Pa28 - preflight actions - change to fuller tank

[PompeyPaul]

I've often wondered about that in the preflight checks. It seems like quite a dangerous thing to do but there's obviously a good reason for it.

Given you've just taxied out on a fuel tank surely you have reasonable confidence that it's all working (free from blockages, no water in the fuel) and so it's safest to take off on that tank. Then change to the fuller fuel tank when at cruise altitude. That way if there is an issue with the fuller fuel tank you can change back in good time.

The change to fuller tanks seems to me to make it more likely to have an engine failure at one of the most critical times in a flight, during take off :ouch:

So why is it advised to do it as an essential pre-take off action?

[Wildpilot]

The idea is to taxi or warm up on one tank (least full) then change tanks to (most full) then do the run up then toff on the (most full) tank that you have just tested.

Then both tanks are tested and you have not interfered with the fuel selector directly before take off.

And no I have not consulted any checklist/poh it is a procedure that is quite common, last plane I used to use it in was the Beaver when only using the front and centre tank.

[sasben]

I may taxi on one tank and switch before the run-up. I stay on the tank I did the run-up with until I hit cruise.

[amishtechie]

Sasben has the right idea.

Pre-flight you must ensure that the fuel from both tanks is available. otherwise the first inflight tank change could get exciting.

Buy switching before the runups you are ensuring that the fuel in the line is burnt and that you are now drawing from the other tank.

As a side note, if you want to know how long the fuel in the line will last try turning the selector off on the ground. At idle you will find it can run up to a minute!

[IO540]

Yes; it is very important to do the fuel tank change well before departure, to ensure that the fuel flow from the last-selected tank is "proved good" before takeoff.

If for some reason you have forgotten to change tanks during preflight, it is safer to depart that way, and change them when at a safe altitude and over some fields :)

I also don't change tanks in flight when over water.

The issue is not just fuel; you can introduce a lot of water into the engine, if the aircraft owner has not bothered to keep the filler cap o-ring seals in a good condition, and there has been heavy rain. This is a very common problem. I have not seen a single drop of water in 9 years in mine, so "condensation" is not the reason for water in tanks.

[dirkdj]

If you ever run a tank dry in flight and fill it up afterwards, your problems are not over yet, as there may be air in the lines between the fuel selector and the tank outlet. This is the reason besides water in the fuel, to start and taxi on one tank and do the runup on the other tank (if you have two).

A friend of mine flew about one hour to a field because the local pump was U/S, ran one tank dry in the air at safe altitude, then filled up and never used this tank again until the third flight after the 'event' .He then switched to the 'fullest tank' ON THE RUNWAY just before takeoff just like it said in the checklist on the panel. There was still air in the fuel line between the tank and the fuel selector, the engine ran just long enough to stop at about 100 ft, but not long enough to put him in the houses at the end of the runway. Six months in hospital and several years before he could walk unaided again. Total loss on the aircraft.

I know from flying this aircraft earlier that it takes about 30 seconds easlly before the engine will restart after running a tank dry in the air.

[Johnm]

The key is to change tanks before the run up having taxied. The checks undertaken while the engine is running at higher revs plus the taxi to line up is then enough to demonstrate fuel flow from both tanks. It's wise then to change tanks about every half hour to stop the aircraft getting out of balance, or every 15 mins on a short flight.

[BackPacker]

I'll be darned. I just checked the PA28 POH I have here and it instructs you to change to the proper (fullest) tank *after* having done the runup. That sounds indeed like a dangerous practice to me.

Changing to the fullest tank *before* the runup, of course, makes perfect sense.

[LeeP-PA28]

Certainly on my checklists, you start with the lowest tank, taxi, then before the power run, brakes on, set 1200, fuel pump on (check pressure), change to next tank, fuel pump off (check pressure), set 2000, carb heat on / off, mag checks and check idle. Then continue to hold and do pre-take off config.

Ensures sufficient fuel flow from both tanks and at high RPM too - by the time you're lining up you have typically done 2 - 3 mins on each tank.

[Intercepted]

Just another example of why we sometimes have to "think" rather than blindly trust what is written down in a manual. As a pilot you should never do anything without understanding in detail why you are doing it.

[Tarq57]

I can report from experience that the carb on an O-540, as used in the PA28-235, holds enough gas for a three minute ground run, a normal runup, and the beginning of a backtrack, having been cleared for take-off. Halfway through the backtrack the engine died. The fuel selector was off.

I would not be inclined to trust that the time spent on a run up guarantees the tank selected is providing viable fuel, unless this can be ascertained by experimenting with actual fuel duration by testing this procedure with the selector in the off position.

It is probably safe. Carbs are of differing sizes, in different engines. Obviously the float chamber in an 0-540 is likely to be a fair bit bigger than that in an 0-320 or 360. But then, the fuel draw will be a bit higher in the larger engine, too.

Test it. Don't rely on an unproven figure. I would not be inclined to change tanks prior to runup/departure just to ensure that they were all feeding correctly, unless that test was comprehensive, and indicated.

[p1andy]

On my checklist you start on the lowest tank, taxi out, apply brakes, set 1200 rpm, change to fuller tank then do the run up to 2000rpm on the fuller tank

[Conventional Gear]

On my checklist you start on the lowest tank, taxi out, apply brakes, set 1200 rpm, change to fuller tank then do the run up to 2000rpm on the fuller tank

That is what it says on my checklist too.

[znww5]

Same here, but I think Tarq57 has a very good point. I doubt that the standard run-up would prove that the other tank was viable as I suspect a typical power check probably takes 40 seconds or so?
Food for thought . . .

[what next]

Food for thought . . .

More food for thought: I looked into the german manual of the Pa28 Arrow (the officially translated one that comes with the aircraft when you buy it). It says "fullest tank" before starting the engine. And then nothing (apart from "switch tanks every 30 minutes in flight"). Nor do all the checklists I have ever seen for various Pa28s that I have flown over the last twenty years. Myself, I never did that switching of tanks on the ground nor have I ever heard of anyone doing it.

Now it would be interesting to compare the singe-tank-fuel-starvation related accident statistics between german and UK Pa28s...

[IO540]

I know from flying this aircraft earlier that it takes about 30 seconds easlly before the engine will restart after running a tank dry in the air.

According to an article by John Deakin, where he mentiones some regs IIRC, the above would not meet current certification for SE.

I just checked the PA28 POH I have here and it instructs you to change to the proper (fullest) tank *after* having done the runup. That sounds indeed like a dangerous practice to me.

Yes; extremely stupid.

[dirkdj]

IO,

the aircraft in question was a Mooney 231. A Moneypit would be a better name.

The way to get the air out of the fuelline is to use the fuel drain and switch tanks while draining, not something you would do in the air.

My friend tried to restart the engine with the starter, 100ft above the runway and dropping. The priority should be to make a forced landing on the remaining runway, push nose down, lower gear and flaps if time permits and leave the engine alone.

[Flyin'Dutch']

My friend tried to restart the engine with the starter, 100ft above the runway and dropping. The priority should be to make a forced landing on the remaining runway, push nose down, lower gear and flaps if time permits and leave the engine alone.

He should have.

However it is not always easy to be wise during the event.

At least he survived it.

[Sir George Cayley]

Or you can ditch the Piper with its idiosyncratic fuel selector and electric pump and fly a much simpler fuel system as found in Cessnas:E

Frank (Clydes friend) Cayley

[znww5]

Indeed I could - but I won't :)

[Big Pistons Forever]

If you ever run a tank dry in flight and fill it up afterwards, your problems are not over yet, as there may be air in the lines between the fuel selector and the tank outlet. This is the reason besides water in the fuel, to start and taxi on one tank and do the runup on the other tank (if you have two).

A friend of mine flew about one hour to a field because the local pump was U/S, ran one tank dry in the air at safe altitude, then filled up and never used this tank again until the third flight after the 'event' .He then switched to the 'fullest tank' ON THE RUNWAY just before takeoff just like it said in the checklist on the panel. There was still air in the fuel line between the tank and the fuel selector, the engine ran just long enough to stop at about 100 ft, but not long enough to put him in the houses at the end of the runway. Six months in hospital and several years before he could walk unaided again. Total loss on the aircraft.

I know from flying this aircraft earlier that it takes about 30 seconds easlly before the engine will restart after running a tank dry in the air.

If the aircraft is a Mooney 231 then it would have had a Continental TSIO 360
which is a fuel injected engine. Fuel injected engines simply can never be allowed to run dry as they may prove to be impossible to restart.

A simple carburated engine will not have the same problem and for aircraft which do not have a "both" position for the fuel selector running one tank dry should be considered if you are getting tight on fuel. It is better to have 3 gallons (ie half an hour) in one tank then 1.5 gallons in two tanks. Considering very low levels are not going to show on the fuel gauge running one tank to 1/4 and then running the other dry will leave you with a known and useful level with to land on. Obviously this should be very much the exception (I have only had to do this once so far) but it is something you should keep in your back pocket.

The procedure is when the engine dies, retard the throttle to idle, mixture full rich, switch tanks and boost pump on if fitted. The engine will pick up immediately and then you can smoothly increase to cruise power and re lean.

[fireflybob]

It's good airmanship to test both tanks feed ok before flight.

If you switch tanks on the ground before take off you must do a power check afterwards! If anyone doubts this try turning the fuel off at 1,200 rpm on the ground and time how long the engine will running before quitting then do the same but immediately after you've turned the fuel off do a power check. In the latter case the engine will quit within a fews seconds of turning the fuel off compared to at least a minute or so when the engine is at 1,200 rpm.

Some checklists that are around are full of holes because they are someone's opinion on how the aircraft should be operated - if in doubt consult the Flight Manual/POH.

The problem with fuel tank selection arises (usually) when the a/c is on level ground when you decide to select the "least tank" prior to start. You then taxi out and park the a/c on level ground for the runup and then many checklists say select "fullest tank" - you then look at the fuel guages and you are already (apparently) on the fullest tank! It would be better if the checklist just said "select opposite tank". Then it would be better in the pre take off check if the checklist said "Fuel - NOT off" etc

[IO540]

Fuel injected engines simply can never be allowed to run dry as they may prove to be impossible to restart.

Why would that be?

If an engine can be started on the ground then it can surely be started in the air.

If one slowed down enough to stop it windmilling then yes it could be a sod to restart... the starter motor would be needed, obviously.

Has anybody else noticed the "quote" button missing?

Sorry for the thread drift but (being in Greece on 3G) I have just opened up a VPN to my office and using that, the problem disappears! Looks like the Greek 3G network (COSMOTE) is eating some bits of pprune's HTML :)

[upupintheair]

After checking both engines, during start-up, taxi and run-up, why doesn't a PA28 have an option of using both fuel tanks at once..? so they will run out equally? does anyone know? is it cheaper or easier to manufacture?

[IO540]

Because it has only one engine? :)

It is a good question though... no idea why.

[stevelup]

If there was a leak in the left fuel tank and you have 'both' selected, you run the risk of losing all your fuel. Maybe that's the reason?

[mad_jock]

I did ask an engineer about that had he seemed to think it was something to do with the low wing tanks and the pressure head on the fuel line.

Cessna high wing types types the pressure head is enough that nothing funny starts happening when you have them combined into a single feed.

[UV]

I can report from experience that the carb on an O-540, as used in the PA28-235, holds enough gas for a three minute ground run, a normal runup, and the beginning of a backtrack, having been cleared for take-off. Halfway through the backtrack the engine died. The fuel selector was off.


Then there is something wrong with your aircraft.
When we do C of A air tests it is a requirement to time how long it takes for the engine to stop using 1200 rpm with the fuel selected Off.
From memory if it is more than 2 minutes then it is a reportable defect.

[Intercepted]

Then there is something wrong with your aircraft.
When we do C of A air tests it is a requirement to time how long it takes for the engine to stop using 1200 rpm with the fuel selected Off.
From memory if it is more than 2 minutes then it is a reportable defect.

In that case I would say that most (possibly all) carbureted PA28s will fail a C of A air test and be deemed unairworthy. :hmm:

[Genghis the Engineer]

The PA28 with an entirely pumped fuel system (no gravity feed unlike, say, a C172) can't be allowed to draw fuel from both tanks at once because if you get to the point that one is empty and the other part full, the pump will try and draw the least viscous fluid available - air. So, you'll get a fuel starvation engine failure whilst there's still fuel remaining.

So, only gravity fed systems are allowed to draw from multiple tanks at once.


UV - I'm fascinated by your statement that you are looking for maximum 2 minutes from a fuel system after shutting the fuel off. I can imagine a type specific minimum for something aerobatic that deliberately uses a large fuel line capacity to ensure a continuous fuel supply whilst in negative g - the Bulldog is like that for example. However, I'm not aware of any airworthiness standard that mandates such a thing, notwithstanding that what you describe seems quite sensible.

G

[mad_jock]

This is theory!!!

Its type dependent Genhis and its written into some aircrafts maintence scheduals.

Basically its to test that the selector valve is working so it does actually shut the fuel off from going through the fire wall. I think its for only some valve types depending if the off position can fail and still allow fuel through.

I think some of them the selector operates a left and right flow or both and when you turn it off its a seperate off valve that closes or you have a three port left, right and blank shut off.

BUt it could be of course to do with your good point that a none gravity feed engine will stop sucking so it won't keep fuel going through the fire wall if you kill the engine.

Its only a theory its been years since I did one for a SEP and i can remeber checking the valve but can't remember if we timed it.

[Pull what]

The reason there are two tanks with two seperate feeds are the same as why there are two magnetos and two sets of spark plugs-duplicity reduces risk.

[UV]

So, only gravity fed systems are allowed to draw from multiple tanks at once.

I wonder why the Super Cub has Left, Right and Off only?
Could it be a Piper thing?

[mad_jock]

There isn't two sets of feeds through the fire wall only one.

And the only time I have seen anything useful done with tank selections was in the US where its quite common to have one tank full of mogas for cruising around on and one full of avgas for departure and landing.

[IO540]

No; you use the fuel selector to draw fuel from one tank preferentially to balance the aircraft if flying with an odd number of people.

A plane will deliver better MPG, and will be easier to fly in terms of having neutral aileron forces, if everything is balanced, ailerons balanced, ball in the middle, and that usually requires the LH tank to have less fuel. Obviously you need accurate fuel gauges to do that :)

[Genghis the Engineer]

I wonder why the Super Cub has Left, Right and Off only?
Could it be a Piper thing?

A while since I flew one, but it may be that it also has a pump in line - which would prohibit a "both" setting, or could just be that the design team preferred it that way or it was cheapest to use the same valve they were already buying for PA28s.

G

[mrmum]

A Commander 114 I fly has a "both" option on it's fuel selector, as well as left, right and off. It's a low-wing. Don't have the POH to hand so can't look at the specific design of it's fuel system or any instructions regarding when to use which selector position.

[Pilot DAR]

why doesn't a PA28 have an option of using both fuel tanks at once..? so they will run out equally?

The answeer to this question lies in the design requirements which says:

Airspaces of tanks with interconnected outlets must be interconnected;

and;

There may be no undrainable points in any vent line where moisture can accumulate with the airplane in either the ground or level flight attitudes

In combination, these requirmeents mean that if you want to have a "Both" fuel selector option, the airspaces of the fuel tanks, (outboard ends of tanks on the PA28) must be interconnected, and the interconnecting vent line must not have an undrainable spot. This would mean that the vent line would have to come up out of the wing, and continue above the wing, right through the middle of the cabin, because of the dihedral of th wing making the outboard ends of the tanks higher than the inboard ends. .

This design requirement is the reason that you nearly never see aircraft with low wing fuel tanks with a "both" fuel selection available.

High wing aircraft usually have the fuel vent line cross up against the inside of the cabin roof, so as to meet this requirement, and allow a "both" selection. Though there are some high wing aircraft which do not have "both" (some Cessna 206 and 210).

[Big Pistons Forever]

Fuel injected engines simply can never be allowed to run dry as they may prove to be impossible to restart.

Why would that be?

If an engine can be started on the ground then it can surely be started in the air.



A fuel injected engine is stopped on the ground by turning off the fuel at the FCU. If you run a tank dry then there will be air from the tank pick up, fuel lines through the fuel pumps, FCU,and fuel spider. The danger is that an air lock can occur which will prevent fuel from reaching the engine. A similar situation can be experienced trying to start a heat soaked engine on a hot day.

re; the no both selection for low wing aircraft, avoiding giving giving the fuel pump the choice of fuel or air is most easily done by positively separating the tanks. As an aside my low wing Nanchang CJ6A has two wing tanks but only a fuel on off valve. It however, has a small centerline collector tank that is lower than both wing tanks and from which the engine draws fuel.

[Pilot DAR]

Nanchang CJ6A has two wing tanks but only a fuel on off valve

... and perhaps is not type certified to CAR 3, FAR 23, or their equivilents?

The danger is that an air lock can occur which will prevent fuel from reaching the engine. A similar situation can be experienced trying to start a heat soaked engine on a hot day.

Don't be too worried about this, the design requirement:


§ 3.429 General. The fuel system shall be constructed and arranged in a manner to assure the provision of fuel to each engine at a flow rate and pressure adequate for proper engine functioning under all normal conditions of operation, including all maneuvers and acrobatics for which the airplane is intended.

.....Would assure that the engine would restart, as being stopped from a dry tank would be a "normal condition" for the purposes of certification. If an engine will not restart from a dry tank, the plane will not be approved.

This is a reason why some injected types (IO-520 and 550) are purged with the fuel pump prior to starting. If vapour has formed in the injection lines while it sat hot, that vapour is purged. Same thing would happen if you'd run a tank dry, or turned the fuel off.

[Big Pistons Forever]

Pilot DAR

I guess I was not very clear in my comment re the Nanchang CJ6. I was agreeing with your comment in an earlier post with respect to ensuring that the engine took fuel from a source that was fully vented under all conditions and that there could never be a situation where the engine fuel pump could air rather than fuel from a tank as long as there was fuel available. Cessna solves this problem by having a gravity feed with a wing side to side interconnected vent system. The Nanchang designers decided to use a centre collector tank mounted below the level of the wing tanks to achieve the same thing. I am not sure what certification standards have to do with this as it is a Military trainer and thus the Chinese air force were not interested in US civil certification. In any case most (all ?) high wing Cessna's with fuel injected engines use the same system for the same reason.

As for your comment about certification standards ensuring a fuel starved, fuel injected engine could be restarted under any circumstances.....well I will bow to your superior knowledge of certification standards. The reason I made the comment was because the C340 that I occasionally fly has a POH note about the possibility of no restart if the engine is fed from a dry tank, and I have seen this caution in other POH's. Perhaps fuel starving a fuel injected engine doesn't come under the definition of "normal operations" that you quoted ?