Flying with both fuel tanks selected?

[peterhemm]

I don't recall seeing anywhere in the Comanche docs a restriction to only use one fuel tank at a time during flight. Sure, it says "select desired tank", but are there reasons not to use both at once? Inadvertent fuel transfer messing with CoG?

[Frank.O]

Hi,

I read somewhere that it is not recommended because if you come into an uncoordinated state fuel could flow from one tank to the other and put the aircraft out of balance. This works in the Comanche as I noticed when doing a walk around with left half empty and right full. When checking for water both tanks must be on and when I was done after about 5 min 8 liters had flown from right to left.

Frank

[Capt. Timeter]

It will hardly put the aircraft significantly out of balance since the tanks are very close to the longitudinal axis. The reason not to do so, at least the one I got reading stuff about the Comanche, is the construction of the bladder tanks and vented caps. If one of the tanks develops a leak subtle enough to be noticed immediately, it can still syphon the fuel out of the both tanks and crossfeeding may keep the level perfectly even preventing you from realising that something is not right until it's too late.

[AKar]

Yes, what happens in the sim is that the tanks soon end up having equal levels, balancing themselves out. I find little reason to see this as undesirable, so I most often fly the A2A Comanche with either both mains or both tips selected. I tried at some point if it was possible to create an out of balance condition by flying uncoordinated, but I ran out of patience and interest before anything noticeable happened in that regard.

-Esa

[Jigsaw]

Besides... one creates a fuel imbalance in the first place when the fuel is being consumed asymmetrically by only using one tank at a time, right?

[AKar]

Yes, obviously. And in the sim, that can easily be balanced by selecting both tanks: if you run another tank half empty, and then select both, they soon balance out at approximately three quarters. That's why I usually don't bother with switching from one tank to another individually.

As more real life consideration, however, it is good to remember that the fuel levels in such arrangement form an unstable system. Think it like having two buckets at the ends of a balanced plank and connected through their bottoms by a hose. Whichever bucket is just a tiny bit lower, perhaps by being a bit fuller and heavier, starts to draw fuel from the other bucket, thus ending up even heavier and even lower and so on, perhaps even overflowing in some cases. Real airplanes just about never sit precisely level nor do they fly perfectly level either, so the conditions for such transfer can exist almost always. In practice, how susceptible different airplane models are to such 'fuel transfers' appears to vary. And the weight of any practical imbalance tends to have little real effect on how the airplane sits, but everybody gets the idea. If it leans, the fuel goes downhill.

-Esa

[Frank.O]

And this is what Scott of A2A (owner of the simulated aircraft) says about it.

[AKar]

And this is what Scott of A2A (owner of the simulated aircraft) says about it.

Personally, I find little applicability for these in sim. Having two fuel tanks that balance themselves out as perfectly as I could ever desire would not make my fuel management any less precise; if anything, it would be the opposite. And, running both tips dry at the same time seems to make no difference whatsoever when compared to running them dry one by one. I never run my mains below the quarter marks (unless specifically doing something stupid), and as they stay well balanced, I've found no risk of running one dry by accident. (The concern is valid in a sense that an open path into a dry tank opens an airway that can disrupt the suction from the other tank, think about puncturing a siphon.)

I do, however, do takeoffs, landings and ground ops on single main, simply to not unlearn the steps I've made my routines of. And, if I was flying a real airplane that I knew acted differently from what happens in the simulator, I'd probably apply similar logic, and would not want to unlearn those routines either in that case. But, if it balanced its tanks out as nicely as the sim one does, I would likely take benefit of that in cruise. Anyways, that's just my point of view.

-Esa

[jeepinforfun]

I have read everything here including what Scott has said in the past, including the old operating manuals, which basically say, "In order to keep the airplane in best lateral trim during
cruising, the fuel should be used alternately from each tank." during cruise and also select the appropriate tank before start and takeoff. So this is what I have done when flying, it also gives me something to do during cruise.

I tried searching for any information on RW Commanche owners as to what they do but couldn't find a thing, I also tried to look up whether or not the Comanche tanks will balance themselves out to no avail. I consider Esa (Akar) a pretty smart fella and often find his input factual and well written and have learned a bunch from him, in this case I think flyers here can do it either way they find comfortable, for me I will continue to do it the way I said above.

[av8erjim]

Been searching for the type of fuel selector we had in N7727P. Can't find it on any of the images. It was just one selector switch, might have been something that was STC'd before we bought it. At the 10 o'clock position was LM, two o'clock was the RM, 8 o'clock was the LT and then at 4 o'clock was the RT. The other thing I've never seen on any other we only had one fuel gauge, but there were push buttons position around the selector switch and if pushed the gauge would show how much for that tank, then push one of the other buttons would change the fuel gauge to that reading. So if we had all four tanks full, we would take off on the left main, then switch to the right tank and run for the same number of minutes, then to the left tip with 15GPH would run about an hour, and then the same on the right tip, then run the left n' right mains switching about every half hour. Never had a problem with our procedure, flew it that way for five years when my partner got transferred and we sold the plane

[sam-tx]

I run both and have never had any issue.