Aircraft with trimmable horizontal stabilizers need some way of sealing the area around the root of the stabilizer, so it can move up and down for pitch trim without opening up gaps in the fuselage that would admit weather and debris and create extra drag.

...Except, it seems, for this DC-10, which appears to have a pronounced open gap in the side of the fuselage just above the stabilizer root:

is that supposed to be open?

(Image originally by clipperarctic at Flickr, via Russavia at Wikimedia Commons; cropped and annotated by me.)

What's going on here?

  • 2
    $\begingroup$ I suppose the thing to note is that the area behind the rear door is home to the rear pressure bulkhead, the area behind that is unpressurized, so having a hole there (as long as it doesn't introduce a lot of drag) really isn't a problem. $\endgroup$
    – Ron Beyer
    Dec 20, 2021 at 4:36

1 Answer 1


Here McDonnell Douglas does not use the roller-type seal. Their own method uses doors that pivot out of the stabilizer's way. From their patent:

enter image description here

In your photo you can see the hinge line of the upper door (42) as well. Their method inherently doesn't fully cover the opening at large deflections, which is "permissible":

While it is important that the cutout portion be covered by fairings or doors during the cruise attitude of the stabilizer, it is less important in the extreme positions of takeoff, climb or landing, all of which occur at lesser airspeeds. Thus, while one of the doors is pivoted inwardly on one stabilizer surface to permit stabilizer movement in that direction, the extreme pivotal movement of the stabilizer may cause a gap or air space between the other door and movable fairing on the other stabilizer surface. However, this is permissible since it occurs only when the aircraft is in a non-cruise attitude.

— Backlund, J., and R. Gibbs. "Fuselage seal." U.S. Patent No. 3,756,529. 4 Sep. 1973. Current Assignee: McDonnell Douglas Corp

You can also see it on the DC-10-derived MD-11:

enter image description here
— Rob Schleiffert via flickr.com [cropped]

Tip: search for photos of landing to get more hits, as this is when the THS is at a high trailing edge up position (taxiing for takeoff may also work).

  • $\begingroup$ Does the DC-10 AFM have restrictions on the use of extreme stabilizer positions in cruise (which could potentially restrict the aircraft's acceptable CoM envelope more than otherwise would be the case)? $\endgroup$
    – Vikki
    Dec 20, 2021 at 0:49
  • 5
    $\begingroup$ @Vikki: that angle is way too big to be needed in cruise, see here. $\endgroup$
    – user14897
    Dec 20, 2021 at 0:52
  • $\begingroup$ Could still potentially present problems in the event of a nose-up trim runaway in cruise, though (well, problems in addition to those inherent to a nose-up trim runaway in cruise). As an aside, I'm hoping there's a maintenance task to periodically clear out anything that's blown in through the gap while on the ground... $\endgroup$
    – Vikki
    Dec 20, 2021 at 1:32

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