Stalls and fuel level

[skidoo]

I was out training yesterday (182T) and my instructor asks me to raise the attitude from a med cruise power for a stall. So, I get the nose up pretty high. I was about 60+ KIAS when he pointed to the fuel gauge. Both tanks started with about half, but the right one began to show near empty. The left one remained the same. Not sure if it would put air in the lines, so I stopped the power on stall maneuver. I lowered the nose, and the fuel gauge returned to normal.

Later in the day, I checked the parts manual and it showed the fuel strainer near the lower rear of the tank and the fuel sender more towards the middle height and slightly front of center. So, it would make sense that with less than half fuel and nose high, the sender could see no fuel, but it would all be back by the strainer outlet. So, even though the gauge indicated empty at that attitude, there was actually less risk of fuel starvation. Both sides appear designed the same.

But I haven't been able to determine why one side showed a great reduction in fuel but the other side did not. Although the nose was high, the wings were level... Any ideas/thoughts?

 

[TangoWhiskey]

I wonder if it has something to do with the way the overflow vent system works; the vent is only on the left tank, with cross-vent tubes to the right tank, if I remember the system diagram correctly. The caps have vent holes, too.

 

[DutchessFlier]

Troy you are correct, and the left tank indications are always different than the right tank due to the venting system. I learned this lesson the first month I flew 71D and tried to make sense out of the fuel gauges. BTW my CFI always had me doing any stall work / slow flight / major attitude deviations within the first segment of any long lesson series. The plane was always filled before every lesson flight, and we never really burned enough avgas to even get to half full...but one of the reasons they kept the planes full was due to stall work and the fuel system vents and cross flow issues.

 

[AdamZ]

Very informative post thanks!

 

[Steve]

How level?

Since the level transmitters are mounted on the inboard ribs of the wing tank, any minor tilt would be amplified by the indicators as fuel settled toward or away the sensors.

 

[jesse]

I would imagine if there were a fuel delivery issue that occurred during reasonable attitudes one would experience while working on basic maneuvers there would be a warning in the POH.

I wouldn't get too concerned about the fuel gauge indications.

Take the above for what it's worth; nothing.

 

[MauleSkinner]

I'd tend to agree with Jesse ...if fuel delivery was an issue, there would be something in the POH, something like the "avoid extended slips" caution that's been in a few of the airplanes I've flown for that very reason (not the same as the Cessna "flap 'n slip" caution).

David

 

[skidoo]

How level?

Since the level transmitters are mounted on the inboard ribs of the wing tank, any minor tilt would be amplified by the indicators as fuel settled toward or away the sensors.

Well, I had thought that we were not banked at all. But, this is a good point. The fuel senders are mounted on the edge where the wing mounts to the fuselage. The fuel pickup is on that same edge but located more towards the bottom and rear.

So, it would make sense that if there was a slight bank say left, the fuel could be away from the left sender and pool up on the right. By the same logic, as long as the fuel selector is on Both, in a bank, nose high attitude, one tank could be void of fuel at the fuel strainer output, but have plenty on the other side to continue fuel delivery.

I am wondering now about steep descents with full flaps. Seems like the fuel could drop away from the fuel strainer outlet, but the sender could see more fuel... Maybe it is just not as important when in a steep decent....

 

[Dan Thomas]

I am wondering now about steep descents with full flaps. Seems like the fuel could drop away from the fuel strainer outlet, but the sender could see more fuel... Maybe it is just not as important when in a steep decent....

That's what "Unuseable Fuel" is all about. On the wing next to the filler the total capacity of the tank is given. On the fuel selector the Useable Fuel is given. The difference between the two is the unuseable fuel. For a 172 it's around two gallons per side. The POH will tell you what it is for your airplane.

Any dipstick had better be calibrated so that it reads zero at the unuseable fuel level. Fuel gauges are supposed to read Empty when the unuseable fuel level is reached.

So now: when you are on approach with full flap and the nose is down, the fuel sloshes forward in the tanks and if there's less than the unuseable fuel left in them, you could get fumes instead of fuel and the engine gets quiet.

Dan

 

[RotorAndWing]

I would imagine if there were a fuel delivery issue that occurred during reasonable attitudes one would experience while working on basic maneuvers there would be a warning in the POH.

I wouldn't get too concerned about the fuel gauge indications.

Take the above for what it's worth; nothing.

You are correct. This is from CAR3 (which the C182 is certified under)

3.433 Fuel flow rate. The ability of the fuel system to provide the required fuel flow rate and pressure shall be demonstrated when the airplane is in the attitude which represents the most adverse condition from the standpoint of fuel feed and quantity of unusable fuel in the tank. During this test fuel shall be delivered to the engine at the applicable flow rate (see §§ 3.434-3.436) and at a pressure not less than the minimum required for proper carburetor operation. A suitable mock-up of the system, in which the most adverse conditions are simulated, may be used for this purpose. The quantity of fuel in the tank being tested shall not exceed the amount established as the unusable fuel supply for that tank as determined by demonstration of compliance with the provisions of § 3.437 (see also §§ 3.440 and 3.672), plus whatever minimum quantity of fuel it may be necessary to add for the purpose of conducting the flow test. If a fuel flowmeter is provided, the meter shall be blocked during the flow test and the fuel shall flow through the meter bypass.

 

[TangoWhiskey]

You are correct. This is from CAR3 (which the C182 is certified under)

3.433 Fuel flow rate. The ability of the fuel system to provide the required fuel flow rate and pressure shall be demonstrated when the airplane is in the attitude which represents the most adverse condition from the standpoint of fuel feed and quantity of unusable fuel in the tank. During this test fuel shall be delivered to the engine at the applicable flow rate (see §§ 3.434-3.436) and at a pressure not less than the minimum required for proper carburetor operation. A suitable mock-up of the system, in which the most adverse conditions are simulated, may be used for this purpose. The quantity of fuel in the tank being tested shall not exceed the amount established as the unusable fuel supply for that tank as determined by demonstration of compliance with the provisions of § 3.437 (see also §§ 3.440 and 3.672), plus whatever minimum quantity of fuel it may be necessary to add for the purpose of conducting the flow test. If a fuel flowmeter is provided, the meter shall be blocked during the flow test and the fuel shall flow through the meter bypass.

How is the manufacturer to ensure that fuel is "delivered to the engine at the applicable flow rate" if the fuel flowmeter is bypassed?

 

[Steve]

I am wondering now about steep descents with full flaps. Seems like the fuel could drop away from the fuel strainer outlet, but the sender could see more fuel... Maybe it is just not as important when in a steep decent....

That is precisely what we believed happened when a pilot friend of ours made an off airport landing following fuel starvation in a T210. The pilot recalled checking the fuel gauges on the way down from 16,000' in an excessively steep descent to do a GPS approach (which were just coming into vogue) and seeing 1/4 indicated on both tanks and upon leveling off watching both needles hit the stops on the E side. Ended up only a couple of miles short of the field after flying for 4.5 hours.

 

[RotorAndWing]

That is precisely what we believed happened when a pilot friend of ours made an off airport landing following fuel starvation in a T210. The pilot recalled checking the fuel gauges on the way down from 16,000' in an excessively steep descent to do a GPS approach (which were just coming into vogue) and seeing 1/4 indicated on both tanks and upon leveling off watching both needles hit the stops on the E side. Ended up only a couple of miles short of the field after flying for 4.5 hours.

I don't think you could attribute the fuel gauges as the cause of this accident.

 

[gismo]

I don't think you could attribute the fuel gauges as the cause of this accident.

Certainly not a cause, but it may be that the pilot incorrectly assumed that there was plenty of fuel given the 1/4 tank indication during the descent. And FWIW, this sounds more like a fuel exhaustion episode than a fuel starvation situation.

 

[gismo]

Well, I had thought that we were not banked at all. But, this is a good point. The fuel senders are mounted on the edge where the wing mounts to the fuselage. The fuel pickup is on that same edge but located more towards the bottom and rear.

So, it would make sense that if there was a slight bank say left, the fuel could be away from the left sender and pool up on the right. By the same logic, as long as the fuel selector is on Both, in a bank, nose high attitude, one tank could be void of fuel at the fuel strainer output, but have plenty on the other side to continue fuel delivery.

Bank angle should have nothing to do with lateral fuel balance as long as the plane is coordinated. Flying in a slip or skid OTOH could result in fuel moving towards one end of a tank.

 

[Steve]

You're correct, Lance. I mis-spoke re starvation/exhaustion. In discussions with the pilot afterwards I was surprised to learn the idea of timing tanks had never been discussed during training. Also, a mis-calibrated fuel servo had the actual fuel flow at ~22 gph vs the 16 planned and combined with large deviations due to weather the options dwindled to one. I'm sure there were other causal factors, too, but over-reliance on the fuel gauge indications seemed to predominate.

Certainly not a cause, but it may be that the pilot incorrectly assumed that there was plenty of fuel given the 1/4 tank indication during the descent. And FWIW, this sounds more like a fuel exhaustion episode than a fuel starvation situation.

 

[MauleSkinner]

In discussions with the pilot afterwards I was surprised to learn the idea of timing tanks had never been discussed during training.

I'd tend to doubt the word "never"...probably more like retaining the standard 20% of what's taught.

Amazing how many times I've mentioned something to the same person for the first time.

 

[Greg Bockelman]

...I was surprised to learn the idea of timing tanks had never been discussed during training.

Well, when a lot of pilots were trained in planes that either CAN'T switch between tanks, (Moi in a C-150) or in planes that can burn off of both tanks at the same time, (C-172) it is a habit that isn't firmly ingrained into one's memory.

 

[skyflyer8]

Bank angle should have nothing to do with lateral fuel balance as long as the plane is coordinated. Flying in a slip or skid OTOH could result in fuel moving towards one end of a tank.

Yes, this theory has to do with someone testing me by setting a styrofoam cup with coffee in it on my instrument panel and challenging me not to tip it over. (I didn't.)

 

[Everskyward]

Well, when a lot of pilots were trained in planes that either CAN'T switch between tanks, (Moi in a C-150) or in planes that can burn off of both tanks at the same time, (C-172) it is a habit that isn't firmly ingrained into one's memory.

But the guy Steve was referring to was flying a C-210 which only has left and right, no BOTH position. At least the one I flew was configured that way.

 

[TangoWhiskey]

Here's a great write up on the Cessna left-wing vent and crosstube system, and why the right tank often stays "full", even when running on the right tank only. It's good info to know if you fly an older Cessna with this setup; if you're somebody who switches tanks based on time, and expect your left tank to have xx hours in it... it might not, if you were previously running on the right tank: your left tank fuel may have ended up in the right tank.

Have you been flying along and watched your fuel gauges show that your left tank is going down while the right tank remains full even with the fuel selector on “BOTH”? This is a common problem with Cessna 172 Skyhawks.

The real shocker is that while the right tank is remaining full, the engine is actually using fuel from the right tank. What is causing the situation is the way Cessna designed the fuel tank venting system. When fuel is used from a tank it must be replaced with something, otherwise a vacuum is created which will either cause interruption of fuel to the engine or cause the fuel tank to begin to collapse.

To avoid this in almost all fuel systems, whether they are in an aircraft, a car, or a lawn mower, fuel that is used from the tank is replaced by air from the outside. On most Cessna 172 Skyhawks, prior to 1997, this venting occurs by connecting the upper outboard portion of the left tank to the “L” shaped vent tube underneath the wing behind the left wing strut. This allows air into the left fuel tank as fuel is used. To vent the right tank, a vent inter-connect line is run from the upper inboard area of the left tank to the upper inboard area of the right tank thus, in theory, venting the right tank to the vented airspace of the left tank.

Unfortunately, wing dihedral, where the wing tip is higher than the wing root, was not sufficiently considered. When the wing tanks are full, the vent inter-connect line is actually submersed in fuel and thus as fuel is used from the left tank, the air coming in from the vent pushes fuel from the left tank through the vent inter-connect line into the right tank, thus replacing fuel that is used from the right tank. And even after enough fuel is used from the left tank to bring the fuel level below the vent inter-connect line, the condition will continue as fuel sloshing in the tank periodically gets into the inter-connect line and pushed through to the right tank.

In really severe cases, fuel usage from the right tank might not be indicated on the gauge until the fuel level in the left tank is as low as 1/3 capacity. The positive thing to keep in mind when experiencing this condition is that fuel is actually being used from the right tank and that fuel being used from the right tank is merely being replaced by fuel from the left tank. This means that even if the left fuel tank should go to empty, you will not experience fuel flow interruption as long as there is fuel in the right tank and the fuel selector is on “BOTH”.

This condition can be minimized somewhat by adjusting the position of the fuel vent behind the lift strut on the left wing, making sure that fuel caps seal tightly so that the “head pressure” in one tank is not altered by a leaking cap, and assuring that the wing strut fairing is sealed against the strut, thus avoiding burbling air right in front of the vent.

However, in the end, the design of the system does not allow for complete resolution of the problem. On some of the later 172 models of the 1980s, the right tank is vented directly with it’s own vent tube behind the right lift strut. This feature which became standard on all 172s beginning in 1997, reduces the uneven fuel feeding but does not eliminate it.

 

[Lawreston]

. . . . .someone testing me by setting a styrofoam cup with coffee in it on my instrument panel and challenging me not to tip it over. (I didn't.)

Ha! That sounds like my dual X-ctry. We had landed at Pittsfield, ME, next stop to be Bangor International Airport(former Dow AF Base). The CFI bought sodas at Pittsfield. I drank mine and recycled the can. On about a five mile final for KBIA the CFI said, "I guess I'll drink my soda," and he reached behind my seat, produced a can and said, "and I don't want to see one drop of that in my lap when you land," as he popped the top.

I set the Skyhawk down behind -- or I should say beyond -- that KC-135 National Guard tanker touchdown and not a drop was spilled. My first experience on 12,000+ feet and I had to taxi at least a thousand feet before getting to the Taxiway Mike I'd been told to exit. Dr. Bruce will relate to the locale.

HR

 

[Lawreston]

Very interesting/informative; but its author should not have used the apostrophe in the first word of the final line of text.

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