How to take off properly using VNAV in a B744?

Question

I'm in a B744 simulator, lined up on one end of a 10k ft long rwy cleared for T/O. i've configured my flaps 10 and a V2+10 (164kts). VNAV/LNAV armed, AT on. airport elevation is 200ft MSL and ATC had cleared me for 8000. I gradually pushed the trottles to 70% and when N1 spool up to 70% i pushed TOGA. 80.. V1.. at Vr I pulled up to 10 degrees and when +ve climb i set gears up. by this time i'm already V2+20! so I pull further to meet the FD. I managed to slow down to V2+10 following the FD. but very quickly I've passed 400 ft and VNAV had engaged. it auto set my AT to 250kts so i intervene it cos i need to manovuer a turn at the most 200kts. but the PFD is still blank (no SPD or THR setting) no matter how many times I press VNAV. the speed just keep spooling up like nobody's business and when I put to AP the pitch went up to 5000 VS and speed drops to 130kts! to prevent stall i level off and managed 200 kts, I press VNAV again and still 5000 VS it goes and IAS drops! Wat is VNAV doing?

Answer 1

Now it's been awhile since I played one of these sims.... but.....

VNAV should be a function of the autopilot. What I'm not sure of is that you may be able to select a speed and have the flight director show you the pitch when you're hand flying. But what I do know is if you have the VNAV bugged at a speed slower than you're going (say 200 in your example) and you're going much faster (250 plus), when you engage the autopilot with VNAV the aircraft will most likely pitch way up to capture the speed and overshoot the speed. That explains to me why it goes down to 130. If you let it go the aircraft might recover and oscillate around the selected speed and eventually capture it. In the plane I fly, we get around that by using Vertical Speed until we get close to the speed we want to fly.

Finally, when you push the TOGA buttons are you sure it's not resetting the paramaters you selected? The plane I'm familiar with we press the TOGA buttons prior to takeoff to set the flight director for Takeoff Mode. On approach and we press we get the Go Around mode. The other time we press it we get Windshear Guidance. It doesn't sound right to press the TOGA buttons on the takeoff role. Have you verified that's correct?

Also, have you double checked the Flaps 10 setting for takeoff? Seems a little low to me.....

Answer 2

Its probably far simpler than you imagine. VNAV is a mode that is designed to allow the aircraft's FMS to calculate a climb or descent angle. If you have something programmed into the FMS AND it matches the altitude preselect, your VNAV should be able to calculate it.

You may need to go into your VNAV configuration and tell it things like climb speed and power rettings and such, but as long as that is all present, it should work.

Answer 3

@250kts retract landing gear