Changing from VNAV -> SPEED

[WebMaximus 372]

Hi,What is the correct way to temporarily change from VNAV -> SPEED in the 747? I tried this on a flight yesterday and no matter what I did I didn't manage to extinguish the VNAV button and instead light the SPEED button.Haven't had that much time to invest in the 747 yet and I'm sure this is in the manual somewhere, just thought if there is a quick and simple explanation to my question?

[macpac22 0]

correct me if im wrong but do you not hit the center of the Speed knob to manually control speed. sorry the autothrottle system is still something im working on learning proper.

[Guest lou ross]

click the vertical speed button (V/S) which will open bothe the v/s and SPEED windows.louross.

[WebMaximus 372]

OK, thanks for your replies guys - I'll sometime soon do some more flying in the 747 and hopefully I'll figure the corrects procedures out then...and most probably with the manual next to me...;-)

[markadeane 274]

Clicking on the center of the speed knob puts you in SPD intervention mode. This means VNAV will function as normal but you can control the speed with the MCP. This method is the best practice assuming all you want to modify is the speed and leave the altitude or climb/descent profile as it is, say for example for a NAT track speed restriction or other ATC speed restriction)SPD intervention is also a great way to regulate your speed on a VNAV PTH descent if you find the airspeed creeping up on you. Possibly one of the best discoveries I have made about the plane after 5 or so months of owning it.

[WebMaximus 372]

OK Mark, thanks alot - will try that on my next flight!

[Guest D17S]

Might also try the FLCH button. Dial up/down the MCP altitude window and press FLCH. That'll get you a climb/descent that will capture in SPD/ALT. You will be entirely out of any Vnav mode. If you later want to go back to Vnav'in, double tap the MCP altitude knob and press Vnav. You'll go right back to VNAV PATH (and all the vertical stuff on the legs page gets recalculated).Careful with that vertical speed mode. It has its place, but it'll get you into lots of trouble. IMO, VNAV SPD or FLCH are the best 'normal' modes to get higher or lower from a VNAV PATH cruise.

[WebMaximus 372]

OK, but have I understood this correctly that once you've selected VNAV and by that having the VNAV button light up you can no longer deselecting VNAV but only replacing it with another thrust mode?Forgive me for all stupid questions, guess the best thing to do is some experimenting...and that's when you're actually lucky you're only in a simulator infront of your PC and not in the real thing ;-)Thanks again for all the tips guys!

[crvm 0]

>OK, but have I understood this correctly that once you've>selected VNAV and by that having the VNAV button light up you>can no longer deselecting VNAV but only replacing it with>another thrust mode? That is 100% correct. You need to tell the AFDS to trasistion from one mode to the next not simply to disengage by selecting the VNAV button. One way to achieve the same thing you are trying would be to hit ALT HOLD. [h4]Randy J. Smith[/h4]AMD 64 4000+|ASUS K8V DELUXE|SAPPHIRE ATI X800XT PE|MUNCHKIN 3200|80 gig SATA|DELL 1905FP 19" LCD|TRACKir PRO|PFC JEPPESEN MOONEY YOKE|CH PRO PEDALS|

[Guest D17S]

One thing to consider too is that on the 747-400, Vnav is a just a pitch mode. The thrust mode works with the AT and can be simply shut off with the AT switch. This is a really, really bad idea though. You'll notice in the MD11 (get that one for sure . . . soon to be remarked on???), the AT switch has been completely removed from the panel. In a more modern AFS (like the MD11), Vnav is ALWAYS both pitch and a thrust mode . . . as it should be. Vnav really needs the AT to work safely. Try some test flights in your XB747. Get into a stable VNAV PATH cruise and shut off the AT with the AT switch. Then pull 'em back to idle. Just let it go and watch the action. Now, thrust to CON power. E-Ticket rides.Even so, Vnav will determine what thrust mode to use when the pilot has the good sense to have the ATs engaged along with Vnav. You can tell what is controlling thrust (and the thrust mode) by looking at the MCP airspeed window. If it's blank and the AT is engaged, Vnav is has control of thrust. Look above the AS tape for the magenta numbers. That is Vnav's target speed. In VNAV SPD, Vnav uses pitch to control airspeed and in VNAV PATH it uses the AT's "SPD" thrust. If the AT is shut off, VNAV PATH is just out of luck. Actually, so's VNAV SPD. Why?Why Boeing would even allow a Vnav mode to engage with out the AT engaged? Maybe someone could help define this logic for me, but so far, I don't get it. You can also override Vnav

[WebMaximus 372]

OK, thanks for the confirmation Randy!

[WebMaximus 372]

Wow Sam...think I will have to print this one out and read it over and over again a couple of times :-)Thanks for sharing your knowledge!

[WebMaximus 372]

Found this info in Boeing's AOM for the 747...doesn't this indicate that you actually should be able to deselect VNAV just the way you can in the 737NG series for example or am I missing something here...?http://forums.avsim.net/user_files/138600.jpg

[Guest D17S]

I think Randy's advice is still on the nose. This comes down definitions here.It seems "

[Qavion 19]

Sam, are you using PMDG's sim to understand how a real 744 would behave in your scenarios... or do you really know how a 744 operates?VNAV has speed protection... It won't let you stall the aircraft (with the A/T functioning). It's not going to let your speed drop to 100kts in VNAV SPD mode with the A/T engaged. The FMC knows the safe speeds (The FMC computer/s in the equipment centre contains the A/T computer and a database of min speeds for various configs).With the A/T off, the real aircraft allows you to operate the thrust manually when in VNAV mode. Tip: If you have real world manuals look for an FMC message "THRUST REQUIRED"Cheers.Q>