These are my questions:

1. I understand that you need to have the following values: distance in NM, time in hours, and speed in knots.

The aircraft performance tables show speed in the following format: “TAS MPH.”

Do I need to convert this to knots, or can I work with this value as well? 

2. When the planned altitude is not reflected in the performance tables, for example, the flight plan is to be carried out at an altitude of 13,500 ft and the tables are 12,500 ft and then go to 15,000 ft, what do you do in that case? 

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You need distance and *ground* speed expressed in the same units to compute time, and then time and fuel flow to compute fuel burn.

Given distances in NM, you need groundspeed in knots (nautical miles per hour) to compute flight time, so yes, you have to convert TAS from MPH to knots and correct for wind. 

When using tabulated data rather than performance curves, at intermediate altitudes interpolation is required.  You won't get precisely accurate values because the variation as a function of altitude is generally not linear, but it'll be in the ball park.  e.g. if the fuel flow is 20 GPH at 12,500 ft and 18 GPH at 15000 ft, the difference is 2 GPH, and at 13,500 ft you're 40% of the way from 12,500 to 15,000, so subtract 40% of 2, or 0.8 GPH for an interpolated value of 19.2 GPH.  Or, if the tab data values are fairly close together, just use the higher of the two fuel burn values understanding that you'll burn a little less.

4 hours ago, Bob Scott said:

You need distance and *ground* speed expressed in the same units to compute time, and then time and fuel flow to compute fuel burn.

Given distances in NM, you need groundspeed in knots (nautical miles per hour) to compute flight time, so yes, you have to convert TAS from MPH to knots and correct for wind. 

When using tabulated data rather than performance curves, at intermediate altitudes interpolation is required.  You won't get precisely accurate values because the variation as a function of altitude is generally not linear, but it'll be in the ball park.  e.g. if the fuel flow is 20 GPH at 12,500 ft and 18 GPH at 15000 ft, the difference is 2 GPH, and at 13,500 ft you're 40% of the way from 12,500 to 15,000, so subtract 40% of 2, or 0.8 GPH for an interpolated value of 19.2 GPH.  Or, if the tab data values are fairly close together, just use the higher of the two fuel burn values understanding that you'll burn a little less.

Thank you for responding.

1. How did you calculate that 40% is 13,500?
2. Should I also interpolate the speed?
3. The weights too, since all the letters are for a weight of 4,630 pounds.

13 hours ago, wilivarob said:

Thank you for responding.

1. How did you calculate that 40% is 13,500?
2. Should I also interpolate the speed?
3. The weights too, since all the letters are for a weight of 4,630 pounds.

1) you need to account for the differential in altitude (+1.000ft) from lower index (12.500) versus the "gap" in the table, which is 2.500ft (12.500ft up to 15.000)
Hence 1000 / 2.500 = 0,4  or 40%. Referencing Bob's example, since 40% of 2GPH is 0.8GPH, you will have a theoretical burn of

20GPH at 12.5000ft

19.2GPH at 13.500ft

18GPH at 15.000

That's the math behind it. 

 

2 + 3) You can interpolate between published values. Just keep in mind the values might not always be fully linear in relationship. So I'd rather err on the safe side, meaning planning with a higher weight than intended.

4.600lbs sounds like a smaller twin engine GA(?) So not that big of a deal. It's not an airliner with a wide weight envelope. Selected power setting (75%, 65%, 55% etc) + altitude is the most important factor when it comes to endurance in these smaller planes.

Finally, any sorts of extrapolation is not tolerated (ref: EASA 033 "Flight planning and fuel monitoring", but probably also valid for other regions), you can only use certified, published data.

 

13 hours ago, SAS443 said:

1) you need to account for the differential in altitude (+1.000ft) from lower index (12.500) versus the "gap" in the table, which is 2.500ft (12.500ft up to 15.000)
Hence 1000 / 2.500 = 0,4  or 40%. Referencing Bob's example, since 40% of 2GPH is 0.8GPH, you will have a theoretical burn of

20GPH at 12.5000ft

19.2GPH at 13.500ft

18GPH at 15.000

That's the math behind it. 

 

2 + 3) You can interpolate between published values. Just keep in mind the values might not always be fully linear in relationship. So I'd rather err on the safe side, meaning planning with a higher weight than intended.

4.600lbs sounds like a smaller twin engine GA(?) So not that big of a deal. It's not an airliner with a wide weight envelope. Selected power setting (75%, 65%, 55% etc) + altitude is the most important factor when it comes to endurance in these smaller planes.

Finally, any sorts of extrapolation is not tolerated (ref: EASA 033 "Flight planning and fuel monitoring", but probably also valid for other regions), you can only use certified, published data.

 

Thank you for responding, Captain.

I already know how to interpolate, but considering that it is prohibited, how should I proceed?

1. Should I always make flight plans at the altitudes indicated on the performance charts?

2. Or should I take the closest one below or above? For example, if the flight plan is set at 13,500 ft, should I take 12,500 ft (below) or 15,000 ft (above)?

There is more to this, unfortunately. If you want to follow proper procedures, you need to have some contingency fuel. The precise minimum amount depends onthe country where you are flying, but 45 minutes flight time would be a good estimate, plus enough fuel to get to the alternate airport. In addition, wind corrections can also be substantial.

Considering this, the difference between fuel usage at 12500' and 15000' is not that critical. I would calculate both and then use whatever is the larger amount of fuel, then you are on the safe side. Now that approach works fine _unless_ your total weight gets close to maximum takeoff weight (MTOW). Only in that case you need to be more precise, or reduce your cargo weight accordingly.

11 hours ago, wilivarob said:

I already know how to interpolate, but considering that it is prohibited, how should I proceed?

Extrapolation is prohibited.

Interpolation is fine, if you account for hazards already mentioned (not always Linear relationship)

What aircraft are you making these calculations for? and are you sure you have the complete performance pages? I say this because fuel calcs are rather straightforward if you have all the information (speaking strictly from GA background)

9 hours ago, qqwertz said:

There is more to this, unfortunately.

Very true, but, my opinion is that it is not that complicated for smaller GA at least (both VFR/IFR) that aren't under part CAT (commercial) nor "performance class A". Sorry, I do not know the equivalent FAA terms, this is pure EASA-land.

Basic fuel scheme:

Taxi fuel: "if significant".

Trip fuel: takeoff/climb/cruise/descent/approach. Those are all in the AFM but need to adjust for ground speed (basically solve how many nautical ground miles for an air mile etc)

Contingency: Small foot print. Either a small percentage of trip fuel or 5min holding 1.500ft AGL.

Interestingly, for airliners/class A, RNAV STARs may impact contingency fuel depending on if you anticipate the full arrival procedure (such as trombone shaped RNAV transitions). If shorter transitions, it goes into trip fuel.

Alternate fuel: straight forward, but adjust for time to anticipated ground speed.

Additional fuel: if you file without alternate this needs at least 15 mins holding at 1.500ft AGL.

Final Reserve Fuel: You need to legally protect this, which is 45 mins for non-turbine IFR. Also easy to obtain (just look up fuel burn/hr for economy cruise at 1500ft Above aerodrome and multiply with 0.75).

Extra fuel: anticipated delays, weather etc

And for sanity checks, Most smaller GA planes will also have an endurance chart for various pwr settings, where a 45 min reserve is included + taxi, run-up and climb. 

It breaks down to this (example from C172 IFR, conservative numbers)

Taxi : 5 litre

Trip: 90 litres / 2hrs

Contingency 5 litres

Alternate: 20 litres / 30mins

Additional: 0

Final Reserve: 30 litres/ 45mins

Extra: 10 litres / 15mins

Fuel required: 160 litres / 116 kg

(Max uplift 212 litres, 200 litres is usable)

But if you move to complex ,turbine driven planes, it gets much more complicated (most of that is beyond my knowledge as a private pilot 😜)

Edited August 28, 2025Aug 28 by SAS443

13 hours ago, qqwertz said:

There is more to this, unfortunately. If you want to follow proper procedures, you need to have some contingency fuel. The precise minimum amount depends onthe country where you are flying, but 45 minutes flight time would be a good estimate, plus enough fuel to get to the alternate airport. In addition, wind corrections can also be substantial.

Considering this, the difference between fuel usage at 12500' and 15000' is not that critical. I would calculate both and then use whatever is the larger amount of fuel, then you are on the safe side. Now that approach works fine _unless_ your total weight gets close to maximum takeoff weight (MTOW). Only in that case you need to be more precise, or reduce your cargo weight accordingly.

Thank you for responding, and I will take your suggestion into account, to do the calculations at 12,500 ft, then at 15,000 ft, and take the one with the most fuel.

On the other hand, I understand the extra fuel that must be carried, but for now the idea is to carry taxi fuel, flight fuel (climb, cruise, descent, approach, and landing), and reserve.

The problem is that whatever calculation I do, even using interpolation, I always end up with a high percentage of reserve, for example, a reserve of 30 pounds and I end up with 50 pounds.

Everything is calculated, altitude, weight, weather (for now only clear weather with no wind), but even so, it seems like too much to me.

Edited August 28, 2025Aug 28 by wilivarob

3 hours ago, SAS443 said:

 

What aircraft are you making these calculations for? and are you sure you have the complete performance pages? I say this because fuel calcs are rather straightforward if you have all the information (speaking strictly from GA background)

It's the C337 Skymaster fairing, and honestly, the POH that comes with it doesn't have any consumption data for climbs or descents, just performance tables.

Some people tell me that since it doesn't have that data, the performance table is already included there.

Anyway, I'll share the PDFs that come with the plane with you.

POH

POH 2

 

One more thing, inside the plane on the dashboard I found some data which I assume is the climb consumption information. Could I use that data for the climb?

Or is the reason for arriving with more fuel than normal because the tables already come with those values and I am adding the ones from the dashboard?

 

4 hours ago, SAS443 said:

Final Reserve Fuel: You need to legally protect this, which is 45 mins for non-turbine IFR. Also easy to obtain (just look up fuel burn/hr for economy cruise at 1500ft Above aerodrome and multiply with 0.75).

Here I would have another problem, which is that the aircraft has a minimum performance chart of 2,500 ft, so would I have to take it from here, either interpolated or uninterpolated?