Since flying the TB20 rather than an Arrow, I've noticed a few differences with the fuel tanks and fuel management.
Firstly, they're much bigger at 320 litres rather than 180 and secondly the fuel gauges seem to work (well at least between full and half tanks). We have a fuel flow meter that tells you the total fuel remaining and below 60 minutes fuel left, starts flashing how many minutes left before you start gliding, but can't determine the proportion for each side. Below about half a tank, you can't easily determine the level by inspecting through the fuel filler hole.
I've also noticed that balancing the fuel on each side seems to be more critical, otherwise can lead to becoming slightly out of trim in the cruise. The POH suggests not more than 75 litres imbalance between sides at any time (especially for landing). Our standard procedure is to change tanks every 30 minutes (so approx 20-25 litres), which should keep things in balance if you start off reasonable so in the first place.
I recall the Arrow POH suggested running one tank for 1/2 hour, then alternate every 60 minutes, presumably to reduce the number of tank changes while keeping <30 minutes of fuel imbalance at any time. I've also had instructors keen to switch tanks every 15 minutes.
I wondered if any imbalance would potentially be a greater problem with larger fuel tanks when the tanks are nearer empty than full, perhaps because it can slosh around more especially when banking. Should I be looking to switch tanks more often, and if so, is this more critical when full or nearer to empty?
In the Mirage it was not more than a 15usg difference. I changed at 10 used, 30 used, 50 used etc so there was never more than a 10 USG difference. I literally wrote columns 10, 50, 90 in the left and 30, 70, 110 in right and crossed off with a time. So if I ever lost the totaliser I knew how much fuel I had used.
In the TB20 a fuel tank imbalance is noticeable via the yoke, as one would expect.
When flying alone, I burn off the LH tank until the LH fuel gauge is about 7mm lower than the RH gauge. That produces the correct configuration i.e. ball in the middle, ailerons equal, horizon straight up, and max speed as a result.
I probably switch tanks every half hour.
Below about half a tank, you can't easily determine the level by inspecting through the fuel filler hole.
That is true for most planes, but is a bigger problem, apparently, on 6-seaters which tend to offer a greater range v. payload tradeoff. G-OMAR is a great example of operating below the visually inspectable level - all the more telling given that the operator was an FTO with a charter AOC!
Personally I almost never depart with either wing below the visually inspectable level - in the TB20 there is no reason to unless you are flying with 3 or 4 unusually heavy people.
The fuel should not slosh around much when banking if the flight is coordinated (ball more or less in the middle).
What I sometimes find is that the fuel warning light (either tank below 8 USG) comes up during taxi, if one is down to say 1/4 tank and taxies around a bend.
How old is your plane? The fuel gauges, if they are the capacitive type, should read correctly all the way down.
Perhaps switching tanks say every 15 mins when below the 1/4 mark is a good idea, but there is the other argument which is that one should run a tank very low before approaching the destination (some pilots say you should run it empty - I'd say you need brass balls for that) and then switch to the one with a decent amount in, for the approach and landing.
On long flights, say 900nm, I run a tank down to when the light comes on and then fly a further say 25 mins to burn off another 4 USG from the same tank, so there should be 4 USG left in it, and then switch to the other one, to land.
I have noticed out of balance tanks can be a slight,though not major impact on aircraft trim - but then so do other factors like is the life raft behind the heaviest of the lightest front seat person.
But, maybe once people have answered the original question, I would be interested in what people think of this:
In order to really know how much fuel you have, would you literally run one tank dry until the engine RPM drops, and then quickly switch over to the other?
My answer is I wouldnt, but I know of one person who employs this method in a PA28 aircraft, when on a long distance fuel level knowledge critical leg, and at a relatively high altitude. The apparent reasoning being that you cant really trust the fuel gauges, and without an electronic totaliser, this is one way to know what you do have (or at least dont have in one tank).
On takeoff in my Saratoga, I toggle the function button on the GTX330 to the timer function and then on every 1/2 hour, and multiple thereafter, I change from one tank to another.
With two tanks totalling 107 usg, by the time you're down to 70 usg, you won't be able to see any fuel via the filler cap, so you then have to rely on
a. The aircraft journey log, which tells you hours flown since the last fill up
b. The edm700 fuel totaliser
c. The mechanical fuel gauges set into the wings
It'd be a cold day in hell before I relied on the electrical gauges on the dashboard...!
Re: running tanks dry, that was pretty much SOP for long flights in my previous aircraft, a Cherokee 6 with four tanks totalling 84 usg - indeed, the procedure was set out in the POH. The reasoning was that to do otherwise might result in an aircraft arriving at it's destination with a perfectly sufficient reserve of fuel (say one hour) but spread over 4 tanks in such a way as to render it un-useable.
Having said that, the factory instructions was to run the mains (the larger inboard tanks) dry before going on to the tips, for reasons of main spar loading. I tended to ignore that instruction as it seemed to leave me with two dumb-bells of weight just where I didn't want it!
I don't have a problem with running a tank dry but the question is why you are doing it. I don't accept it really let's you go max range as if your instrumentation is so bad, how do you know how long to fly on the other tank?
Other than in an emergency, you shouldn't be anywhere near unusable fuel in both tanks.
Having said that, the factory instructions was to run the mains (the larger inboard tanks) dry before going on to the tips, for reasons of main spar loading. I tended to ignore that instruction as it seemed to leave me with two dumb-bells of weight just where I didn't want it!
Surely that is exactly where you ant the weight? In terms of structure it is definitely better outboard. On early King Air 90s there are mods that increase the MAUW by adding weights on the wingtips.
If you have a fuel totaliser and an iPhone or ipad there's a free app called Tank 2 that will track the amount of fuel in each tank. I don't know if there's an Android equivalent.
