Fuel gauge accuracy

[cristi.neagu]

Hello,

I read some posts on this forum stating that the reason why a Comanche pilot should use only one tank at a time is because the fuel gauges are inaccurate. Is this modelled in the sim? Or are the gauges 100% accurate?

Thanks.

[AKar]

They're accurate.

-Esa

[cristi.neagu]

Thanks for the reply!

[William Hughes]

There are other reasons to use one tank at a time beyond inaccurate fuel gauges.

[cristi.neagu]

Now that you mentioned it, can you please explain more?

[scottb613]

Hi Folks,

As most pilots know - as per the FAA - the fuel gauges in real planes only need to be accurate when empty - lol - any other accurate times are just gravy...

Regards,
Scott


Sent from my iPad using Tapatalk

[mallcott]

Hi Folks,

As most pilots know - as per the FAA - the fuel gauges in real planes only need to be accurate when empty - lol - any other accurate times are just gravy...

Regards,
Scott


Sent from my iPad using Tapatalk

A dipstick is in every A2A pilots flight bag for a very good reason.

Actually that's a thought for Accusim - inaccurate fuel gauge which becomes less inaccurate (or accurate, if you must) if the dipstick is used on each tank during the walkround...

[Scott - A2A]

I can only speak about the A2A Comanche in terms of fuel gauge accuracy. The main fuel gauges are very accurate and the tip tank gauge shows exactly 2x the amount of fuel in the tanks. So when the tip tank is right on F, the tank is 1/2 full, when on 1/2, it's 1/4 full, and so on.

The reason for running one tank at a time is so you can more accurately know better how much fuel is in each tank. For example, if you are in a steady cruise using 12 gph, you can switch to a tank and run a timer say, for 30 min. And when the timer is up, you know you have used approximately 6 gallons of fuel. And so on. This is just a better way to manage your fuel.

Scott.

[William Hughes]

A bit more detail as requested:

The fuel system is different on different types of Comanche singles. But in this 250 with tip tanks there are two 3 position valves in the floor. Each valve can shut off the fuel, switch to a main, or switch to a tip. These are connected to a common line which feeds to the pumps. So you can connect two tanks together if the two valves are set to a tank at the same time.

One main reason to use one tank at a time is that you can end up starving the engine of fuel unexpectedly. If connected to two tanks, one of which is empty, there is much less pressure across the empty line. The pump could end up drawing air from that line and very little fuel from the other. So the engine quits, but a gauge still shows fuel in the tank.

Another is uneven fuel flow. Each tank is vented in a different place on the aircraft. Depending on air flow around the aircraft this means that different pressures can exist in the tanks. So if two tanks are connected together at the valve you can end up pushing fuel around in ways you didn't expect, including possibly venting fuel overboard. One tank shows full and the other drains a bit faster than you expect.

Another is the uneven fuel flow again. Due to the different lengths of fuel line, valve arrangements, and venting the fuel will be drawn unequally from the tanks. Given that the mains gauges (irl) are rather iffy a person wants to track the fuel carefully. Emptying one tank at a time gives you four known data points where the pilot can know, for sure, that the tank is empty. Logging that means you can calculate your fuel burn pretty closely.

During startup, taxi, and run up I switch to the full left main and leave it there. I know for a fact there is a few hours of fuel in there and I have proven it works for several minutes before takeoff. I don't touch that valve until I am well up in the air and have the aircraft cleaned up. If the engine stops it isn't because of fuel. Plus there is only one valve to reach for if I have to shut off the fuel in a hurry in the event of a forced landing. One tank, one valve, always the same one.

I fly a 250 with tip tanks irl, very similar to the a2a simulation, and I'll take it on 5 hour legs. Switching between fuel tanks every 30 minutes, and keeping a careful log, means I can double check fuel burn rates against the flow meter in the air, and lets me keep careful track of the fuel. This isn't necessary for short fun hops or a sightseeing flight but I do it anyway as good habits are hard to form. But on a 5 hour leg, with six hours fuel on board, it becomes very important. Drawing from two tanks at once means the tracking becomes more vague and this is not a good idea.

More than one Comanche has ended its service life because the pilot lost track of fuel in the tanks. Some of the later singles have six tanks, and the twins can have eight. Those aircraft were force landed due to fuel starvation while there was still fuel in the aircraft. Totally unnecessary.

My aircraft has the Brittain tips and I've found that, for now, the tip tank gauge is spot on. The mains are mostly a notional guess. At high altitude the left main gauge shows nearly full until it suddenly drops to zero. Both of them vary with temperature, altitude, and I am pretty sure some evil deceitful spirit affects them from time to time. Not just fuel quantity.

Anything you can do to make the fuel management simpler is a good idea. One tank at a time for Comanches.

Note that these ideas here are specific to this type of low wing aircraft with this arrangement of fuel tanks. Other aircraft have other issues, and this isn't so much of a problem for high wing aircraft, for example. It's largely an issue where fuel has to be sucked up from the tanks.

[cristi.neagu]

Wow. Didn't know that. Thanks!