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Are their two fixed reserves while doing fuel calculations?

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shuyaib created the topic: Are their two fixed reserves while doing fuel calculations?

On page 90 of the performance book it says fixed reserves are reserves used for holding. On page 101 it says holding fuel is another fixed reserve. Looking at the example and exercise 4.14 i can see that the 15g of fixed reserve is used, and the additional holding fuel is calculated using the minutes and added to the total. This means that their are two fixed reserves? Also what is the purpose of the first fixed reserve (45mins) if the second fixed reserve is being anticipated ? It would make sense if the the holding was more than 45mins from which we would then add extra fuel to the existing fixed reserves?Thanks for the clarification. .

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  • John.Heddles
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John.Heddles replied the topic: Are their two fixed reserves while doing fuel calculations?

Fair questions.

fixed reserves are reserves used for holding.

Yes, but the intent is for serious emergency holding prior to landing, not routine traffic or weather holding

Consider the case where you arrive at your destination/alternate and there are no problems - you proceed and land. The remaining fuel will be the fixed reserve plus whatever else you might have left. Proceed to the hotel for the overnight and all is well.

Now, consider the quite different case where you arrive and, as you are manoeuvring to land, the previous landing aircraft has a major malfunction while decelerating and ends up at the intersection of the only two usable runways and blocks the whole airport. Ergo, you go to the miss. Now, unfortunately, the remaining runway lengths are too short for your aircraft and you don't have fuel for an alternate, so you are stuck. What the fixed reserve gives you is some breathing space before everything goes temporarily quiet so that the folks on the ground have an opportunity to drag the disabled aircraft clear for you. That's the purpose of fixed reserve - gives you a bit of fat when things go very horribly wrong at the end of the flight. What happens if the good folks on the ground can't drag the other aircraft clear ? Then, you really do have a problem.

The following MMA F28 story gives you an idea of such a circumstance.


Does this sort of thing happen all the time ? No, but sufficiently often that most of us have had the occasional nail-biting end to an otherwise routine flight.

How about variable reserve ? That's to cover for the minor ups and downs in routine sector flying - wind a little stronger than expected, minor diversions around buildups, and so on. It is there to provide a reasonable chance that you won't end up going below the remaining fuel expected after the sector burn (plus variable) before you are in a position to land, using the last of the flight planned fuel to do so.

Weather and traffic holding ? Most folks probably wouldn't call this a reserve (although that really is what it is) rather an allowance to cover reasonably anticipated delays during the flight.

So, if you wish to call holding fuel a "fixed reserve", that's fine but it's NOT the fixed reserve we generally talk about. Two different animals altogether.

Engineering specialist in aircraft performance and weight control.

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bobtait replied the topic: Are their two fixed reserves while doing fuel calculations?

It was never the intention of the text to give the impression that the fixed reserve was intended specifically for holding. If that was the message to took from the text on page 90 it was not what was meant. I'll rewrite that paragraph to make sure it's clear that, the fixed reserve is there to cater for an emergency situation such as an undercarriage problem or an obstruction on the runway after you have arrived. That could occur even after you have burnt your holding fuel. I once arrived at a fairly short homestead strip to find a dead cow in the middle of the runway. CASA have determined that 45 minutes is a reasonable time for dead cow to amble off the runway! If not, your in trouble.

I like to put it this way to classes.

Fixed reserve accounts for things that are a 'nasty surprise' that you weren't expecting. (obstructed runway etc).
Variable reserve accounts for things that are no surprise but could happen en route. ( unexpected headwinds or diversions).

If you are planning on continuing on, without refuelling, to another destination after landing, you always assume that the variable reserve will not be available because it would be no surprise if you did burn it en route. However, you may carry the fixed reserve over for the next leg since you do not expect that you will need it (if you did use it and you departed with minimum fuel, you would have to re-plan the flight).
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