Fuel Calculations

[Lagentium]

Good Evening All,

I was wondering how accurate the fuel consumption figures are in the aircrafts POH are just out of interest, as the Warrior POH I was looking at stated 10US Gallons P/H in the cruise. Am I right in assuming the aircrafts engine won't burn fuel as well as it did when it was brand spanking new?

Cheers, James :ok:

[IO540]

You are correct, in general.

An engine with a knackered camshaft, for example, will need more fuel for a given power output or, alternatively, will not make rated power and will thus fall short of book takeoff performance, during "full power" takeoff.

Also you sometimes get a different prop fitted, to the one which the POH is based on :)

The only way to be sure of fuel flow for a given IAS is to do some tests. Do two flights which are identical except for the length of the stabilised cruise segment, and start and end each one with a full tank. Then you can work it out. Or have a fuel totaliser installed :)

[FlyingStone]

I wouldn't worry too much with age of an engine. It can be a bit "tired" (usually near TBO when the owner doesn't want to "waste" the engine resource by overhauling the engine sooner than TBO, even if there is proof that the engine should require overhaul), but usualy the problem is that you achieve far less than rated power - the fuel consumtion should stay more or less the same. On the other hand, the most common problem with people having higher fuel consumption / less range than the POH figures is because they don't read the "small print" on the performance charts.

For example, the POH for Warrior III suggests a fuel flow of 9,2 USGal/h for 65% power. Right next to the fuel flow it says "Best Economy Mixture" - this is the important part! You won't get this figures by leaning to max RPM, and than moving a mixture half an inch forward for every flight instructor that said "don't touch the red knob" to you. If you then look at the Cruise section of Normal Procedures, you will see that the required procedure to obtain the noted fuel flow for best economy is with full throttle, and mixture leaned to achieve RPM stated in the table. Otherwise very, very strange method, especially with carburetted engines at low altitudes.

Basically, operate the aircraft as the book says, note IAS/TAS which you achieve and compare it with the burned fuel and you'll be able to see where you stand.

[Big Pistons Forever]

For planning purposes when flying your average flying school beater, I tell students to flight plan for a TAS 10% less than the POH and a fuel burn correctly leaned of 10 % more. This will often produce some amazingly accurate real world results.

[IO540]

Flying full rich does about 20-30% less MPG than flying at "best economy" which is peak EGT.

So yes leaning correctly is critical to get anywhere near the right figure.

Time for another LOP thread? ;)

[2high2fastagain]

LOL. Ah, just learning about LOP. Isn't that where you lean to the last cylinder to peak and then trash all the others? My engineer will shoot me if i try any LOP tricks. But seriously I keep my o540 CHTs to 380/390F and she flies bang on the POH for Fuel flow and speed. As the engine is 30 years old I consider myself very lucky. I swear by my EDM. A miraculous bit of kit. It's a damn sight more useful than the mode s I've just bought.

[Pilot DAR]

Unless the flight manual, or the engine operating manual provides instruction for Lean of Peak operation, I would suggest you don't. I know people who do (I suppose they can affort the cost of the engine).

I recently did an STC approval project with the new Lycoming IO-390, 210 HP engine. It's operating manual (from Lycoming as opposed to airframe related) prohibited lean of peak operation. Before you laugh that off as the engine manfacturer being conservative, it has the same legal implications as the airframe manufacturer saying that aerobatics are prohibited in a plane. It's unfortunate that often pilots do not get to see the engine operating manual.

If a worn camshaft is affecting any engine characteristic detectably - don't fly the plane at all!

Run the engine the way the manufacturer says, yes, including the fine print on the performance tables.

[dublinpilot]

What the POH says you'll use in fuel, what you'll use unleaned, what you'll use properly leaned will all be different.

What you will use, when you lean however you do it, and what another pilot will use when they lean will also be different (even thought it shouldn't be, experience has shown that it is).

One of the most interesting things that I did in flying, was to calibrate an accurate dipstick for the aircraft that I fly, and then use it measure my fuel consumption on each flight.

Simply dip that thanks before and after each flight and work out how much fuel you used.

Then compare that to your flight times and see what you get.

I compared it to my startup/shutdown duration, my in flight duration and my tacho movement. To my surprise it was the tacho movement that was the most consistant indicator of fuel usage.

Do the same for yourself. It won't take all that many flights (5-7) before you start to see consistant readings, and you'll be able to predict your fuel remaining to which a few litres.

It's a very enlighting exercise to do.

dp

[IO540]

Isn't that where you lean to the last cylinder to peak and then trash all the others? My engineer will shoot me if i try any LOP tricks

Do his knuckes touch the ground, by any chance?

http://www.happehtheory.com/wp-content/uploads/2010/09/SecretsOfLife10_GorillaSmall.jpg

Times have moved on.

LOP, BTW, does not in itself deliver better MPG than peak-EGT, because anywhere at or past peak you are burning all the fuel that there is to burn.

For most Lyco engines you cannot be at/above 75% power at peak EGT or LOP.

That's the short version :)

[Big Pistons Forever]

LOP operation is irrelevant to the question posed by Lagentum, the thread starter.

He was asking about POH fuel burn for a Piper Warrior. It is impossible to lean a carburated Lycoming 4 cylinder engine to LOP and still have it run smoothly. It will start to run rough and otherwise indicate its unhappiness well before you are truely lean of peak because the mixture distribution varies so much between cylinders.

Plus to do LOP operation without hurting your engine you need an engine analyzer which gives continuous readings of the EGT and CHT of each cylinder.

If anyone can find even one example of a VFR Warrior available for rent with an engine analyzer, I will eat my copy of Kaz Thomas's outstanding book "Fly the Engine" :}

[2high2fastagain]

IO - Not a bad likeness, though mine generally looks angrier than that. Must be a brother.

BPF makes a fair point for the warrior, but my interpretation of the original question was that lagentium was looking for a generic answer. I think we've got as far as 'you can expect some degradation but if you have an engine management system then you stand the best chance of achieving or getting close to the book values without mucking up your engine'

[Big Pistons Forever]

OK Generic advice is lean as per POH and don't expect to get exactly what the POH says if you are flying a typical high time club aircraft. Like I said earlier applying a 10 % penalty to POH numbers seems to work out quite well in practice as a way to account for the inevitable inefficiencies that build up over the years.

[Gertrude the Wombat]

I round up the POH fuel consumption figures, then I round up the planned time for the flight, I plan to land with a full hour's fuel even for a single VFR circuit and even over East Anglia when there's always a choice of a dozen runways within twenty minutes, then if the aircraft has one gallon less than what I've calculated I need I add fuel.

One the one hand I do occasionally get told I'm being over cautious (I accept that people who are better at planning and flying than me won't feel they need the same margins), but on the other hand I've never (yet) run out of fuel.

And people do run out of fuel, as we all know, and sometimes they die.

[A and C]

When looking at the fuel burn across the four Lycoming engined ( not injected ) aircraft that I own I would say that the engines all the have about 18-20 % less fuel consumption when leaned IAW the Lycoming instructions and that is slightly RoP.

As for fuel planning VFR I use TRIP FUEL + DIV FUEL + 45 MIN HOLD + 5%.

TRIP & DIV are calculated at the power setting to be used with the HOLD calculated at 55 % that equate to the best L/D speed then 5% of the whole burn.

If it is a long trip (over 4.5 hours) and there are a lot of airfields that could be used as an en-route alternate then I will reduce the 5% to 5% of DIV+45 MIN HOLD. As long as the fuel plot stays at what is expected then it is OK but if after two hours or so the fuel plot drops below the expected line you will have to pick one of your en-route alternates and land, this will ensure that you land with a (very) safe amount of fuel but it will require a lot of discipline to avoid the temptation to try to push on in the hope of the fuel plot coming back towards the line.............it usually won't !!