roundsaturn

Autopilot Altitude Hold Question/setting

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Hi guys,

 

wondering the proper procedure for climbing out/taking off in a commercial jetliner (regarding autopilot), as I must be doing something wrong;

I have noticed that on some aircraft, when I take off, as I get a positive climb I put up the gear and turn on the autopilot. I already have the speed preset to 250 and the chosen altitude set, along with the rate of climb.

However, the plane will dive down, instead of continuing to climb once the autopilot is engaged.

Some planes fly fine under these procedures, others want to drop down and then the autopilot "recovers them" (if I was already high enough up in the sky to allow it) and they start to climb back up again.

Thanks for your thoughts,

Todd

 

 

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It's really advantageous to have that aircraft in proper trim before engaging AP. Good ones won't even let you until it is.

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When you release the controls (assuming you're not using the keyboard), what happens. If you're using the keyboard, you don't really get a good feel of what the plane does, but you want the controls at neutral before you engage the autopilot.

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I have never experience this problem with any default or 3rd party aircraft.  If I did, I would probably do a Numpad+5 key to center all controls before engaging the A/P.

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Posted (edited)
2 hours ago, roundsaturn said:

Some planes fly fine under these procedures, others want to drop down and then the autopilot "recovers them" (if I was already high enough up in the sky to allow it) and they start to climb back up again.

Perhaps provide a list of those that do and those that do not.  When an altitude is set and armed, some autopilots capture the current vertical speed at the instant the AP is engaged and if not told otherwise carries that VS to the set altitude. If an Alt or Arm or VS key is pressed in the interim the AP instantly reacts, reading it as new input.

Example: at a significant VS, if the AP reads a key or button input as a new request, such as to hold the current altitude, it identifies the altitude at that instant.  The aircraft with its established vertical speed cannot adjust instantly.  It will overrun that new altitude because of the vertical speed, and will need to then correct for that overrun of altitude by descending back to what it interprets the new altitude request to be.

This is  more prevalent with autopilots in general aviation aircraft, but it good reason to provide a list of the aircraft exhibiting the issue you describe.

Edited by fppilot

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Thanks for the input so far. I was using a project opensky 737 and I believe I was climbing nose up 10 degrees, speed set to 250, going to 18000 and using a vertical climb set to +1500. Once I left the ground and pulled the gear up I turn the master autopilot switch on and I already had the autothrottle on and all the parameters previously mentioned set. Everything was fine until I hit the altitude hold. Instead of continuing to climb, the plane went nose down for a while and then (for lack of a better term) caught itself and started to climb back up. That's what I'm wondering about. Hopefully this is more specific. Thanks guys. Maybe when flying the default 737 or POSKY 737 one of you could go through your takeoff procedure and climb out? Thanks again for any help.

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Posted (edited)

Altitude Hold should cause the plane to hold the current altitude..

SO, if you hit ALT while in climb, the plane will descend and then attempt to hold the altitude you were at when hitting ALT..

Edited by Bert Pieke

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Posted (edited)

Seems to me, the Default type AP requires ALT to be engaged in order to capture the preset altitude. Not right but that's how it always worked. There is no LVLCH or Vnav. The way I use to do it with the default AP, was set the clb airspeed to say 280 (SPD) and vary the VS to maintain it at a given N1 of like 92%. That's kinda how a VNAV or LVL equipped aircraft works.

Edited by Adrian123

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Not sure I quite understand that answer Bert because that would mean on a commercial flight, let's say from LAX to Vegas, the pilot wouldn't have the altitude hold on until he got to say 39000 ft? I can't imagine that, but maybe I'm wrong? So you're saying altitude hold isn't engaged until you're at Cruise level?

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Posted (edited)
27 minutes ago, roundsaturn said:

that would mean on a commercial flight, let's say from LAX to Vegas, the pilot wouldn't have the altitude hold on until he got to say 39000 ft

On a commercial aircraft with the real AP/MCU, FMC system. They use VNAV and or LVL change to capture the preset altitude and ALT is used as "hold" current altitude. As I stated, the default type AP doesn't work that way. ALT must be activated in order to capture the preset altitude, it acts more like an altitude mode.

Edited by Adrian123

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Guys, thank you for the help. I think I found what is "hopefully" the correct answer on another forum. There, somebody asked the same type of question and the answer was; "to make sure the flight director was on, turn on the altitude hold BEFORE turning on the autopilot so that the flight director indicator and the actual angle will be identical before turning the autopilot on, causing the plane to continue at the same climb rate/angle." I can't wait to get home from work to try this, cheers!

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Posted (edited)
3 minutes ago, roundsaturn said:

altitude hold BEFORE turning on the autopilot

"the Default type AP requires ALT to be engaged in order to capture the preset altitude"

Your next challenge will be to keep the engines out of the red and the proper climb airspeed. switch to Mach above FL240.

Edited by Adrian123

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All of the 737s I fly are POSKY 737-800.  Like I said before I have never seen what you describe.  Before takeoff, everything is dialed into the A/P including altitude given by Departure Control, runway heading, 250Knots IAS.  I turn on the A/P before takeoff and after wheels up and flaps starting to come up I engage ALT, HDG, and autothrottle.  It works correctly every time.

I don't recommend using A/T for takeoff.  I set N1 to 92% and engage the N1 button and then switch to IAS hold after wheels up.

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Posted (edited)
15 minutes ago, charliearon said:

All of the 737s I fly are POSKY 737-800.  Like I said before I have never seen what you describe.

No, you wont see those on a default type aircraft. Only on realistic payware. PMDG, Aerosoft etc. I was trying to describe how it works in the "real world". It can be manually emulated as it was on early commercial aircraft.

Edited by Adrian123

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1 hour ago, roundsaturn said:

Not sure I quite understand that answer Bert because that would mean on a commercial flight, let's say from LAX to Vegas, the pilot wouldn't have the altitude hold on until he got to say 39000 ft? I can't imagine that, but maybe I'm wrong? So you're saying altitude hold isn't engaged until you're at Cruise level?

For a real airplane/autopilot, yes.

You would climb in VS mode or SPD mode.

As others have pointed out, the default autopilot in FS works differently, but I would hope that your addon airliner is more sophisticated than that..

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33 minutes ago, Adrian123 said:

No, you wont see those on a default type aircraft. Only on realistic payware. PMDG, Aerosoft etc. I was trying to describe how it works in the "real world". It can be manually emulated as it was on early commercial aircraft.

I'm thinking he meant that with the Project Opensky 737, he isn't seeing the behavior that the original poster is describing.

26 minutes ago, Bert Pieke said:

For a real airplane/autopilot, yes.

You would climb in VS mode or SPD mode.

As others have pointed out, the default autopilot in FS works differently, but I would hope that your addon airliner is more sophisticated than that..

The Project Opensky 737 is really just a freeware model, it uses the default 737 panel, so no, his add-on airliner is not more sophisticated than that.

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THANKS EVERYBODY for the help!

-Question to Charliearon; not trying to be stupid or to ask a stupid question, but when you say " I don't recommend using A/T for takeoff.  I set N1 to 92% and engage the N1 button and then switch to IAS hold after wheels up."  When I try to do that, the N1 button won't turn on or engage unless the A/T is also on.......

As far as your other instructions, I did it just like you said and it worked perfectly! Thank you so much!!!!

 

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Yeah, sorry I do mean that the AT is on but using the N1 for takeoff and not IAS hold.  I didn't have enough coffee in me at the time. 😉

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Not to bug you too much more, but I do have a couple follow-up questions for Charliearon; so are you setting the n1 and then using that autothrottle/autopilot setting instead of moving the actual throttle itself? Also, when I tried it, since autopilot was on, my joystick was disabled and I was unable to rotate.....thanks in advance for your wisdom....

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Yeah, come to think of it, I never do move the throttle on my joystick so I guess the thrust levers don't move either.  Keep in mind that 92% N1 is fine for takeoff, but after wheels up and engaging A/P for heading and altitude, I then engage Speed hold for IAS set to 250 knots for climb to 10,000 feet.

I think I forgot once or twice to set speed to IAS and at 92% N1 thrust, the aircraft will overspeed very quickly.  I'll try to remember to keep an eye on the thrust levers next time I fly my 737.

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To be fair, if you want to try to be as realistic as possible with the default panel, you would be maintaining a constant thrust, so you'd leave it on N1. Of course, in order to maintain the speed you'd want, you'd have to constantly adjust your vertical speed, which on the more complex add-ons and also the actual plane, VNAV would take care of that for you.

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