Who has measured their actual flow rate?

[IO540]

Has anybody here (excluding those with a Shadin or similar flowmeter) does an actual measurement of their plane's fuel flow rate in cruise, when a) full rich and b) leaned to peak EGT?

It needs two flights: one with a short enroute section and another identical one with a longer enroute section; the difference gives you the enroute power setting fuel flow.

If you haven't done this, how do you work out your fuel planning?

[Flyin'Dutch']

I suspect most will go by the POH figure and apply that to their own flying; possibly without leaning the engine.

[rustle]

I suspect most will go by the POH figure and apply that to their own flying; possibly without leaning the engine.

Why would you suspect that?

My own suspicion would be that:

for hirers, they would tend to believe what the club/school tell them is a sensible hourly burn rate, and/or a review of the tech log would soon illustrate fuel uplifted -vs- hours flown and there might be a clue there ;);

for owners/sharers, they would have evidence or tried it out.

[Mariner9]

I kept (and still keep) a careful fuel log for my Pioneer, after 200-odd hours I can come up with a pretty accurate estimtion of consumption at any standard power setting. (To be honest I need to given the cr@ppy fuel gauges)

On the odd occasion where I rent something bigger, slower, and far less efficient:hmm: , I use POH +10% for wear & tear +10% for the wife & kids;)

[Pitts2112]

I just did mine by keeping track of fueling figures with hours flown. Worked out to about 33 liters/hour in the cruise (full rich) and about 45 liters/hour for aerobatics (mostly full-chat). Discussed said figures with other Pitts pilots and the numbers were in broad agreement, so confirmed my maths.

Sorties are fairly short, though, so I treat the numbers with a great pinch of my own salt. To date I haven't run out of fuel and I haven't been overly surprised at how much was actually left in the tank after my estimates for a flight. I don't keep tracck like I used to but do a quick mental check every once in a while to make sure the numbers are still in agreement with actual peformance

Pitts2112
PS - having a calibrated dipstick then, makes it easy to translate fuel in the tank to minutes of noise from the front.

[Zulu Alpha]

I have an AEIO360 which does 41.58 liters per tach hour. This is calculated from a spreadsheet with tach hours and fuel purchased in it over the past 200 hrs.

I suspect the cruise is a lot less and aeros a bit more.

As my guauge reads empty with anything from 0 to 18 liters I work out an approx tach reading for empty before flying and make sure I land before I get there.

Using a spreadsheet makes it easier to do the engine, prop, aerobatics and total time for the aircraft logbooks.

[IO540]

What I didn't make clear in my original Q is whether people have determined their actual cruise fuel flow rate.

This is vastly different to some overall figure determined by dividing # of litres used in a year by the # of hours flown that year.

One needs to know the cruise fuel flow, for a given engine power setting (either RPM or, with a CS prop, the MP/RPM setting) and a given altitude, to do proper flight planning for longer legs.

I saw some awful practices during my PPL training and wonder how far these extend into the post-PPL world. It's quite easy to do it right, with a couple of test flights.

[englishal]

Does this count :)
http://www.digital-reality.co.uk/ts5.jpg

[BEagle]

Is that Imp GPH or US GPH?

Poorly marked gauges which don't state whether Imperial or US gallons are being used are a distinct hazard, to my mind!

[BEagle]

But lots of US aircraft also fly in the UK - where Imp galls are used.

All it needs is 'US GPH' to be used on that display and things would be unambiguous.

Ask any group of UK PPL students how much 100 US gallons of AvGas with a SG of 0.72 weighs and I guarantee that many will say 72 lbs! Most of them confuse density with specific gravity; not such a problem with imperial or metric units, but a definite problem with US gallons!

The only aircraft I flew which allowed assessment of fuel flow in the cruise was a Rockwell 112. By the time you'd tweaked RPM, MAP, cowl flaps and mixture to optimise the fuel burn and EGT it was almost time to land again!

The Bulldog had a flowmeter; but since the RAF usually flew it at 2600 RPM, best power mixture and MAP for IAS, it wasn't terribly relevant. Although I once flew from Abingdon to St Mawgan with low RPM, high MAP (just short of overboosting) and lean of peak mixture (fortunately we had CHT gauges) - saved a lot of fuel!

[SNS3Guppy]

Has anybody here (excluding those with a Shadin or similar flowmeter) does an actual measurement of their plane's fuel flow rate in cruise, when a) full rich and b) leaned to peak EGT?



You should be doing this every flight, all the time. You need to be keeping real-time track of what your actual fuel consumption is doing, comparing it to your gauges, and deducting it from what you really know is in the tank. If this is an aircraft in which you can dip the tanks, you're best off doing that before every flight, so you have a known quantity from which to work.

As far as the calibration of the tank...check your aircraft flight manual. It will tell you.

[IO540]

You should be doing this every flight, all the time. You need to be keeping real-time track of what your actual fuel consumption is doing, comparing it to your gauges

Not sure how one would be doing that without an accurate fuel flowmeter, and I excluded those from my original Q (because it's an obvious answer).

I am curious about imperial gallons. In what context do they appear in the UK? All US stuff is in USG, and all pumps I have seen in Europe are in litres.

I work in USG in flight planning, have USG-marked tank scales, and the Shadin is thus set to display USG. To check the Shadin system for accuracy (which I do at every fill-up) I convert the Shadin USG to litres (x3.78). And further convert litres to kg (x0.72) on the very rare occassions I need the weight of the stuff for W&B.

Beagle - was that an analog flowmeter? They tend to be close to useless when it comes to accuracy. Mine certainly is, but is required equipment :) They just measure fuel pressure. Proper flow measurement requires a turbine, or doppler/ultrasound, or similar.

[Mariner9]

Most of them confuse density with specific gravity; not such a problem with imperial or metric units, but a definite problem with US gallons!


Dens/SG causes lots of confusion whatever the volume unit!

Prize then for the first correct answer to the following:

What is the weight in air (in kilos) of 1000 litres of Avgas @15C with a specific gravity of 0.7523? :8

[IO540]

752.3kg surely.

But is Avgas ever 0.752? I thought it was close to 0.71/0.72 most of the time.

[BEagle]

At the risk of half-remembered A-level physics from 1968....

Density of water at 15 deg C is 0.999099 g/cm**3

One litre thus has a mass of 999.099 g

If the SG of the reference fuel is 0.7523, then a litre will have a mass of 999.099 x 0.7523 = 751.62218 g. 1000 litre will have a mass of 751.62218 kg..... Don't know what the weight would be as you haven't specified the location's local gravitational constant!

Some UK aircraft still have Imp gall fuel gauges. SG of 0.75 means that the fuel would have a mass of 0.75 lb/gall - because a gallon of water has a mass of 10 lb. Equally, a litre would have a mass of 0.75 kg, because (as we have seen) a litre of water has a mass of 1 kg as near as dammit.

But a weedy US gallon of water only has a mass of 8.333 lb. So mixing up SG and density is a greater risk - at a SG of 0.7523, fuel has a density 6.269 lb/USG - or 7.523 lb/Imp gall......

I'd readily support standardisation on this issue!

[Flyin'Dutch']

Less conversions, less scope for error.

I stick to liters. That's what you get out the pump too.

Fuel gauges in GA aircraft are approximators at best so the amounts indicated are useless.

Fuel flow meter indicates liters too.

[dublinpilot]

IO540,

To answer your question, I have not done it as you have suggested.

What I have done, is take a period of months where I recorded the start up time, take off time, touch down time and shut down times, (so that I could determine airborne time, and ground run time), record the cruise altitude that I flew most of the flight at, recorded the tacho movement, and the fuel used (by using a dip stick that I calibrated myself).

I recorded this over a period of months in a spreadsheet. What I found was that the average fuel consumption for airborne hour varied more than I expected, but his was probably likely to flights having longer or shorter ground runs. Being based at a large airport ground time can be long, or short depending on the day, runway in use, how busy the airport is, which controller is on duty etc etc.

I found fuel used per tacho unit to be very consistent though. Using this I can usually predict my remaining fuel to within 5 litres or so, on a typical (for me) 2 hour leg.

In the absence of a fuel flow meter, it's as good as I can do, and as long as I flew with my usual power settings, & not outside my normal altitude bands, it seems to work pretty well.

dp

[Mariner9]

I asked for weight not mass BEagle as you rightly pointed out. Sufficient info was provided to calculate it. You're on the right lines, but remember the SG's quoted at 60F not 15C :E

Density and SG are close together (but not the same) around the density ranges of Avgas/Gasoline, for the quantity of fuel a light aircraft can carry (whether calibrated in US gallons, Imperial gallons or litres) the differences are pretty insignificant. If the volume in litres is multiplied by density & SG, the difference would only be ~2Kg per 1000 litres. (Incidentally, both would give incorrect answers by the way :E :E)

[BEagle]

Actually, my value for water density was for 15 deg C!

[IO540]

Come on, this isn't the ATPL ground school, with its CAA-exam word plays ;)

[BEagle]

Well said!

Only point I was making was to be careful whenver 'gallons' are quoted!

The flowmeter in the Bulldog was indeed a simple fuel pressure gauge; I think the one in the Rockwell was somewhat different?

[IO540]

Most fuel flow gauges are pressure sensing devices.

If you saw something like this (http://www.shadin.com/products/ff_mgt/microflo/index.html)then that would use a remotely mounted turbine transducer, typically a Floscan 201B, which sends pulses back (of the order of 30,000 per USG) which are totalised. Such systems can be accurate to 1% or better, if they are correctly installed and calibrated. All TB20s made after 2000 or so had their Shadin totaliser out of cal by anything up to 25%; there is a U.S. STC fix for it but it's not legal to implement it on a G-reg :yuk:

Dublinpilot's "tacho unit" method is interesting. I guess this tacho simply counts engine revolutions which, in typical usage and with a fixed pitch prop, is going to approximate total fuel used. It should not be that close however because prop thrust is the cube of RPM, so fuel flow should also be the cube of RPM, whereas the tacho is just counting straight RPM.

[dublinpilot]

You maybe correct IO540, but you're forgetting that most of us pilots are creatures of habit! We fly generally fly with the same RPM or RPM/MP combination!

I reckon that's why it works, and am very conceous that if I vary that or fly well beyond my usual altitude bands, then all bets are off.

That was also why I was determined to get my own fuel usage figures. They are relevant for how I fly the aircraft, and not some figure for how someone else flys it (or has remembered from someone else who told them, who heard from someone else who ought to know, etc etc).

dp

[FlyingForFun]

To answer the original question, no I haven't.

Do I consider this to be a problem? No, because I am always conservative when it comes to fuel planning. Does this prevent me from flying "longer" legs? Yes, but in the type of flying I do at the moment, I have no need to fly longer legs, so I am perfectly happy with my conservative fuel-planning.

On the other hand, if I had any reason to fly longer legs and push the endurance of the aircraft close to its limits, of course I would make sure I knew exactly how much fuel my aircraft used. But I would guess that very few piston-engined aircraft (whether flown by a PPL or commercially) are operated in this way.

FFF
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[IO540]

I wasn't suggesting that cruise settings vary.

Mine never vary either (10.5GPH, LOP) unless I am above about 10,000ft and then I am happy with what I get with the throttle on full-bore :)

What I was getting at is that one has to do something to separate the fuel usage during climb/descent, so one can get a decent grip on one's enroute/cruise fuel flow.

On most flights, one can estimate the climb usage fairly easily (straight out of the POH; all levers are fully fwd anyway), and the descent one similarly. It is the cruise one which needs to be determined extra carefully in order to do accurate flight planning.

[Mariner9]

Actually, my value for water density was for 15 deg C!
...which was why you got the wrong answer :E

The correct method is to convert SG@60F to density at 15C in vac, you can either use a formula similar to BEagles above (but using dens water at 60F and then using volume factors to convert to 15C) or simply accept it from me that in the gasoline range the difference is 2 Kg/m3. Thus for an SG@60F of .7523 the density@15C would be 0.7521 in vacuum. To get the density in air you should then subtract a further 0.0011 from that, giving a density @15C in air of 0.7510. Thus 1000 litres would weigh 751.0 Kg.

Of course, for aircraft sized tanks the above is irrelevant as the differences are insignificant :8 :)

[IO540]

I hope they don't make you learn this pointless detail in the JAA ATPL.

And if they do, why?

[Mariner9]

I hope they don't make you learn this pointless detail in the JAA ATPL.

And if they do, why?

As a humble ppl, no idea :)

But supremely important in my job (oil industry) :8

[S-Works]

I have a FS450 in my aircraft linked to the GPS. When I get to the top of the climb and trim for the cruise I put cruise config and finish the lean (I lean in the climb to airways height), at this point I check the fuel flow against the laminated excel sheet I created from the POH. At 10,000ft indicated airspeed of 120kts I am doing doing 32lph. This is bang on with book figures. The FS450 is accurate to a fraction of a liter when I refuel.