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Actual Fuel on Board (FOB) versus Computed FOB (Fuel Transducer)

I have just fitted the EI CGR-30P system in a TB20 and was wondering how other people have their fuel flow systems calibrated.

The one thing that you don’t want is your system gauges showing more fuel than is actually in the tanks. To that end I was wondering how accurately other people have their systems set up.

I am currently re-calibrating the fuel transducer K-Factor every time the tanks are filled up. This has only happened once since the refit and required changing the K-Factor significantly. It is my intention to get to a point where the FOB as calculated by the CGR is 1 to 2% less than is actually in the tanks. In other words there is 1 to 2% more fuel in the tanks than is showing on the instruments.

The fuel temperature and expansion/contraction is going to affect the accuracy of the system. Full fuel left in a parked aircraft for several hours in very hot weather will expand, or even vent out of the tanks. The opposite is true as well if fuel is loaded at say 15C and then the aircraft is flown in the cruise for several hours at -5C.

How do other people have their own systems set up?
Is a 1-2% under reading of FOB a reasonable assumption?

EGLK, United Kingdom

I have an EI unit and chased the K-factor around for a while, but in the end achieved the better than 2% result you’re looking for. On my most recent fill up I was within 0.1 USG, which is obviously partially due to luck. But it’s generally pretty good.

I think whether you’d prefer the transducer nearly accurate or conservative is a matter of philosophy.

I doubt you will get it better than within a 2% band – see this

… and that is filling up mostly at Shoreham, where the pumps will be as accurate as anywhere. If you are using equipment in the “south of Europe” then you will find errors of 5% quite routinely.

If you like, you can adjust the K-factor so the errors are on the “safe” side, but it doesn’t really matter much because, in most of Europe including Greece, you want to be landing with some 20USG at the alternate. I have gone as low as 12, in a scenario with plentiful alternates along the route.

Administrator
Shoreham EGKA, United Kingdom

I set mine up as accurately as possible. 1 or 2% isn’t going to change an outcome. If you really want a margin like that, increase your planned fuel burn by 1-2%. (I use 5%)

Instead of adjusting the K factor after each refuel, sum the last 6 or 8 fill ups and do the calculation on that. i.e. as many fills as possible. That way you will reduce the errors from the aircraft not being level, different people stopping refuelling at different levels, temperature etc.

Spending too long online
EGTF Fairoaks, EGLL Heathrow, United Kingdom

When I installed transducers and calibrated them I flew on the rear tank until the gauge showed zero and then switched tanks. After landing I drained the rear tank to find there was still 5 or 6 litres left. That was good enough for me. (All bar one litre being useable)

Last Edited by Stickandrudderman at 27 Oct 16:01
Forever learning
EGTB

Peter wrote:

I doubt you will get it better than within a 2% band – see this… and that is filling up mostly at Shoreham, where the pumps will be as accurate as anywhere. If you are using equipment in the “south of Europe” then you will find errors of 5% quite routinely.

Good point about pump variation, and I think depending on the aircraft tank configuration, minor ground slope differences can be significant when filling up. When setting up my EI K-factor, I did all the calibration checks based on filling in the same spot on the tarmac, from the same truck. Fill-ups elsewhere I didn’t use. It seems to be within 2% now, sometimes better, with occasional abberations on single filllups that I ignore.

I like to have my totalizer as accurate as possible, versus building in a safety factor, with the idea that the data is the best available but could vary either way. Otherwise I think you have a temptation to utilize what you believe to be the safety factor. I like the same thing in cars – when the gauge hits empty I prefer that it run out, not go another ill-defined number of kilometers that I’m tempted to use… even though I can’t remember exactly how much extra fuel remains when the needle hits empty!

Last Edited by Silvaire at 27 Oct 18:08

My JPI EDM 700 was installed and the factory calibrated K factor was accurate within 1-2% straight away. Compared to the 39 year old Archer Gauges and previously dipping tanks with calibrated hollow Perspex sticks, that was like a minor Miracle!! My range is now determined by loo stops! I totally agree, it would only fiddle with the K factor after (say) 10 fills to allow all of the noise to cancel out.

Now I need to install one on that Wine Box!

United Kingdom

I have a fuel totaliser in my TB20 and I love it. It’s been very accurately set and when my plane is refuelled the litres for which I am charged almost exactly equate to the the difference between the reported fuel level on landing and full tanks. Perfect.

…well, not exactly perfect. The system reports total fuel on board. What I’d really like to know is the fuel level in each of the plane’s two separate tanks during flight. In the TB20 these tanks are completely separate supplies (one cannot transfer fuel from one to the other) and a fuel outage in one tank will lead to fuel starvation and the engine stopping, unless the fuel supply is switched to the tank with fuel before one of the tanks empties completely. Other planes are similar of course – the PA28 series planes being a case in point.

So, my questions are:

1. Are there fuel totalisers for planes like TB20s and PA28s that measure flow from each tank separately? Are these prohibitively expensive?

2. More importantly – what happens if fuel becomes exhausted from one tank during flight and the engine stops- how easy is a restart of a hot, rapidly cooling engine during flight? Is there a procedure that is known to work?

Howard

Flying a TB20 out of EGTR
Elstree (EGTR), United Kingdom

Howard wrote:

So, my questions are:

1. Are there fuel totalisers for planes like TB20s and PA28s that measure flow from each tank separately? Are these prohibitively expensive?

2. More importantly – what happens if fuel becomes exhausted from one tank during flight and the engine stops- how easy is a restart of a hot, rapidly cooling engine during flight? Is there a procedure that is known to work?

1. I haven’t seen one. They normally work later in the fuel system so don’t know what tank you are on. That is what the fuel gauges are for.

2. Read your AFM but normally change tanks and turn fuel pump on. Should just restart.

EGTK Oxford

My JPI EDM 700 was installed and the factory calibrated K factor was accurate within 1-2% straight away

That is actually how it is supposed to work. Shadin buy the Flo-scan 201 transducer from the boating trade manufacturer and put it on a rig, with avgas (I assume) going through it and they engrave the K-factor on the housing; normally about 28000 pulses per USG.

And that value “should just work”

But often it doesn’t, and that’s probably due to installations where there is turbulence in the inlet pipe, because it has a kink in it. If this variation is more than a few % then it will be dependent on the flow itself, and the pressure (i.e. whether or not the electric pump is on) and you will never get it right.

So if the engraved K-factor is way out, I would smell a rat.

Are there fuel totalisers for planes like TB20s and PA28s that measure flow from each tank separately? Are these prohibitively expensive?

Never heard of such an STC but you could fit one of these into each pipe from the tanks.

I don’t think it would be useful, however:

2. More importantly – what happens if fuel becomes exhausted from one tank during flight and the engine stops- how easy is a restart of a hot, rapidly cooling engine during flight? Is there a procedure that is known to work?

I think a certified SEP is supposed to restart within X seconds (20?). Uncertified… all bets are off w.r.t. to the fuel system and various funny stuff does go on.

In reality the TB20 capacitive gauges are very accurate and I use them to switch tanks.

On long flights, the LOW FUEL light comes on when the lowest tank has 8 USG in it, and that is pretty accurate – I have measured it with a pump fill. So I run till I see the light come on and then I have just enough balls to draw another 4 USG out of it before switching to the fuller one

Administrator
Shoreham EGKA, United Kingdom
19 Posts
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