14
$\begingroup$

This question pertains to trijets with two engines on the wings and one at the tail, like the Lockheed Tristar. It does not pertain to trijets with all engines at the tail, like the Tu-154. (And it doesn't matter if the rear engine has an S-duct or not.)

How is fuel stored and routed on a trijet that has only one engine at the tail? Does the rear engine have totally separate fuel tanks and lines?

$\endgroup$
  • $\begingroup$ The fuel system needs to be able to supply any engine from any tank and ideally also move fuel between tanks, because otherwise an engine failure would lead to a shift in center of gravity. Mounting a tank far aft also requires the other tanks to be moved forward, but if you want to use the wings as tanks, this means shifting center of lift against center of gravity. $\endgroup$ – Simon Richter Sep 14 '16 at 12:50
25
$\begingroup$

enter image description here
(Own work based on MSFS)

On an MD-11 the tail mounted engine is fed from tank 2, which is two halves as shown above -- the two halves are connected via vent lines.

Sequence

The center tank on an MD-11 is called aux, and it feeds all the engines until it's empty -- the aux tank actually fills the other tanks and the other tanks do the feeding (systems display shown below). Tank 2 (being bigger than 1 or 3) then carries on filling tanks 1 and 3 plus feeding all the engines. Once tanks 1, 2, and 3 have the same fuel quantity, each tank then feeds the respective engine.

enter image description here
(Own work based on MSFS)

Stabilizer tank

Fuel is transferred to and from the stabilizer (tail tank) to manage the CG. It is fully consumed by the time the plane reaches its top of descent -- as a forward CG is more economical for descending. The exact sequence is managed by the fuel system controller. The tail tank also feeds the tail engine as part of the CG automation.

The sequence (and scheduling) is automated even with an engine, or even two, out.

Summary

The #2 engine is fed by the aux, tail, and #2 tanks. If an engine fails, then cross-feeding is involved.

$\endgroup$
  • 4
    $\begingroup$ Great picture. Definitely shows what you are describing. $\endgroup$ – wbeard52 Sep 14 '16 at 0:45
  • 1
    $\begingroup$ Can you elaborate on why "a forward CG is more economical for descending"? $\endgroup$ – pericynthion Sep 14 '16 at 5:10
  • 4
    $\begingroup$ @pericynthion In forward CG you are already nose down. Thats what you need to descend. $\endgroup$ – vasin1987 Sep 14 '16 at 9:27
  • 2
    $\begingroup$ @pericynthion - that's a great basis for a whole new question! You should ask it. $\endgroup$ – FreeMan Sep 14 '16 at 14:15
  • $\begingroup$ @pericynthion: It helps keep you from pitching up unexpectedly, making it harder to stall the plane (especially important at low speed, such as when coming in for landing) and easier to recover if a stall does occur. $\endgroup$ – Sean Mar 28 '18 at 19:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.