This is the bottom of the fuselage of a DA40NG aircraft and where the right wing attaches to the fuselage. What is the thing I'm pointing at with the red arrow in the picture?

As it is written in the photo, we see the fuel cooler inlet and outlet sections. But towards the right, where the wing is very close to joining the fuselage, there is a hole and a metal protrusion coming out of it.

Does anyone know what it is and what it does? At least from which source can I learn this information? Because I could not find this information in AFM and Airplane Maintenance Manual.

  • $\begingroup$ The fuel cooler is odd. It must be for certification using mogas or something. $\endgroup$
    – John K
    Commented Mar 9, 2023 at 21:36

2 Answers 2


That is a fuel sump drain valve. Water is heavier than fuel, so it sinks to the bottom of the tank. The sump drain valve is at the bottom of the tank so any water in the tank can be drained using the valve, this is done using a cup with a stick pointing out of it - you push the valve open using the stick and fuel and/or water drains out into the cup for visual inspection.

Airplanes often have multiple sumps, sometimes more than one per wing, either because they have multiple tanks, or multiple low points in a single tank. IIRC (and I could be wrong as it's been awhile since I flew one), the DA40 has 5: 2 per wing and one under the engine compartment. Most light singles have 3.

A pilot should drain fuel out of the sumps as part of the pre-flight checks to make sure there's no water contamination.

  • 1
    $\begingroup$ So, how is this fuel sump drain valve different from the fuel tank drain section on the left? Because before the flight, we check whether there is sediment or water by taking a fuel sample from the fuel tank drain section. $\endgroup$
    – pilot162
    Commented Mar 9, 2023 at 17:27
  • 1
    $\begingroup$ There's more than one @pilot162, I'll edit with more detail. $\endgroup$
    – GdD
    Commented Mar 9, 2023 at 17:28
  • 2
    $\begingroup$ @pilot162 The 172 I fly has 6 per wing and 2 on the belly. Makes checking for contamination...tedious $\endgroup$
    – Jamiec
    Commented Mar 9, 2023 at 17:33
  • $\begingroup$ We use only 1 drain hole for each fuel tank. I just learned that others do the same. $\endgroup$
    – pilot162
    Commented Mar 9, 2023 at 17:37
  • $\begingroup$ 6 drains in a 172? I've never seen more than 1 per wing on a 172, I guess that means they were made before the lawyers took over @Jamiec! $\endgroup$
    – GdD
    Commented Mar 9, 2023 at 17:48

Schematic illustration of Drain Valve:

enter image description here


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