Fuel selector procedure prior to TO
 

The procedure I'm aware of is to start on the lowest tank and change to the fullest tank prior to power checks. Having witnessed how long it takes fuel to run through the lines it's always concerned me that water may get through to the engine at the most critical time just after take off. Do others follow this procedure or do you tend to switch tanks earlier?

That's the procedure I use; it's an interesting point - it would be enlightening to try turning the fuel off and doing power checks on any given aeroplane. If the engine stops before the checks are done, you have your answer, if it doesn't, you also have your answer.

G

I've always considered this a pretty dangerous procedure and don't do it. Not only might you have water in there (although you should have checked for that), if there's any obstruction in the line from the tank you just switched to it'll manifest itself at the worst possible moment - just after becoming airborne. If you absolutely feel you have to ensure that both tanks feed to the engine, then start on one, taxi halfway to the runup and then switch.

It's also a very good idea to know how long the engine will run on YOUR airplane if the fuel selector is switched to off. Gives you an idea how long you need to run the engine to ensure fuel flowing the way it should.

It depends on the design of your fuel system. I would be very surprised if there are any common carburated aircraft where just the the fuel in the lines would be sufficient to conduct a complete set of runup checks, the pretakeoff checks and a takeoff. You will IMO certainly be drawing actual fuel from the selected tank if it were selected before the start of the runup.

However many fuel injected airplanes including virtually all of the high wing Cessna's (including new build C172 R and S models) have a 1 gal or so collector tank between the engine and the fuel tanks. The selector valve determines which tank will feed the collector tank. The one gal of fuel is more than enough to get you through a runup and into the air before it would be depleted. Selecting each tank with this style of fuel system as a ground check is meaningless as you will always first have to use all of the collector tank fuel.

This is interesting in that firstly, it presupposes that the standard checks for water (drains - and on some 172s an awful lot of them) are not 100% succesful. Does anybody have experience that is the case ?

I have never read it in an accident report (I ain't perfect of course) other than on one occasion when there was so much water it completely filled the fuel tester and thus with a single amorphous liquid fooled the guy testing into thinking it was fuel.

Secondly, as a principle it can be applied differently to different aeroplanes. A PA-28 can only be run on left or right. A 172 (for example) can be right / left or both.

Typically I have been running the 172 on both right through taxi, test and tak-off, however (as an engineer) I am well aware that does not mean (head losses in hydraulic systems etc.) that the engine is always drawing half its demand from each tank. Thus if my "firstly" can give rise to a problem, the manifestation of the problem would depend on when the engine started to draw from the tank with water in it.

If the aeroplane had been left with the selector on both and was across a slight slope, that could well give a gallon or two more in one side that the other. If it was the lesser side with the water in, it could well be quite some time until that water got to the important bits. Murphy says that would be at a Bad Time.

Hmmmm. does this give cause for concern ? Discuss.

Testing a rebuilt Auster J5L with a Gypsy Major engine I did a standard fuel test during run-ups - all fine. Took off, got airborne, discovered halfway through the sortie that the fuel tap had been mis-assembled and either way was drawing from the same tank only. My plan to use 60% of available fuel on the sortie suddenly was rather inadequately conservative!

Landed with about half a gallon of fuel left on board, and a bad mood.

G

Okavango, it may be an obvious question, but what does the POH say?

I would expect the authors of that document would have taken your worries and the actual layout of the fuel system (how many tanks, length/diameter of fuel piping, collector tank or not, typical fuel usage during taxi/runup, ...) into account and written their procedures accordingly.

The PA28-161 Cadet POH (1989 edition) I have here for instance only specifies that before starting the fuel selector should be "moved to the desired tank" (without specifying what the desired tank is or should be), and before takeoff that you should "check the fuel selector to make sure it is on the proper tank (fullest)".

There is no mention anywhere of the practice of starting on the least full tank and switching to the fullest before the runup.

This is why you should check the color of the liquid - just hold the fuel tester against a white surface - if it's blue, it's 100LL, if it's transparent then you either have water or JetA :eek:

As most airplanes are either white or have a white surface somewhere, the above is pretty easy to accomplish.

I cannot get my head round this thing that water can be mistaken for fuel, even if it is all water. Emptying the sample on concrete or tarmac or your hand in the absence of either it is obvious. Water collects in globules, fuel does not. Water has "surface tension"? fuel does not. Fuel smells awful & tastes worse. :yuk:

Regarding POH thread - similar to the text given and in my experience none have been specific - the changing of tanks in the manner described just seems to be a generally followed practice though.

On discussing water, I'm in agreement on the fuel testing and would also be interested to hear of anyones experience of this not being fully conclusive (I have heard of ice forming in the tanks overnight then subsequently melting during flight causing an issue). Also remember however, it could also be some dirt or debris that might cause difficulty.

Except that the envirofundamentalists won't like that and will squeal "Polluter - you'll kill off all the water voles / natterjack toads / water beetles......"

Whereas an EFATO and accident following water in the fuel wouldn't?

Always do a water check - and the colour is readily observable against a white checklist. I've seen a fuel tester fill with water only once (which the student brought to my attention, good lad!) and have no idea how it could have happened - the aeroplane had been hangared overnight and it hadn't rained for several days.....

...or the new colourless UL91. If you are not sure about the difference between water and colourless fuel I would recommend to fill a fuel tester with water and notice the difference of smell, surface tension and the feeling on the skin. A JetA fuel tester filled with water will still smell JetA though :ooh:

I have mainly run on Mogas - and the colour can vary quite a lot.

I have had one instance where the 'standard' check of the tanks and gascolator did not show water but during subsequent power checks the engine stumbled. It recovered but I was not feeling happy and so tried all the usual checks - ending with draining the gascolator again and removing a lot of water. So it can happen. All of this caution was because the strip I was based on was a little 'dificult' with pronouned slopes and also obstructions at one end. So power checks were pretty important.

My present aircraft will only run for about a minute with the fuel cock shut. The Terrier I was flying when the incident above occured, would run for long enough to taxi to the strip, carry out power checks and then get a couple of hundred metres before dying. So one of the things I do with each aircraft I operate is try this and then bear it in mind.

if your getting a full tester please get a engineer to check the aircraft before flying. The have been one noteable engine failure in Scotland when an instructor pulled multiple full samples out then went flying.

Also as well there is no point testing if you have moved it or refueled it within the last hour.

Its pretty obvious when you have a mixture of water and fuel its not so simple when you have an old plastic tester and full of either. You can get testers which we use on Jet A which is a syringe with a plastic thing which goes black on the top if water is present when you suck through it.

I have only found water in fuel in any qty on two occasions in 25 years. In both cases it was a lot of water and the colour was slightly lighter than normal. Now I run mostly on mogas with the odd bit of Avgas. I would not recommend the colour method – pour it over your fingers if you are uncertain and it is instantly obvious. Blocked drains, allowing the area around the inset filler cap to fill with water on an AA5 with perished seals can get you 20L plus of water in one tank after two weeks of rain. If the water has been in the tank for some time the colour difference is less.
I taxi and take off on the tank with the most fuel in, get to cruse altitude and then switch tanks. If the fuel pressure drops unduly I switch back and return to base. This reduces the likelihood of EFATO which is much more of an issue than a drop in pressure at 3k feet with an airfield nearby. I also carry out a how long does it take for the engine to miss test. Results vary a lot from aircraft to aircraft. Many aerobatic aircraft have header tanks containing significant amounts of fuel which changes the game significantly as you can get airborne with the fuel switched off from engine start.

LAA engineering considers most aircraft issues to be fuel or firewall forward. Understanding your fuel system, which may be much more complex than it first appears, is vital to understanding what is going on.

Rod1

This topic was well covered in a previous thread - fill your boots!

Pa28 - preflight actions - change to fuller tank

1. Back to my original question.... has anybody ever found that despite the normal drain checks for water ... there was water that somehow escaped being found ?

(unless of course you had just refuelled or otherwise agitated the fuel in the tanks)

2. I appreciate that Avgas should be blue and smell of petrol and evaporate when poured on the skin and so on and despite everybody thinking it couldn't happen to me ... it did to somebody ... I just cannot find the report at present.

I hadn't thought about Mogas (which in my mower is a pale straw colour) and other colourless inflammable liquids.

I reckon a fuel tester that has had diesel in and the diesel is replaced with water, will still stink of diesel.

I think the forced landing out of CUM had the pilots draining water out until it was clean and then the engine still quit.

Don't know if the report is out yet.

Dave Gittens: An old trick used when you are unsure whether the sampler tube is full of water instead of fuel is to put some of your SPIT in it. If its fuel it should sink - with water it floats on the surface!

:ok: and of course it works irrespective of the colour of the fuel. Suck a smartie first and it'll be even more obvious.