J41 - autopilot issues

[marcstares 0]

HiPlease could you advise how to set the autopilot on the PMDG J41. The problem occurs when changing altitude. For Example: Heading is set to desired heading, altitude set at 10,000. ATC gives clearance to 16,000. I select ALT SEL and ‘dial-in’ 16,000, active the VS button and use the thumb dial to increase the VS value. At this point the plane will either go into a steep climb or dive and the autopilot will disengage. After re-establishing level flight, selecting autopilot simply puts the plane back into the dive or climb.Comments greatly received

[Paul_Smith 92]

Why are you using V/S and what values are you entering? If you enter -1000 you will descend at 1000ft per min regardless of what altitude mode you have selected. Best climb is at 170knots so select IAS and then roll the speed down from current to a minimum of 170 in steps. E.g. if you are starting from 225knots when you select IAS, dial it back to 200 and let it settle. If you are happy, leave it alone but if you want to climb faster, dial it back to 185, let it settle and then and re-evalute.Polite Note:- You really should use your full name if you want support on this forum

[marcstares 0]

Why are you using V/S and what values are you entering? If you enter -1000 you will descend at 1000ft per min regardless of what altitude mode you have selected. Best climb is at 170knots so select IAS and then roll the speed down from current to a minimum of 170 in steps. E.g. if you are starting from 225knots when you select IAS, dial it back to 200 and let it settle. If you are happy, leave it alone but if you want to climb faster, dial it back to 185, let it settle and then and re-evalute.Polite Note:- You really should use your full name if you want support on this forum

OK - so let me get this right - you have to use the IAS (green bug?) to retain level flight. If I want to descend I need to set the speed I wish to descend and then dial-in the required altitude and select ALT SEL?Marc Stares

[VIPERGTSR01 22]

IAS is probably what is causing the climb or dive on cruise, it is NOT for cruise or retaining level flight.IAS AP mode is to for the AP to hold IAS (Indicated Air Speed) using pitch, for example: If you're crusing at 210knots and want to descend (with IAS) you press the IAS button (will show green speed reading on the bottom right of the EADI) then reduce the power levers (throttles), this causes you to lose speed (less power, less speed) and the auto pilot compensates by pitching the nose down. The further you pull the power levers back the steeper the AP will pitch nose down to hold the selected speed. If you wish to change the IAS speed you can use the wheel (same as the VS wheel) next to the AP/YD button.Same goes for ascent/climb. If want to IAS to hold 170konts then press IAS and select 170knots with the wheel (if you're arn't already doing that speed when you pressed it), if you have given the engines suffiecent power the AP will pitch up to hold the 170knots. ALT SEL isn't even required unless you want the autopilot to capture the alt you selected, as you get close to that alt the AP will start capturing and change to ALT (alt hode) mode and cancel IAS (or VS if you used that).Remember IAS is not for cruise (unless you don't mind the AP changing the ALT all the time like you have been experiencing to hold the speed like a Concorde) and the AP doesn't know or care if you want to climb or descend it will just hold IAS so you have to control the pitch the AP commands with the throttle (less power = nose down, more = nose up). Just setting a VS on climb can cause stall problems if you are not paying attention and adjusting VS rate because the AP will just hold it even if the aircraft can't support it at high alts.

[Guest BlueRidgeDx]

Marc,No, the airspeed bugs on the EADI are for reference only, and do not control any aspect of the Autopilot or Flight Director.The proper procedure for initiating a climb or descent is to set the new altitude, press ALT SEL, then select a vertical mode and adjust the thumbwheel as necessary to achieve the desired rate of climb or descent.When you choose Vertical Speed mode, the mode will engage at the present VS value - that is, if you were in a 500fpm climb when you press the button, the FD will continue to command a 500fpm climb. You may adjust the selected VS using the thumbwheel. This mode is straightforward, but does not offer stall protection so you have to make sure you decrease the VS as you climb and airspeed bleeds off.IAS mode also engages at the airspeed present upon selection. In IAS mode, the FD attains and maintains the selected airspeed by commanding changes in pitch attitude. The pilot influences the the commanded pitch attitude by manipulating the power levers. For instance:If you are in level, unaccelerated flight at 200kt (with the AP on) and you engage IAS mode, the airplane will continue to fly straight and level. Without touching the power levers, if you roll the pitch wheel to an IAS of 210kt, the airplane will begin a shallow descent. Now, if you rapidly increase power, the nose will rise and you will begin a brisk ascent to maintain 210kt.You also need to be gentle when selecting a new target IAS. Any large changes in commanded airspeed will rapidly result in excessive vertical speeds. For instance, if you're descending at 250kt and ATC asks you to reduce speed to 200kt, spinning the thumbwheel to 200kt will result in a zoom climb with >6000fpm followed by a "pushover" at the top. Not good. Note that in the first example, the airplane vacated its original altitude, then performed both a shallow descent and a steep climb all without ever selecting a new altitude, or arming ALT SEL. Unlike most airplanes, the IAS mode does not care whether your selected altitude is above or below you...it will use whatever pitch attitude is necessary to achieve the commanded airspeed, even if it means going in the wrong direction.Thats why the proper procedure is to "Set it, Arm it, Do it" - SET the new altitude, ARM ALT SEL, DO the mode selection (VS/IAS).From your description it sounds like you did things correctly. Just verify you're setting an appropriate VS value, and if you're using IAS mode, be sure that you're using the power levers appropriately.EDIT: I walked away for a few minutes while replying and somebody beat me to it.

Edited August 9, 2010 by BlueRidgeDx

[Paul_Smith 92]

OK - so let me get this right - you have to use the IAS (green bug?) to retain level flight. If I want to descend I need to set the speed I wish to descend and then dial-in the required altitude and select ALT SEL?Marc Stares

No, that is not what I said. You use IAS to CHANGE level, not to maintain it. Remember that the JS41 AP does not have autothrottle or VNAV and only two limited vertical modes. IAS mode tells the autopilot to adjust pitch as necessary to reach and maintain target airspeed. V/S mode tells the autopilot to adjust pitch as necessary to reach and maintain a rate of climb (or descent). Whichever mode you select, when ALT SEL altitude is reached, the AP switches out of vertical mode and into ALT HOLD mode, and speed will change until power matches air-resistence. If you want to change alt, up or down, dial in the new alt and select ALT SEL. This tells the AP to fly staight and level when it reaches the selected altitude. Then select alt change mode (V/S or IAS). If V/S is selected, it will start at 0, if IAS is selected, it will start at your current airspeed, this is why it is important to be straight and level to start or to adjust it correctly before engaging the AP! Then change power, full power if you are climbing, idle power for descent. Check your mode value is still correct and engage AP if not already on. Then change the mode in the direction you desire, but be gentle, small changes only! If you are in IAS mode and want to climb, use the dial to reduce the IAS. If you dial from 225 to 170 in one go, the nose will pitch up until 170 is reached, but the engines can not hold that angle so the speed will drop well below safe while the nose is still coming back down. The speed will drop so low the nose will have to go well below the horizon to try and regain it. Not good! You will get similar results in V/S mode if you enter extreme values. A 2000ft per minute climb from 10,000ft will very quickly drop your airspeed below stall, forcing the nose down and disconecting the AP. This might have been what you were seeing. Dial in a small change and evaluate the results. Then, if needed, dial in another small change.Edit - answered better and faster above.

[marcstares 0]

Thanks for all the great feedback and advice. All makes sense now Thanks againMarc

[Lucass 3]

Hi,

 

I fly this bird on FSX:SE. Everything worked fine except autopilot IAS mode, but this is known issue on Steam Edition.

 

Few weeks ago another issue has appeared... on apporach, J41 doesn't stop descending at set altitude (for example I set 3000 to descend for ILS, but J41 shoots this altitude and continue descending). There is also problem with APR mode... JS41 'catches' localizer but can't descend on GS - consequently I have to fly manually all approach.

 

What is going on? What should I check/do to resolve these issues?

 

Ps. I installed FS2Crew for JS41 - do you think that could be an impact on AP modes?

 

Lucas

[chintzo 2]

Hi Lucas.

I find that if V/S is selected as well as APR the V/S will have priority. In your example I would aim for 3000 before ILS becomes active, so I would be at level flight before reaching the glideslope.

 

Hope that helps

 

Roy

[Lucass 3]

Thanks for response, Roy.

 

But even if V/S has a priority, aircraft should stop descending at set altitude (3000 ft) - right?

[chintzo 2]

You would think so. But I find that when APR takes over before the set altitude is reached the set 3000 is ignored like you would expect but V/S stays in command.

 

I'm not sure if it's a FSXSE only thing, which is what I use.

I will check it out tonight and let you know how I get on.

[chintzo 2]

Hi Lucas.

I've just tried a few test flights and couldn't replicate the problem. The V/S always disconnected. I'll keep an eye on it when I fly over the weekend and let you know what I find.

Sorry I couldn't help more.

Roy

[Lucass 3]

Thanks Roy. Maybe I will try reinstall JS41...

[Lucass 3]

Hi,

 

problem still exist. Reinstalling JS41 didn't help...

If anyone meets similiar problem and know how to resolve it - please help...

 

Lucas

[ckyliu 1,749]

But even if V/S has a priority, aircraft should stop descending at set altitude (3000 ft) - right?

This might be stating the obvious, but... have you got ALT SEL armed in yellow? The autopilot will not level off without it!

 

...I find that when APR takes over before the set altitude is reached the set 3000 is ignored like you would expect but V/S stays in command...

Typically you would engage NAV mode first, then APR when the glideslope is alive (and you're underneath it, so the autopilot can "catch" it)