Airborne_Again wrote:
If I may speculate as to why light aircraft normally use volume and not mass is that temperature differences during operation are not that great.
Probably correct. That said, the OEM fuel flow meter in our C210 is calibrated in lb/hour. The EDM, however, shows USG/hr, so in a way we have it both ways. In any case, I have yet to come across a fueler who measures 100LL in lbs and not in gls.
The gauges used in the F-16 (and most jets of any kind for that matter) measures capacitance or resistance (don’t remember). This is dependent on the mass inside the level of where it is measured
I am pretty sure that is not correct, otherwise the lotsa-$$$$$ business around true mass flowmeters (one of them having been invented, I am told, by a guy I was at univ with – way smarter than I was, too) would not exist.
OEM fuel flow meter in our C210 is calibrated in lb/hour.
The analog indicators usually show the pressure coming out of the 7th hole in the fuel distribution spider, which sort of works 
I guess quite a few of those ultra complex fuel tank arrangements come from the fact that the original airplane was designed with too small fuel tanks to begin with. So most of these 4-6 tank arrangements are either after market or later additions to make a short-legged airplane more attractive for better range.
If you end up with 6 fuel tanks which need to be burned off in a particular succession not quite obvious to the new operator or even worse contraptions such as baggage tanks which flow into one particular other tank while pumping fuel over board from return lines, the question is if a plane like that is good advice to the average low time pilot. If you add to that tanks which don’t have fuel gauges or whose fuel gauges need to be switched to show the correct amount, then maybe it is not a bad idea to look for a type with less exiting arrangements.
Looking at fuel schedules for some more complex types they can cause head scratching. E.g. some airplanes need to empty one particular tank or set of tanks first in order to provide space for return line fuel from the engine(s). Only then may auxiliary tanks be used. Some airplanes have tip tanks which empty by gravity into other tanks while others need to be switched manually. Some fuel valves for left and right are augmented by secondary valves which then define what left and right actually feed from and all that without an actual means of checking individual quantities at the same time. I’d say, some would make the FE panel of a 747 look pale in terms of practicality. And it may make you feel jealous of the nice airliner representations which have nice EFIS based fuel pages where you can see all tanks and what feeds from where.
However, I fail to see why the use of a normal fuel computer should be any different. Usually, fuel computers derive their function from a flow transducer fixed somewhere between the fuel valves and the engine, so as long as fuel is flowing, they will provide flow and whatever else they can provide. The paramount thing is to know exactly how much fuel you have to start with, something which is not always very easy to determine unless tanks are full. Likewise, there are airplanes with long range tanks which won’t show the correct amount on the gauges until a substantial amount has been burned off, Mooneys with aftermarket Monroy tanks are like that for instance, so in such cases the FC may well be the only half ways reliable source of information on how much fuel you have on a given point in time.
I won’t lie to you, I am perfectly happy with a left/right arrangement without additional quirks, so if I ever were to upgrade to a twin of my dreams, there might go my ambition or a Twin Commanche with 6-8 tanks in favour of a Cougar or Seneca, which have dead easy systems.
I’m a big fan of the six tank system in the C 310 I used to fly. When used properly, it gave you great confidence as to how much fuel you had, and where it was. But, you had to use the fuel system as it was designed. Selecting the aux tanks before there was room in the mains for return fuel, would be at the very least, embarrassing and expensive.
When I flew the C 303, I liked the fuel system much less. Sure, it was more simple, one tank per engine, but when you see the fuel quantity indicator, powered by a junky quantity transmitter, bouncing an eighth of a tank around the quarter mark, how much fuel to you actually have? This concern weighs more heavily, when you can’t see Iceland yet!
As said earlier, understand the fuel system in the plane you’re flying, and use it as designed!
Peter wrote:
I am pretty sure that is not correct,
I’m pretty sure it is
Mass flow meters are always handy though, IF they are accurate, or much cheaper than any competitor at the same accuracy, or smaller, or easier to install and so on. But mass flow can be many things, gas, liquid, solid material (powders typically), and mixes of all three. Where they are most obsessed with it is in the food industry, measuring the mass flow of powders and mixes, that’s tricky.
Pilot_DAR wrote:
As said earlier, understand the fuel system in the plane you’re flying, and use it as designed!
Absolutely, that is the key.
I am not sure how the fuel system in the C310 works, do you have a gauge for each tank? And how is the valve switching?
I’ve seen one system (not sure I believe on a Baron) where you had master fuel selector which had the main and aux position, then a 2ndary fuel selector which selected the actual aux and some additional tanks which (not sure anymore) emptied into the aux tanks. The fuel gauges had to be switched from one to another by a different selector and only showed the fuel of the selected tank. Personally I did not like the complexity very much.
I also remember a Setup in a Cessna 210 which had an additional baggage tank which needed transfer in flight into a specific tank. One one flight the fuel pump broke leaving the airplane out of fuel despite having some considerable amount on board.
You are very right about the fuel gauges. I have seen some Cessnas which had massively inaccurate gauges up to the point where it showed zero when there was more than 1/4 tank left… I am pretty happy with the fuel gauges in the Mooney however, in the C they work really well. In newer models you need separate gauges (on the wing) to see the fuel on ground vs in the air, as the airplane sits quite nose high and therefore the cockpit gauges won’t show correct unless in flight.
The thing I don’t get, totaliser or not, is how can you possibly know where the fuel is, on a multi separate tank system, unless you let them go dry in turn.
A better way would be a means of being sure a tank is empty. Presumably that is how auto tank switching works; they detect some low level in the tank. The traditional GA fuel gauges are pretty useless.
Peter wrote:
The thing I don’t get, totaliser or not, is how can you possibly know where the fuel is, on a multi separate tank system, unless you let them go dry in turn.
Like Emir said in post #15 for the DA42 case. When there is enough room in the main tanks, you transfer all the fuel from the aux tanks into the main tanks. So in a sene you run the aux tanks dry by not by running the engine from them but by transferring them into the main tanks. It’s of course more difficult if you have an aircraft where you can’t transfer between tanks.
Peter wrote:
The thing I don’t get, totaliser or not, is how can you possibly know where the fuel is, on a multi separate tank system, unless you let them go dry in turn.
You need to keep a fuel schedule and ideally use a similar worksheet as I recall them for the FE’s on large jets in ages past. In practice, you get a sheet with each tank listed separately (and possibly graphically) where you put the actual loaded fuel per tank down before start up. The totalizer needs to be manually set to the combined figure. Thereafter, every time you switch a tank you calculate and put down the fuel used from that tank (taken from the Fuel computer and crosschecked with the gauge) and substract it from the figure that tank had at the beginning. Then you do the next and next and next tank, which also allows you to balance the fuel system. It will get more complex where you manually transfer fuel from one tank to the other, but in general, you need to keep your bookkeeping and crosscheck it with the gauges in level flight.
A better way would be a means of being sure a tank is empty. Presumably that is how auto tank switching works; they detect some low level in the tank. The traditional GA fuel gauges are pretty useless.
Auto tank switching is quite a lot more complex but bases on the same principle. The manufacturer will give you a fuel schedule, with which you need to work. Which tank needs to used first, which 2nd and so on, what is the optimum CG and how do you have to transfer fuel from one tank to the next, e.t.c. Concorde was known for CG being moved by manual fuel transfer, Airbus has trim tanks in the tail which also get operated with the goal of arriving at an optimum CG. The difference is that airliners of today do all this automatically, while those who had complex systems did have a flight engineer.
You can run tanks dry but you need to be careful. Easiest is to keep your eyes on the fuel pressure gauge when you approach emty and switch at the first sign of the pressure dropping. That will leave a small quantity in the tank but is better than making your pax nervous with temporarily starved engines. Some manufacturers actually reommend running tanks dry.
A French Staggerwing landed in a field a few years ago because one of the fuel selectors had broken. It had 5 fuel tanks, a main and 2 each side. It transpires that instead of a single selector (Off-1-2-3-4-5), it had 4x (Off-L-R) selectors inline, making 81 possible combinations. I’m surprised they’re still flying.
