No issues, they both work the way they should.

Well then there's some issues...

There are no issues. The MCP altitude set determines where the VNAV is limited to. When you are climbing, it won't climb higher than the altitude set, and when you are descending, it won't descend lower than the altitude set. This means that you have to remove this limit in order for the airplane (737,747,MD-11, etc) to descend or climb past what was previously set. If you don't lower the MCP altitude before T/D, then you are going to be up there until you run out of fuel.

Peter, Unfortunately, I don't think your understanding a fundemental principle of the Boeing Autoflight philosophy. All current production aircraft including the 747-400, 777, 767, 757 and 737 series ALL have autoflight programming designed to "Restrict" Vnav operation to an altitude dialed into the MCP Window. If operating in VNAV, the Autopilot should NEVER violate an altitude dialed into the MCP as it is a Limit to the Autopilot (with the exception of some approach modes). This is the reason you get the "ADJUST MCP ALT" message when approaching TOD and also the reason the a/c will level off on departure if you fail to select a higher altitude during climb (assuming you set the Departure Procedure altitude in the window). If you watch any number of videos online of Cockpit procedures, ATC will many times assign an altitude other than what is programmed into the FMC during the departure or arrival. What you should see when these altitude restrictions are provided by ATC is that the PNF reaches up and dials in the assigned altitude into the MCP. This ensures that 1) Both Pilots are aware of the restriction as it is annunciated on the MCP and both speed tapes, and 2) The Autopilot now knows that the aircraft has been restricted to that Altitude regardless of what is programmed into the FMC. If, before reaching the altitude, the assigned altitude is changed, the crew simply dials in the new altitude to the MCP and the A/P continues flying in VNAV to the new restriction. Please take a read through the FCOM on VNAV operation that we provided with the 737NGX or even the FCTM and I think you will find it quite enlightening.

Well then there's some issues...

meaning with my simulation...

What issue are you having? The way you've described it so far, your ngx is acting as it's supposed to.

Well then there's some issues...

Think we found the issue...

But the whole point of VNAV, Vertical Navigation, is that it should calculate when the aircraft climbs and decends, how fast it does this, and it shoudl do this regardless of what altitude the MCP is in. In the legs section, it shows the altitudes that the aircraft should be at while pasing through every waypoint. If VNAV requires the changing of the MCP input, it is no different than flight level change. VNAV is supposed to work somewhat liek an ILS system, where it descends on a pre-determined path, not by manual input. Because if I want to be at say 8,500' for the FAITH intersection for an ILS into SFO's Rwy 28L, it should bring me to that altitude on time, rather than require me put in 8,500' then put in 7,000' feet for DIVEC when I start the ILS.

I think you're misunderstanding the the VNAV. Let's look at it this way: VNAV is a servant, a skilled one, able to let you know that for instance, to be able to arrive at this airport, you need to start your descent at this point. Also able to do the actual descending for you, but you're the boss, so VNAV, being the obedient servant that it is, still waits for you to give it the permission to descend (after all, you, the pilot could fall asleep, and it would just keep descending until you drill a hole in the ground). It's a safety feature. Now, let's say that on reaching your TOD, you first descend to FL260, then FL220, then FL180, but ATC tells you to maintain FL180 until further instructions then you'd dial in FL180 before the TOD, (meaning, "hey Mr VNAV, go ahead and descend at TOD, but make sure you don't go lower than 18,000 feet") VNAV would then keep all the altitude changes at the waypoints until reaching FL180, and level off, waiting for your next permission. Or you can just dial in your lowest altitude (altitude where you should be before capturing the glideslope, and it will follow every altitude at every waypoint until reaching say 1900', which is the GS capture altitude. I stand to be corrected if I'm wrong EDIT: Jet tech explained it better. I didn't see that before I posted.

Edit: Smasher77th beat me to it, and I like his explanation better than mine in any case!

Edited September 25, 201114 yr by JRBarrett

But the whole point of VNAV, Vertical Navigation, is that it should calculate when the aircraft climbs and decends, how fast it does this, and it shoudl do this regardless of what altitude the MCP is in. In the legs section, it shows the altitudes that the aircraft should be at while pasing through every waypoint. If VNAV requires the changing of the MCP input, it is no different than flight level change. VNAV is supposed to work somewhat liek an ILS system, where it descends on a pre-determined path, not by manual input. Because if I want to be at say 8,500' for the FAITH intersection for an ILS into SFO's Rwy 28L, it should bring me to that altitude on time, rather than require me put in 8,500' then put in 7,000' feet for DIVEC when I start the ILS.

I think you don't understand VNAV. First, VNAV will never descend below or climb above the altitude dialed in to the MCP altitude window for safety reasons. Second, FLCH (flight level change) will not respect altitude/speed restrictions of waypoints in the flightplan, while VNAV mode will climb or descend on a flightplan/vertical path without busting speed/altitude restrictions. of course, I could be completely wrong

But the whole point of VNAV, Vertical Navigation, is that it should calculate when the aircraft climbs and decends, how fast it does this, and it shoudl do this regardless of what altitude the MCP is in. In the legs section, it shows the altitudes that the aircraft should be at while pasing through every waypoint. If VNAV requires the changing of the MCP input, it is no different than flight level change. VNAV is supposed to work somewhat liek an ILS system, where it descends on a pre-determined path, not by manual input. Because if I want to be at say 8,500' for the FAITH intersection for an ILS into SFO's Rwy 28L, it should bring me to that altitude on time, rather than require me put in 8,500' then put in 7,000' feet for DIVEC when I start the ILS.

Does "RESET MCP ALT" ring anjy bells? That's the FMC saying,"You programmed me to go down, but you have to tell me it's okay."

I think you don't understand VNAV. First, VNAV will never descend below or climb above the altitude dialed in to the MCP altitude window for safety reasons. Second, FLCH (flight level change) will not respect altitude/speed restrictions of waypoints in the flightplan, while VNAV mode will climb or descend on a flightplan/vertical path without busting speed/altitude restrictions. of course, I could be completely wrong

No, you're not. That's the actual key point here. All this "VNAV is like LVL CHG" discussion - it's NOT! Seriously, get your FCOM out of the cobwebs as you've some catching up to do. Do you actually understand the VNAV modes SPD vs PTH? Get these nailed down first, and then on your next flight during your (hopefully correct!) VNAV operation, watch the FMA and think about what the plane is doing by that time. You could think of VNAV SPD being a KIND OF level change thingy - but in fact, and again, it's not. As level change doesn't exactly care about what your flight plan says, but VNAV most certainly does.

In real you will never climb or descent without a clearance from ATC. It will be that cleared level/altitude you put in to the MCP. When you start an approach you will in most cases not get descent clearance allt the way down to where you are supposed to pick up the ILS at once, it will be more of a step by step descent depending on how the airspace around that airport is organized. If you use the FSX default ATC, RadarContact or fly online it will make more sence. Or even better, if you live close to some airport, get yourself an airscanner and listen to real ATC. There is a lot to learn from that. The VNAV function is more of an aid, its still you, by selecting the correct alltitudes in the MCP depending on valid ATC clearance, who fly the airplane! RegardsJohnny Holmgren

Only occasionally will my VNAV arm or engage weather I'm on the ground or above 400 ft. LNAV works perfectly, comes on while on the ground and tracks the FMC course perfectly. But VNAV, most of the time will never arm or engage. FMC is complete with altitudes and speeds on the legs page and matches a STARS chart perfectly. Both flight directors are on and A/T is armed. TO/GA works perfect as well. I just don't know why VNAV will not engage or arm.I followed the tutorial and got the VNAV to come on once but that its. Tried again using the tutorial and it wouldn't come on . Also, on one of my own flights I got it to work but only once. Does anyone have any ideas. Tried both Collins and Honeywell.Thanks-Brian

To go back to the original post, I'm pretty sure LNAV/VNAV won't arm on the ground unless the first waypoint is within 4 miles of the runway +-20 degrees runway heading or something along those lines.

FMC is complete with altitudes and speeds on the legs page and matches a STARS chart perfectly. Both flight directors are on and A/T is armed. TO/GA works perfect as well. I just don't know why VNAV will not engage or arm.

Altitude and speed predictions or constraints on the LEGS page doesn't exactly make FMC initialization "complete". For VNAV to engage or even just arm, make sure your PERF INIT is correctly fed into the system AND EXECUTED. No PERF INIT, no VNAV. Simple as that.