Flight Plan determinations for AC Speed and Fuel Needed

Pro Member First Officer
Steve (SpiderWings) First Officer

Anyone else have this problem?

I always make a flight plan for both VFR and IFR flights. And I also keep a log in word pad of all my flights. At the top of each flight in the log I make notes from the flight plan so that I can remember what to expect along the way and also so I can compare actual experience to the plan - especially in the area of fuel consumed.

As a result I have noticed that the flight plan for fuel almost never agrees with what I actually experience. Flying from Kaua'i to Maui and back to Honolulu, the plan had me using more fuel than the Baron could hold, so I figured I'd have to land in Maui and refuel. But I didn't need to in fact, I landed at Honolulu with 51% fuel.

Then yesterday I filed a flight plan from Honolulu to New York City in 747 and it indicated I'd only need 75% of a full load of fuel. I got to Salt Lake City and noticed that I was down to 14% fuel. So I refiled for Denver and that was almost a mistake. I came in on fumes and ran out of gas as soon as I got off the runway and onto the taxiway. The winds were out of 258* by the way.... tail wind right?

I flew much of the flight at FL290. Is that not a good place for economic fuel usage?

Now for speed.

For jetliner flights the flight planner will usually tell me a speed that I never can achieve in the actual flight without the Overspeed warning. On the Honolulu to Denver flight in the 747, it showed that I should fly at 505 knots to get to the destination in 5 hr and 46 minutes - 2917 nm. That is what I came up with using my Flight Computer also... close... 507. But then once I'm at cruising altitude I can't get beyond 368 without the overspeed warning.

Also I noticed that as I went higher the KIAS would get lower even at full throttle. Thinner air = lower KIAS perhaps?

3 Responses

Pro Member First Officer
ARD-DC First Officer

Sounds very familiar to me...
I have no idea how the flightplan calculates fuel consumption and distance, but it is always off.

This is my preferred method; (assuming you don't use a 3rd party addon to perform calculations), set yourself up for a typical flight and write down some numbers:

Fuel used on ground
Fuel used during takeoff and climb to a normal cruise level (FL34 gives much better fuel consumption than FL29)
Fuel consumption per hour of cruise flight
Fuel used for a descend / Appr. / Landing

If you figure out these numbers once, under average conditions in a fully loaded plane, you'll be able to use them again and again; the only number that is different each flight is obviously the duration of the cruise flight.

Anyway - this is how I always calc. my est. fuel requirement; Obviously this isnt very precise, and therefor I always end up with a (positive) difference, though it is hardly ever more than 1000 - 1500 lbs, which is quite acceptable to me when flying a B737. At least I now have enough for a go-around ... Wink

Pro Member First Officer
Steve (SpiderWings) First Officer

Thanks ARD-DC,

I guess I do a similar thing with my logs where I estimate fuel ahead based usually on the last time I used the same aircraft. At the end of each flight I always compute the acutal %used per hour.

I was just wondering if I'm doing something wrong with the flight planner.

I did realize one thing on my Honolulu to Denver Flight mentioned above. When I later took off from Denver to continue on to NYC, I seemed to not be able to hold my speed as I climbed to FL290 as instructed by ATC. I felt that I was not trimmed well. So I doubled checked the flaps and gear - even checking from Spot Plane view. Everything looked fine.

Then I clicked open the throttle quadrant (747) to see if the flap settings were full up and they were. But then I noticed something... the tan lever to the left of the throttle levers. It was partway down. For some reason when I hover my mouse pointer over things it doesn't come up with a label. And I couldn't read the lettering on the lever but it could easily be "Speed Brake" from how it looks and so I assumed thats exactly what it was. I pushed it all the way up and my speed problems went away.

I also realize that this would explain the high rate of fuel consumption on the previous flight. And in fact, the flight to NYC ended up using a bit less fuel than estimated by the flight planner.

However, I still have the problem with the flight planner speed and what I'm actually able to get on the A/C before getting the Overspeed Warning.

I wonder about a couple of possible explanations.

1. Is the flight planner speed in knots or mph or kph?
2. Does the KIAS depend on air density? If so then would the speed be lower at high altitudes.

I need to experiment with both of these theories.... check to see if actual resulting time of flight bears a relationship to the planned speed despite the KIAS readings.


Pro Member First Officer
ARD-DC First Officer

I believe, although Im not certain, the flightplanner assumes a typical cruise speed between point A and point B to compute the flight duration. Obviously, the first +/-20 minutes and last +/- 30 minutes your airspeed may not even be half that value, so there's no way you can complete the flight in the time given by the flightplanner without exceeding speed limits, except perhaps when you do a 10 hour flight while pushing the barber pole all the time 😂

On the other issue; if you weren't able to hold your airspeed, you must have been pushing your engines to their max N1 continuously I presume..?
That in itself would be a good indication something is causing excessive drag on your aircraft... Not sure about the B744, but the B739 I fly requires about 84 / 86% N1, depending on alt. and wind conditions.

For the fuelplanning-business, obviously you can get some addon to do the work for you if you want, there bound to be a few available here and there.

Personally I just have never bothered with it; I quite enjoy working it out as well as I can "by hand"🙂

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