Sucking a tank dry

denverpilot

denverpilot

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From another source today, I saw a comment that caught my eye.

I know some folks "run a tank dry" in aircraft with multiple tanks sometimes.

The comment seem said, "FAR 23.951 says you can't do that."

Well... The FAR actually says the fuel system must be designed in such a way that it can never suck in air...

So this guy is making a leap and saying if the FAA doesn't want the system to ever suck air, you MUST use the system as-designed and switch prior to empty. Big jump.

What say y'all?

-----

(a) Each fuel system must be constructed and arranged to ensure fuel flow at a rate and pressure established for proper engine and auxiliary power unit functioning under each likely operating condition, including any maneuver for which certification is requested and during which the engine or auxiliary power unit is permitted to be in operation.

(b) Each fuel system must be arranged so that—

(1) No fuel pump can draw fuel from more than one tank at a time; or

(2) There are means to prevent introducing air into the system.

(c) Each fuel system for a turbine engine must be capable of sustained operation throughout its flow and pressure range with fuel initially saturated with water at 80 °F and having 0.75cc of free water per gallon added and cooled to the most critical condition for icing likely to be encountered in operation.

(d) Each fuel system for a turbine engine powered airplane must meet the applicable fuel venting requirements of part 34 of this chapter.
 
denverpilot said:
From another source today, I saw a comment that caught my eye.

I know some folks "run a tank dry" in aircraft with multiple tanks sometimes.

The comment seem said, "FAR 23.951 says you can't do that."

Well... The FAR actually says the fuel system must be designed in such a way that it can never suck in air...

So this guy is making a leap and saying if the FAA doesn't want the system to ever suck air, you MUST use the system as-designed and switch prior to empty. Big jump.

What say y'all?

-----

(a) Each fuel system must be constructed and arranged to ensure fuel flow at a rate and pressure established for proper engine and auxiliary power unit functioning under each likely operating condition, including any maneuver for which certification is requested and during which the engine or auxiliary power unit is permitted to be in operation.

(b) Each fuel system must be arranged so that—

(1) No fuel pump can draw fuel from more than one tank at a time; or

(2) There are means to prevent introducing air into the system.

(c) Each fuel system for a turbine engine must be capable of sustained operation throughout its flow and pressure range with fuel initially saturated with water at 80 °F and having 0.75cc of free water per gallon added and cooled to the most critical condition for icing likely to be encountered in operation.

(d) Each fuel system for a turbine engine powered airplane must meet the applicable fuel venting requirements of part 34 of this chapter.
Click to expand...

Given that the quoted FAR pertains to system design, and your comment pertains to operation, I don't see how the two are related.
 
I always run my auxes dry if I'm on a run that's gonna use the auxes.
 
How would a system be designed to be air proof?

Fuel pickups must have a valve installed to shut off access to the fuel line if the tank is run dry?
Fuel pickups must remain in the fuel mass if the plane is unloaded in flight by staying in the fuel by some magical means?
Design a collapsing tank that will keep air in the tank to zero as fuel quantity decreases?


Part 23 is airworthiness standards, not flight operations. Have them show you the rule in P91.
 
Depends on the airplane. I used to run tanks dry everyday in some twins, Aztec, Twin Beech, for example, but in others, C-404 or Navajo/Chieftain I didn't. The reason to run tanks dry is, in an airplane with multiple tanks, fuel has to be consumed in a certain order, and, when you get right down to it, the only times a pilot can be sure of the fuel quanity aboard is either when he's just fueled it and...when the tank runs dry. Not every airplane has such a fuel system. I never ran a tanks dry in a turbo-charged airplane as it's very hard on the engines (results in bootstrapping - wild RPM fluxuations). I have never flown a turboprop that required tank switching. Doesn't mean there aren't some - but I wouldn't run them dry either.
 
denverpilot said:
From another source today, I saw a comment that caught my eye.

I know some folks "run a tank dry" in aircraft with multiple tanks sometimes.

The comment seem said, "FAR 23.951 says you can't do that."

Well... The FAR actually says the fuel system must be designed in such a way that it can never suck in air...

So this guy is making a leap and saying if the FAA doesn't want the system to ever suck air, you MUST use the system as-designed and switch prior to empty. Big jump.

What say y'all?
Click to expand...

My thoughts are:

It's the reason you don't see a "both" selector for low wing aircraft, when it comes to simple fuel systems. If one tank runs dry (sucking air), fuel can't be sucked from the other tank.
 
There are only two times you really know how much fuel is in a tank. when it is full, and you can see it is, and when it is empty.

unporting a fuel tank in some aircraft will kill you.

the Fairchild 24 will not feed fuel after the air enters the system by running the tank dry. It simply vapor locks the delivery line and must be started again by disconnecting the delivery line from that tank, and blowing all the fuel out of the line and allowing it to drain down with out air entering the line.
 
Of course it depends. I used to run a Bo basically dry. Just watch the fuel flow as soon as it starts fluctuating switch. You can do it without the passengers knowing and for all intents and purposes you know where the remaining fuel is.

The other related question is how much fuel does your tank(s) really hold? Run em dry and then fill them is the only way I know to verify.

Between the two, especially with a totalizer, the fuel situation should be under control.
 
L.Adamson said:
My thoughts are:

It's the reason you don't see a "both" selector for low wing aircraft, when it comes to simple fuel systems. If one tank runs dry (sucking air), fuel can't be sucked from the other tank.
Click to expand...

Stock Navions only have a both selector. In fact they only have one fuel gauge and one filler.

Some people swear that you'll suck mud or something at the bottom of the tank and the engine will never refire.

John Deakin in several of his articles points out that having the engine refire after one tank has gone dry is mandated by the type certification and you never trully know how much fuel you've used until you run one dry.

Due to the way the tankage works in my plane, only the tip tanks are ever run dry safely and I've done it on a number of occasions without incident.
 
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flyingron said:
Some people swear that you'll suck mud or something at the bottom of the tank and the engine will never refire.
Click to expand...

A basic lack of understanding of the tank and outlets, that one. The tank's outlet to the engine is a bit above the bottom of the tank. You don't want the outlet right at the lowest point; that's where the sump drain goes to let the dirt and water out. Because of this, it's pretty much impossible to actually suck the tank "dry."

But the engine will still quit.

Dan
 
Dunno what you were flying, but Cessnas exhibit the same smooth run-down and re-light whether NA or TC.
j1b3h0 said:
I never ran a tanks dry in a turbo-charged airplane as it's very hard on the engines (results in bootstrapping - wild RPM fluxuations).
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At 10,000ft ish, a Navajo engine would quit for a second, then, when resuppied with fuel start again and surge strongly, 3 or 4 times after switching tanks. And, if one was foolish enough to let that happen above 18,000ft, lore had it the engine might not restart at all. Remember, it's not just the engine surge per se, but all that air being pumped through the exhaust-then a pair of 20,000rpm turbos...then through the engine, then the turbos. Not my favored method. The 404 didn't require any switching of tanks on the routes I flew it.
 
denverpilot said:
From another source today, I saw a comment that caught my eye.

I know some folks "run a tank dry" in aircraft with multiple tanks sometimes.

The comment seem said, "FAR 23.951 says you can't do that."

Well... The FAR actually says the fuel system must be designed in such a way that it can never suck in air...

So this guy is making a leap and saying if the FAA doesn't want the system to ever suck air, you MUST use the system as-designed and switch prior to empty. Big jump.

What say y'all?
Click to expand...

Is this the same guy who thinks that taking the reverse hi-speed without prior permission is prohibited by .... oh, never mind. ;)
 
Dan Thomas said:
A basic lack of understanding of the tank and outlets, that one. The tank's outlet to the engine is a bit above the bottom of the tank. You don't want the outlet right at the lowest point; that's where the sump drain goes to let the dirt and water out. Because of this, it's pretty much impossible to actually suck the tank "dry."

But the engine will still quit.

Dan
Click to expand...
Study the picture, note the tank outlet is right next to the bottom sump drain,
 

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Running tanks dry is a fools errand.

I'm working for an operator at the moment that uses a policy of planning to return with half tanks. If anyone lands with quarter tanks, they are required to submit a report explaining why.

I like that policy.
 
I'd be very nervous without a third of the main (an hour reserve).
The rear aux I always run dry (it just transfers to the main).
I'll run the tips as low as I can get them and I've run it dry a few times. It takes less than a second to hit the boost pump and turn the fuel selector (though it seems like an eternity). My plane refires quickly once you're back pulling from a tank with fuel.
There's no real issue with sucking air temporarily (other than the engine stops). Sucking vapor might be an issue, but it takes longer than I'm going to coast with no engine running.
 
We've never had that policy, but maybe the difference is that we haven't had trouble hiring pilots that are smart enough to land with adequate reserves.

For domestic ops I would immediately fire whoever came up with that drivel. But maybe we fly in places with many more friendly places to land.

DouglasBader said:
Running tanks dry is a fools errand.

I'm working for an operator at the moment that uses a policy of planning to return with half tanks. If anyone lands with quarter tanks, they are required to submit a report explaining why.

I like that policy.
Click to expand...
 
I look at it like this, if I can avoid a fuel stop that saves 1 hour from the trip time and depending on the aircraft a few gallons lost climbing back to altitude. That's a lot when you think about it. If you have a 6 tank bird and leave 3 in each one that's 15 gals somewhere other than the last main. Say an hour in a single and thirty minutes in a twin.

Slowing down, determining the altitude depending on wind, leaning, and fuel tank management can really stretch the range in a piston and is always faster than the guy that has to stop.
 
Depends on the airplane...
In my Apache I routinely run the outboard tanks dry when on a cross country (at altitude) Engine starts to sputter and and you get a big yaw which wakes you up, switch tanks to mains, blip the electric pump, engine is back on before it actually quits, reset timer, go back to sleep knowing there are 3 more hours in the mains plus an hour of reserve...

Now, I never run the mains dry and I have a one hour policy I treat like religion... There will never be less than one hour of fuel onboard at any time, no exceptions to the rule... I have landed 20 minutes from my home airport for fuel because that would take the mains below one hour... The guy with me was incredulous, thought I was a nut case... As I was entering the pattern he was arguing with me that he was going to be late for a dinner party with his wife because I was stopping... He was pizzed - and he never flew with me again...
He has since banged up an airplane running it out of fuel... (Can you hear me NOW Jack!)
 
The problem with reducing power to make range is that it must often be done starting with the first hour rather than the last hour. Lots of evidence in the NTSB to prove that many pilots are incapable of such planning or unwilling to accept slower ground speeds that result. Running a tank dry to insure that all the fuel is in one tank is a no-brainer, especially when most of those can be accomplished over an airport if the lever happens to break off in your hand.

Alexb2000 said:
I look at it like this, if I can avoid a fuel stop that saves 1 hour from the trip time and depending on the aircraft a few gallons lost climbing back to altitude. That's a lot when you think about it. If you have a 6 tank bird and leave 3 in each one that's 15 gals somewhere other than the last main. Say an hour in a single and thirty minutes in a twin.

Slowing down, determining the altitude depending on wind, leaning, and fuel tank management can really stretch the range in a piston and is always faster than the guy that has to stop.
Click to expand...
 
wabower said:
The problem with reducing power to make range is that it must often be done starting with the first hour rather than the last hour. Lots of evidence in the NTSB to prove that many pilots are incapable of such planning or unwilling to accept slower ground speeds that result. Running a tank dry to insure that all the fuel is in one tank is a no-brainer, especially when most of those can be accomplished over an airport if the lever happens to break off in your hand.
Click to expand...

True. I bet you also hire pilots capable of flight planning as well as fuel management.:)

With an FMS integrated totalizer and onboard weather it really is pretty easy if you just take the time early in the flight to work with it. Worse case you can plan a fuel stop early somewhere with cheap(er) fuel.
 
You might be surprised by the number of jet pilots who say a Citation 500/501 won't make Salt Lake City from Dallas. They're right if they try to cruise at 340-350 for the first two hours and then pull back for the last hour. They're wrong if they pull back to 310 after reaching altitude.

Alexb2000 said:
True. I bet you also hire pilots capable of flight planning as well as fuel management.:)

With an FMS integrated totalizer and onboard weather it really is pretty easy if you just take the time early in the flight to work with it. Worse case you can plan a fuel stop early somewhere with cheap(er) fuel.
Click to expand...
 
Part 23 is addressed to aircraft manufacturers, not pilots.

Bob Gardner
 
After running an hour out of the left main tank I would routinely run the aux tanks in my K35 Bonanza until the fuel flow gauge started to twitch indicating the tanks were empty. The aux tanks held 19 gallons usable, none of which could be used for takeoff or landing. Because the fuel injection system pulled gas out of the aux tanks at about double the cruise fuel flow rate and returned the excess to the left main tank, the 19-gallon aux tanks would be empty in about 50 minutes (cruise fuel flow was only about 11.5 gph). Meanwhile, the amount of fuel in the left main, which could be used for landing, was increasing!

So every extra minute I could coax out of the aux tanks meant two extra minutes of usable endurance.

FAR 23.955(e):
(e) Multiple fuel tanks. For reciprocating engines that are supplied with fuel from more than one tank, if engine power loss becomes apparent due to fuel depletion from the tank selected, it must be possible after switching to any full tank, in level flight, to obtain 75 percent maximum continuous power on that engine in not more than—

(1) 10 seconds for naturally aspirated single-engine airplanes;

(2) 20 seconds for turbocharged single-engine airplanes, provided that 75 percent maximum continuous naturally aspirated power is regained within 10 seconds; or

(3) 20 seconds for multiengine airplanes.
Click to expand...
 
L.Adamson said:
It's the reason you don't see a "both" selector for low wing aircraft, when it comes to simple fuel systems. If one tank runs dry (sucking air), fuel can't be sucked from the other tank.
Click to expand...

I have flown certified low wing aircraft with a both selector. For example, the Rockwell Commander 114 has a both selector. If I remember correctly, the only time you have to select an individual tank is during an engine restart in flight.
 
As stated, the Navion fuel selector is "on-off" only. Both wing tanks drain to a small accumulator in the wing roots which feeds the engine. There's only one filler which is why you have to fill it slowly (give it time to feed over to the other side).
 
Good discussion all.

As I suspected, the commenter from the e-mail "newsletter" (actually a recurring advertisement for a so-called "Master" CFI that also happens to contain flying tips) is off his rocker as far as the relationship of this FAR to flight ops.
 
Tom-D said:
Study the picture, note the tank outlet is right next to the bottom sump drain,
Click to expand...

That would not appear to be a FAR 23 tank. The reg for sumps, which presumably means a volume that can't get into the fuel outlet, goes like this:

23.971 Fuel tank sump.

(a) Each fuel tank must have a drainable sump with an effective capacity, in the normal ground and flight attitudes, of 0.25 percent of the tank capacity, or1/16gallon, whichever is greater.

(b) Each fuel tank must allow drainage of any hazardous quantity of water from any part of the tank to its sump with the airplane in the normal ground attitude.


(c) Each reciprocating engine fuel system must have a sediment bowl or chamber that is accessible for drainage; has a capacity of 1 ounce for every 20 gallons of fuel tank capacity; and each fuel tank outlet is located so that, in the normal flight attitude, water will drain from all parts of the tank except the sump to the sediment bowl or chamber.


That last bit, "except the sump," implies that the sump has the minimum volume specified in (a) that cannot drain through the tank outlet in normal flight attitude. That requires the fuel outlet to be well above the sump.


Dan
 
flyingron said:
As stated, the Navion fuel selector is "on-off" only. Both wing tanks drain to a small accumulator in the wing roots which feeds the engine. There's only one filler which is why you have to fill it slowly (give it time to feed over to the other side).
Click to expand...

I was looking into that yesterday. Does the Rockwell Commander, as mentioned in the previous reply, use something of the same? If you take a direct feed from right or left, into a both selector...........you WILL have serious problems without some kind of accumulator or header tank. Fuel will move from one tank to the other, and if one runs dry (or is unported), it will stop flow from the other. This can be tested by sucking through two staws and two glasses. Pull one straw out, and see what "no longer happens".

L.Adamson
 
Tom-D said:
There are only two times you really know how much fuel is in a tank. when it is full, and you can see it is, and when it is empty.\
Click to expand...

This is true of float type sending units of old. But most, if not all modern aircraft have very accurate fuel quantity measurements.
 
Ever been on a flight when you really wondered how accurate? In an emergency fuel situation, knowing you've already run the tank dry will prevent one from even considering switching back to that tank, is one good arguement for the 'run til dry' method.

I think most of us take it on faith that we really know how much fuel our tanks hold, or for that matter, how much our engines really consume, the way we operate. In order to really know, one would have to keep a running tally...power settings/time and a stack of fuel receipts over time, for each airplane we fly. Ostensibly, aircraft manufacturers make such information available, in the form of POHs and the like, but the only way to verify it is to do the math. Sure the gage is supposedly accurate, but if I pumped the fuel into the tank: I know it's full. If I run it dry, not so much.
 
Inverted said:
This is true of float type sending units of old. But most, if not all modern aircraft have very accurate fuel quantity measurements.
Click to expand...

There are still a lot more of the old than the new in the overall fleet. That'll change over time, but probably just barely in my lifetime. If even then. Depends on how many are wrecked beyond economically feasible repair and how many are destroyed by neglect.

Production of most light aircraft in the 70s was measured in the many thousands. Production today, the GA manufacturer's association counts numbers in the few hundreds as a "good year".
 
In all fairness I wasn't talking about just GA aircraft. but even older GA aircraft can have pretty accurate fuel gauges. And of course it is all what you consider to be GA or not. There are 757s used for GA...
 
j1b3h0 said:
Ever been on a flight when you really wondered how accurate? In an emergency fuel situation, knowing you've already run the tank dry will prevent one from even considering switching back to that tank, is one good arguement for the 'run til dry' method.
Click to expand...

Its one thing to time a tank and things like that. But its a whole other to just watch the gauge and when it hits zero you start timing. I have always been amazed that people say how inaccurate gauges are, yet they trust when they hit E. Clearly if we are having this conversation it means that we have all come to terms with the fact that even though these types of fuel quantity gauges are only certified accurate when empty, but we all know they aren't, why are we allowing them to go to E at all. You could have 30 seconds or 16 minutes, and it can vary. It would be one thing to time it 50 times and have the tank go empty at the exact same time every time. Some of the aircraft I fly that have inaccurate gauges, are all over the place when it comes to what they display. I have ran outboards dry depending on my level of boredness, and sometimes they sputter before E, sometimes it can take 30 min on E before it starts to go.
 
denverpilot said:
There are still a lot more of the old than the new in the overall fleet. That'll change over time, but probably just barely in my lifetime. If even then. Depends on how many are wrecked beyond economically feasible repair and how many are destroyed by neglect.

Production of most light aircraft in the 70s was measured in the many thousands. Production today, the GA manufacturer's association counts numbers in the few hundreds as a "good year".
Click to expand...

After flying with old unreliable gauges, new gauges, and then an integrated totalizer. One of the first modifications I would do to any older GA piston is to add a totalizer. It just never gets old flying close to max range into a headwind knowing you'll still have enough gas when you get there vs. being nervous the whole time.
 
That's why I carefully said "light aircraft" in my comments. ;)

As far as transports and commercial ewuipment go, the desert boneyards aren't getting any smaller. Rotting corpses abound with bad fuel gauges there.
 
Alexb2000 said:
After flying with old unreliable gauges, new gauges, and then an integrated totalizer. One of the first modifications I would do to any older GA piston is to add a totalizer. It just never gets old flying close to max range into a headwind knowing you'll still have enough gas when you get there vs. being nervous the whole time.
Click to expand...

You bet! It's one of the best items I added to my plane. Always accurate within a quarter of a gallon on every fuel up. It's also connected to the GPS, to display calculated use & reserve for destinations entered into the GPS.
 
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