I've recently read some insightful articles on piston engines by Avweb columnist John Deakin. He advocates the practise of running a fuel tank dry in-flight (as long as another tank contains fuel) and vehemently dismisses the notion that doing so can damage the engine and/or components of the fuel system.
I'd really like to know the mechanical implications of doing this and the answer to the question of whether not it has a quantifiable negative effect on the engine. Letting combustion stop once you're airborne is, to me, rather bold - especially when you consider that the turning of a knob five seconds earlier would have kept the fire going. I am however of the opinion that behavioural principles should result from something tangible rather than an overly-cautious outlook.
If the consensus is that running a tank dry is harmful, how much fuel needs to remain for satisfactory function? Would it be any less harmful to change to a fuller tank just as a reduction in fuel pressure is indicated, or would the hypothetical damage have already been done at this stage?
When there is a good operational reason, and you are properly prepared, running a tank dry is a good idea. At the first moment the engine begins to stumble fuel from another tank should be appied in accordance with the flight manual procedures.
When ferrying aircraft I used to do this regularly on alternaing tanks, to determine how long the engine would run, once "empty" was indicated. This was helpful to the other ferry pilot who took the plane long over water distances on the next legs.
When I flew the C310, I would do this regularly to confirm that tanks were empty, to optimize range.
As long as the engine is not trying to develop high power for long on no fuel, it will be fine. These things are considered during certification. Keep it to a minimum though.
In terms of a caution, do not begin this process, and forget you are doing it. It can be really disconcerting to have it quit turning base, or entering a hold, because you forgot what you were doing. If your phase of flight changes, or the workload goes up, select the fuller tank, and leave the experimentation to another flight. Also, warn your passenger that is what you are doing, before the engine stumbles, so as to not alarm them.
A couple of points. Both Continental and Lycoming do not recommending running the tank dry for fuel injected engines as the engine my not immediately restart.
For carburated engines there appears to be no real issues although I do not think it should be made a regular practice. I teach to save this practice for a minimum fuel situation. In this case it is for example much better to have 4 gals in one tank then 2 in each tank (eg Pa 28) as you are approaching your destination. So running one side dry to keep a good supply in the other is a good idea. of course it is better to not get into this situation in the first place......
If you do run the tank dry bring the throttle to idle before switching to the other tank and then when the engine restarts you can smoothly bring the power back up.
Legal day VFR minimum in Canada is 30 mins reserve, as I believe it is in the UK. The Pa 28 at 55 % power burns 6.0 Gals per hour therefore 4 gals = 40 minutes of fuel. You should not be in this situation by choice but hands up for all those perfect pilots that never once got tight on fuel due to unexpected headwinds or overly optimistic flight planning.
I stand by my assertion that in this situation it would be better to have 4 gals all in one tank than 2 gals in each wing tank......and you had better be at the stage of the flight where you are looking at your destination runway.
My thoughts too. One nearly empty tank with a depth of fuel is a better option than two very nearly empty tanks with a smear of fuel sloshing along the bottom.
Running a tank dry isn't a big deal, in my experience. In some types you can even get a low fuel pressure warning, change tank and not have the engine miss a beat. In others without the benefit of a low fuel pressure warning you can become familiar enough with fuel consumption to have your hand on the selector and ready a few minutes before the tank is due to run dry. At the 1st cough, change tank. Usually with little or no disruption.
An added benefit of running a tank dry is that it can give you a fairly accurate known fuel quantity, irrespective of what the gauges read. Consider a plane with a main and and aux tank, both full (or a known quantity in each prior to start). Start & take off on MAIN. A few minutes after start you establish in a cruise climb, note the time & change to the AUX. You will have good idea of fuel on board at that point - the margin of error for fuel used during the few minutes it usually takes to get airborne is very, very small. When the AUX tank runs dry you will again know fuel on board but still with the same negligible margin of error used for the fuel burnt for start & take-off.
Does running a fuel tank dry damage the fuel pump or increase the likelihood of vapour lock? I ask these questions with a carburettored Lycoming or Continental in mind. As an aside it seems that the general consensus on the Internet is that running a car's fuel tank dry can cause damage.
I have read Deakin's articles too, and while technically one cannot disagree with him on this one, I personally don't do it, because it tends to follow that on the long trips on which I need to be precise with fuel I tend to be crossing barren terrain (or better still mountains ) and you don't want to tempt fate over that.
But if one is flying the standard type of "useless fuel gauge" GA aircraft (most of GA basically) with no fuel totaliser installed, then there is a much better case for doing this.
I suspect that with cars pretty much all of the fuel in the tank is technically usable, while most AC have a fair amount of unusable fuel in them due to the shape constraints. What that means is that to drain a car tank dry, you will also suck any dirt from the bottom of the tank through the system, with an aircraft there is space for the dirt to settle where it wont be sucked in. No experience with this - so take this guess with a pinch of salt!
In the PA32 Cherokee six, you have 4 tanks (2 x 17 usg, 2 x 25 usg) with a total of 84 usg. You can select each tank in turn, but there is no facility to select "ALL".
To get maximum endurance safely, the POH advised running some of the tanks dry, in order to avoid the situation where you arrived at a destination with sufficient fuel in the tanks for reserves, but insufficient in any one tank to reliably cover the fuel outlet.
The advice, which seemed counterintuitive, was to run the two inboard tanks dry first, and then run on the outboard tanks; the reason given was to unload the main spar.
IME, the process was a non-event just so long as
a. you were prepared for it and switched tanks smartly (remembering to disengage the altitude hold on the autopilot)
b. You had sufficient altitude to deal with the exercise safely. Typically,the time between the engine starting to cough and full power being restored was 7-10 seconds.
Quite important to let any passengers know what is going on too, lest a 'brown-trouser' moment result. I only forgot once, but had quite a memorable response.....
The reason that it's not a good idea to run a car's tank dry is that the catalytic converter can be damaged. Whether or not that is the case I don't know but that is the common advice from manufacturers in the handbooks of all cars I've either owned or driven.
Aircraft of course don't have cats so that removes that problem. As for deliberately running a tank dry, personally I wouldn't do it unless I was in major bother and trying to squeeze every last drop out of the tanks to get home, shouldn't be a problem in ye olde carburetted Yank flat fours most of us sit behind but I'd still be worried about the engine not restarting...
Another issue with cars, as mentioned earlier, has to do with fuel injection. If the injector lines are run dry, it might take a significant amount of time before they're filled again with fuel. Not funny when you're in the left (or in the UK: right) lane with cars close behind you. Diesel is even worse in this respect. Don't know the details, but you might have to bleed the whole fuel system before you can get it going again.
Not to mention the fact that most cars only have one fuel tank. If it's dry, you're out of fuel period. Coasting to a stop in the inside lane of the motorway is a recipe for disaster on its own, regardless of the internal damage to the engine. So the advice on not running your tank dry in a car is sound for a number of reasons.
Anyway, I fly an aerobatics aircraft which doesn't have an inverted fuel/oil system. Under any zero or negative G load, the carbs simply flood, and the fuel pickup in the tank is sucking in air instead of fuel. Simply close the throttle (preferably in anticipation) and gently open it again when positive G is restored. The engine will pick up in two to three seconds.
If you forget to disengage the altitude hold whilst the fuel flow re-establishes itself, you can very quickly find yourself going very slowly indeed, as the autopilot struggles to hold altitude at the expense of your airspeed.
I can't say that i've ever lost very much (if any) height during this process, but it is critically important for the pilot to be in control, rather than the 'idiot savant'...
Last edited by wsmempson; 27th Feb 2012 at 12:49.
Reason: dodgy punctuation which muddled the meaning of the post...
We don't do it with the wing tanks, which are rubber bladders, but we do run the tip tanks dry. Our configuration has six wing tanks, from inboard to outboard there's the mains (30g each), the auxes (15g each) and the tips (15g each) giving a total of 120g and a potential problem with keeping track of where the remaining fuel is if you are not careful. There are two fuel selectors, one for each wing. If you do not close one of them, fuel can crossfeed via the sump and you won't know where the fuel is. We keep a detailed fuel log with the amounts used from each tank, using our fuel computer. These amounts are checked by dipping the tanks and by noting the uplifted amounts. Thus we have a good idea when a tank is supposed to run dry, almost to the liter. The fuel computer is a bit conservative so it may take a minute or so longer, but when we plan to run a tip tank dry we are ready. The moment the fuel pressure drops, we switch. The engine is windmilling and has no trouble restarting. It takes quite a while for the plane to slow down. Even when the switch is to the other wing tank, with the longest route for the fuel to reach the engine, there is no altitude loss. The disadvantage of running the tank dry is that you won't have remaining fuel in the line, so it will take longer for the new fuel to reach the engine. But the advantage is that you don't have fuel sloshing around that is of no use and sits in the worst position for balance.
Running a fuel tank dry will have no mechanical implications regardless of which fuel or metering device. Running out of the current fuel supply a) in the fast lane of the motorway or b) turning finals is not a good idea. I'd also like to dispell the myth that running a car low on fuel will "suck the muck". The fuel is drawn from the bottom of the tank regardless of tank quantity and so any contamination be thus ingested.
How many of you suffered there from a wandering of the mind when I said "suck the muck"?
A slight digression if I may, but on a related subject. Why are aircraft fuel gauges (i,e. Cessna models, 150, 152 etc.,) probably other makes too) so very innacurate and unreliable? Are newer aircraft types much better?
Another reason for not running a (modern) car out of fuel, other than having to bleed the fuel system, is that if it runs out while you're still moving, you'll lose pressure in the power steering and brake servo's, which could easily catch one out if you're not expecting it.
Are any aircraft fuel tanks baffled? I could see a baffled section at the lowest point of the tank being useful for accurate content measurement, and for placing the fuel pump in an area which should empty last.
The injector lines are emptied every time you shut the engine down after flight with the mixture lever and starting up the next time is straightforward enough with the boost pump. The fuel system is often completely drained during maintenance and again, there is no difficulty in getting the pressure up/starting.. The boost pump might sing for a couple of seconds as it's drawing fuel but that's it.