variable reserve ( ... / 1.1) from all the elements of the fuel? By dividing all by 1.1, we're extracting 10% from Startup and Taxi for example.

Thanks,

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 541

### John.Heddles replied the topic: Safe endurance calculations

SE = (Total fuel - Fixed reserve - Startup/Taxi - Holding) / 1.1 / Fuel Flow

You might like to note that you will need to be able to account for different flight classifications in respect of variable reserve per the CAAP as well as any margin fuel (by whatever title) and alternate fuel requirements.

the way the SE formula is set out, would this not be taking the variable reserve ( ... / 1.1) from all the elements of the fuel? By dividing all by 1.1, we're extracting 10% from Startup and Taxi for example.

No .. subject to my first comment, you are misreading your own formula which removes the non-TF elements before applying the VR factor. It might help you a little if you rewrite the formula a little differently -

fuel available for trip fuel and VR ={ total fuel - (FR + S/T + hold + whatever else other than trip fuel)}

VR factor = 1 + VR/100 (eg, if the VR is 10%, this becomes 1 + 10/100 = 1.1)

trip fuel = fuel available for trip fuel and VR / VR factor.

This is equivalent to what you are calling safe endurance.

Your post highlights one of the problems with formula use - one needs to know all the ins and outs of the formula if one is to get it right each and every time.

The alternative to using the formula is to use a table similar to the CAAP story, fill in the bits you know, and then figure out the bits you don't know. That's all the formula tries to do for you.

Engineering specialist in aircraft performance and weight control.

• Posts: 11

### GeeEss replied the topic: Safe endurance calculations

I understand that variable reserve is not always 10% and referencing the CAAP for fuel requirements is essentials and to adjust as required.

To the best of my understanding - the formula is incorrect? Despite being in the textbook and mentioned in other forums?

• Posts: 11

### GeeEss replied the topic: Safe endurance calculations

John,

I walked away from my desk for a break and came back to your reply with a fresher set of eyes.

The question that I was working through when I picked up on the issue I'm trying to resolve now had the following variables:

Total fuel = 320L
Fixed Res = 42L
S/Taxi = 10L
Holding = 28L
Fuel Flow = 76l/h

So, by doing it with my old method, I would have done this:

SE=[Total fuel - Fixed res - S/Taxi - Holding] / 1.1 / Fuel flow
SE=[320-(.75*56)-4-6-(0.5*56)]/1.1/76
SE=[320-42-10-28]/1.1/76
SE=240/1.1/76
SE=2.87 hours

And if I am to use your method:

1. Fuel available = [Total fuel - (F/Res + S/Taxi + Holding)
2. Var Res Factor = 1 + 10/100
3. Safe endurance = Fuel available / Var Res factor

1. Fuel available = [320 - (42 + 10 + 28)] = 240L
2. Var Res factor = 1 + 10/100 = 1.1
3. SE = 240/1.1 = 218.18L

If I had then divided by the fuel flow of 76l/h, I would arrive at the exact same answer had I used the formula above.

So again, I have divided out 10% of the start up and taxi.

And I'm asking the same question I was at the very start. Why am I taking 10% (or whatever the CAAP dictates) from the startup and taxi? Shouldn't 10% only be taken from the trip fuel?

Thanks.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 541

### John.Heddles replied the topic: Safe endurance calculations

I'm not quite sure that I follow where your confusion arises ? Perhaps you can be more explicit ?

the formula is incorrect?

Not so much incorrect as incomplete as I endeavoured to explain previously.

The question that I was working through when I picked up on the issue I'm trying to resolve now had the following variables:

A bit late at night for an old chap to wade through all those numbers - I'll have a looksee over the weekend and come back on that bit.

And if I am to use your method:

Actually, I wasn't suggesting a method, per se, rather trying to lead you gently to your error.

So again, I have divided out 10% of the start up and taxi.

And, there again, be your error. Now, I have no idea what your background in algebra might be but you probably might benefit by reviewing algebraic order of operations ? That should lead you to seeing the error with some more clarity than you appear to be doing at present. Specifically, have you applied the VR reduction to the original quantity of 320, or to a lesser quantity of 240 ?

Engineering specialist in aircraft performance and weight control.

• Posts: 11

### GeeEss replied the topic: Safe endurance calculations

Perhaps the second reply I added to the forum would answer most of the questions regarding using your approach/method and arriving at the same result as I had the first time around.

I have divided 240 by 1.1 in both your method and the method I was first using - and again, they both arrive at the same result.

Thanks.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 541

### John.Heddles replied the topic: Safe endurance calculations

OK, the discussion has gone right off the rails, so let's start again, shall we ?

In your first post, you state a concern that the VR factor has been taken from all fuel elements. Your equation does NOT do that. Why do you think it does ?

Your concern is both incorrect and totally without any basis.

Engineering specialist in aircraft performance and weight control.