Fuel calculations

[fish4life]

I tried a search and couldn't find this so if it is here somewhere let me know.

With all the talk of fuel totalizer's and calculations lately I was wondering at which point do you start to teach people to start calculating fuel burns and loads in lbs/hr vs GPH. Most of the gauges in light aircraft tend to just show a fraction of how much fuel you have (eg 1/4, 1/2, 3/4) anyway. If the book fuel flows are in GPH do you stick to using GPH for calculations in flight planning and just switch it over to lbs when it comes to weight and balance? Or do you figure out the lbs/hr fuel burn and use it the whole way through from the start. This came up because it seems like GPH would be good for the PPL guy but when it comes to doing your commercial license most aircraft you will get a job on will do everything in lbs/hr so would it not be good to start practicing your fuel calculations in lbs/hr right from the start of your CPL? or would this cause confusion and math errors?

[Hedley]

Keep It Simple:

BSFC for pilots:

If you have a 150 hp engine, knock off a zero, and it's going to burn 15 gph on takeoff, which is 100% power. We normally cruise at 65% power, so two-thirds of 15 is 10gph in cruise. We can do a little better than that if we lean expertly, and reduce our power setting.

With 300 hp, you should see 30 gph on takeoff, 20 gph cruise.

With 450 hp, expect 45 gph on takeoff, 30 gph cruise.

These numbers work very well for airplanes that you are not familiar with. As always, the POH supercedes these very rough rules of thumb!

Let's work backwards. An airplane I fly, which produces thrust but zero horsepower at the start of the takeoff run with the brakes on, burns 330 gph on takeoff, which is probably around 3,300 hp. Low power, low altitude cruise is around 160 gph or 1,600 hp

This was actually on the CPL flight test, many decades ago. I have no idea if it still is.

[PanEuropean]

I think that the best course of action would be to stick with whatever unit of measure is presented in the performance charts in the AFM.

If the AFM speaks 'gallons per hour', use that (with appropriate caution about whether it is US gallons or Imperial gallons). If the AFM speaks pounds per hour, use that (with appropriate caution considering that the fuel is sold by volume, but you are making calculations by weight, and fuel temperature has a very significant effect on what a given volume of fuel will weigh).

Michael

[Big Pistons Forever]

By the second or third flight I will teach a student that they must know two things. The takeoff weight of the aircraft and the fact that it must be less than the MGTOW.
Implicit in this is they must know the weight of the fuel on board. I then get them to translate this fuel weight into endurance based on a block training fuel flow (60 lbs/hr for the C 172), and compare this to the expected duration of the flight. I want them to think of fuel as "time in the tanks" right from the beginning of their training.

[Geko]

By the second or third flight I will teach a student that they must know two things. The takeoff weight of the aircraft and the fact that it must be less than the MGTOW.
Implicit in this is they must know the weight of the fuel on board. I then get them to translate this fuel weight into endurance based on a block training fuel flow (60 lbs/hr for the C 172), and compare this to the expected duration of the flight. I want them to think of fuel as "time in the tanks" right from the beginning of their training.

Whenever I read your posts on flight training I feel like I should ask for a refund for at least 50% of my dual PPL costs.

[Hedley]

That's the difference between what BPF and I provide - "Boutique" flight training - and Walmart flight training.

I met a guy recently, he did 7 very expensive flights at a fancy FTU with all sorts of great paperwork. Then, he did one flight with a senior (10,000 TT) instructor, and he said that he learned more in that single flight, then he did during all 7 flights combined at the (clearly Walmart) FTU. Unfortunately the FTU did not price itself like Walmart. I am unsure as to whether or not he really got good value during his Walmart training.

[Shiny Side Up]

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[Hedley]

why it took seven flights to determine you're not happy with a product

Like most consumers, he didn't know any better. It was his first exposure to flight training, so he concluded that's just what it was like.

I am unsure if his flight instructor either simply wasn't very good, or intentionally milking him - don't know the guy. Could have been either.

[Geo]

Like most consumers, he didn't know any better. It was his first exposure to flight training, so he concluded that's just what it was like.

As a current PPL student I can add to that. Not only would we not know better, we're so thrilled with just being in the air *flying a plane* that it can be hard to distinguish between good instruction and bad (except perhaps in really egregious cases). Not so much a "wow" factor as a "whee!" factor.

One other thing, I remember clearly how intimidated I was on my first flight - I didn't even want to talk on the radio! (my instructor pushed me to do it once anyway - not as scary as I thought.) I didn't conclude anything from my first flight except for "I love this" and "Let's do it again!".

Hmm, and so my instructor, having been the one to "introduce me" to this wonderful world, probably gets a halo effect as well.

g

[Hedley]

Flight Instructors have an enormous responsibility to set a good example and make the right first impression with low-time students. I am frequently guilty of not doing a very good job of this.

I jump into an aerobatic aircraft - which has no checklist, it would just blow away - start up, immediately taxi out, check the mags on the way out, and when my CHT is in the green at the end of the runway:

http://www.pittspecials.com/movies/takeoff.wmv

I try not to think about this, too much. I suspect the other instructors standing there with their students, say, "Don't do that", which seems somewhat lame. I remember doing my first ICAS evaluation with Andre Lortie, who told me that I was going to be setting an example. I didn't understand him, at the time - I just wanted to fly the frikken airplane as well as I could.

[Shiny Side Up]

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