FMC take off and climb speed in autopilot?

[Guest cliffie1931]

We dial the FMC calculated take off speed into the autopilot prior to take off.That presumably means that after take off we'll climb at that speed until a new, faster speed is dialled in.At what stage do we dial in the higher speed please?Cliff

[Guest ivo]

Well, you might want to set the MCP speed at V2+15 or so. Once the gear is up and you have established yourself in a climb, you can accelarate and retract some of the flaps. You should have green markings on you speedtape telling you when to retract the flaps and to how many degrees.

[cowpatz 243]

Gidday Cliff,Have a look at page 40 of the FMC user manual.Make sure you have entered all the apppropriate information in the performance page.On the Takeoff page you might like to try entering 20/1500 next to where it says flaps (1500 is in small font). You will have to type in 20/1500 and line select it to that key. Beside E/O acceleration height enter a typical value of 1500 (or higher if terrain is a factor) Once again you will see it at 1500 in small font and you will need to manually type it in. Beside Thrust reduction enter the altitude or flap setting that you would like the autothrottles to set climb thrust ie. 1500 or just 5 to select flap 5. This will cover most noise abatement procedures. On the right hand side you enter your TO speeds that you have calculated (V1, VR and V2. You could just use the values that are there in small font (they wont appear until you have entered something in the flap setting at the top left). If you press the keys next to those speeds they will come up in large font and "activate". Set the MCP speed window to the V2 speed. Ensure VNAV is armed ( and usually LNAV as well) and you are now set to go. If you entered 1500 as the thrust reduction point you will observe climb thrust happen and the aircraft will acelerate towards the FMC programmed speed or the next flap limit speed (whichever occurs first). You could also manually set a speed in the speed window on the MCP.CheersSteve

[Guest cliffie1931]

Thanks mate. That gives me something to chew on.Cliff

[Guest D17S]

Here's the way the system works to establish initial airspeeds: For take-off (in TOGA pitch mode), the MCP airspeed setting only positions an airspeed bug on the PFD's airspeed tape and does not provide AFS(/AT) FD/CMD flight guidance. In TOGA pitch mode, the MCP airspeed setting does not control airspeed. In TOGA pitch mode, the AFS uses the greater of:1) The FMC's calculated V2, or2) An adjusted V2 based on lift-off (rotation?) airspeed, or3) An averaged speed occurring after lift-off (rotation?)Prior to acceleration altitude, Vnav engagement will direct airspeed to the FMC

[Guest D17S]

There was some discussion that Vnav was just too complicated to use. Here I was arguing that,

[Vauban 0]

>If armed, at 400 feet Vnav will engage. Pitch adjusts to>maintain the adjusted V2 or V2+10, whichever is greater.>There

[downscc 1,327]

<

[Guest Hawkeyeted]

The VNAV/LNAV on the NG can't be "armed" as in the 747-400. When you engage the A/P (above 400 AGL), you will be in SPD climb mode on the runway heading until you engage VNAV/LNAV. That said, has anyone noticed that on the NG if you set the MCP speed to V2, upon A/P engagement, it will reset the Magenta speed bug on the PFD to V2+10? This frees you of having to "Bug-up" during the climbout. Once VNAV is engaged, the FMC will "bug-up" for you, but limit speeds based on flap settings.

[Vauban 0]

Thanks Ted, I'd heard of 'arming' VNAV before and thought maybe I'd missed something on the 737 NG.Geoff

[Guest D17S]

Don't know about the NG, but the 744 can arm V/Lnav prior to TO. The little light is the light in the AP switch light that comes on that lets me know the AP has engaged. Glad you're enjoyin' all this, Me too!

[Guest D17S]

Sounds like the 737 might work the same way as a 744. As an experiment, set the MCP to 250kts. Take-off and let the AP/AT control the airplane in TOGA pitch/THR REF thrust. You'll probably get one of the 3, possible V2 speeds. The greater of: 1) FMC's V2, 2) Liftoff speed, or 3) An averaged speed since lift-off. It probably will not go to 250 kts. Now engage Vnav and watch that the speed target STILL goes to either FMC's V2+10, choice #2, or choice #3, whichever is greater. It's likely the MCP airspeed window setting just won't be part of the picture. Like the 744, sounds like the MCP may also have no authority over speed control in the take-off TOGA pitch mode. This is the way the MD11 works too, except the procedure is to always set 250 knots in the MCP airspeed window for TO. Bit more honest, if you ask me. (Ahhh, IMHO!)