747F VNAV Approach Speeds

[tstiegler 0]

Hi,if I want to set a VNAV speed restriction during approach using the FMC LEGS page (by entering something like "190/") sometimes I experience the following problems:1) I can't enter the restriction at all (although the waypoint I want to assign the restriction to is not the active one).2) I can enter the restriction but VNAV does not follow it. I have to disconnect VNAV and to use LVLCHG instead of it.If 2) happens the plane will also not recognize any speed restriction entered into the VNAV FMC page.As written above, it's not always the case. Probably I do something wrong I'm not aware of. Any hints are appreciated.Thanks, Thomas

[downscc 1,327]

Flying VNAV during approach doesn't work out very well for a number of reasons. Regardless, I only use VNAV for climbout and cruise, rarely for descent and never in terminal airspace.I've read posts by others that use LVLCHG but I prefer to use SPEED/VS directly controlling my descent. This works best with ATC constraints as well as avoids the limitations of the PMDG FMS not providing "descend-through" altitude constraints (the aircraft always levels off at the constraint) and provides me much better control over deacceleration.

[tstiegler 0]

Hi Dan,understood, but is this the only way an approach should be handled? A VNAV approach should be possible as well -- and it is as long as I don't touch the predefined flightplan during the approach.Regards, Thomas

[tunnelcat 1]

Thomas,Try this when you enter an altitude restriction, e.g. 190/2500A, putting an 'A' after your altitude to indicate 'at or above'. You can also use a 'B' if you want to be 'at or below' your desired altitude for that waypoint.Kim

[cowpatz 243]

Thomas try entering the restrictions as Kim has explained above. Try to ensure that they are "reasonable and can be achieved". Make sure that the FMA (Flight Mode Annunciator) at the top of your PFD Primary Flight Display or AH if you will) is indicating VNAV PTH. If it is displaying VNAV SPD then it will not adhere to the altitude or speed restrictions. VNAV PTH means exactly that. That you are on the PROGRAMMED PATH. Any other mode means that you must do something to regain the path or that you maintain the path yourself. Look at the magenta path deviation indicator on the ND (Nav display) or select PROG page 2 on the FMC and this will indicate to you how far from the path you are.VNAV approaches actually work very well in the PMDG 747 and I recommend them. I have just about finished putting together a guide on Non precision approaches and when I have finished I will post it on wiki ops.In a nut shell what I do is make sure that the approach altitudes are reasonable. Just before commencing the approach wind down the MCP (Mode control panel on the glareshield) to the missed approach altitude or the nearest 100 ft below. Make sure you are annunciating VNAV PTH. If not then position the aircraft back on the path using another mode. Push the speed selector knob to open the speed window and control the speed manually. Let LNAV track the aircraft and VNAV control the descent. When you are more than 300 ft below the MISSED APPROACH altitude set the altitude on the MCP to the missed approach altitude (only exception to this would be if you intend to circle whereby you would leave it at circling minima). Try to do this quickly or the aircraft might try to level out as the indicator passes the actual altitude of the aircraft. If it does capture it (VNAV ALT indicated) then:1. quickly select Vert SPD and keep the descent going.2. set the altitude selector to Missed approach altitude.3. reselect VNAV4. ensure VNAV PTH annuciated or if not then do something to regain the path.Make sure you monitor the ground base nav aids for correct tracking.Good luck.