Richman
JC’s Resident Curmudgeon
Wouldn't have made any difference if they had. DC-9s/MD-80s/717s don't work like that.
MD-87 Down KTME 10/19
- Thread starter fholbert
- Start date
Wouldn't have made any difference if they had. DC-9s/MD-80s/717s don't work like that.
fholbert
Mod's - Please don't edit my posts!
Rich, What does that mean?
Hacker15e
Who am I? Where are my pants?
It means the control wheels in the cockpit are connected to control tabs, not the actual elevators. The elevators are free floating when they're not acted upon by aerodynamic forces.
Thus, a "control check" does actually check if the elevators move or not.
The video on the previous page explains it.
charlie1017
Well-Known Member
Yep. At the old shop we'd use the ELEVATOR POWER ON light to check whether the elevators were locked up or not. You would push full forward on the control column, and then once the angular difference between tab and elevator exceeded 10 degrees (iirc), the elevators would actually move to nose down by hydraulic power. Not seeing this light with the yoke set at full nose down was bad juju.
knot4u
Repeat Offender
The elevators aren't connected to the yolk by cables, they have servo tabs to move them and geared tabs to assist. I can remember trundling around KBUR in the early '90s in a fuel truck and seeing them with one elevator up and its partner pointed down on the same airplane and thinking it seemed odd. I've flown as a passenger more than once on them without issue, it seems like a system without any sort of redundancy unless the preflight includes a scissor lift to actually check the freedom of movement regardless of control column position which this design depends on. It's hard for me to imagine how this was ever certified. It truly was a rodeo back then.
Roger Roger
Bottom of the list
Kinda weird how we went 60 years or whatever without this being an issue and then we have 2 in a few years.
knot4u
Repeat Offender
Can you please provide some information about how that worked? I'm not contesting your knowledge or experience I'm still just trying to wrap my head around how the elevator system works on a DC-9.
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charlie1017
Well-Known Member
It was a system designed to augment elevator controls in a stall (high AOA) situation. But during the "flight control check" we tested it, and no illumination of the light during a proper check was a no go item. Not only due to its necessity for aerodynamic stall recovery, but due to the very potential indication of jammed elevators. I wish I had a good reference (outside of just Googling). Its been years and a few types ago.
CFI A&P
Exploring the world one toilet at a time.
Almost as if they had routine maintenance when in 121 service and once entered private hands they started skipping out on maintenance.
knot4u
Repeat Offender
Again, I'm not questioning your experience, I'm just trying to learn. The elevators are "free floating" and controlled by servo tabs, with static dampers and assisted by geared tabs. I'm wondering how the control column position can possibly be used to absolutely accomplish a control check? Again, I'm not questioning or attacking you or the airplane, I'd just like to understand how this thing works, it seems to be a recurring situation and I like to learn as an effort to avoid any further issues. Thanks.
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charlie1017
Well-Known Member
Hahaha the airplane is horribly designed (sorry @SlumTodd_Millionaire ). The short answer is (DC9/MD80) you cant check the actual movement of the surfaces. Just the tabs. The ELEVATOR POWER ON check only gave you *some* confidence that the elevators aren't jammed. Again I'm years and a few type ratings removed from it. I wish I had actual reference material with me.
This verbatim from an MD88 POM:
"A hydraulic power augmenter system provides elevator control for additional nose down capability in extreme high angle of attack flight conditions. Supplied by the left hydraulic system, the power augmenter system activates during an extreme stall condition after elevator control tabs have been displaced approximately 10 degrees airplane nose down with respect elevator surface. (This augmenter system hydraulically displaces the elevators full down.) A blue “ELEVATOR POWER ON” light on the overhead annunciator panel illuminates when hydraulic pressure is being applied to the elevator."
The ELEVATOR POWER ON light just shows hyd power has been applied to the elevator augmenter but indicates nothing about actual elevator position.
"some confidence" ? Perhaps.
charlie1017
Well-Known Member
Matter of factly, yes. Lets just say I had an experience with this that is better shared over beers. It was obviously not at a Legacy carrier with their top tier maintenance and infrastructure.
T/O w/FSII
Well-Known Member
It’s almost like getting old sucks or something
bustinminimums
Well-Known Member
I just remembered something about the elevators on the 80. If you have wind gusts greater than 60 knots then you must inspect the elevators for free movement .Even with the dampers doing their thing if you exceed that number it can cause damage.