Fuel Gauges

[Deano777]

How reliable are the fuel guages in say a PA-28? during my training my instructor told me never to trust what they said as they are inaccurate, then when he started air charter work I had another instructor who said "oh yes, you should always look and rely on them", basically who is right? at the moment all my fuel planning is done manually using my planned data etc.
What if I wanted to substitute some fuel weight to enable me to carry more pax? if you didnt use the gauges how can you asscertain how much fuel is in the aircraft an how much more to put in, as after the last flight the aircraft is topped up right away by the fuel truck, basically I want to take my wife and my brother & sister in-law in a warrior to the Isle Of White sometime and Im just thinking maybe the plane could be overloaded with 4 adults and fuel to tabs

thanks in advance for you time to reply

Dean :)

[Chilli Monster]

Do not trust the gauges. Get a dipstick made or buy one that someone else has already done the calibration for.

You don't say whether it's a group or club aircraft. If it's club and it flies just before you then I'd suggest doing your weight and balance beforehand and then loitering around for when it gets back - making sure only the fuel you want is put in. Either that or get accurate weights for you plus pax - you might find tabs fuel is ok.

[willbav8r]

I was told that the only time to trust a fuel gauge is when it reads empty!

No reason to go fly with empty fuel tanks? And if stopping somewhere in most lighties, look inside to verify there is fuel. Leaks have been known to happen! Use a dipstick. C182 and others with landing weight restrictions really need a dipstick to back up the gauges. High wings are tough to visually check, so dipstick is again quite important.

As you know what the fuel burn is, and how long you have been turning and burning (add a 15% fudge factor) you can mentally verify what the tanks (and gauges) should be telling you.

Oh yeah, W&B is MUCHO important - especially with 4 up in a warrior. Hope your passengers are all svelte!

[Onan the Clumsy]

I wouldn't rely on them, though the ones in the a/c I fly seem to be quite accurate.

In any event, if the gauge says you have 4 gals left (20 mins in a 182) are you really going to push it another quarter hour?

Wasn't there a case in England recently where a doctor put a twin into a house because it ran dry? It was a rental and the owners wouldn't allow full fuel between renters.

[Tinstaafl]

Between your calculations, dipstick & fuel gauges trust whichever one gives the least amount of fuel remaining (for endurance & range). Trust whichever one gives the most amount of fuel (for weight & performance)

[Deano777]

Thanks for the comments guys, yes it is a club aircraft, and the W +B issue is why Im asking the question, we may just be inside the W + B limitation for takeoff but are definately inside it for landing, this much I have asscertained (subject to adverse wind / ground speed on the day of dep), its only a 40-50 minute flight to the IOW from my departure aerodrome so no need to refuel whilst there, being too heavy will only be an issue on the way down :)

Thanks for the tips :ok:

Dean.

[Genghis the Engineer]

Below I've pasted the total requirements (apart from deleting some paragraph cross references) from FAR-23, which is the safety code against which most light aircraft worldwide are certified, that affect fuel gauge accuracy. It wouldn't do to expect more than the aircraft was certified to do...

G



23.963 (e) Each fuel quantity indicator must be adjusted, as specified ... to account for the unusable fuel supply...

23.1305(1) A fuel quantity indicator (must be supplied) for each fuel tank

23.1337(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition:

(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined


23.1553 Fuel quantity indicator.

A red radial line must be marked on each indicator at the calibrated zero reading,



23.1551 (1) For fuel systems having no selector controls, the usable fuel capacity of the system must be indicated at the fuel quantity indicator

[tmmorris]

On the PA-28's I've flown (Warrior and -180) full fuel was 50USG and tabs was 34USG. Handily enough, four average adults leaves you room for 34USG or so of fuel - i.e. as a rule of thumb fuel to tabs is OK unless one or more of your adults is on the heavy side. I guess that's why the tabs are where they are (remember this would have been calculated in the 60's when people were perhaps lighter!).

Of course you should still do a W&B calculation the first few times you travel 3- or 4-up - but you may well find that the above works for your aircraft too.

Tim

[Julian]

As Genghis points out, the only time a fuel guage must legally be accurate is when its reading empty...by them I think you will have other things on your mind!!!

ALWAYS check your fuel quantity using visual and dipstick.
There was a previous discussion thread running saying that on some models this can be very hard to do anyway.

[Genghis the Engineer]

Incidentally, a lot of aircraft use capacitance based gauges, which read differently with different brands of fuel, water content, cycle of the moon, etc. anyway.

G


N.B. Re-reading my previous post, I find myself wondering how you achieve level flight with an empty fuel tank anyway? Slightly spurious requirement one might argue !

[PPPPP]

ALWAYS check your fuel quantity using visual and dipstick. I religiously use a dipstick when flying the 172s but having recently been checked out on a Warrior, I am having some difficulty finding a dipstick for the PA28 series. Is it a case of self-calibration of a suitable length of wood or does someone sell them?

[IO540]

Deano777

Never trust fuel gauges for ANYTHING. They are usually either completely useless, or nearly so. Flying school/club planes are often maintained to the minimum standard legally possible and gauges aren't important.

The way to do it, always, is to work out how much fuel is needed for the proposed flight, and make sure you have it (plus a decent reserve), using a PHYSICAL (visual) check. Always visually check the tank content before every flight. Never accept anyone else's word for it - not even that of an instructor! Even if an instructor says there is enough for 3 hrs, the tech log says 3 hrs, and the daily flight log says 3 hrs, still check it because all of these could be wrong (I've seen it myself).

If permitted by W&B, always fill up to the top. Then you have a known endurance (hours airborne) at a given engine power setting. Make sure you know what these are (the endurance in hours, and the cruise power setting in RPM or whatever, and the mixture setting if not full rich).

If you cannot fill up to the top, be extra careful. Regardless of how you have done your calculations, ALWAYS visually check what's in the tank before departure. I know on many planes the fuel level isn't visible through the filler cap once at or below say 50%, and on some the W&B are so severely limited that those planes often "have" to be operated below 50%, but then you are taking on a very big risk. The incident referred to in this thread was partially caused by the fact that the fuel level could not be checked.

The reason fuel gauges are usually c**p and few people seem to care is that one should not need them for flight/fuel planning, and once airborne they are no more than handy for knowing when to switch tanks. For the latter alone they ought to work but usually they aren't even OK for that; when I used to rent I used to carry a kitchen timer set to 30 mins.

Genghis

I have capacitance gauges and they are highly accurate, in fact as accurate as I can read the dials. These are the only fuel gauges which I have seen (on about 10-15 diff planes) which are actually usable.

They should not be susceptible to fuel grade etc unless they were defective to the point of falling apart. Have you any specific information that diff brands of fuel affect these gauges?

[FlyingForFun]

Capacitance guages are good - I have one on my Europa, and it is essential because it's not possible to visually check the fuel level. But be careful - the scale is not linear! It is vital that the guage is calibrated properly, and you must read the calibration data before you start interpreting what the guage is telling you.

FFF
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[S-Works]

I recently changed the gauages and senders on my Cessna for new ones and have found them to be totally accurate. I have a FS450 fuel computer which is totally accurate and the gauges compare bang on with the fuel computer.

But it still does not stop me from dipping the tanks on every flight. I guess the fear of falling out of the sky after omitting a visual check that the gadgets are telling the truth is still as strong as it was when I first learnt!!

[Genghis the Engineer]

I believe it's down to alcohol and water content, which can vary a lot depending upon the particular fuel company's procedures, and the airfield's storage of fuel. Increasing the water content fuel alters the dielectric constant of the fuel.

I heard this authoritatively from the engineer who signed off the design of your aeroplane FFF !

G

[hoey5o]

It continues to be an absolute scandal that after all these years the majority of mainstream light aircraft still have poor fuel indication systems. What is the point in having a fuel gauge fitted if you carnt trust it ?
With the technology available now, why o why can we not now have cheap but reliable sytems available for such a vital area.
Fuel starvation incidents continue abound every year so come on
piper/cessna/authorities etc, as the decades go past can we sort it out !!

[FlyingForFun]

I heard this authoritatively from the engineer who signed off the design of your aeroplane FFF !Well, I won't argue with him! He did also include a reserve tank in the design, to give me an hour or so of extra fuel if the worst happens, which is a very handy safety blanket - even though the guage been totally reliable in my experience so far, I don't particularly like not being able to see the fuel level visually.

FFF
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[Deano777]

what a wonderful response thanks alot guys :ok: theres nothing I can really add here apart from the fact that I always do the visual check on my walk around, and if it aint tabs or higher I dont fly (well I do, I get them to top it up :D)
Tim
Obviously W+B is very important with 4 up (it is with any No. of pax) and is a legal requirement, I have an excel spreadsheet which does the calculations for me so I print off 2 copies, one comes with me in the a/c and one stays at home, just incase ;)

If anyone wants a copy of this spreadsheet dont hesitate to ask and I will send it to you or upload it to some webspace, its bloody marvelous :D
oh btw its for a PA-28 Warrior II and I also have one for the PA-28 140 Cherokee:)

Regards


Dean.

[Gertrude the Wombat]

Always visually check the tank content before every flight. Never accept anyone else's word for it Well, if you're picking up the aircraft from the pump and you've seen the refueller's signature in the book it might seem somewhat untrusting to climb up and look inside the tank whilst the guy's watching you ... but one should still do it.

[IO540]

Well, if you're picking up the aircraft from the pump and you've seen the refueller's signature in the book it might seem somewhat untrusting to climb up and look inside the tank whilst the guy's watching you

Any pump operator who thinks this is un-trusting should be working at a petrol station, not at an airfield :O

[Onan the Clumsy]

It continues to be an absolute scandal that after all these years the majority of mainstream light aircraft still have poor fuel indication systems. Not so the Grumman Yankee. The tanks are the main spar and they have a sight glass right there in the cockpit, so you know exactly how much fuel you have. On the down side though, I wonder what happens if you crash :uhoh:

Well, if you're picking up the aircraft from the pump and you've seen the refueller's signature in the book it might seem somewhat untrusting to climb up and look inside the tank whilst the guy's watching you I do it all the time, every time, right in front of them. It's not untrusting at all and they don't take it that way either.

[englishal]

I read an interesting accident report once.....The refueling guy refueled the aircraft (C172 or something) and left the fuel caps off EXPECTING the pilot to check the fuel. The pilot and his mate wandered out to the aircraft, and decided not to bother going flying, so they left again. Next day, after the aircraft had been sat on the ground over night in the rain with the fuel caps off, another pilot came out to go flying. It was a cold freezing morning, and the pilot saw the caps were off and assumed the aircraft had just been refueled. He drained some fuel and everything looked dandy so off they went. 30 minutes or so into the flight, the engine failed and they crashed. Apparently the tanks got a load of water in, which then froze and so didn't show up when the tanks were drained. During the flight the water melted and stopped the engine.

I also read another report along the same lines. Pilot saw aircraft being refueled, didn't check the fuel. Fuel guy left caps off for pilot to check....except he didn't. Took off, fuel sucked out, engine stop.....you get the drift.

On the subject of accurate fuel guages, most new aircraft have accurate fuel guages, and most panel mounted GPS's have a facility where you can enter your calculated duration / range and it'll keep a running tally on how much fuel you've got left.....All calculated of course, but better than nothing.

I'm paranoid about fuel, for reasons best not discussed in public :O

EA

[FlyingForFun]

The tanks... have a sight glass right there in the cockpit, so you know exactly how much fuel you haveAre you familiar with the fuel guage in the Piper Super Cub (and I think some earlier variants too)? A sight glass, right there in the cockpit, so you know exactly how much fuel you have... as long as you're in exactly the attitude which the designer wanted you to be in. The aircraft's attitude is, apparently, important enough for Piper to have put two scales on the fuel guage, one for the 3-point attitude on the ground (which will be similar to the attitude you have during slow flight, although it doesn't say so), and one for "straight and level" whatever straight and level meant to the designer. The difference between the two scales is about 1/4 of a tank.

A sight guage can't lie to you, but it can certainly mislead you if you don't know how to read it.

FFF
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[IO540]

Englishal

I have sen the same problem with filler caps on, where the rubber seals had perished. This is common in flying school planes, particularly PA38s. I once drained out about a pint of water (!!) after a night of heavy rain.

Re the fuel totaliser, one needs a flowmeter, e.g. a Shadin, coupled to the GPS. The GPS gets an RS232 data stream from the flowmeter, giving the flow rate and the remaining fuel, and it projects the reserve at the flight planned destination. It's a nice feature, though not essential to flight safety because I always fly with at least 2hrs reserve. I suppose it would be handy after a series of emergency weather diversions...

I don't think a GPS can do this alone (without a flowmeter) - certainly it would not be accurate because it won't know the flow rate.

[hoey5o]

Englishal, I accept that new aircraft are better but the vast majority of light aircraft being used now are most certainly not new. Can there surely not be a postmod available to refit these aircraft ?
You also mention GPS which is I suppose is something, it is however totally dependent on what you type in and it can only calculate from what you have told it. Aircraft with 2 or 3 FMS's have still run out of fuel eg Gimini Glider, Hapag Airbus etc.

[DFC]

Deano,

For the PA28-140, 180 and Arrow II;

Full Tank - 25USG
Tab - 17USG
Base of the Outer Tank Wall - 10USG

Unuseable - 1USG per tank (this can be less in pre 1973 models so check the book!!)

Don't think anyone will bother going anywhere with less than 9USG useable!!!......or if one does, it won't be very far !! :D

The only thing I really use the gauges for is to detect a leak!! and even then the chances are that if it is a big leak, the tank will buckle and freeze the gauge!!

Regards,

DFC

[englishal]

it is however totally dependent on what you type in
Very true...you enter estimated fuel flow, fuel onboard etc at the start of the flight, and it gives you a tally of how much you *should* have left.....

No substitute for a proper fuel guage, though it can draw your attention to a possible low fuel state or remaining range. Ideally it would have a flow sensor on the fuel so it knows really how much you've used...

EA

[TheAerosCo]

A sight guage can't lie to you, but it can certainly mislead you if you don't know how to read it

A word of caution - a sight gauge can lie!
If the gauge is of the flexible pipe design and the pipe takes a relatively tortuous route from the tank, then trapped air bubbles can form giving a false reading. This is particularly applicable to aerobatic aircraft such as the Pitts. I am aware of at least one engine stoppage resulting from the pilot believing he had more fuel than he actually had for exactly this reason.

[FlyingForFun]

I stand corrected, then. What you've described makes perfect sense.

FFF
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[kabz]

I always like to check the caps and drains after being fuelled, and it seems sensible to check how much you get fuelled with after a flight, so you can get a rough fuel burn.

Example: the twin I fly burns about 18 gal / hour in general training, and at 65 % power. That means about 2.5 hours are in the mains, and 3 hours in the aux tanks.

Typically the left main gauge indicates 1/4 full when it is really at 3/4s. The right main indicates correctly. I'd be pretty unhappy with this if I was on a long flight and switching back to a low-looking main with a descent and approach to shoot.

[BigEndBob]

The pilot should always be the last one to check and fasten the fuel caps even after refuelling, then he has only himself to blame if they come off in flight.

I once ran a fuel tank dry on a TB10, took a few moments to realise what was going on.
Having been distracted by an aircraft being towed and needing to get past me at the hold forgot to change to the higher tank for dep.! One hour into flight the engine started to stop.
i may have noticed the lack of fuel in that tank if the fuel gauges at worked. They always read 1/4 full.
I knew the fuel gauges were faulty and they are built as one unit with i think from memory oil press./temp gauges. The unit was going to cost £1000 to replace back in 1992.
So as a club we just put up with it.
A disaster waiting to happen.

We would'nt put up with a us airspeed indicator so why to be put up with crappy fuel gauge systems. I've known Cessna fuel gauges show full when empty and virca versa. This usually happens when the refueller accidentlty pressurises the tank by not releasing the nozzle handle quick enough because not paying attention to the fact the fuel is reaching the top of the tank.
All the cars i have driven have always had very reliable fuel gauge systems. But there again most of those aren't 25 yrs old.

[LowNSlow]

I've got a rotating bezel driven by a float in the Auster, ie completely mechanical. It's the original 1946 gauage as far as I know. On the ground it underreads by 3 gallons and in straight & level flight it underreads by 2 gallons. I've verified this by putting a known quantity of fuel in it. At least it errs on the safe side! It's nearly as accurate as the wire on a stick that is fitted to the J3 / L4 Cub.

[Sliding member]

Imust admit to never really taking fuel gauges seriously (on a 152), I've always flown with at least 1/2 tanks and generally fly for one hour. What amazes me is a few clubs don't seem to have any fuel record on their tech logs. When I learnt to fly the t/l was annotated when tanks filled making fuel calculations easier. Also I wonder howm many of these cut down broom handles I see are calibrated in any way?

[IO540]

Sliding member

What amazes me is a few clubs don't seem to have any fuel record on their tech logs

That sort of record is worth less than the paper it is written on. Always do a physical check, and if one is not possible (because due to W&B you "have" to fly with the fuel level below where it is visible) then take on enough fuel at the pump such that even if the tank was empty to start with you could complete the planned flight with reserve. Might need to re-calc W&B though.

The recent well publicised incident (G-OMAR) involving an unsuccessful CAA prosecution shows these records are completely and utterly worthless. Relying on them does appear to be legal though!! Perhaps because airliners do it routinely (but they have good equipment) and the CAA could hardly admit openly that most light planes have useless gauges...

I have seen two cases of "3.5hrs" in the logs and empty tanks, and that was on PA28s. No idea how often it happened on Cessnas; I trained on them but never checked them (if going solo, always went to pumps and got a gollocking from the CFI occassionally...)