Low Fuel Annunciation question

[gkaplan]

Hello,
On a recent flight, I started a fuel transfer from right to left tanks and got distracted. By the time I noticed, I was down to about 4 gallons in the right tank and 11 in the left - well within the 9 gallon imbalance limit. I was pretty close to the destination. As I got closer to destination, I started getting low-fuel warnings. To see if I could stop the annoying warning, I moved a little more into the left tank, leaving about 3 gallons in the right - maybe there was 5 in the left. That didn't solve the problem. This made me wonder, is the low fuel warning considering the level in the right tank? I thought it was possible to drain the right tank completely and fly off the left (within balance limits). Does anyone know what the parameters and levels are for low-fuel warnings in the DA40-NG?

[chili4way]

The low fuel warning only senses the fuel level in the left/main tank. See the AFM page 4B-16 and this note. When the left/main tank gets low, you'll experience intermittent warnings if you slip the airplane, e.g., during a cross-wind landing from the left or an uncoordinated turn to the left with insufficient left rudder.

There is a low fuel sensor in the right/aux tank. It turns off the fuel transfer pump, similar to what the high fuel sensor in the left tank does. See the AMM (maintenance manual) section 28-20-00, page 5. Many pilots avoid entirely draining the right tank.

A handy diagram of the DA40NG fuel system is on section 28-00-00, page 3.

[Boatguy]

Many pilots avoid entirely draining the right tank.

I avoided emptying the right tank because then the engine strip on the MFD shows a big RED indication for the right tank which is distracting. So I would leave about a gallon.

When the left tank gets low, it's pretty easy to trigger the "Low Fuel", especially when taxiing.

[gkaplan]

Thank you to both of you. Appreciate the detailed response.

[Lance Murray]

I’m just curious. Why does the NG have or need the transfer pump?

[chili4way]

I’m just curious. Why does the NG have or need the transfer pump?

TL:DR - The NG's heated fuel return into the left tank makes it simpler and less error-prone to manage fuel levels and balance by transferring fuel than trying to do this using the fuel selector switch.

The design of the fuel system is different. One crucial difference is the heated fuel return from the high-pressure pump (HPP) and common rail system. In typical operation, one of two electric fuel pumps draws the fuel from the left/main tank. It is fed to a high-pressure fuel pump (HPP) that feeds the common rail injection system. The HPP compression heats the fuel and produces more fuel than the injectors require. That excess fuel is returned to the left/main tank through pipes running through the right/aux tank, warming that fuel.

The fuel transfer pump moves fuel from the right/aux tank to the left/main tank. If the fuel transfer pump fails, an emergency position on the fuel selector causes the engine to draw fuel from the right tank. However, the excess heated fuel is still returned to the left/main tank. This means it is possible to run the right tank dry (not good for the HPP) and still have fuel in the left tank. The AFM has warnings about this emergency mode of operation.

Operating the fuel transfer pump has a reduced risk of pilot error than managing the fuel (flow and balance) with the fuel selector. This is why the fuel selector is only used in an emergency.

See the fuel system diagram in a previous post in this thread.

[Boatguy]

If the fuel transfer pump fails, an emergency position on the fuel selector causes the engine to draw fuel from the right tank. However, the excess heated fuel is still returned to the left/main tank. This means it is possible to run the right tank dry (not good for the HPP) and still have fuel in the left tank. The AFM has warnings about this emergency mode of operation.

To further clarify, the Emergency selector position should be treated like an emergency fuel pump switch, not like switching tanks on a DA40. About 90% of the fuel pumped to the engine from the right tank will be returned to the left tank. Thus the Emergency position should be used for no more than 5-7 minutes at a time.

[Soareyes]

Operating the fuel transfer pump has a reduced risk of pilot error than managing the fuel (flow and balance) with the fuel selector. This is why the fuel selector is only used in an emergency.

Trying to wrap my head around the fuel system in the DA40NG. Isn't it still possible to forget to transfer and run the left tank dry (while ignoring CAS messages)?

[gkaplan]

Yes. The same is possible in a non-NG if you forget to switch fuel tanks - you’ll eventually run the tank dry and starve the engine.

[gkaplan]

One thing I have wondered is regarding the fuel totalizer - is the unused fuel that is returned to the left tank accounted for in the totalizer? Where does the totalizer measure fuel used? The returned fuel will ultimately run through the measuring sensor again - in my mind, this means the total fuel used quantity calculated by the totalizer would indicate more than was actually used since some was returned. Hypothetically I could imagine filling the tanks full (say 39 gallons), running them completely dry, and having the totalizer show 43 gallons used. (Hypothetical of course)