I read about the fuel systems on aircraft for a project I'm working on, and I was wondering about the "fuel drain ports" mentioned, which the document stated were used for complete drainage of fuel tanks and sampling. On a 747-300, do they exist? What do they look like, where are they, and how are they controlled or opened/closed?


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Disclaimer: The B747-300 belongs to the B747-100/-200/-300/SP family. I have detailed knowledge of the B747-400 & -8 which differ quite a bit from the originals. That being said I would be very surprised if Boeing changed much regarding sump drains.

All tanks in transport category aircraft have individual sump drains. That is a requirement in FAR 25.971. In the B747 the sump drains are fully mechanical poppet valves. The poppet is held in place by a spring pressing down on it sealing it against the valve seat. To drain it you press against the poppet unseating it and therefore allowing fuel to flow.

There are two versions:

  • direct type (fuel intake directly above the valve)

    Used in no. 2 & 3 main tanks, the center wing tank and all surge tanks. Located under the wing in the lowest locations of said tanks.

  • indirect type (fuel intake connected to the valve via a fuel sump drain line)

    Used in no. 1 & 4 main tanks and in no. 2 & 3 reserve tanks. Located under the wing inboard of said tanks. That's due the lowest point of the tank being above the engine strut and the associated fairing.

Cross-cut of a direct type sump drain: direct type sump drain

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    $\begingroup$ I can’t help but wonder what the draining device looks like, and what the draining interval is. I am picturing someone out there with a ladder and a GATS jar every morning. $\endgroup$
    – Max R
    Oct 14, 2022 at 5:14
  • $\begingroup$ @Max for the draining interval, see here: How often is the fuel sampled in turbine aircraft? $\endgroup$
    – Bianfable
    Oct 14, 2022 at 6:56
  • $\begingroup$ @Max the draining device is a transparent container with a long tube attached to it so you can easily reach the drains without a ladder. $\endgroup$
    – Chris
    Oct 14, 2022 at 10:00
  • $\begingroup$ Whew! I was thinking about the poppet drain for the oil filter in my car. I could only envision the poor sap who had to do this getting soaked in Jet A every time... :) $\endgroup$
    – FreeMan
    Oct 14, 2022 at 15:38

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