SuseJ772
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Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 3:29 pm

Alright, I know this isn't a place to discuss Flight Sim, but this question really isn't Flight Sim, but derived from my MS Flight Sim experience.

Scenario: I am flying on Final Approach with lets say NAV, ATL, and SPD activated with a V/S of -800. I come to a point where I want to take manual control of the aircraft so I disengage the autopilot. At this moment I usually have some degree (it varies) of pitch up or pitch down (depending on my approach angle). So my question is this: does this happen on real aircraft or this just an error in the FlightSim software. I have never been on an aircraft that felt like a sporadic pitch up/down except during turbulence.

Second part of the question, when you disengage autopilot, is there a procedure to it or do you just turn off the master autopilot control. I have noticed on FlightSim if I disengage the ALT setting, before hitting the master autopilot, the pitch up/down is usually less of issue?

Hope that post made sense.
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wilco737
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 4:28 pm

Quoting SuseJ772 (Thread starter):
Scenario: I am flying on Final Approach with lets say NAV, ATL, and SPD activated with a V/S of -800. I come to a point where I want to take manual control of the aircraft so I disengage the autopilot. At this moment I usually have some degree (it varies) of pitch up or pitch down (depending on my approach angle). So my question is this: does this happen on real aircraft or this just an error in the FlightSim software. I have never been on an aircraft that felt like a sporadic pitch up/down except during turbulence.

on the 737 this doesnt happen! The aircraft is trimmed well by the Autopilot and when you take over manually then you shouldnt feel a thing...

Quoting SuseJ772 (Thread starter):
Second part of the question, when you disengage autopilot, is there a procedure to it or do you just turn off the master autopilot control. I have noticed on FlightSim if I disengage the ALT setting, before hitting the master autopilot, the pitch up/down is usually less of issue?

At Lufthansa there isnt... You can fly manually as much as you want to... It depends on weather, time of day, approach etc... I try to fly a lot manually... thats the fun about flying, isnt it?

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Curmudgeon
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 4:42 pm

The autopilots on most jets are very good. The 737-200 would occasionally bump a bit when disconnecting the autopilot, especially just after selecting flaps 15, (which creates a bit of a pitch change anyway).

Closer to the runway, (737-300 and later), in a two-channel coupled approach, the autopilot starts to introduce a nose-up trim to aid in the flare maneuver. The closer to the ground that you disconnect the more nose-up bump you will feel.

I don't know anything about your program, but if it's supposed to be realistic, a two-channel approach should give you flashing glide slope and localiser indications at 1500', which is the self-test feature of the autoland. At 500' you should see "flare" in white on the EADI*, then green as the flare is performed.

*PFD on 737NG

I hope this helps.

Oh, by the way...I fly manually a lot too. Wink
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Legs
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 5:44 pm

From my (admittedly limited) contact with modern flight control software, on shutdown of the autopilot there is a function that slowly returns the control surfaces to their commanded position, in order to reduce the effect that you describe. For example, if your commanded aileron position differs from where the autopilot had it, the aileron command is introduced over maybe a second or so.

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BAE146QT
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 5:59 pm

Quote:
At this moment I usually have some degree (it varies) of pitch up or pitch down (depending on my approach angle).

I think what you've got there is that the aircraft is out of trim for some reason. Perhaps there is slack in your controls, (joystick, whatever), or they're over-sensitive, which the autopilot compensates for while it's engaged, but which reverts when you switch it off. But we probably shouldn't really discuss that here.

Incidentally, this is a major problem with MSFS in general. Even with the laughable force-feedback sticks, you really don't get a feel for trim condition. You end up chasing the VSI all over the place because the subtle information you should get from the control surfaces through the cables just isn't there.

OB-TechOps: Airbus drivers; how does that work for you? Are the simulated forces in the sidestick realistic when it comes to trimming off while you're hand flying?
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Starlionblue
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 9:07 pm

SuseJ772, are you using "stock" MSFS or an add-on like PDMG? The plain vanilla install is much simplified in terms of autopilot. If you use a well-designed add-on you will definitely get more realistic behavior.
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YYZAeroEng
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 11:42 pm

From what I recall of my Aircraft Performance courses, if you disengage the auto-pilot and leave the plane to its own natural responses, there should be a tendency to pitch down (if there is some sort of disturbance to the a/c).

From the data I've seen, most commercial aircraft are design with a negative moment coefficient.

However there is a world of difference from theory and practice. The above posts seem to illustrate that nicely.
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SuseJ772
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Wed Dec 06, 2006 11:46 pm

Quoting Starlionblue (Reply 5):
SuseJ772, are you using "stock" MSFS or an add-on like PDMG? The plain vanilla install is much simplified in terms of autopilot. If you use a well-designed add-on you will definitely get more realistic behavior.

Yeah, I am using stock MSFS, I'll have to check ou the PDMG addon. Thanks for the suggestion, I appreciate it!

Everyone else, thanks for the responses. Like I said, I was less concerned with how to fly better on MSFS, and more concerned with what real flight was like. This is why I love this place!
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speedracer1407
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Thu Dec 07, 2006 5:02 pm

Quoting SuseJ772 (Reply 7):
Yeah, I am using stock MSFS, I'll have to check ou the PDMG addon.

Stock PC flightsim autopilots, as stated above, are generally pretty rudimentary. I've found that they often mix a bit of elevator with stab trim to achieve a stable glide path. Disconnecting the AP a few hundred feet from the ground bring the elevators back to neutral, resulting in a bit of pitch up or down. This may or may not be the problem you are experiencing, but try using an outside "chase" view while disconnecting the AP---you might be able to see which surfaces move to cause the erroneous pitch movement.
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Starlionblue
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Thu Dec 07, 2006 9:44 pm

Quoting SuseJ772 (Reply 7):
Quoting Starlionblue (Reply 5):
SuseJ772, are you using "stock" MSFS or an add-on like PDMG? The plain vanilla install is much simplified in terms of autopilot. If you use a well-designed add-on you will definitely get more realistic behavior.

Yeah, I am using stock MSFS, I'll have to check ou the PDMG addon. Thanks for the suggestion, I appreciate it!

Made a spelling booboo. It's "PMDG", not PDMG. Here: http://www.precisionmanuals.com/

Note: Read the manual! Big grin
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ThrottleHold
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Fri Dec 08, 2006 7:13 am

Quoting BAe146QT (Reply 4):
Airbus drivers; how does that work for you? Are the simulated forces in the sidestick realistic when it comes to trimming off while you're hand flying?

There aren't any forces in an Airbus sidestick. When hand flying, trimming is still done auotmatically by the ELAC's.
 
BAE146QT
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Fri Dec 08, 2006 7:28 am

Quoting ThrottleHold (Reply 10):
When hand flying, trimming is still done auotmatically by the ELAC's.

Ah - thank you. So you point it where you want to go, and it trims off for you? That's pretty neat.
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Starlionblue
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Fri Dec 08, 2006 10:16 am

Quoting BAe146QT (Reply 11):

Ah - thank you. So you point it where you want to go, and it trims off for you? That's pretty neat.

It is pretty neat. Also, in normal mode, aileron movement is not proportional to stick movement. Movement of the stick commands RATE of roll, not surface deflection.
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BAE146QT
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Fri Dec 08, 2006 7:23 pm

Quoting Starlionblue:
Movement of the stick commands RATE of roll, not surface deflection.

That must take some getting used to when transitioning from a type with proportional controls. Over-correcting must be common in the beninning.

I'm trying to imagine what it would be like if you had a similar system in a car...
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Starlionblue
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sat Dec 09, 2006 12:44 am

Quoting BAe146QT (Reply 13):

I'm trying to imagine what it would be like if you had a similar system in a car...

The new BMW steering is non proportional. It's basically speed dependent steer by wire.

Quoting BAe146QT (Reply 13):
That must take some getting used to when transitioning from a type with proportional controls. Over-correcting must be common in the beninning.

Just to add to the fun, when not in normal mode (close to the ground) it IS proportional.
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BAE146QT
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sat Dec 09, 2006 2:18 am

Quoting Starlionblue (Reply 14):
The new BMW steering is non proportional. It's basically speed dependent steer by wire.

I thought it was drive by wire but still proportional. Speed sensitive, yes, but that's pretty common. Seems to me that making steering non-proportional would be far more dangerous on the roads than in a plane. Less (physical) room for error.

I've never driven one though, so I have no practical experience.

Quoting Starlionblue (Reply 14):
Just to add to the fun, when not in normal mode (close to the ground) it IS proportional.

Sounds like a prime example of why type-specific pilot certification is required...
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Starlionblue
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sat Dec 09, 2006 2:24 am

Quoting BAe146QT (Reply 15):
Quoting Starlionblue (Reply 14):
The new BMW steering is non proportional. It's basically speed dependent steer by wire.

I thought it was drive by wire but still proportional. Speed sensitive, yes, but that's pretty common. Seems to me that making steering non-proportional would be far more dangerous on the roads than in a plane. Less (physical) room for error.

I've never driven one though, so I have no practical experience.

Hmmm. I'm not actually completely sure. You may be right.
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BAE146QT
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sat Dec 09, 2006 3:01 am

Quoting Starlionblue (Reply 16):
Hmmm. I'm not actually completely sure.

I could look it up but... Meh. It strikes me that you would see BMWs piled up in heaps less than a mile away from the dealership if it were true, but it could be that it's only a very minor deviation from proportional control. Or perhaps it's only active when at parking speeds? Dunno.

Anyway, better say something Tech/Ops (and thread) related:

What tail surfaces does the trim wheel on a 747 control? I'm thinking trim tabs because that's the obvious answer, but flying with a tail config that looks a bit like this;

__/)

...seems very inefficient.

My searches on the subject suggested that it's the elevators that get moved by the autopilot, but that just seems odd. How often do 747s get flown by hand in cruise? Rarely, I would think - so where would be the point in the trim tabs? For climb/descent? Have I just answered my own question? Again?
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Starlionblue
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sat Dec 09, 2006 9:37 am

Quoting BAe146QT (Reply 17):

What tail surfaces does the trim wheel on a 747 control? I'm thinking trim tabs because that's the obvious answer, but flying with a tail config that looks a bit like this;

Elevator trim on the 747 moves the entire horizontal stabilizer using a gynormous (sic) jackscrew. You can see here the flat spaces on the fuse corresponding to the movement of the front of the stabilizer:

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speedracer1407
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sat Dec 09, 2006 10:17 am

Quoting Starlionblue (Reply 14):
The new BMW steering is non proportional. It's basically speed dependent steer by wire.

BMW's "active steering" is an option, and a rather maligned onw at that, according to journalists at least.

On cars equipped with it, the system uses accelerometers and preset algorithums to adjust the steering ratio based on a number of perameters, such as speed, slip angle, etc. Though I've not driven one, it apparently feels fairly unnatural, atleast in the early versions. Not sure if BMW has improved the system.

Speed-sensitive steering is common on many cars, and simply adjusts the steering effort, usually higher as speeds rise.
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sccutler
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sat Dec 09, 2006 2:03 pm

...and yet no one has directly answered the question.

In most advanced autopilots, including those used on every currently-active airliner of which I am aware, trim is automatically adjusted and an autopilot disconnect should not result in a significant pitch excursion, up or down.

On the other hand, while some general aviation autopilot systems do have auto-trim, many do not, and if one is lax about ensuring that the aircraft is in trim before engaging the a/p, they will have a significant pitch shift upon disconnect.

Example: One is climbing to cruise altitude in, say, a Bonanza with a Century 2000 autopilot, pitch mode. You reach your desired altitude and, instead of disconnecting the autopilot, trimming for cruise speed with no climb or descent, you just punch the "ALT" button. The A/P will dutifully hold altitude, with the pitch servo commanding downward pitch on the elevators (as if you were pushing the yoke to hold the altitude). As the airplane accelerates to its final cruise speed, its natural urge to climb will become greater and the autopilot will impart greater nose-down force to hold the altitude steady. Hence, you are flying level, but with a great deal of aerodynamic force from the trim tabs, trying to push the elevator control surface up.

Tap that "disconnect" button and the servo stops holding the elevator in the position that maintains a level pitch attitude; instead, all that aerodynamic pressure pushes the elevator to its trimmed state, and the plane pitches up, usually with great gusto and a quick, "Aw crap" from the pilot.
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concordegboad
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RE: Pitch Up (or Down) After Disegage Of Autopilot

Sun Dec 10, 2006 10:09 pm

Remember, your joystick position is at x:0, y:0 when you disengage the autopilot. If the sim's AP adjusts control surfaces (elevators) in addition to trim, these will return to neutral on disengage.

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