RESEARCH - Airliner fuselage "dimples" at stabilizer roots, etc.??

I’ve not had good luck digging up a quick explanation of this one. I’m hoping some of our Doper aeronautics people can help.

On newer airliners (say, ones built in the last 10 years or so) you sometimes observe an indentation in the fuselage (or I guess more appropriately in the empannage) right at the horizontal stabilizer root. The root of the stablizer is actually “set in” if you will, several inches below the mean surface level of the rest of the sheet metal, with the indentation taking on the general shape of an ellipse.

I have to assume that this is somehow related to fuel efficiency - i.e. altering laminar flow characteristics around the airfoil where it meets the empannage, etc. But I haven’t been able to find any good information about what these things are there for. Mostly, I think, because I haven’t the foggiest idea what they’re called.

Any help on this one?

Don’t suppose you’ve got some links to pictures have you?


The most obvious examples that I can find of what you are talking about are on the 767 and A-300/310, but many others have this.

I think that this design is a function of horizontal stab movement. Most large jets use the stab as a longitudinal trim device. The elevators are still controlled by the yoke, but when changing the trim the entire stab moves up or down. Very efficient, because the entire surface area of the stab can be used instead of just the elevator. This also allows a very large CG range. The “dimples” you see are where the fuselage must be flattened to accomodate movement of the leading edge of the stab. Occasionally you will see marks on the front end of these dimples to indicate the limits of stab travel.

Airplanes that do not taper very much near the empennage (ie narrowbodies like the 737) can just smooth the tapering for the length of the stab and all that is visible is an “aisle” for the forward control arm to move through.

Airplanes with high-mounted stabs (T-tails) need no tapering because the vertical stab is symmetrical anyway, but the area where the horizontal stab can move is still flat - it’s just not that noticeable.

I hope this is what you were talking about!

Thanks, I think pilot141 answered it. I never even thought of the issue of mechanical clearances before.

And Skogcat, I’d post a link to a picture if I could find one handy. My bad luck at finding out basic info on this extends to images. I haven’t found any airplane pictures to date that are that detailed.

Now, is there a name for these things?

Dang, beat me to it. Yeah, what appears to be elliptical cutouts are really areas where the fuselage flattens towards the tail to allow for the stabilizer moving with trim changes.

I know this post is redundant, but I come bearing pictures:

777 front view, looks a lot like cutouts.

777 side view, you can tell it’s really just the flat taper of the fuselage.

As for links, try these:

A VERY obvious example on an A-340:

Another example on an MD-11:

TVGuy, as to what these are called: I have no idea. I’m sure some Boeing engineer has a name and even an acronym for these things, but in my experience they are considered so integral to the design that the names don’t trickle down to the operators. However, I learned early on that any device, component, section, or unit that is on an airplane but cannot be described by any other word is a HOODANKUS.

Pronunciation: HOO (Who) - DANK (dank, sank, prank) - US.

This can be inserted into the conversation in place of the dubious item’s name. “Well, Boeing does a much better job with their Hoodankuses - hell, you can’t even paint an Airbus Hoodankus! What’s the point of painting an airplane if your giant Hoodankus sits out there with it’s silver metal messing up the whole look? If you ask me, it ain’t a real Hoodankus unless it can be painted!”

BTW, if you haven’t tried out, you should do so. It’s by far the best place on the web for airliner pictures. (I don’t have any ties to the site, it’s just a great resource!)

OK, I found a picture - not terrific quality, but it’ll do

Airbus A320 tail photo

This isn’t the best example, as on some airplanes, this structure looks WAY deeper than it appears here.

But, I can sure see now what pilot141 is talking about - it’s obvious once someone provides a little enlightenment.

I learn something new every day on the SDMB.

(But I’d still love to find out what this structure is actually called…)

I love a triple simul-post!!

Thanks guys. JoeyG, THAT’s it!! That 777 picture is exactly what I was talking about. I guess a lot of it depends on the lighting - sometimes, when you’re sitting in the gate area and the sun’s hitting the plane just right, you’d swear that these things are three feet deep.

pilot141, you need to spend some time in the broadcast engineering world. We have more HODANKUS’s than you could imagine.

Thanks for the help folks.

I have 2 guesses of what you may be talking about.

  1. The stab is basically a one piece wing, something like the wings in balsa wood airplanes. It is hinged at the back and the forward can be raised or lowered by jackscrew. This is how airplanes maintain flight attitude. The hole through the empennage is shaped like a piece of pie. Close out panels are installed on the upper and lower stab surfaces, the forward edges of these panels ride in slots to provide air deflection. I have done stab rigging on the 737 and 777 model airplanes. This is the same jackscrew that was at fault for the Alaska Airlines MD-80 crash a few years ago.

  2. Or you may be talking about the APU ram air inlet. This is located on the right side of the empannage just forward and below the right hand stab. Under normal circumstances the APU is not needed in flight. But in case of an emergency, the ram air inlet has an air deflector that deploys to air start the APU. Air pressure is also tapped for pneumatic and bleed air operated equipment.

Here is a picture of a 737 prior to installation of the stab. It is hard to see but you can see the slot that allows the stab to move up and down. On the 737, this slot is about the just forward of the midpoint of the stab.

In thispicture you can almost see the APU ram air inlet. It is just aft of the rear service door and cuts into the narrow blue line.

This is where I work. The factory looks a lot different today, the airplanes are now nose to tail.

Excuse me?

The flying stabilizer is still around, and using the same jackscrew as the Alaska MD-83? Does the FAA know about this? (rhetorical - the FAA allowed that MD-83 to be ignored to the point of failure).

Is the jackscrew still a “jesus joint”?


The flying stabilizer will be around as long as jets are around. If you want to fly on an airplane that does NOT use the horizontal stab for trim, fly on a prop or some other airplane with a very narrow airspeed range.

The jackscrew on the stab is one of the most frequently inspected parts on an airplane. Just because the same part is being used on airplanes today does NOT mean it’s unsafe.

Let’s say you put some brand-new Firestone guaranteed Flying-Tread tires on your staion wagon. The NHTSA determines that the tires you are using are UNSAFE! So you replace them with Goodyear guaranteed Flying-Tread tires. Oh My God! You have replaced Tires with TIRES! Stop driving! TIRES are unsafe!

You know, of course, that your new tires are different than the unsafe tires. Much like an airline knows that its jackscrews are different than the unsafe ones…but they are still jackscrews.

The jackscrew CAN be a “jesus point” if you horse around with it enough like the Alaska guys did. If you take what you get (a stab stuck in one position) and fly the airplane to the ground you will be fine.

No one wants to see airplanes crash, especially airlines. They walk the fine line between ticket prices and profitability, and unfortunately there are many places that will cut safety to improve the bottom line. Hopefully passengers will shun these airlines and will pay for the assurance that their airplane is the best possible specimen…

Or maybe I’m just dreaming…

Actually, even some props use a flying stab. Beechcraft Sundowner is one, if I recall correctly (haven’t flown one since 1996, so my memory may be somewhat faulty) And the Mooney moves the whole empennage for trim.

Yeah, you’re dreaming. Passengers aren’t willing to pay for anything that doesn’t effect the quality of the peanuts they get on the trip. That includes fuel, maintenance, safety equipment and security.

IMO, the only reason they’re putting up with the new TSA rules right now is that 9/11 is still to recent and fresh in everyone’s memory. Give it a few years and I’ll betcha that we’ll be right back in the $99 ticket wars, passengers bitching about an extra five minutes and yelling at flight attendants.

  • by the way, GREAT website over there at I’m amazed and the number and quality of photos on there. What I’m really amazed at is that so many people shoot pictures of airplanes that you can pull up multiple pictures of INDIVIDUAL AIRCRAFT by tail number from places all over the world. Thanks for the link earlier.