VNAV Challenges...informational post

[Guest D17S]

Did you dial down the altitude in the MCP altitude window? The FMC will ask permission to descend with the prompt Check MCP Altitude. Dial in your 1st altitude constraint and wait. The AFS will begin the Vnav descent when its ready without any further pilot input. Watch the ND as the airplane approaches that green T/D dot. Down you'll go. It magic!

[kevinh 409]

Where on earth did you get that idea from? Are you suggesting that for a 20,000 foot flight level change, FLCH will attempt to get there in 125 seconds?? That's 9,600 fpm.FLCH will give you a climb rate corresponding to the pitch required to maintain the MCP speed target. Time to climb has absolutely nothing to do with it. In fact FLCH does not set a climb rate. It controls airspeed with pitch. The resultant climb rate is just that, a result, not a target. If you select a 2000 foot climb or a 20,000 foot climb the ROC will be the same. The only difference being that for a small step climb, you will capture the new altitude fairly soon, thus beginning to reduce the ROC. In a climb, FLCH will always select the reference thrust (CLB), but for a small step change, CLB power might not be reached. In a descent it commands idle thrust then controls pitch to achieve the MCP speed target.Using V/S mode means you have less airspeed protection. It's OK for small, gentle altitude adjustments, but not safe for larger changes.It seems many people think climb should be controlled as it is by the default 737/747 FS autopilot. That AP relies on VS mode for climb/descent rate and autothrottle for IAS control. With a real autopilot, in a climb vertical speed is determined by climb thrust and the airspeed selected. Airspeed being controlled by pitch, not power.Kevin

[Guest adameke777]

>Where on earth did you get that idea from? Are you>suggesting that for a 20,000 foot flight level change, FLCH>will attempt to get there in 125 seconds?? That's 9,600 fpm.If the thrust required for the level change is less than CLB or greater than IDLE, i.e. for small level changes, FLCH will set the thrust to get to new level in 125 seconds while maintaining MCP SPD. Otherwise it's either CLB or IDLE to get to the new level as soon as possible (at MCP SPD).Don't take this explanation for granted though.A.G.

[Guest D17S]

KevinA.G. has it right on the nose.So, AHHG. You

[kevinh 409]

All A.G. is saying is that for small altitude changes less than CLB thrust might be selected. A.G. does not confirm your theory that this applies to greater level changes. How can it? Once the engines are at CLB thrust all the FCC can do is adjust pitch to maintain the selected airspeed. This does not vary by the way. In FLCH, the MCP speed target is a pilot input.When I asked where you got the idea from, I didn't want or need such a verbose, slightly patronising reply. Your last paragraph answers my question. Using a PC flightsim to derive how the aircraft might be designed to work is not likely to be very accurate. Although not a pilot, I have often used FLCH mode in Full Flight Simulators, which do accurately represent the aircaft. In my experience FLCH operates very simply. It maintains the MCP speed with pitch and applies CLB thrust, though not always immediately, I agree.There are differences between aircraft types. For example, the 757 will set CLB thrust for any altitude change. The 767 will apply thrust in increments as necessary, up to the CLB limit.Kevin

[Guest D17S]

"Using a PC flightsim to derive how the aircraft might be designed to work is not likely to be very accurate." Depends which one you're usin' ;) Next time you are in the big sim, give it a try. lt us know.

[Guest hmslion]

I remember way back when the PMDG guys released their 757 for Fly2k (dating myself here) that a real 757 driver said the very same thing about VNAV -- squirly and unpredictable. Many ##### moments in the cockpit.So, sounds pretty real as it gets to me!Colin

[Guest langevine]

Dave,I haven't tried this with the PSS model, but in RWF, the runway heading should autoselect on the runway selected in the FMC. This is a good thing, as in t he event of an engine failure you'll need to get out of TOGA and into HS to go straight out -OR- fly the appropriate engine out profile for the departure. Pre-selecting a heading other than RW heading is discouraged. Remember, in TOGA the FD will command a track you had at lift off -- and track isn't heading. LNAV can likely do all this for you, again, if you have everything set up correctly. Otherwise, your list of instructions seems pretty much right on.Good job.---leo---

[Guest FlyinJ]

What a GREAT thread! Very informative and enlightening! I really don't have anything to add except thanks to Dave, Sam and all the others who, through their meticulous dissertations, have helped us all to understand more about the complexities and nuances of VNAV, FLCH, the FMC and MCP.Happy Flying!Jeff

[Guest langevine]

Yes, reset the MCP altitude, select the VNAV DESCENT page and select DESCEND NOW. If it works correctly, you will start down at a rate computed to intercept the VNAV profile and should NOT result in excessive descent rates. FLCH does NOT recognize the VNAV profile and if the altitude change is significant, the rate will be higher.I often used DESCEND NOW, even when not to comply with ATC because it results in a less significant rate transition from cruise to the descent profile.---leo---