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Usable fuel/margin fuel question

• ray2310
• Topic Author

ray2310 created the topic: Usable fuel/margin fuel question

Hi,
I sat my PPL test the other week and passed with 87% I am now studying up on my CPL Nav. I had a question come up which was along the lines of (can't remember the exact numbers):

The usable fuel on board is 68 gallons and the flight time is 160 minutes with a fuel flow of 12gph
What is the Margin fuel available at destination if fixed reserves remain intact. (Use Fuel Flow to work out Fixed Reserve of 45 mins)

According to my KDR I'm guessing I got it wrong.

I First worked out the flight fuel 12gp @ 160 mins = 32 gallons. I think I then subtracted 32 from the usable fuel which gave me 36 gallons.
I wasn't sure if usable fuel included the fixed reserves or not. Because the question stated

use cruise fuel flow to work out fixed reserve for 45 mins

. I then worked out the Fixed Reserve to be 9 gallons and got 45 gallons for margin at destination (Added 9 to 36) . I obviously did something wrong, I probably over complicated things.

The question threw me off a bit as it stated to use the cruise fuel flow to work out the fix reserve. Is usable fuel purely flight fuel or are reserves included in that.
What would the correct method of working this question out be?

If someone can shed some light that would be appreciated!

Cheers

• Posts: 1914

bobtait replied the topic: Usable fuel/margin fuel question

Firstly, useable fuel means all of the fuel hat is available to the engine. That includes the fixed reserve. The fixed reserve is useable fuel - there would be no point having a fixed reserve if it was unusable would there? Assuming the examiner didn't give any allowance for start up and taxi and no mention was made of a variable reserve, the working would be as follows..

Useable fuel at take-off 68 gallons
Less flight fuel 32 gallons
Less fixed reserve 9 gallons
Margin fuel 68 - 32 - 9 = 27 gallons.

Sometimes we can 'overthink' these questions.

Bob

• ray2310
• Topic Author

ray2310 replied the topic: Usable fuel/margin fuel question

Thanks for that, Bob. Makes sense now!

Cheers

• ray2310
• Topic Author

ray2310 replied the topic: Usable fuel/margin fuel question

Hi Bob,
I came up stuck on this same type of question studying for my CNAV. In the Exam Prep 1 Question # 11.

A PVT flight takes off with 65 gallons of useable fuel.

For a planned flight time of 2 hours and 40 minutes at a fuel consumption rate of 14 gph, the margin of fuel available at the destination assuming the 45 minute fixed reserve remains intact is -

I am still a bit vague as to why you subtract the fixed reserve when calculating the Margin? Isn't the margin all fuel available for the next leg of the flight?

I worked out the Flight Fuel to be 38 USG and then simply subtracted that from the Usable fuel of 65 Gallons to give me a 27 USG Margin, which is wrong as you are further meant to subtract the fixed reserve. I can't wrap my head around why we need to subtract the fixed reserve when it's asking for the margin (fuel available to use?).

Help would again be appreciated.

Cheers

• Posts: 1914

bobtait replied the topic: Usable fuel/margin fuel question

This question says you are PLANNING the flight. That is, you haven't taken off yet. When you consider the margin fuel for the next leg, you must assume that the reserves will be used. You cannot claim to be carrying a fixed reserve if you are then going to assume that it will not be used.

The variable reserve accounts for the the things that are likely to happen, such as unexpected headwinds or diversions around poor weather.

The fixed reserve accounts for the unexpected, such as the runway at the destination being blocked by a landing accident, or maybe your undercarriage not coming down when you select it down. If you arrive at the destination and the flight has gone exactly according to plan, there is nothing to stop you recalculating your fuel, but you can not do that before you take off.

• 794400

794400 replied the topic: Usable fuel/margin fuel question

Hi bob, in regards to the 9 gallons of fixed reserve is that figure calculated at the 45mins at 12gph?

Cheers

J

• Posts: 1914

bobtait replied the topic: Usable fuel/margin fuel question

The fixed reserve is not necessarily a specific amount of fuel. All that is required is that you can remain airborne for an extra 45 minutes after you arrive at the destination. However, some operators specify a fixed amount of fuel that would represent 45 minutes at a fuel consumption rate that is less than that for normal cruise. For most light aircraft however, the 45 minutes is often calculated at normal cruise consumption rate. In the case you have mentioned, 45 minutes at 12 gallons per hour would be 9 gallons. An exam question will have to stipulate whether you are to carry the fixed reserve at normal cruise rate or at some reduced rate.

• Skivvy

Skivvy replied the topic: Usable fuel/margin fuel question

Hi Bob,

I came across a similar question in the casa exam.

A PVT flight takes off with 65 gallons of useable fuel.

For a planned flight time of 2 hours and 40 minutes at a fuel consumption rate of 14 gph,
Fixed reserve is required calculated at cruise rate, but taxi and variable is included.
calculate fuel available over and above the flight time.

fixed reserve at 20 min + 2 hours and 40min flight time= 14gph x 3hr = 42 galloons.
65 gallons - 42 gallons = 23 gallons.

I'm concerned that this may have been a fixed wing question and I am studying for my CPL Helicopter licence.