Coordinate that turn to final

[pelmet]

At least when the fuel quantity is very low. Otherwise the fuel may not make it to the engine....

"C-GDZA, a Cessna 172G aircraft operated by FD Air Tours, was conducting a local VFR photo flight from Summerside, PE (CYSU) over the island. After approximately 4 hours and 10 minutes of flight, the aircraft returned for landing at CYSU. In the turn from right base to final approach for Runway 23, the engine lost power. The pilot set up for a short field landing and landed in the grass, approximately 500 feet short of the runway. There were no injuries or damage to the aircraft. The pilot noted that there was 20 litres of fuel in the right tank and the left tank was empty. Once the aircraft was refueled, the engine operated without any issues. The pilot believes that, in the turn, the engine experienced fuel starvation due to the low fuel level."

Of course, the exact details of this accident are unknown but...…….sideways g-loading(or whatever the exact terminology is) can cause problems whether done in flight and on the ground in certain circumstances. A low-wing LSA(non-certified) aircraft that I have been flying apparently can have a power loss during a sideslip if the tanks are not fairly full depending on which wing you lower. Therefore the procedure for a sideslip in such a situation is to push the rudder on the same side as the selected tank resulting in the selected fuel tank not having its fuel feed line unported. Can be a ground issue as well with the Bonanza V35 POH warning that says "turning type takeoffs and takeoff following fast taxi turn prohibited." I assume that is more of a problem when the tanks are not full.

[photofly]

Even better would be not to empty either tank completely, and in the 172 make sure the fuel selector is set to “both” before landing.

[digits_]

Even better would be not to empty either tank complete

Why not? It is the most accurate way to find out how much fuel you have left on long legs.

If you don't do that, due to inaccuracies in the system you could for example end up with 5 liter on each side thinking you have 15 on each side left.

[photofly]

Even better would be not to empty either tank complete

Why not?

One reason: if you do, a crosswind landing to the wrong side will cause the engine to quit due to fuel starvation. It wouldn't be much fun to attempt a go around in strong gusty winds only to find out that the tiny fuel strainer bowl had just reached empty.

A second reason: fuel in two tanks gives you an element of redundancy in case of a blockage or leak. If you deliberately fly for a time with one tank dry you've thrown that benefit away. I don't see there's much return for that loss.

t is the most accurate way to find out how much fuel you have left on long legs.

The most accurate way would be an electronic fuel flow gauge and totalizer.

If you don't do that, due to inaccuracies in the system you could for example end up with 5 liter on each side thinking you have 15 on each side left.

I can't say continuing a flight with even 15 litres (3.9 gallons) on each side, in a 172, thrills me. If I had, exceptionally, to make that length of trip, I wouldn't rely on draining a tank to be sure of how much fuel I had left. There are better ways to calibrate your fuel flow.

Alternately again, if you like to run a tank dry, then in a 172 returning to the "both" setting should allow the levels slowly to equalize, giving you back some redundancy and side-slip capablity to both sides.

[photofly]

In respect of the 172 about which this thread was created, a mandatory placard in view of the pilot include (for the 172M, at least):
"BOTH TANKS ON FOR TAKEOFF AND LANDING"

In section 2 of the POH (Restrictions) it states:
"Takeoff and land with the fuel valve selector handle in the BOTH position."

The BEFORE LANDING checklist includes (item 2):
"Fuel selector valve -- BOTH"

It also says (Section 7):
"The fuel selector valve should be in the BOTH position for takeoff, climb, landing, and maneuvers that involve prolonged slips or skids. Operation from either LEFT or RIGHT tank is reserved for cruising flight."

The pilot could hardly say he wasn't warned.

On the subject of measuring the fuel by flying on one tank: the POH goes on to say:
"It is not practical to measure the time required to consume all of the fuel in one tank, and, after switching to the opposite tank, expect an equal duration from the remain-ing fuel. The airspace in both fuel tanks is interconnected by a vent line and, therefore, some sloshing offuel between tanks can be expected when the tanks are nearly full and the wings are not level."

Recall that when the gauge reads zero, there are still two gallons of fuel in the tank, which is considered not to be "useable in all flight attitudes". Those two gallons remaining in the "empty" tank is your insurance policy against suffering what happened to the pilot of the incident airplane. When you calculate your range, you deliberately *don't* count those last two gallons each side. There's a good reason for that.

I hear the prophesy that "there will be no new causes of aircraft accidents this year" ringing loudly in my ears.

[Zaibatsu]

I hope you don’t fly a Piper Cherokee.

[digits_]

Owkay. When I talk about fuel in the tank, I mean usable fuel. That's what you measure and what you calculate with anyways. Even if you run a tank "dry" on one tank, the unusable amount of fuel will still be in there (unless you do really weird stuff and fly uncoordinated for half an hour or so)

A fuel totalizer might be nice, but if you don't have that in your plane, it won't help you. If you are planning to land with 5 USG at the end of your flight, the pilot should be familiar with an average fuel flow and the actual fuel in the tank. Know any quirks or issue related to the fuel system. If you then switch tanks every half hour, and keep track of fuel consumption, you should be fairly accurate when estimating the amount of fuel left in the tanks. Much more than when you just use the gauges, or keep it running on both the whole time and hope your initial calculation was correct.

Some deductions from the CADORs:
- it was a commercial flight (not a lot of people fly 4 hours locally for fun)
- there was over 5 USG on board the airplane

The pilot was merely flying the plane to it's maximum endurance. It is not determined wether he ran one tank dry intentionally, or if there was a venting issue/weird fuel flow issue that burned off one tank before the other.

Selecting one tank in flight and running it dry is not prohibited by Cessna. If I recall correctly, it is even specified in the POH how long it will take for the engine to start running again after you run it out of fuel. I believe it is under 20 seconds.

You do need to switch to BOTH for take off and landing, so if he didn't do that, then that was a mistake. Other than that, I see no issues with what the pilot did. Even flying uncoordinated should not be a problem at minimum fuel. After all, you should still be able to do a crosswind landing and a a go-around close to min fuel.

[photofly]

Owkay. When I talk about fuel in the tank, I mean usable fuel. That's what you measure and what you calculate with anyways. Even if you run a tank "dry" on one tank, the unusable amount of fuel will still be in there (unless you do really weird stuff and fly uncoordinated for half an hour or so)

That's not correct. You don't understand the Cessna 1XX fuel system, and you don't understand what "unusable fuel" means, either.

Let's look at 14 CFR 23.959:

§ 23.959 Unusable fuel supply.
(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.

The "unusable" fuel isn't unusable - it's "unusable in all flight attitudes" - that is, it cannot be relied upon during all the manoeuvres for which the aircraft is certified. That means that in level flight, it's entirely useable. Where is that "unusable" fuel? It's the last two gallons in the tank. When you run the tank dry in level flight, both gallons of your "unusable" fuel has been used.

Know any quirks or issue related to the fuel system

That's an excellent idea. Let's read the C172M POH, again:

"When an indicator shows an empty tank, approximately 2 gallons remains in a standard tank, and 2 gallons remains in a long range tank, as unusable fuel."

Have you ever drained a Cessna fuel system and replaced a Cessna fuel tank? I have. Let me promise you, there is nowhere in a 172 to hide 2 gallons of "unusable" fuel. Where is it? In plain sight, at the bottom of the tanks. This should be really obvious to anyone who thinks about it, in fact. Picture two gallon jugs of fuel. See how big that is? Where else is it going to be?

Don't run dry tanks.

[Big Pistons Forever]

The CADOR stated that after the aircraft was refueled the engine operated without any issues. I do not see any plausible scenario where with the fuel selector on both and 20 litres of fuel in one tank that the engine would stop due to fuel exhaustion.

The most obvious conclusion is that the pilot had the empty tank selected although of course I do not know this for a fact.

The bottom line is that it is important to follow the POH. With respect to selecting an individual tank the POH is clear that the purpose of the left right selection is to balance the fuel qualities in cruise flight to help keep the aircraft laterally balanced. During any other phase of flight the fuel selctor should be on both

It is also quite clear that the "running one tank dry to know how much fuel you have in the other tank" doesn't work because fuel can move between the tanks i the vent line even if only one tank is selected.

I think it is also important to note that the fuel system in the new C172 R and S models is significantly different than the older models with carburated engines. As a general rule fuel injected engines like the one in the new Cessna C172's, should never be fuel starved as the engine may not start. As an aside I am dismayed at how many FTU's running R and S model 172's have a checklist that specifies selecting each tank as a check before take off, a completely pointless exercise as even a casual glance at the POH fuel system schematic will show.

[digits_]

When the CADOR talked about an empty fuel tank, how do you interpret it then? How did they determine the fuel tank was empty?

Going back to your definition of unusable fuel:

FAA definnition:

§ 23.959 Unusable fuel supply.
(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.

Your interpretation:

The "unusable" fuel isn't unusable - it's "unusable in all flight attitudes" - that is, it cannot be relied upon during all the manoeuvres for which the aircraft is certified. That means that in level flight, it's entirely useable. Where is that "unusable" fuel? It's the last two gallons in the tank. When you run the tank dry in level flight, both gallons of your "unusable" fuel has been used.

This does not mean or imply that unusable fuel can be used in level flight, or even that there is any way at all for the fuel to be used. It only means that they found one attitude at which the fuel is not used.
Granted, it does not mean that it can't be used either. That part is undefined.

Assuming the commonly used dipsticks are correct, the unusable fuel will be hard to register. If I dip a tank and I manage to get the fuel indicated on the dipstick below the zero, I'd say the fuel tank is empty as well, even if it might still have some drops left.

I don't think the 2 USG would be in the fuel lines, merely on the bottom of the tank, mabye stuck in a corner due to an incline in the tank or parking surface, or the level that you have when the fuel drops to below the fuel pickup hose. There might be exceptions, but the fuel line hookup is not supposed to be on the lowes part of the fuel tank, that is where the fuel drains are supposed to be. So in level attitudes, there should be some fuel left below that line hookup. If that is always 2 USG, or 0.1 USG, I don't know.

[digits_]

As an aside I am dismayed at how many FTU's running R and S model 172's have a checklist that specifies selecting each tank as a check before take off, a completely pointless exercise as even a casual glance at the POH fuel system schematic will show.

Why would that be? I've looked at an R model schematic, and it isn't obvious to me.

[photofly]

but the fuel line hookup is not supposed to be on the lowes part of the fuel tank, that is where the fuel drains are supposed to be.

The fuel hookup is within about a half of an inch of depth of the fuel drain, and they are both at the inboard end of the tank. There are two hookups, one at the front and one at the back, so the fuel will feed in both a nose up and nose down attitude. If you can wait an hour, I'll take some photos for you.

This does not mean or imply that unusable fuel can be used in level flight, or even that there is any way at all for the fuel to be used. It only means that they found one attitude at which the fuel is not used.
Granted, it does not mean that it can't be used either. That part is undefined.

Only a muppet would design a fuel system with some fuel that cannot, ever be used.

When you drain a tank your unusable fuel is gone. It's that simple.

When the CADOR talked about an empty fuel tank, how do you interpret it then? How did they determine the fuel tank was empty?

The same way anyone determines an empty tank. You open the drain at the bottom of the tank and no fuel comes out. I don't understand why you would ask such a question.

Other than that, I see no issues with what the pilot did. Even flying uncoordinated should not be a problem at minimum fuel. After all, you should still be able to do a crosswind landing and a a go-around close to min fuel.

I see a big issue with what the pilot did. He ran one tank dry, which is two gallons below "zero fuel". He deliberately ensured that the minimum amount of fuel necessary to do a crosswind landing and a go around wasn't available. Cessna told him what minimum fuel is, which is when the gauge reads zero and there's still two gallons left. That's the point of the gauge reading zero. He used up those two gallons. I don't really know how much more stupid he could have been.

[photofly]

As an aside I am dismayed at how many FTU's running R and S model 172's have a checklist that specifies selecting each tank as a check before take off, a completely pointless exercise as even a casual glance at the POH fuel system schematic will show.

Why would that be? I've looked at an R model schematic, and it isn't obvious to me.

Injected engines are required to have a header tank. The R and S models are no exception. There are about 2 quarts of fuel stored after the fuel selector valve, so unless you idle for about eight minutes on one side, you haven't proved that side main tank is feeding: you're running off the fuel in the header tank, and there's no way to tell if it's refilling from the wing or not.

On my STC'd injected engine the header tank is at floor level in the cabin, behind the pedestal. With the fuel set to "off" there's just enough fuel in the header tank to start up, taxi to the threshold, and take off. Several aircraft with this engine have been lost that way. That's one reason why the fuel shutoff is a different control from the left-both-right control in the R and S model 172s. Less likely to set it to "off" by accident.

[digits_]

This does not mean or imply that unusable fuel can be used in level flight, or even that there is any way at all for the fuel to be used. It only means that they found one attitude at which the fuel is not used.
Granted, it does not mean that it can't be used either. That part is undefined.

Only a muppet would design a fuel system with some fuel that cannot, ever be used.

Seriously?
It's one of those limitations that would cost a lot of money to fix, and just isn't worth it in a small airplane. I believe fighter jets use expensive pressurized fuel bladders to use almost all fuel on board. Kind of overkill to install that on a 172, no? It would be much cheaper to have 2 USG unusable fuel.

When you drain a tank your unusable fuel is gone. It's that simple.

When the CADOR talked about an empty fuel tank, how do you interpret it then? How did they determine the fuel tank was empty?

The same way anyone determines an empty tank. You open the drain at the bottom of the tank and no fuel comes out. I don't understand why you would ask such a question.

How do you determine how much fuel is in a tank? You dip the tanks, that's what you would do before the flight. If I landed/crashed with a suspicion of fuel quantity issues, I'd dip the tanks to see how much fuel I have left. I'm not going to drain all vents in a bucket after landing. It doesn't matter if I find 0.1 or 1 USG in it, it would mean that the tanks were, for all practical purposes, empty.

Other than that, I see no issues with what the pilot did. Even flying uncoordinated should not be a problem at minimum fuel. After all, you should still be able to do a crosswind landing and a a go-around close to min fuel.

I see a big issue with what the pilot did. He ran one tank dry, which is two gallons below "zero fuel". He deliberately ensured that the minimum amount of fuel necessary to do a crosswind landing and a go around wasn't available. Cessna told him what minimum fuel is, which is when the gauge reads zero and there's still two gallons left. He used those two gallons. I don't really know how much more stupid he could have been.

IF he ran a tank dry, that is not prohibited by the POH, as long as he lands with fuel selector in BOTH. Note that this has not been proven yet.
It does not say the pilot needs to keep 2 USG in the tank at all times, it only says that if you try to get fuel out of a tank with less than 2 USG in, you might not get it. If you try, and you don't get it, the engine will restart after switching tanks.

One note I found that is remotely applicable to this, is:

When the fuel tanks are 1/4 full or less, prolonged
uncoordinated flight such as slips or skids can uncover the
fuel tank outlets. Therefore, if operating with one fuel tank
dry or if operating on LEFT or RIGHT tank when 1/4 full or
less, do not allow the airplane to remain in uncoordinated
flight for periods in excess of 30 seconds.

Crosswind landing should still be possible if you don't flare for more than 30 seconds.

[photofly]

You say how important it is to be conversant with the details of an aircraft system, then you spend a lot of time arguing with someone who knows the details of the 172 fuel system.

Here's a photograph of the inboard aft corner of a Cessna fuel bladder. I took it right now, just for you. The outlet at the bottom is for the tank drain; the outlet on the right is where the fuel finger screen inserts. The outlet is arranged so that when the screen sits right at the very bottom of the lowest part of the tank, so the very last drop of fuel can be used. You can, quite literally, run the engine until the tank is dry, as long as the aircraft is in a reasonably level attitude. (There's another finger screen at the front corner, so you can run it dry nose up or nose down.) The dihedral in the wings makes the fuel run to the inboard end on both sides.

IMG_2322.jpg (106.75 KiB) Viewed 2127 times

Not coincidentally, there's no fuel in the tank when the drain is emptied either, The outlet and the drain sit at the same level, which is at the bottom of the tank.

When the fuel gauge reads zero, there's no useable fuel left, per the definition, but there are still two gallons of "unusable" fuel in the tank. If you don't have your unusable fuel left in the tank, you can't do crosswind landings. It's explicit in the very definition of "unusable" fuel that if you don't have it, the engine will quit in some manoeuvre that it shouldn't.

This is not controversial. The guy ran a tank dry and had a lucky escape. Don't run a dry tank in a 172.

[photofly]

Here's an image from the inside of a C182 tank, during installation. This is the forward inboard corner of the right tank. You're looking at the forward finger screen, which is in the wing cavity above the forward corner of the passenger door. The tank is mid install and hasn't been stretched out, hence the wrinkles and sags.

The point to note here is that the screen is right at the bottom corner of the tank, at the lowest point (due to the dihedral.)

Untitled 3.jpg (198.07 KiB) Viewed 2113 times

There is no fuel that cannot be drained into the engine, when the aircraft is in level flight. But two gallons of that fuel are still considered "unusable" and have to be left in the tank to meet the specification not to cause fuel starvation when the aircraft is in some attitude as part of one of its certified manoeuvres.

[photofly]

it only says that if you try to get fuel out of a tank with less than 2 USG in, you might not get it. If you try, and you don't get it, the engine will restart after switching tanks.

How well did switching tanks work for this guy?

Don't run an empty tank in a 172.

[digits_]

it only says that if you try to get fuel out of a tank with less than 2 USG in, you might not get it. If you try, and you don't get it, the engine will restart after switching tanks.

How well did switching tanks work for this guy, when he tried to get the fuel out of the empty tank? Not so well, huh?

Don't run an empty tank in a 172.

Who says that he tried to switch tanks in time? Who says he was running on both or L or R? You don't know that from the CADOR. He might have exceeded the 30 second rule I quoted by crabbing all the way on final.

I have not read any proof here that shows that Cessna does not allow you to run a tank dry. On the contrary, my quote even refers to Cessna saying what would happen if you fly uncoordinated on a dry tank. They never specify or predict what happens if you fly unauthorized manouvers (probably for liability reasons). Ergo, running a tank dry is allowed on a 172.

[photofly]

You’re welcome to run a tank dry if you don’t mind losing the engine when you sideslip. Very much your call, Cessna doesn’t forbid it, so go ahead.

Now if he or she had been running on “BOTH” for any length of time, as the POH required before landing, the fuel would have crossfed into the empty tank, and he wouldn’t have had the problem.

You point out that this was a commercial flight, I would expect a commercial pilot to know his or her fuel system better. Shabby.

[digits_]

You’re welcome to run a tank dry if you don’t mind losing the engine when you sideslip. Very much your call, Cessna doesn’t forbid it, so go ahead.

Only if you sideslip it for more than 30 seconds.