This question is mostly for ATPs or PMDG pros. So, if I set up a vertical profile in an FMS, IE 10K at wayoint A, 20k by waypoint B, 35k by waypoint C, D, E, etc. how does the autopilot follow that? At what rate of accent would the AP track the vertical profile? Or does it vary by different systems? Basically, how would I set up an autopilot to follow the vertical flight plan profile?

Thanks 

In a simple hack, the FMS is going to calculate each leg to meet those altitudes. The FMS actually has a performance database that it will use to calculate the climb. Depending on the altitudes and distance to the points, you might get the max climb rate attainable at climb power and climb speed schedule. You may get a lower power climb if the hard altitude requires a shallow climb. You might not be able to make it at all if the hard altitude is close and too high. The FMS will feed the required climb power to the engines and the required climb rate to the AP for each leg. Nothing cosmic at all. if there are no hard altitudes, you will basically get climb power and the rate attained by pitching for airspeed, similar FLCH.

For most systems I know of, climb is simply get there full throttle and hold, it the distance between wptA and wptB is far enough, it'll climb to FL200 and hold until wptB and climb to FL350 right after clear wptB.

If it's too close for aircraft to climb on full throttle and normal climb speed, it'll show a message on MCDU: UNABLE NEXT ALTITUDE (for most Boeings), you can try switch to MAX ANGLE to climb, but it also won't guarantee success out side the aircraft's capability.

 

Descend is more complicated , for 737 along , there are 2 different airline option:

1) If you fly C>B>A, it'll fly over wptC at FL350, hold FL350, until it feels (based on informations you have putted in), it can descend right at FL200 at wptB, then it's idle thrust and all the way down.

2) Another option is just go straight between waypoints on descend, it's called geo-descend option on 737 and by default on A320s, consume more fuel to control the path, but more stable on low altitude and approach.

if it's too steep befween C and B, MCDU'll show "STEEP DES AFTER C" before it reach wptC, for some 737, it'll try to stay on a straight path between wptC and wptB. but the speed will creep up to even overspeed, for some other 737 and A320 if speed is too fast , it'll rise the nose to keep speed in check but you'll not able to reach wptB at FL200.

 

 

6 hours ago, Gary1124 said:

This question is mostly for ATPs or PMDG pros. So, if I set up a vertical profile in an FMS, IE 10K at wayoint A, 20k by waypoint B, 35k by waypoint C, D, E, etc. how does the autopilot follow that? At what rate of accent would the AP track the vertical profile? Or does it vary by different systems? Basically, how would I set up an autopilot to follow the vertical flight plan profile?

Thanks 

Side track from big Boeings or Airbusses: On my light aircraft with Garmin Avionics, you have a dedicated VNAV menu.

You select the desired VNAV descent profile (slope angle or V/S)  and what altitude MSL you want to be at Waypoint X (note: or AGL - great feature for arriving at small/unknown airfields during emergencies, especially at nights or in bad weather).

Then you have the option to specify a distance in NM if you want to level off prior, at, or after the waypoint (there can be reasons why you want to do this).

The computers in the plane knows  current position, track, groundspeed, flightplan Legs and where the computed path from Waypoint X will intersect current altitude. Top of Descent marker appears on map display and "Time to VNAV" countdown also shows up as reminder.

Edited August 9, 20223 yr by SAS443

22 hours ago, SAS443 said:

Side track from big Boeings or Airbusses: On my light aircraft with Garmin Avionics, you have a dedicated VNAV menu.

You select the desired VNAV descent profile (slope angle or V/S)  and what altitude MSL you want to be at Waypoint X (note: or AGL - great feature for arriving at small/unknown airfields during emergencies, especially at nights or in bad weather).

Then you have the option to specify a distance in NM if you want to level off prior, at, or after the waypoint (there can be reasons why you want to do this).

The computers in the plane knows  current position, track, groundspeed, flightplan Legs and where the computed path from Waypoint X will intersect current altitude. Top of Descent marker appears on map display and "Time to VNAV" countdown also shows up as reminder.

So. In the A2A Bonanza GTN equipped will the GTN just give cues? I use the A2A Skylane a lot also with 550. Neither has auto throttle. I know GTN can record a vertical profile but without autothrottle. I want to get into learning about the 737 8 also. Zibo 737 in XP. Been following the Flightdeck2Sim vids. Good replies guys. Thanks.

1 hour ago, Gary1124 said:

So. In the A2A Bonanza GTN equipped will the GTN just give cues?

Can't speak for the specific product since I don't own it. 

But in general a GTN should be able drive a compatible garmin autopilot system. 

I make use of the integrated GPS in the G3X avionics, and VNAV and all its submodes are under FD guidance and also slaved to the AP (Garmin GMC507 Autopilot), and obviously no AT. Only speed on pitch (IAS) is available, but vertical path is honored while in VNAV, which is a descent mode only on this plane.

Edited August 10, 20223 yr by SAS443

On 8/10/2022 at 2:57 AM, SAS443 said:

Can't speak for the specific product since I don't own it. 

But in general a GTN should be able drive a compatible garmin autopilot system. 

I make use of the integrated GPS in the G3X avionics, and VNAV and all its submodes are under FD guidance and also slaved to the AP (Garmin GMC507 Autopilot), and obviously no AT. Only speed on pitch (IAS) is available, but vertical path is honored while in VNAV, which is a descent mode only on this plane.

Im going to experiment with VNAV in the GTN 750 which my copy of the Alabeo 421C is configured with. The 421 is a good plane for that as occasionly RXP GTN in the demanding A2A models can cause P3D CTD. It my ATI drivers. Hope to get a 2070 soon and dump radeon.