Too many tabs: Servo, anti-servo, control, balance, trim...

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Hengelo

Pre-takeoff checklist
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Hengelo
Ok, studying the many, many tab types here and wondering if there is a resident authority. Found this old thread but it seems more confused than ever.

Here is what I have:

Balance, servo and trim tabs all move opposite to the control surface, the difference is in what moves them.
Trim tabs are moved by the trim wheel/knob
Balance tabs are attached to the control linkage
Servo tabs are also attached to the control linkage
balance and servo tabs seems to be the same thing
Anti servo tabs move in the same direction as the control surface and are connected to the control linkage
Ground adjustable tabs are just that; bendy metal things you tend to leave the hell alone unless you know what you are doing

thanks in advance.

sources:

www.boldmethod.com

How The 4 Types Of Trim Tabs Work

Trim tabs are a secondary flight control surface that helps you reduce (or eliminate) the need to place pressure on the yoke or rudder to keep your airplane flying straight and level.
www.boldmethod.com www.boldmethod.com
en.wikipedia.org

Servo tab - Wikipedia

en.wikipedia.org en.wikipedia.org
PHAK chapter 6 https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf
 
Control tabs… think DC-9.
 
The PHAK states a servo tab "drives" the primary control, while a balance tab is moved mechanically in conjunction with the primary control surface. However, different manufacturers use these terms differently. Embraer says the ERJ has "servo tabs" but they meet the FAA's definition of balance tabs. Other manufacturers refer to tabs that drive the primary control as "control tabs".
 
I've never understood how servo tabs were ever allowed to become a thing, like on the DC-9/MD-80 series. There is no mechanical linkage to move the elevator, and no way to do a proper control check on the ground. This has led to two MD-80s in the last decade having high speed aborts and running off the end of the runway causing damage and injuries. In both cases there was undetected wind damage to the elevator, causing it to jam, but no way to see it during a preflight. I'm surprised the whole fleet didn't get grounded, what's left of it.
 
Actually, there is a way to see it on the ground. If simple criteria isn’t met, you literally gotta get up there and move them manually…

And no, those poor dudes didn’t know that beforehand. SCARY.

Edit: if simple criteria is met, you’re supposedly good to go…. Ya, not super comforting.
 
Last edited:
Tools said:
Actually, there is a way to see it on the ground. If simple criteria isn’t met, you literally gotta get up there and move them manually…

And no, those poor dudes didn’t know that beforehand. SCARY.
Click to expand...
Have to add a scissor lift to my flight bag for preflight, right next to the flashlight and fuel sump tester.
 
midwestpa24 said:
I've never understood how servo tabs were ever allowed to become a thing, like on the DC-9/MD-80 series. There is no mechanical linkage to move the elevator, and no way to do a proper control check on the ground. This has led to two MD-80s in the last decade having high speed aborts and running off the end of the runway causing damage and injuries. In both cases there was undetected wind damage to the elevator, causing it to jam, but no way to see it during a preflight. I'm surprised the whole fleet didn't get grounded, what's left of it.
Click to expand...
You do an elevator check at 80kts on the takeoff roll. You push forward and look for the dip in the nose. If you don't get it, you reject.

The (much larger) DC8 has a similar, but not separated, un-powered elevator. You did the same check and you also had a small gauge that showed elevator movement that was checked during taxi.

Both systems had balance tabs (I think that's what they were called) which adjusted based on stabilizer position (the stab was trimmed, as in most jets, not the elevator) so that the neutral point of the elevator would be streamlined with the stab.

The DC9 also had control tabs on the ailerons as those were similarly unpowered. Both airplanes had powered rudders. The DC8 had powered ailerons as well.
 
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