MD90 oscillating?

Tantalum

Tantalum

Final Approach
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San_Diego_Pilot
So..anyone else ever noticed this? Happened once during latter half of climb, for about 10 minutes, there was a noticeable up and down wave oscillation... felt like 5 seconds or so between up down. It went away in cruise and I didn't think about it again until my most recent MD90 flight it happened again... this time the entire 2.5hr cruise. You could actually see it if you looked out the window and used the wing as reference

Anyone experience this or know its cause? I must say that I didn't enjoy the thought of there being some component in the elevator constantly trimming back and forth...
 
Dutch roll?
 
Maybe a crappy autopilot? I'll usually climb in V/S mode above 250 knots because my plane tends to chase the speeds and makes large corrections.
 
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Yes, it's common. I think it's the autopilot trying to hold altitude. I'd say Phugoid effect if it stopped after a bit, but not the entire cruise.
 
ktup-flyer said:
if it stopped after a bit, but not the entire cruise.
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It would start mild, then over a span of 15 minutes progressively get worse and then get better and go away entirely for about 10 minutes, then the cycle would repeat.

I assumed crappy autopilot, but really?! It's a commercial airliner, I would assume the autopilot is a little better than an stec hunting around...
 
Tantalum said:
It would start mild, then over a span of 15 minutes progressively get worse and then get better and go away entirely for about 10 minutes, then the cycle would repeat.

I assumed crappy autopilot, but really?! It's a commercial airliner, I would assume the autopilot is a little better than an stec hunting around...
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Out of trim?
 
Tantalum said:
It would start mild, then over a span of 15 minutes progressively get worse and then get better and go away entirely for about 10 minutes, then the cycle would repeat.

I assumed crappy autopilot, but really?! It's a commercial airliner, I would assume the autopilot is a little better than an stec hunting around...
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It's an OLD commercial airliner. I'm betting it's the AP climbing in speed mode.
As Jordan suggests, that has the potential to be uncomfortable.
 
Kritchlow said:
This can be a bad habit....
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Curious why? I've heard of VS mode being frowned on by others who fly the big metal in favor of letting climb/descent rate be dictated by the commanded speed and altitude, but I'm curious why that is..
 
Tantalum said:
Curious why? I've heard of VS mode being frowned on by others who fly the big metal in favor of letting climb/descent rate be dictated by the commanded speed and altitude, but I'm curious why that is..
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Commanding 2000 fpm, if not monitored very closely, can result in a high altitude stall. The speed will continually bleed off to maintain a set VS. it has happened several times in history.
 
Okay, that makes sense. I wonder, would an Airbus do the same and eventually get to alpha floor? Or would it just max out the angle at some point before stall..
 
Tantalum said:
Okay, that makes sense. I wonder, would an Airbus do the same and eventually get to alpha floor? Or would it just max out the angle at some point before stall..
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I fly the Bus, and it does get complicated as to what exactly will happen. Going into Alpha Floor is likely would be my guess, but that's not a guarantee against a stall.. just another layer of prevention.
That said, who wants to write a report to the company explaining why the AC entered Alpha Floor??
 
I should add... in the Bus VS overrides other commands. It is unclear what happens to VS in Alpha Floor however.
The only certainty is TOGA power.
 
I learn so much here, thanks. I know the Airbus philosophy irks some folks but the envelope protection and FBW logic is impressive. I remember when Air France went down I must have read an encyclopedia worth of information on it
 
Tantalum said:
I learn so much here, thanks. I know the Airbus philosophy irks some folks but the envelope protection and FBW logic is impressive. I remember when Air France went down I must have read an encyclopedia worth of information on it
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It can do a lot, but the airplane is built for third world. Doing something as simple as overriding a speed because you are slowed down can mess with the entire logic of the profile decent. Let it do its thing and it's usually good enough. Start overriding portions of the automation... can of worms.
 
Kritchlow said:
This can be a bad habit....
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Sure, if unmonitored, it can end badly, which is why I'm my eyes are glued to the A/S indicator when I do it. It's a much more comfortable ride doing it in VS mode as opposed to speed mode in my plane.
 
Kritchlow said:
I should add... in the Bus VS overrides other commands. It is unclear what happens to VS in Alpha Floor however.
The only certainty is TOGA power.
Click to expand...

Not sure how it might have changed on the A320, but on the 300 and 310 Alpha Floor only changes the autothrottle mode and doesn't do anything to the V/S mode. Since V/S is the default mode, I don't imagine anything would change at all after it got too slow and kicked the autopilot off. The FD would still be directing the climb to match the rate dialed in the V/S window.
 
I'm also a V/S after 250 guy. Speed mode makes people annoyed after awhile of up, down, up, down movement.

ERJ is a lot worse then the CRJ. Both have crappy autopilots. LOC intercepts are the worst. Nothing, nothing, nothing, HELLO 30° BANK as I get butt slapped by the stall bar!!
 
ARFlyer said:
I'm also a V/S after 250 guy. Speed mode makes people annoyed after awhile of up, down, up, down movement.

ERJ is a lot worse then the CRJ. Both have crappy autopilots. LOC intercepts are the worst. Nothing, nothing, nothing, HELLO 30° BANK as I get butt slapped by the stall bar!!
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Everytime! Plane blows through the LOC, makes this massive correction, then blows through it on the other side. About half of our planes can intercept in white needles and it does a much better job.
 
Why is guys flying airliners coming to POA to find out whats wrong with the airplane making me nervous? Lol.

EDIT. Oops. I didn't catch he was a passenger.
 
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jordane93 said:
Sure, if unmonitored, it can end badly, which is why I'm my eyes are glued to the A/S indicator when I do it. It's a much more comfortable ride doing it in VS mode as opposed to speed mode in my plane.
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Trust me... one little distraction such as a traffic call, caution message, or even a freq change, can take your mind off of it. Stuff happens. I have worked for a major outfit in the past that had SOP's NOT to do it.
 
Hacker said:
Not sure how it might have changed on the A320, but on the 300 and 310 Alpha Floor only changes the autothrottle mode and doesn't do anything to the V/S mode. Since V/S is the default mode, I don't imagine anything would change at all after it got too slow and kicked the autopilot off. The FD would still be directing the climb to match the rate dialed in the V/S window.
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I do believe we are prettty much saying the same thing, just using different words....
 
ARFlyer said:
I'm also a V/S after 250 guy. Speed mode makes people annoyed after awhile of up, down, up, down movement.

ERJ is a lot worse then the CRJ. Both have crappy autopilots. LOC intercepts are the worst. Nothing, nothing, nothing, HELLO 30° BANK as I get butt slapped by the stall bar!!
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and I thought this only happened in FSX and X-plane, lol.
 
LOL at the FSX comment.. unless I already had the glideslope perfectly dialed in manually the APPR or VOR/LOC features on the virtual AP always seemed to over correct. Glad I wasn't the only one

@luvflyin haha I'm just a lowly PPL and Cirrus/Skyhawk driver (depending on what's available at the club and purpose of flight). Flying is in my blood but I chose not to go to the airlines ultimately.. that story will be for another thread

GlennAB1 said:
it was commonly fixed by re-nulling or replacing stabilizer position sensors
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Thanks! So the AP doesn't "know" the stabilizer's position so it tends to hunt more. Makes sense, but still disconcerting and hate the thought of components cycling back and forth... all that wear and metal fatigue!
 
@luvflyin haha I'm just a lowly PPL and Cirrus/Skyhawk driver (depending on what's available at the club and purpose of flight). Flying is in my blood but I chose not to go to the airlines ultimately.. that story will be for another thread[/QUOTE]
Lol. I scanned the thread way to fast. It actually seemed to me an MD90 driver was asking what's wrong. My bad.
 
yes.. that would be somewhat unsettling!
 
wsuffa said:
Allegient aircraft?
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Heh - you'd think so! But I think Delta is the only domestic carrier flying MD-90s these days.
 
It was a Delta bird, which recently I have found their planes to be all very clean and updated, at least on the routes I fly theit 757, 737, and MD90 fleet tend to be in very nice shape
 
Tantalum said:
It was a Delta bird, which recently I have found their planes to be all very clean and updated, at least on the routes I fly theit 757, 737, and MD90 fleet tend to be in very nice shape
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Yes, Delta does some of the best maintenance in the business. At least the domestic business.
 
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