Pa28 - preflight actions - change to fuller tank
 

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?

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.

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.

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!

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.

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.

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.

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.

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.

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.

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.

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.

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 . . .

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...

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.

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.

He should have.

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

At least he survived it.

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

Indeed I could - but I won't :)