V2 Replaced by V2+10 on PFD with E/O at 400'

[John H Watson]

on all engines seemed to indicate that VNAV engaged at FMC V2+10 regardless of the initial climbout speed flown.

A long time ago, an pilot ran some tests for me in a big sim, engaging VNAV after takeoff after varying the initial climbout speed.
With a (relatively low) V2 of 146, he achieved 150kts during initial climbout, but then pulled the nose back until speed was pegged at 146kts, VNAV was engaged and the target became 156kts. At 1500' (accel height), the bug and command speed jumped to the max for Flap 20 config., and continued as normal after that (in VNAV).
Using the same V2 for a second takeoff, he pushed the nose down and let the speed build up in the initial climb. Engaging VNAV, the speedtape and command speed jumped to V2+25kts.

Unfortunately, he made the following statement which I didn't understand:

Soooo ... the 'beast' works basically as I expected and the V2 bit is indeed
a product of VNAV and NOT the FMC/MCP.

The mentioned "5 seconds" in his email. I recall this was the minimum time period for the automatics to register the speed.

[Kieber]

Yes Peter, for an all engine take off you are right. As far as I can remember and according to my OM, the flight director commands a pitch attitude to maintain a speed of not less then target speed V2+10 at lift off with all engines operating. But if current airspeed remains above target speed for 5 seconds, target airspeed is reset to current airspeed limited to a maximum of V2+25!

John, this confirmes your V2+25 observation! 

Just for the Simmer-Pilots:
Normally, if rotation rate is correct, the target body attitude results in an airspeed of approximately V2+10 kts for normal gross weights. If the vertical speed is acceptable, adjust the pitch attitude smoothlly,as required, to maintain an airspeed of V2+10 to V2+25 kts to acceleration height. Any speed between V2+10 to V2+25 will not significantly affect the takeoff profile.

Cheers
Walter

[emerydc8]

Hi Peter,

I think this is correct with all engines operating.

Jon D.

[Britjet]

Just did it again at 396T - V2 180. (All engines)
Initial climb out at 196kt max and the VNAV engaged and showed 196 on the PFD.

Peter

[emerydc8]

That's with all engines operating, right? If an engine had failed on you, shouldn't it open at 190kt (if greater than V2+10 (you were at V2+16), the PFD command speed would be V2+10 when VNAV goes active)?

[Hardy Heinlin]

Just did it again at 396T - V2 180. (All engines)
Initial climb out at 196kt max and the VNAV engaged and showed 196 on the PFD.

PSX works in the same way (with all engines). In the next update, for E/O, I just have to apply the same flexibility and set the +25 limit down to +10. It's all already modelled in TOGA mode (incl. the 5 second feature), but not quite so in VNAV mode.

[emerydc8]

Hi Hardy,

With the next update, if you lose an engine at V1 and you maintain V2 on the climbout (as you should), will the command speed on the PFD appear as V2 or V2+10 when VNAV goes active?

[Hardy Heinlin]

As V2.

[emerydc8]

Excellent. Thanks!

[Hardy Heinlin]

Jon, what do you set on the MCP SPD? V2 or V2+10?


|-|ardy

[emerydc8]

Always V2.

[Hardy Heinlin]

Modification now available in PSX 10.0.9-beta17:

http://aerowinx.com/board/index.php?topic=3685.0


|-|ardy

[emerydc8]

Thanks, Hardy. I've been looking forward to this mod.

Jon