The International Cessna^® 170 Association

An email from AOPA states: "You know that you should check your new fuel load for water or other contaminants. But doing so immediately after refueling may not work. How long should you wait? "According to industry standards, the minimum safe settling time for aviation fuel is 15 minutes per foot of depth of water. Therefore, if your aircraft has fuel tanks that are 6 inches in depth, the minimum wait time should be 7.5 minutes,"

If that is so, then we should consider the distance from the filler neck to the bottom of the tank...and the distance to the wing-sump point. Our three-point stance would likely lead us to the conclusion that about 30 minutes might transpire before all the water that might be delivered might arrive at the drain in the wing tank.
This might explain why I've sampled fuel and found no water, then sampled again several hours later and found some.

Additionally, if one goes flying, and the inside walls of the tanks collect water from in-flight condensation (the cool, moist air entering the tanks to replace fuel consumed may deposit water droplets upon the interior walls at the ullage, which fuel then sloshes down into the fuel)... it's possible to sample fuel during pre-flight and find nothing...then sample fuel after a flight and find water. That fuel load you just took on may not be the source of water found subsequent to the flight...depending upon when you sampled the fuel.
Just something to think about.

George, I'm astounded you didn't know that you needed to wait for the water to settle out of the fuel before you would be able to see it while sampling. All the information that flows out of your head and that little piece of info never made it in there.

I learned this in the Army where I seem to remember it would take about 2 1/2 hours for water to completely settle out of JP-4 in a UH-1H tank.

I shake my head and laugh when I see people fill their tanks, usually with enough pressure to thoroughly mix any water not already mixed into the fuel, then immediately take a fuel sample. It's a wast of time.

Another time sampling fuel is a waste of time and fuel is when your aircraft with the fuel in it has been sitting outside for sometime in temperatures under freezing. Frozen water doesn't flow out the valve.

N9149A wrote:George, I'm astounded you didn't know that you needed to wait for the water to settle out of the fuel before you would be able to see it while sampling. All the information that flows out of your head and that little piece of info never made it in there. ....

Oh, no you don't....you're not gonna get me THAT easy, Bruce. You've made an incorrect assumption or suffered a memory failure. I never indicated in the post above that I was unaware of the time necessary for water to settle.

It was a couple of years ago (2005) when I discussed this issue at : http://cessna170.org/phpBB2/viewtopic.p ... ater+drain where I said, "...When refueling, it can take almost an hour and/or over 16 ounces (about a pint) of fuel to be drained from the gascolator before water in the new fuel reaches our fuel sampling cups. Even more might have to be drained to observe it in the cup itself, and even then the water wouldn't have been purged from the fuel system....."

and : " if you pour a quart of water into your fuel tank..... it will take about 30 minutes and draining of almost a quart of fuel....before you even see a trace of water at the gascolator..... and EVEN THEN...only if you've somehow LEVELLED the airplane!!! (The 170 fuel tanks drain fuel from the center of the tank...not the rear, where the wing sumps are located.) The wing sumps may not show water for several minutes, and the gascolator will not show water until the airplane is levelled sufficiently to allow the water to find the fuel line leading to the gascolator."

Based on the back & forth on the subject of water in the fuel, it would seem to make sense to fuel first at fuel stops, rather than get lunch, take a break and then fuel before departing. After three and one half hours in the airplane most of us are more interested in getting out of the airplane, into and air conditioned building and putting something in our stomachs than taking the additional time to fuel first. I think I will make it a priority to fuel first from now on and sump when I come back to the airplane.

Bruce I strongly disagree with your coment that sumping fuel imediately after fueling is a waste of time. I did an emergency landing one day in all liklyhood because of my failure to do just that. My 170 ragwing sat out in the rain for the first time after I purchased it and I found water in the sump of one wing. I shook the wings and got a little more out and then proceded to take off on a flight of 3 hours. The plane ran fine and I landed for fuel. Wind was blowing about 25 knts and temp was in the upper 20s. Did a quick turn and headed for home. About two hrs latter the engine quite like I shut the mag switch off with no warning. I could not get it started and managed to land at a small grass strip on a 1400ft runway in DE. Upon looking into the problem I drain a large amount of water from the gascolator and a small amoun from the wing sump. Thinking ther might have been ice in the system I had the FBO put the plane in a heatted hangar for the night and check the carberator fuel bowl for water and some was found there. Then we flew home and found more water in the gascolator. We then pulled the sumps and completely drained the tanks. Flew the plane again and still found a few drops in the gascolator. That continued for several flights and then the samples completely cleared up and I have not seen water for the last ten years. I have concluded that just because there is water in the tank does not mean it will find its way to the sump. I took a lot more water out of the gascolator then I did out of the sumps. The moral to the story as I see it is if you find water in your tanks BE VERY CAREFUL AND YOU CAN NT SUMP TOO OFTEN EVEN IF YOU JUST FUELED UP. Although I haven"t done it yet I feel that fuel cap mod for the umbrella type caps are an excelent mod for the rag wings. Bill K

Bill, while I am not disagreeing with someone who sumps the tanks immediately after fueling (presumeing they are immediately going flying), I do not see how your msg persuades us that sumping immediately after fueling is somehow more beneficial than waiting a while (as you seem to imply.)
Your experience seems to tell us your airplane was already thoroughly contaminated with water in it's fuel. You found a lot of it, and continued to find a lot of it for quite some time. The story does not prove that fuel purchased during the trip had any water in it. The re-fueling action during the stop likely kept existing water stirred up and it's unknown whether you 1-sumped following the refueling or 2- that such re-sumping immediately after refueling would have produced any more water or 3- that such actions would have removed all the pre-existing water and/or prevented the emergency landing which later occured.
While it's never wrong to sump the fuel immediately before flight, it is less convincing that sumping immediately after refueling provides any real protection against failure due to water in the fuel. IMHO

Bruce stated that "sumping fuel immediately after fueling is a waste of time." That is what I was in disagreement with. If you plan on departing immediately it is still a good idea to sump your tanks and gascolator. I do not believe nor did I say that it would not be better to wait half an hour. From my experience I believe that our planes and particularly ragwings can carry quite a bit of water in the tanks that does not come out when you sump the tanks. That water does come out of the gascolator hopefully you don't get so much in it that it chokes the fuel flow to the engine. Bottom line if you must depart immediately after fueling as I see many pilots do it is definitely not a waste of time to sump your tanks and your gascolator. If I had sumped my gascolator and tanks at my fuel stop I believe I would have found quite a bit of water in the gascolator and a little in the sump. Probably would have prevented my problem that is my point. We just lost a 170 recently and the NTSB site indicates contaminated fuel as a likly cause that is why this is an important issue. Any one who believes that sumping your tanks will remove all water regaurdless of how long the plane has set needs to rethink that philosophy. That said longer is definitely better. I dont know if this is true but I was told by an older pilot that if I had pulled the nose of the plane up the water in the carberator would have rolled to the back of the bowl and allowed fuel to reach the jet and alowwed the engine to continue to run as long as the plane was in a climb attitude allowing the plane to climb until it reached an airport and then do a power off landing. Not sure if there is anything to that but if I were ever to get in that situation again and I had a little altitude to spare I might give it a try but I would not risk it close to the ground. Bill K

Bill if you are going to sump your fuel then do it BEFORE you fuel and mix the water you are sumping for into the new fuel your putting in.

The gasolator is a different story and I wasn't thinking of it but thinking of the tanks where the new fuel and water would be mixed. That wouldn't happen in the gasolator just from adding fuel. If you think about it your unlikely to find water in the gasolator that had actually settled out of the fuel that was just inside the gasolator. Water in the gasolator is most likely there because it came from the tanks during flight.

BTW while testing for alcohol in mogas I noticed something curious. When alcohol was present the water/alcohol separated out of the fuel sample nearly immediately while a sample known not to have alcohol took about 5 minutes for the water to separate out. Now I'm in no way advocating using mogas with alcohol to help separate out the water. It's just an observation.

George I'm so glad to hear you knew previously about the time it takes for water to settle out of fuel. I'm just getting over the thought that you might not have known. All is well.

I see your point better after reading that response, Bill. I can now better see what you were thinking.
I believe it's a good idea to sump before every flight, as long a period as possible after re-fueling. I can also see Bruce's logic in sumping before re-fueling ...if you are concerned about water already existing in the present fuel load. As fuel passes by the gascolator bowl, any water present is given an opportunity to drop out and settle into the lower recess of that bowl. The gascolator is a reservoir for precisely that purpose. It is lower even than the carburetor, so that water, if present, must travel up-hill in order to arrive at the carburetor under normal flight attitudes. (Water can also accumulate in carburetor bowls even if none was present in the fuel. How? Remember that the carburetor's job is to atomize fuel. That process deeply chills the carburetor body. The carburetor has a bowl that is vented to atmosphere. Although small, the amount of atmosphere that enters may contain moisture which, over time, may accumulate to noticeable amounts. That is one reason (among several) why 100 hour/annual inspections call for the carb bowl to be drained. When draining that bowl, do not re-tighten that plug too much or you will crack the bowl. Remember that very little pressure exists on that plug and it does not need to be tighter than finger-tight plus a 1/16th turn. Then safety-wire it.)
The FAA does not like glass gascolator bowls. I do, however. After a flight, we should all be walking around our airplanes performing a post-flight inspection even if we suspect nothing has occured during the last flight because bird strikes, loose hardware, oil and fuel leaks, and water in a gascolator bowl is best discovered at that time. Although I may not actually drain my gascolator at a post-flight, I do glance at it while looking at the area for oil/fuel leaks, to see it's condition and if noticeable amounts of water has collected there. If I re-fuel I drain it before the next take-off.
Did you guys catch what I am endorsing? Post-flight inspections! They are as important as pre-flights.

Bruce, the separated material you see in the bottom of the glass (after using the alcohol-test procedure) is a mix of water and alcohol. The alcohol has a strong affinity for water and the resulting mix has a higher specific gravity than gasoline so it rapidly sinks to the bottom of the beaker. That's the reason for the test, to take advantage of that reaction.
But that does not mean that alcohol in the fuel is a good thing. It's a bad thing. Why? Many reasons exist, but one is because of this exact behavior....the water may separate out at such a rate that it may overwhelm the gascolator and shut the engine down in flight! Especially at altitude when the temperature of that fuel cools in the pipes on the way to the gascolator. (Not to mention the absolute prohibition of using alcohol in our fuel found in the Type Certificate, page one.)

Yes George you are correct I just hadn't thought why the alcohol-water would separate faster.

Post flights are just the start of your preflight.

And slightly off topic......

gahorn wrote: But that does not mean that alcohol in the fuel is a good thing. It's a bad thing. Why? Many reasons exist, but one is because of this exact behavior....the water may separate out at such a rate that it may overwhelm the gascolator and shut the engine down in flight! Especially at altitude when the temperature of that fuel cools in the pipes on the way to the gascolator. (Not to mention the absolute prohibition of using alcohol in our fuel found in the Type Certificate, page one.)

I do not disagree with the above statement and have stopped using mogas at the present time because mogas does not exist in my area without alcohol.

But I can't help but wonder what we are going to think in the future (maybe not to distant) when avgas is $20 a gallon and not readily available and E85 is available on every corner. I also wonder how those airplanes that are currently STC'd for E85 fuel overcome these problems.

Just wondering..........

I don't know much about how long water takes to settle in the sumps, but several years ago Steve Grimsley and I were flying back to the Fort Worth/Dallas area from a flyin in central TX. We stopped at Lampasas, TX to fuel up with auto fuel. In the haste to leave, neither of us sumped our tanks. When Steve got home he checked his and then called me to ask if I had done so (which I hadn't). He got all kinds of crude out of his sumps and gasolater. I don't remember how much now, but I am thinking close to a quart. I walked back over to the hangar and checked mine and sure enough I had the same stuff although not as much. Mine was confined to the tank sumps, none in the gasolater. Kept a sample for several years thinking I would get an analysis of it, but never did. Later purchases at that apt were all OK. I assume had we sumped after fueling that we probably would have found the stuff. Good thing was it didn't result in any problems for either of us.