OK Guys... educate me... C172 Fuel Problem

[speed2height]

I came close to running out of fuel today in a C172 due to a leak in the RH wing tank. I am an ex-miltary guy with little experience in GA, and I took a club aircraft on a nav.

Before I explain what happened I want to say that these GA guages (fuel, heading, fixed card ADF) are rather innacurate and difficult to use accurately.

I departed AAA with full fuel (5hrs endurance) bound for BBB then CCC (3hrs 10 total flying) with a stop at BBB to pick up a a pax.

90 minutes to BBB saw me land with 1/2 left and full right tanks.

Picked up the pax and departed for the 90 min flight to CCC.

After 30 min the right tank level went from full to 1/4 in the space of 5 minutes. Left was indicating 1/4. A look out the right showed a little fuel staining from the fuel drain valve.

Decided to play it safe and landed at DDD (30 miles short of destination). Lucky I did as THE RIGHT TANK WAS DRY and the left had 3 LITRES in it.

My calculations show 45 litres or 1.5 hrs of flight fuel went missing.

Guys this is unsatisfactory for a bunch or reasons... your comments please

[haughtney1]

After 30 min the right tank level went from full to 1/4 in the space of 5 minutes. Left was indicating 1/4. A look out the right showed a little fuel staining from the fuel drain valve.

This comment is a little telling to me...and begs me to ask a question.

Did you confirm that the fuel drain(s) were correctly seated after checking the fuel/water etc..? (not trying to teach you to suck eggs etc, just wanting a clearer picture of what you did) As this method of leakage is a trap for the unwary.
Secondly, on preflight inspection did you notice any fuel stains running away from the drain areas...and potentialy also on the leading edge of the flap, and or flap rod?

Otherwise if you'd done all of this...I need a better idea of where else the leak may have originated.

BTW I'ver never trusted a Cessna fuel guage

[ITCZ]

Welcome to GA.

Siphoning out of filler caps and fuel drains is unlikely to give you the loss rate you experienced.

It has been 10 years since I flew a 172 and seven since I flew a cessna 200/400 series, but the following spring to mind.......

1. They are crap guages. Float and potentiometer sensor, 1960's DC electrics and cheap ammeter calibrated in gallons. Not the capacitance gauges you may have been used to. You really need to know that aircrafts hourly burn from the club records, not from the book, and work to that plus a fuel log. Compare with guages. Believe the worst figure.

2. They might be bladder tanks and not lying flat due to the bonding of the bladder to the wing having come adrift, or perhaps only flat when a certain weight of fuel is holding them down. Also possible problem with vents being blocked or partially blocked and bladder is 'deflating' rather than allowing air in and fuel level going down relative to float sensor.

3. Was it filled on a level surface?

4. I'd believe a fuel dipstick, however crude, over the guages alone.

5. Try filling to known points. Cant remember on the 172, but the 206 and 210 had 'tabs' in the filler inlet. You could go a combination of full as a known quantity, tabs as a known quantity, and one full/one tabs as a third option for a known quantity.

6. If you rented it from a flying school/club, they have to keep fuel burn records for each type. Ask to see them. Also, anything on the maintenance release?

7. How did the start fuel + fuel added from bowser reading compare with the final indicated fuel load? I flew a C206 once with total capacity of 280 litres. I thought I had landed with 50 minutes (30 litres per side) and then proceeded to fill it to capacity for the empty leg home. BP bowser reading? 260 litres!

I agree, there are a number of things you should be concerned about.

In the meantime as our colleague notes... Never trust a Cessna gauge!

[speed2height]

Aweome guys, thanks for replying.

Yes the fuel drains were correctly seated after testing for water. There was about a 6" wide fuel stain on the underside of the flap, yet there was no visible fuel leak on the ground or in the air. This was not evident on the pre flight but there was a small amount evident on a pre flight I did on the same aircraft a month before.

The leak appeared to be forward of the drain valve as it was exiting the panel joint and the drain valve hole.

I dip sticked the acft at full tanks at departure from AAA.

I admit now in hind sight I may have had an indication of this problem on the previous flight that was 50 min long (30ltr/hr burn rate), and I put 66ltr of fuel in the tanks to refill. But as I hadn't filled the acft initially myself and it was dipped sticked a bit less the full tanks I wasn't sure it was full. I also had run the engine a bit rich so I disregarded the higher than normal burn rate. (As I said hindsight).

[speed2height]

Originally Posted by ITCZ

Welcome to GA.

Thanks

[ITCZ]

I shouldn't be too hard on GA.

The problem you ran into is maybe not so much one of fuel systems and cheap gauges. Its probably more a 'culture shock' as you transition from one operating culture to another.

Good GA operators are just as disciplined as other operations. Just different. You will not receive anywhere near the support you may be used to in your previous life. A lot of the ex-service pilots I sit next to had the benefit of some excellent training. A lot of the ex-GA people I sit next to are equally good pilots but have highly developed cunning and self reliance! They are used to equipment that is often not of the same quality as may be found in a PC9 and bigger. The people renting you the aeroplane may or may not be well trained and knowledgeable.

The discipline with fuel on navs is to know the engine management techniques, know the recorded fuel burns and know the vagaries of the GA fuel systems, which include...

- wet wings vs bladder tanks that lift and change shape.
- rocking the wings to let water drain to the drain points
- tipping the drained fuel over your fingertips to feel if it is Avgas and not just a whole bottle of water you just pulled out of the wing (tropical pilots know it well)
- making sure the drains are properly seated and aren't leaking after you drain them
- ALWAYS dipping the tanks at each stop to confirm your fuel log = dips = gauges.
- Knowing the expected quantity to be added in litres before you start the bowser.
- Knowing your engine handling and leaning techniques.

Other little things that come to mind.

The plumbing in the Cessnas runs from the inboard of the tank, down through the forward door pillar, under your feet and then to the fuel tank selector, then onwards to the firewall.

I believe the rotary fuel tank selector is the same valve in all 100 and 200 series Cessnas, allowing for selection of LEFT, RIGHT, BOTH and OFF except in some models the BOTH (pointing forward) selection is not available due to the design of the centre console. Unless of course when you are in real trouble and you grab your leatherman, undo the handle and use the leatherman to turn the spindle to a BOTH position

Do the club thing. Kick around on saturday afternoons etc, meet the pilots that fly them and especially the engineer that maintains them. Find out where they keep the fuel burn records, etc. Don't be too fast to tell them all YOUR war stories, because like most civvy pilots, they will shut up to be polite (or shut up and hope you will eventually go away) and think that a warrie about a piper warrior does not compare to a warrie about a caribou in the highlands. You will learn a lot if you let them tell you THEIR war stories!

[speed2height]

Great advice ITCZ thanks.

[Runaway Gun]

I think that Speed2height should be congratulated for having the thought to ask for everyone's advice in such a polite manner, and for responding so well.

You'll soon learn that GA holds some very knowledgable and talented pilots and engineers that have entered the flying game via a very different route, and that we all can learn from each other.

Yes, there's some cowboys out there, but it's the same wherever you go.

Just learn the limitations to be aware of in the civvy world, and you'll continue to enjoy aviation.

[185skywagon]

Originally Posted by ITCZ

The plumbing in the Cessnas runs from the inboard of the tank, down through the forward door pillar, under your feet and then to the fuel tank selector, then onwards to the firewall.

The Main Fuel pipe from each tank, actually runs down the rear of the door frame, with a supplementary pick up at the front of the tank.
Earlier (1960's 100 series) Cessna only had the rear outlet resulting in a quantity of fuel being unavailable in all attitudes (about 1/4 of tank unusable except in level flight). the main reason that bladders may lift off the bottom of the tank is that the underwing vent is blocked or the little rubber flap on the underside of the filler cap is stuck and not venting.
Pretty sure that most 172's had either metal tanks or a wet wing configuration. Wet wings can develop leaks all over the place along lap joints due to failure of the tank sealant. Wet wings are prone to this in rough operations like Seaplane Ops. Bladders are usually the choice in Cessna Seaplanes such as the 185 and 206.

I believe the rotary fuel tank selector is the same valve in all 100 and 200 series Cessnas, allowing for selection of LEFT, RIGHT, BOTH and OFF except in some models the BOTH (pointing forward) selection is not available due to the design of the centre console. Unless of course when you are in real trouble and you grab your leatherman, undo the handle and use the leatherman to turn the spindle to a BOTH position

Some if not most 206/210's have only left and right selections due to the design of the fuel return system, associated with the fuel injection system.
"Both" is not an option regardless of whether the console blocks this option.
I believe the last 206/210 of the 1980's had Both tanks available at once.

My old 185A (fuel injected) had the best system of all, ON or OFF. You couldn't park it on a slope though, as all the fuel would run from the high tank into the low tank and out the underwing vent onto the ground.
My latest one (1979) has left/ right/both.

Speed2height, Uneven fuel burn between tanks, is a normal thing in most cessna aircraft. it can be dependent on how well you keep the machine in balance.
185.

[Capt Claret]

A word of caution re fuel dips

After an unpleasant fuel related incident in a Gonad (N24) my then employer instigated a 'dip before and after flight' process as they investigated said incident.

On a trip from Tindal to Delamere to Darwin the dips showed that the Gonad burned signifficantly more fuel Tindal to Delamere than Delamere to Darwin. As best I can recall Tindal Delamere is about 1/3 the distance of Delamere to Darwin.

On another occasaion after dipping the tanks prior to refuelling, about 60+ litres of fuel went into the tanks over and above what the dip sticks indicated.

I am not saying they're useless, just that I remain ever wary of their accuracy.

[*Lancer*]

All these tips are exceedingly valid, but has something been missed?

The 172 had an expected (even better than that) burn for the first 90 minutes of flight. Other than an actual, serious fuel leak, or a seriously distorted bladder tank ex AAA (AND BBB), what could have changed during the second sector to cause such a discrepancy in expected vs actual burn?

speed2height dipped the tanks 3/4 full ex BBB...

[Sunfish]

With respect, a dipstick is essential, the gauges cannot be trusted. The planning figure we use is 36 l/h for a C172. You might like to suggest that the school do a couple of short flights with a known fuel load and dipstick to confirm fuel loss is real and not finger trouble before taking matters further.

Early C172's (down to N?) have steel internal tanks, the new ones (with fifty million drains) has a wet wing, I think, and not a bladder. If it was a bladder, a clogged vent line has been known to partially collapse the bladder, resulting in a "full" indication when in fact very little fuel has been added.

Unless the engine is gulping fuel (which I doubt) your options are a bung cap, bung drain, bung tank or bung plumbing (including gascolator drain and fuel pump diaphrams). I think I understand that a bung cap seal would produce an undetectable major loss of fuel in flight (sucked out and vapourised with no stain).

As a total newbie, and being used to the "brace and suspenders" approach of Boeing commercial jets, I was amazed at GA engineering both pleasantly and not so pleasantly, having said that, I found the safety margins in certain military aircraft bits to be simply scary. I guess its the devil you know...

Good luck and please let us know how you go.

[185skywagon]

Another question ( not intended to cause embarassment) :

Did you fill the aircraft yourself before starting out on your trip????
Was it really full??
In some Cessnas, it is quite possible to get another 10-20 litres per side into the tanks when the level is within an inch from the top.

[turnarounds]

Tank size

Mmmmmmmmm there are several different fuel tank sizes on 172's (short range, log range, integral .........)

Are you sure it had the endurance you thought?

[Mr.Buzzy]

Silly question.
Are you certain of the tank capacity?

Have seen many aeroclub disagreement over which 172 has which tanks fitted.

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[Keg]

Originally Posted by haughtney1

BTW I'ver never trusted a Cessna fuel guage

Amen brother. Herein is the most important aspect of flying Cessna lighties I reckon. It was pointed out to me on my very first flight. I can still remember the instructor telling me about it and stressing the importance of ensuring the fuel cap was secured because it's harder to see the fuel venting because of the high wing.

Lancer I reckon it's a function of the fuel cap not being secure. Some of the older seals aren't that flash and to get the cap 'flush' sometime required a bit of fiddling.

Glad it all worked out OK for speed2height.

[bushy]

Something missing

There is something missing here. Aeroplanes and engine behaviour is predictable.

I remember flying a chieftain for the RFDS some years ago. We used to keep the main (inboard) tanks full, and the outboard tanks half full. On one ocasion I was flying along happily, using the outboard tanks for cruise, when I noticed the right tank guage reading about three quarters full, when it had only been half full when I took off. It kept increasing as I burned fuel.

I snagged the guage on the MR, and talked to our engineers about it. They checked it and found nothing wrong. It did it again on the next flight, and I talked to the engineers about it. They found nothing. We both walked away mumbling to ourselves. When I checked the aircraft and refuelled it on the ground, it behaved normally. But it did it again in flight. I asked the engineers to check the venting system. They said it was ok. It did it again, in flight and everyone thought I was nuts.

Finally, the engineers fitted a new seal to the fuel filler cap, and everything returned to normal.
I believe suction from the low pressure area where the fuel cap is, was sucking the bladder up so it lifted the float and increased the fuel guage reading. When the aircraft was on the ground for a while, the pressure equalised, and the symptoms were gone. Maybe the vent was partially restricted as well.

This may appear to be un connected to the 172 story, but there is a connection, and it is this. There IS A REASON why this happened to the 172, and it should be found. There is no black magic. The reason may be obscure, but it is there somewhere.

"Don't ever trust fuel guages" is a copout. They would not be fitted if they were no good. If they are well maintained they are useful. Many are not well maintained, and fluctuate too much. Pilots have let aircraft run out of fuel when the fuel guages have been telling them it was about to happen, and they "did not trust the fuel guages". Like everything else in aviation you double check. Using dipsticks and fuel records is normal good practise. In the case of this 172, the fuel guages warned the pilot, and he took appropriate action.

[Captain Condom]

Did you lean the mixture? I had a similar event just after I'd got my PPL when I was building hours for my CPL. The fuel burn at full rich, compared to correctly leaned, can be significant on a long flight, and the fuel burn figures quoted in the flight manual are for when the mixture is correctly leaned.

[kiwiblue]

Originally Posted by bushy

"Don't ever trust fuel guages" is a copout. They would not be fitted if they were no good.

Sorry Bushy, I disagree. The fuel gauges in most common GA types are shockers. They are inaccurate. whatever else may be said float-type gauges as found in most of the GA fleet give every indication of having been supplied and fitted by the lowest contract bid.

The comments/reasons as to the possible excessive fuel burn have been excellent! Many many possible causes there. I would like to present another possibility:

The Lycoming engine as fitted to many C172's does have one interesting little feature that once very nearly claimed me... if the aircraft is being operated at or very near to full-throttle, as in an extended climb, the mixture is put into an 'auto-full-rich' setting, regardless of the pilots manipulation of the mixture control.
This is easily verifiable by an 172 drivers out there... simply climb to an appropriate level, set your cruise RPM and lean to peak, rich of peak or lean of peak, wherever it may be you operate your aircraft. Note the reading of your EGT gauge. Then simply slide full throttle in and watch what happens to your EGT.

It is possible to operate the engine at or close to full RPM without reaching the auto-full-rich, but you need first to know about it, before you know how to avoid it.

In an extended full-throttle climb, this alone can almost double the flight-planned fuel burn.

Hope this is of some benefit.

Regards

RS

[Howard Hughes]

Originally Posted by Captain Condom

Did you lean the mixture? I had a similar event just after I'd got my PPL when I was building hours for my CPL. The fuel burn at full rich, compared to correctly leaned, can be significant on a long flight, and the fuel burn figures quoted in the flight manual are for when the mixture is correctly leaned.

I am with condom, my first thought was "did he lean the mixture"?
Also, what altitude was the trip done at?

Gotta admit it's been a long time since I flew a piston and probably ten years since I flew a 172, but if I was guessing I would say a mixture of perhaps not quite full tanks and a rich mixture may have been the cause.

Cheers, HH.

PS: I think we all have a story about the inaccuracy of Cessna fuel guages!!