As a little background, although I don't work in the industry I have a degree in Aircraft Engineering, and have an avid interest in commercial and military aircraft.

On a recent flight on _______ Airline my flight was delayed due to maintenance issues on the aircraft that we would eventually take to our destination. After a 2hr delay we were told the aircraft was fixed and in route. When it arrived and taxied to the gate I noticed the port side control surface on the horizontal stabilizer standing almost straight up in the air and the trim tab also seemed to be at full deflection. The starboard side control surface was in a neutral position. This caught my attention.

As they moved the jet bridge and deplaned I watched the port control surface return to neutral, then back to this near vertical position several times. The starboard side never moved. No other control surface on the aircraft moved during this 3-4 minute time period. Finally the port side horiz. stab. control surface returned to full upward position and stayed there.

Eventually an older gentleman in a pilots uniform deplaned and I got his attention. I motioned him over away from people, gently nodded out the window and asked if we were still having issues with the aircraft. He said replied no. I noted that I had a bit of background and fly a lot, and have never seen that behavior in control surfaces before. At that point he gave me a 'shut up you idiot look' and told me that it is normal, the surfaces on that aircraft move deferentially, its normal. I told him that I watched that surface move between full up, and neutral and the opposite surface never moved. He gave me condescending smile and said "it's normal, it is how airplanes fly" and walked away.

I was uneasy, thinking that with a Horizontal Stab that high above aircraft center line, imparting a differential control input would induce a pitch, roll, and probably yaw moment. Granted I don't spend a lot of time thinking about MD80's and perhaps I'd fallen behind a bit in tech, or they were running a specific test that he didn't want to reveal I let it go. The flight was uneventful.

Tonight, right at the end of the 60 minutes spot on Allegiant air, they showed an MD80 from the rear and both horizontal stabilizer control surfaces moved through the full range, together. It got me thinking again.

What say the experts?

Not an expert or an airline pilot, but No way in hell I am getting on any aircraft after I see the horizontal stabilizer move. That just ain’t gonna happen. Elevator, okay. Stabilator, okay. Horizontal stab, not happening.

There are probably other configurations (leaving aside v-tails and their ruddervators), but the horizontal tail feather configurations I am familiar with are: 1) horizontal stabilizer and elevator where the horizontal stabilizer is a fixed surface (like the wing, but smaller) and the elevator is the movable control surface. and 2) stabilator where the whole surface is movable.

Okay, I think I know what happened. Googling for MD-80 control surfaces, I found this link https://www.youtube.com/watch?v=XcgChCAN26U, which wasn’t useful, but a related video talking about the rudder, elevators and stabilizer was. Turns out the two separate elevators are not mechanically interconnected and are controlled by geared tabs. If I understood this correctly, the elevators free float and the pilot doesn’t directly mechanically control them like on most small planes. Instead, the pilot controls the tab, which controls the elevator. Assuming the video is correct, the elevators could be blown around by whatever wind they experience when the plane is sitting still (or moving slowly) on the ground. Given that the elevator surfaces are not parallel and are on opposite sides of the rudder, it is easy to believe the two separate elevators would experience different wind and behave differently.

It would be a dick move for a pilot to give you a shut up stupid look, but I can understand how a pilot might feel challenged to quickly and succinctly explain how that is normal.

I never flew the Mad Dog, but I've been told the elevators are not connected like a standard elevator, but simply balanced. The trim tabs move and impart force to move the elevators, or some such PFM like that.

I watched this video, which made what I observed even more scary... https://www.youtube.com/watch?v=-iZoKLbvFnc

The thing that caught my attention was that the stabilizer was in what I'll call it's normal ground position (no different than the other aircraft parked at neighboring gates. The port elevator (control surface) was standing up at a high angle, the center trim tab was also extended upwards.

According to the video, this is not possible as
A. The elevator is moved by aerodynamic forces from tab actuation. Since the aircraft was parked there would be no way to move the elevator

B. The trim tab was extended (up) in the opposite direction from what would be required to move the elevator upwards (tab down)

How did that happen? the elevator moved at least 6 times from neutral to full up (or what I assume was full up as I could see the entire control surface)

quote:

Originally posted by Rightwire:How did that happen? the elevator moved at least 6 times from neutral to full up (or what I assume was full up as I could see the entire control surface)

Natural wind at the airfield is enough to individually move the elevators. I've been behind MD-80 series aircraft waiting for takeoff and observed different movements between elevators.

quote:

Originally posted by Tailhook 84:

quote:

Originally posted by Rightwire:How did that happen? the elevator moved at least 6 times from neutral to full up (or what I assume was full up as I could see the entire control surface)

Natural wind at the airfield is enough to individually move the elevators. I've been behind MD-80 series aircraft waiting for takeoff and observed different movements between elevators.

Ok, fair enough. Thanks!

My thought is if it was behaving in that manor in flight, we probably wouldn't be having this discussion now...

I have worked flight controls on F-4 (C/D/E) F-15 (A/B C/D and E), T-38, T-37 and they do some "odd looking" movements in various situations of internal/external power situations and while engines running and generator cycling.

And lastly, but most importantly, we don't have feathers and the fact that we get our fat asses off the ground in shit that we weigh in metric tons is why we have officer's and enlisted clubs and Air Force bases named after pilots in the second place...

The MD-80 uses two independent hydraulic systems (left and right), each with their own resorvoir. The left system serves the elevators, but each elevator has independent valves and actuators. Elevators are not hydraulically linked.

When the hydraulic systems are not powered, the elevators are free to float independently and are not commanded to a particular position. There is nothing to cause them to act together if there is no hydraulic pressure or system rigidity.

Short story is that what you saw is normal. As system pressure decreased, the actuators moved under residual pressure, depending on which side of the actuator had greater pressure.

You can see a quick run-down on the hydraulic system below, with a simple schematic.

http://www.hydraulicspneumatic...ble-and-easy-operate