I've installed the PMDG737-600/700/800/900 with all the service packs but now i've really got serious problem witht the VNAV. Can anyone help me? Much thanks.1.Even though everything is set correctly in the FMC with FD and A/T armed, the VNAV still cannot be armed on ground. 2.VNAV occasionally fail to be engaged after airbone, or cannot coordinate with N1 Limit, causing aircraft to lose control and crash.3.FMC calculated descend path is very wierd. Cannot follow the altitude restricitons entered in FMC LEGS page. When VNAV mode engaged, FMC Message 'DES PATH UNACHIEVABLE', then Autopilot jumps to either LVL CHANGE or ALT HOLD mode.

1. This is correct, you must wait till 400 feet AGL to engage VNAV.2. Not sure there, make sure you read a tutorial (there's several in the AVSIM library) to make sure you're using the correct procedure.3. DES PATH UNACHIEVABLE means just that, you're asking the plane to perform a descent that it can't physically do within the constraints of the flight profile (speed etc).

Than you for your reply sir but I've got query about the No.3 question. I'm sure the descend is normal for the aircraft, and I've done that with the PMDG744/744F. I"ll try illustrate it here.Suppose my cruise altitude is 5000ft,FMC LEGS PAGEGUAVA 250/5000SOKOE 220/3000LIMES 200/2000NLG21 180/1700VNAV PTH mode is engaged. I set the MCP to 1700ft before T/D. However, after passing through T/D and started descending, the FMC descend path indicator on the ND says i'm below the calculated descend path...and though i level off, i still find the indicator saying im too low(and the nubmer is increasing). Finally, the VNAV disengaged itself and FMC Message 'DES PATH UNACHIEVABLE '

It's normal for the VNAV to command a descent that is either below or above the verticle path indicator, at least initially. The provided info isn't enough to visualize the problem, except to say that if you "level off" then VNAV can't do it's job.The 737 VNAV routine is very old and not the best... myself and others prefer to use the VNAV as a reference and fly the descent with the MCP in either LVL CHG or V/S modes. This will always achieve better results. The most glaring problem is VNAV tends to lock up after passing a constraint (witnessed by a magenta bug in the verticle speed tape), and it always commands a "level off" at a constraint even if what is required is a "descent through" the constraint. There's more... but the point is I use VNAV for climbs and MCP for descents.