VNAV ALT - FMC speed creeps downwards

[PikalaxALT 0]

Scenario: ATC tells me to descend and maintain a certain altitude, say, 6000.  I don't get a further descent until well after reaching 6000 feet.  While maintaining 6000, the PFD annunciates "FMC SPD | LNAV | VNAV ALT".  However, the speed bug starts dropping down by 1 kt per second until it hits the flaps UP speed.  Is this the intended behavior?  And if so, how do I set up the aircraft to inhibit this deceleration?

[scandinavian13 5,192]

However, the speed bug starts dropping down by 1 kt per second until it hits the flaps UP speed.  Is this the intended behavior?  And if so, how do I set up the aircraft to inhibit this deceleration?

 

Yes - this is correct. It's called energy compensation, or similar.

 

Basically, it's realizing that you're now getting above your VNAV path and the angle required to descend back to it is going to require it to descend at a rate where you're going to pick up speed. If the red line is only a few knots above, that won't be possible. If the red line is 80 knots above, then you have 80 knots of descending faster than your idle-constant-speed descent to use to get back on path.

 

In order to override it, just hit SPD INTV and set the speed you prefer to hold.

 

I believe this is referenced in the intro manual, which should be a go-to any time you think there's problem or bug. Usually, it's not.

[PikalaxALT 0]

Thanks for the explanation.  I figured that's what it was doing, and it's actually kinda cool that PMDG simulates that detail.