FLCH

[apilot53@aol.com]

There is an issue using either flch or vnav to initiate a descent.  It is not related to the .84 mach issue because when I try to descent using either mode the aircraft dives and starts descent at 6000fpm, even if it is already at the target speed.  obviously it then accelerates way past mcp speed and it pitches up to get back to mcp speed.  After that it works fine, it seems the problem is only when it is initially engaged.

[mmerelles]

22 minutes ago, apilot53@aol.com said:

There is an issue using either flch or vnav to initiate a descent.  It is not related to the .84 mach issue because when I try to descent using either mode the aircraft dives and starts descent at 6000fpm, even if it is already at the target speed.  obviously it then accelerates way past mcp speed and it pitches up to get back to mcp speed.  After that it works fine, it seems the problem is only when it is initially engaged.

Agree here.

Watching this behavior "i think" i have identified 2 issues during cruise and engaging FLCH to initiate the descend.

 

ISSUE 1

When you hit FLCH the aircraft starts retarding the throttles to idle while keeping itself leveled. This makes the aircraft to drop a few knots below present target speed. Then the aircraft dives the nose hard down to reach target seed reaching 6000 fpm rates.

 

ISSUE 2

While the aircraft is heavily nose down 6000 fpm, it does not anticipate reaching the target speed which casuse the aircraft to actually overshoot the target speed, until it finally rises the nose up to stabilize the descend on target speed.

 

It seems FLCH requires fine tuning to me

Edited May 11, 2016 by mmerelles
misspelling

[xplana]

agree, same here

[jimbim]

I had this behaviour one time as well. I thought I did something wrong...

 

Sebastian