Hazards of Running a Tank Dry

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pelmet
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Hazards of Running a Tank Dry

Post by pelmet »

To be honest, I have never done this myself but have seen it done once as a passenger in a Marchetti). I can see the potential advantage of doing so. But perhaps there are hazards that can happen in certain aircraft types/engine/fuel system combinations.

Does anybody here have any advice, especially cautions about certain aircraft based on their experience.

"C-GSGQ, a privately owned Cessna 340A, was conducting an IFR flight from Ottawa Macdonald-
Cartier Intl Airport (CYOW), Ontario to Toronto/Buttonville Municipal Airport (CYKZ), Ontario, with

one pilot and two passengers on board. During descent from 5000 feet, the RH engine (Teledyne
Continental TSIO-520-NB) failed due to fuel starvation, as the RH auxiliary fuel tank had been
emptied. Attempts to restart the engine after selecting the main tank were unsuccessful, and the
pilot arranged with ATC to divert to nearby Peterborough Airport (CYPQ), Ontario, where an
uneventful landing was made at 1543EST. Vapor lock was the suspected cause of the failed restart
attempts while airborne. On the apron at CYPQ, the RH engine was restarted successfully and a
run-up was conducted which revealed no anomalies. The pilot departed at 1615EST and arrived
uneventfully at the planned destination (CYKZ)."
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co-joe
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Re: Hazards of Running a Tank Dry

Post by co-joe »

I only have about 500 hours in twin cessnas so I'm not the guru by any stretch, and it was a long time ago almost in another life so I'll do my best here. The twin cessnas I've flown (310, 402, 414,and 421) all shared a virtually identical fuel system. It carried about 1200 lbs of fuel in 6 tanks on the earlier models. It was by far the most complicated fuel system to manage that I've ever used, due in large part to the shape of the very thin laminar flow wing.

You had to run the mains down I think 200 lbs first, because the aux tanks supplied more fuel than the engines would burn and the extra always went back to the mains. So you would run the mains down 200, then run the aux tanks till then engines quit (called "blowing a tank"), then the mains would be full again from the aux overflow. If you screwed this up and started running off the aux tanks too early, the extra fuel vented overboard through a sniffle valve and you could watch it vaporize from the back of the tip tank.

Now the best part were the wing locker tanks. You can't run the engine off the wing lockers because they only had transfer pumps that transferred their fuel to the mains (I think 200 lbs each) They worked great down low but were somewhat unreliable up high so you wanted to transfer their contents first. Plus their transfer system wasn’t fast enough to actually supply an engine demands but would fall behind so you had to have enough main fuel to augment their feed.

The only reliable way to ensure you used every drop was to run a tank by stopwatch until the engine coughed, manifold pressure would drop, or sometimes you'd hear the props come off the governor and the engine surge. At that point you flip the boost pumps to high and quickly switch tanks. Fuel flows are never the same so the engines would usually do this 5 minutes or so apart but theoretically it could happen at the exact same time.

It was definitely a dance, and blowing a tank always wakes the pax up so you have to smile and say everything is cool. If only one wing locker worked you'd end up with a pretty serious imbalance to deal with involving crossfeed as well. The later models improved by only having mains and wing lockers, but still a fair bit to manage on long flights. I think the early models had something like 8 fuel pumps because the tip tanks needed a feeder pump for steep climbs . The later models only had 6 pumps, unless they didn't have wing locker fuel. To my knowledge all of the 310 - 414 class had 6 tanks, only the 402-421 C models had the integral tanks.

Vapour lock can be a bitch. The funniest part of the movie 2012 for me is when John Cusak jumps in the 414 hot, on a quick turn and just fires it up first try. :lol: The 310 I flew one of the engines you had to count blades, It was something like exactly 7 blades and let go of the started and she starts every time...(If you did the mixture full rich, boost pump high for 3 seconds, then idle cut off, hit the starter, boost pump low, bring the mixture up...then ICO, then up and gradually increase it and maybe click the boost pump 2 or 3 times...) If you screwed it up, go in side and make a pot of coffee while the engine cools down.
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anofly
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Re: Hazards of Running a Tank Dry

Post by anofly »

I only have experience with RSA (bendix?) fuel injection. there is no "return line for excess fuel " like on cessna twins.
You can run a tank dry, but you best be up high, and planning for it when it happens. It seems to take a long time to "prime all the lines and get it flowing to the engine again" but its like 15-20 secs. That is no fun down low, doing an approach in IMC, with a crosswind at night!. So I say plan it for the cruise part of your flight, be patient, it will restart.... select the new tank, run the boost pump, mixtures up and throttle maybe a bit back of full, once it fires up, boost pump off, throttle , and mixture to where you want them. AT night, imc etc, dont run em dry....you have enough to worry about...
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JasonE
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Re: Hazards of Running a Tank Dry

Post by JasonE »

I've run the aux tanks dry quite a few times on a Comanche single. You can watch the fuel pressure start to drop before it actually runs out. 4 tanks to feed 1 engine. When I'm cruising at higher altitude I like to drain the outboard tanks first.
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2R
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Re: Hazards of Running a Tank Dry

Post by 2R »

I cannot find any reference to running tanks dry in any POH/AFM in my library . Can anyone post an approved procedure from an approved POH/AFM .
Please and thank you :)
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J31
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Re: Hazards of Running a Tank Dry

Post by J31 »

Cessna 340 the auxiliary tanks are for use in cruse only. Each engine has a respective main and auxiliary tank.

It is very easy to manage the monitor the auxiliary and main fuel level with respective gauges. If you want to run the auxiliary tanks very low it is easy to catch, as the fuel flow gives notice. If you miss all that, the surging will get your attention! Then all you have to do is simply select the main tank, auxiliary pump on for a few moments to stabilize the fuel flow.

If you run the auxiliary tank completely out of fuel before switching to the main tank and do not use the auxiliary pump, the respective engine will quit due to fuel starvation.
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captain_dc
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Re: Hazards of Running a Tank Dry

Post by captain_dc »

I have not flown multi Cessna's and will not speak to the safety of running the tanks dry in them.

I will speak to what I know and is common practice in commercial flying.

There is really only one way to know if you have taken all of the fuel available from a tank, and that is to blow the tank. If you are quick and recognize the signs you can usually switch the tank before anyone notices

C206: Having the ability to draw only on 1 tank at a time and unreliable fuel gages means getting all the fuel from your first tank is critical on some flights. Usually you can catch the change in the prop RPM quick enough and switch before anyone notices.

PA31: Standard is outboard tanks for cruise and inboard mains. Although there are low fuel pressure indicator lights, I've never seen them function properly on a straight ho, chieftains seem to function more reliably. If you keep a keen eye on your Fuel pressure you can catch the blow out before you hear the prop start falling out of sync and your passengers notice.

BN2A: With the outboard tanks on the islander selected in cruise you can start to see fluctuations in fuel pressure long before you will blow the tank...if you really want to get everything out of it you have to let it blow, the prop comes out of sync, you lose power it gets a little western for a few seconds even after you select the main.

In 20 years of flying and blowing tanks I still get a shot of adrenaline when that engine starts to wheeze for fuel, however I've never actually had an engine quite because of it.
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anofly
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Re: Hazards of Running a Tank Dry

Post by anofly »

I would further mention that on an RSA (bendix fuel injection) like on a piper single or twin comanche, when the system starts to pass both fuel and a bit of air as the tank runs dry, there is a chance if you are looking , you will see first, an increase in "indicated" fuel flow spiking upward. On an RSA system fuel flow is "inferred" from a pressure reading taken from the flow divider.
If you catch this upward "spike" on the fuel flow meter, you can swap tanks and get by with barely a stumble, and there is so close to no fuel left in the tank you are trying to MT it isnt funny.
As a side note, a plugged injector will cause an increase in the " indicated fuel flow" due to higher pressure at the divider. If you have a few gallons per hour of extra flow indicated to one engine, at similar power settings to the other engine, it is often a plugged injector. If you have an engine monitor it is easier to find where to start....
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anofly
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Re: Hazards of Running a Tank Dry

Post by anofly »

I would further mention that on an RSA (bendix fuel injection) like on a piper single or twin comanche, when the system starts to pass both fuel and a bit of air as the tank runs dry, there is a chance if you are looking , you will see first, an increase in "indicated" fuel flow spiking upward. On an RSA system fuel flow is "inferred" from a pressure reading taken from the flow divider.
If you catch this upward "spike" on the fuel flow meter, you can swap tanks and get by with barely a stumble, and there is so close to no fuel left in the tank you are trying to MT it isnt funny.
As a side note, a plugged injector will cause an increase in the " indicated fuel flow" due to higher pressure at the divider. If you have a few gallons per hour of extra flow indicated to one engine, at similar power settings to the other engine, it is often a plugged injector. If you have an engine monitor it is easier to find where to start....
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