Teaching Fuel Sampling

[Big Pistons Forever]

There is another thread going regarding the unfortunate crash of a Grumman Goose caused by fuel contamination. There is an excellent discussion about the correct procedures for fueling from drums which should be reviewed by anyone planning a bush flying career or flying up North.

Within the context of flight school operations it is very unlikely that a student will be doing drum refueling but I would suggest that there is more to teaching students how to check for fuel contamination than just showing them how to stick the sampler in the quick drain.

Points that should also be covered

1) It can take up to 20 minutes for water to settle to the bottom of the tank so sampling the fuel right after it has been topped up may not capture water in the fuel. I am not shy about requesting the fueler to do a clear and bright test before filling the airplane and always will do it at least once with a student to show them how the test works

2) I always show students what water actually looks like by using a clear glass with some fuel in and showing what a few dropped look like, then enough water to show a meniscus and then pouring out all the fuel and just adding water to show that you can mistake a sample jar full of water for a clean sample if you just take a quick look

3) I make a point of checking the fuel tank cap seals when you take the cap off to check the fuel level. I have found numerous caps with deteriorated, flattened, cut or even the seal missing entirely as well as caps that don't close tightly. All of the above can allow water to enter the tank when it rains. Caps in poor condition should be snagged

[goingnowherefast]

I love it. Especially point 3.

One can be excellent at detecting water. But if nothing is done to address the leaky fuel cap, it's just gonna be the next pilot when the engine gets fed an unhealthy diet of water.

[pelmet]

I love it. Especially point 3.

One can be excellent at detecting water. But if nothing is done to address the leaky fuel cap, it's just gonna be the next pilot when the engine gets fed an unhealthy diet of water.

A leaky fuel cap does not necessarily mean that the resulting problem that you will encounter will be due to contamination.

I used to fly a particular Citabria(that was used for taildragger currency) that was quite well equipped and as a result, had a fairly small useful load. It had been decided the maximum takeoff weight would be strictly adhered to. As a result, whenever I flew it with the other pilot, we had about 20 minutes flying time until we down to our half hour VFR reserve. Therefore, always short flights when the two of us were flying ad all my flights were with the two of us and short.

I have only ever flown one other Citabria and unlike that other Citabria(which had a fuel guage for each tank), this particular Citabria only had pone fuel guage, which was for the right tank. I have never been able to get confirmation about the legality of that.

This particular aircraft enjoyed life in a heated hangar year-round. Eventually, it was sold and I was designated to fly it down to the new owner. I did some performance calculations and discovered that I should be able to do a nice cross country flight for 2.5 hours(or so) with a comfortable reserve.

I had the tanks topped off and departed for my flight. I was a little surprised to see that not long after levelling off, the right tank was down to 3/4. Obviously, I could not be sure what was in the left tank. That right tank kept on decreasing faster than expected and I was down to half in less than an hour. Seeing as it was close to 1/4 tank after perhaps an hour, I decided to divert to an enroute airport.

After landing, I checked the right tank and it was as low as the guage was indicating. The left tank was completely full. As I walked behind the aircraft, the angle of the sunlight reflecting off the dark coloured fabric showed some sort of a evidence of a stain. The fuel cap was leaking and as the low pressure on top of the wing sucked out of the left tank, it was also sucking fuel from the right side to the left through the crossfeed tube. It didn't surprise me as I had once left a fuel cap off of a Citabria for a flight and the tank without the cap had much more fuel after the flight than the tank with the cap.

Anyways, the seal in the cap was either gone or faulty(I can't remember now) and so I had to fuel up at a couple of locations to ensure that I got to destination with sufficient fuel.

[jamesel]

AFAIK, a gauge for each fuel tank was an option from the 1st Citabrias in 1964 up until Bellanca bought the line in 1970.

[pelmet]

I suspect that the leak had been there for quite a while, but the short flights had masked any unusual gauge indications, as not too much had leaked to be obvious( from an indication point of view or a refueling point of view, if one takes note of such things). And keep in mind, based on my Citabria experience, that uneven fuel tank use is normal.

A question to consider is…….What if it had been a leak from the right tank instead of the left. Keep in mind that like a lot of high wing Cessna’s, fuel does not decrease evenly in Citabria fuel tanks(at least in my experience with one other of the type).

With a leak from the right tank, and the only fuel gauge being for the right tank, the right tank gauge would have continued to accurately indicate full with fuel transferring from left to right until the left tank was empty. The extended full indication on the right tank would have seemed strange and I might have made an assumption about the serviceability of that gauge and decided to ignore it and just go on time and estimated fuel burn. Or I might have assumed uneven fuel feed.

Then once the left tank was empty, the right tank indication would go down more quickly than normal, potentially confirming my belief that the gauge was faulty or uneven fuel tank burn. Quite possibly, I would have just assumed that the left tank quantity was fine, the right tank gauge was intermittent, and planned on mentioning this issue to the new owner and not worry about the right tank having what I assumed to be a faulty low indication, or decided that even if the right tank eventually was low, fuel would then feed from the assumed to be partially full left tank to the engine.

An interesting scenario. I have done plenty of flying in light aircraft with faulty gauges, where one simply uses a dipstick or can confirm fuel levels. In those cases, the gauges are ignored and considered of little value. But all goes out the window, if one encounters a leak. That is where accurate gauges become worth their weight in gold. Fuel gauges are not just for fuel burn but also detecting leaks. If you don’t have accurate gauges, a mental note that you have reduced leak detection may be wise.

I suppose if one is in a situation where they do not trust the gauges and plan to use other methods for fuel management, they should be even more alert than usual to potential leaks. What is the reason for fuel staining on wings(is it from an issue in the past that has been resolved - as I have seen - or is it a current issue). Does the fuel cap seal appear to be in good condition.

[digits_]

These pictures are interesting:
https://www.sumpthis.com/cessna150andce ... 24x768.htm

It takes about 2 liters of water contamination before it starts to show up in a drain sample of a C152.

[Big Pistons Forever]

The photo array above doesn't make sense to me. It doesn't take much water to kill the engine so I can't believe Cessna's have been flying around for 50 years with 2 liters of undrained water in their tank. As I understand it the wing dihedral and wing incidence will result in the sump being the low point of the tank and therefore the heavier water will end up collecting there.

God point about the danger of a loose fuel cap allowing fuel to siphon in flight. It is another reason to check the cap every time you check the fuel and service any cap that is loose or has a perished gasket.

[pelmet]

These pictures are interesting:
https://www.sumpthis.com/cessna150andce ... 24x768.htm

It takes about 2 liters of water contamination before it starts to show up in a drain sample of a C152.

I have seen that article many years ago. In the end, there is not much I can do with the various light aircraft that I fly but a good consideration would be to rock the wings of your aircraft if there is a suspicion of water in the fuel. This can be done many ties and may allow water to move around things such as baffles or other obstructions. In fact, some aircraft like the C182 with bladder tanks(that can have a wrinkle on the bottom) have this sort of recommended action. I think it is called rock and roll and also involves pushing the tail down while rocking wings.

Of course, different aircraft can be different. I had an ongoing issue with water in a C337 that would not resolve itself despite maintenance checks for where water could be coming from. I would always get some water on one side. it had a complicated fuel system with four interconnected tanks in each wing acting as a single tank for the pilot. Never did figure out the issue before it was sold.

[digits_]

The photo array above doesn't make sense to me. It doesn't take much water to kill the engine so I can't believe Cessna's have been flying around for 50 years with 2 liters of undrained water in their tank. As I understand it the wing dihedral and wing incidence will result in the sump being the low point of the tank and therefore the heavier water will end up collecting there.

I think it's more to show that the wing drain is quite useless (perhaps added to satisfy regulatory requirements?) and that the most important one is the fuel strainer.

A lot would have to go wrong for an aircraft to collect 2 liters of water in a tank in normal use. Condensation, leakage or even broken or missing fuel cap seals only collect so much. 2 liters is a lot.

There were some newer models (I think the c172 SP) that had 7 drains on each tank or something silly, probably to overcorrect on that issue a bit.

And then there are other aircraft such as commanders with 11 (?) interconnected bladder tanks that barely have any drains.

[Big Pistons Forever]

IMO saying "The wing drain is quite useless" is an inappropriate message for the flight training forum. I have found water when draining Cessna wing drains on numerous occasions so I will always drain them as part of my preflight.

It is also important to note that a carbureted engines will pass a small amount of water but fuel injected engines can be stopped with only a few drops of water.

With respect to the later model Cessna's with the extra drains this was undoubtedly a consideration as well as the fact that they have wet wing fuel tanks. Unlike the removable rectangular aluminum fuel tanks in legacy Cessna singles, the wet wing tanks have internal stiffeners which could provide a trap for water and prevent the water from draining to the inboard end fuel drains which is why there are extra drains in the outboard ends of the tank.

My message to students is simple. Sample all of the fuel drains as part of your preflight

[digits_]

IMO saying "The wing drain is quite useless" is an inappropriate message for the flight training forum.

Yes you're right. It's too broad of a statement. I am sure there are different types where it is much more critical. I am also not advocating not draining the tanks, but more from a "i suppose it doesn't hurt" point of view than an "oh god this will save my life guaranteed"

I do think a lot of pilots are flying around with water in their thanks, blissfully unaware because the drain sample was clear.

I can't recall any accidents where a small amount of water was an issue in an otherwise well maintained aircraft. The accidents that come to mind, and one that I have witnessed, were caused by fuelling with pure water or other extreme scenarios.

[pelmet]

I can't recall any accidents where a small amount of water was an issue in an otherwise well maintained aircraft. The accidents that come to mind, and one that I have witnessed, were caused by fuelling with pure water or other extreme scenarios.

I would be willing to bet that there have been a significant number of accidents where contamination was caused by leaky fuel caps.

A likely case is this one…..

https://data.ntsb.gov/carol-repgen/api/ ... 193736/pdf

https://www.youtube.com/watch?v=VGtK4AlCOP0