Auto pilot descents.

[arrowspace90 1]

When using V/S, I can get about the rate of descent I need thanks to the map with the arc (aka the "banana").  But if I have to get down as quickly as possible, what rate should a push of the "DSC", descent key, produce?   I tried it a couple of times and it works, boy does it!  I was getting a 4000 fpm rate and the monitor was shrieking "pull up, pull up".  Well of course I did, but why would it give me that much?  In the real 737, the Lvl Chg button never produces 4000 feet per minute.  Something like half that would be more appropriate.

Is this something I can set?  I'm perplexed.  Where is the documentation?

[mobeans10 164]

I noticed in both the Airbus and the 737 that a flight level change using autopilot in level change mode, not vertical speed, will give you a descent flight path to maintain the airspeed in the speed window even when autothrottle is disengaged. This has caught me out a couple of times with excessive descent rates when descending from cruise using autopilot and disengaging auto throttle and reducing to idle thrust. Need to remember to adjust speed hold value before doing even though autothrottle is disengaged

Edited September 1 by mobeans10

[Bobsk8 11,873]

Need to plan your descent.

[LRBS 891]

1 hour ago, arrowspace90 said:

When using V/S, I can get about the rate of descent I need thanks to the map with the arc (aka the "banana").  But if I have to get down as quickly as possible, what rate should a push of the "DSC", descent key, produce?   I tried it a couple of times and it works, boy does it!  I was getting a 4000 fpm rate and the monitor was shrieking "pull up, pull up".  Well of course I did, but why would it give me that much?  In the real 737, the Lvl Chg button never produces 4000 feet per minute.  Something like half that would be more appropriate.

Is this something I can set?  I'm perplexed.  Where is the documentation?

The autopilot or FD is programmed to adjust the aircraft's pitch and throttle to maintain that attitude. The rate of climb or descent during a FL CH is determined by several factors, including present speed, the aircraft's weight, performance, and the desired altitude difference. As an example, smaller altitude differences will give a slower rate of climb or descent. When you engage FL CH mode, in VNAN, first it will open window speed at your present IAS/MACH, and it will target that speed during climb/descent, not to be confused with V/S mode which maintains a specific vertical speed. That 4000 FT/MIN it's not a common situation unless it's a rapid descend most attributed to dialing a higher IAS, speed brakes extended, etc. 

Edited September 1 by LRBS

[LRBS 891]

58 minutes ago, Bobsk8 said:

Need to plan your descent.

As a fact, in the current environment, even if you plan your descend for a certain point (T/D) it might not work due to traffic. Many times ATC will keep you high or just descend you way early. Even during STARS/SIDS they might take you out of your profile. It boils down to how you can adjust to comply with a clearance via FL CH/VS or just reject the request by stating unable due to performance.  

[Orlaam 267]

You dial the speed up high enough in level change, you'll probably hit 4,000 fpm.  On average, from my understanding, the 737 aims to descend at 3000 fpm, but that is based on planning, cost index, ATC instructions and so on.   It's more about maintaining a speed and attitude, rather than fpm.  I'm not sure what you are trying to do.  Some add-ons handle RNAV better and that should be your go-to method unless you need to intervene.  Intervene when ATC deviates from your path or you need to adjust the angle for another reason.  The 737 has a nice little feature in the FMC DES page that will inform you the rate needed to hit a waypoint restriction.  There are ways to manage being too high, but in some cases, S-curves or a hold/turn is needed.  Not sure what plane you are using that has a "descent/DSC" key.  The King Air has this in the AP, but it's based on power settings and speed.  Obviously, smaller planes don't do as well with steep descents.

ETA: I see you mentioned it was the King Air, so I replied on that thread.  You were going too slow and pressing DSC commanded a speed 10 knots below the current barberpole, thus the plane dropped rapidly to hit a much faster speed.  Use V/S in a case like that or be quick to select PITCH-SYNC mode on the yoke.  The King Air takes some effort to get used to and you have to really baby it IMO.

Edited September 1 by Orlaam