I've been having some AP issues lately... obviously a PEBKAC problem but I'm trying to figure out what I'm doing wrong. Most of my flights are with very light payload and fuel... usually 1/3 fuel and 40-50% loading. I usually get around 135-140 V2 speed and set that in the MCP. Perhaps this is my first mistake? Am I to be setting V2+20 or V2 into the MCP? Second, after takeoff I pitch up to about where the command bar tells me (normally 12-15 deg) and after 400 ft engage CMD A with VNAV and LNAV armed. Since I'm so light the thing sometimes rockets upwards and if I've got a low altitude set in the MCP (and FSX will bust it for sure) I basically have to override the AP with control input and then the alarms blare at me and the AP disconnects.In a nutshell I don't trust the AP especially when I'm flying online and altitude deviations matter.Is it possible to fly this thing or rather, have the AP fly it when the plane is very light?After this all happens there is no way to re-engage the AP. I acknowledge all the alarms and turn everything off, but I am unable to turn on CMD A. It will flash briefly, but the light never stays on. How to re-engage AP?

Small altitude changes using pitch-for-speed modes at very light weights with high thrust are pretty much always going to do this - it happens in real life too. Are you derating your takeoffs at all? That will help if not. Also you should probably consider handflying if there's such a low initial restriction - I very much doubt a real pilot would turn the AP on in a mode like VNAV or LVL CHG when the restriction is so close. Those modes just do what they're told, they command thrust to the full N1 limit and they pitch up to maintain the selected speed.I'm not sure why you're not able to reengage the AP after - do any failures show in the FMC failures page? V2 is correct in the speed window too - do the tutorials if you're unclear on basic stuff like that.

Hey,If you fly ligth, set a serious derate (choose a standard one or use the assumed temp method).As Ryan says: if your likely to be restricted in altitude, fly it by hand or use a vertical speed mode (or level change when your derate is good).If you cannot re-engage the AP, is it maybe because you are giving input with your controls? (When you are struggeling to level off the plane maybe?)I can also advise you to do manual touch and goes with different weights. That way you get a better feel of manual steering and power input.Good luckBert Van Bulck

If you cannot re-engage the AP, is it maybe because you are giving input with your controls? (When you are struggeling to level off the plane maybe?)

Or accidently break the FCCs. This can happen if you apply too much control force to the yoke (>110 lbf), read the intro doc for further information. Quite a cool "feature", but nasty when it happens to you. You might want to check the failure pages for broken FCCs. Just one of the possible reasons why an AP might not connect.

To be sure about broken FCCs, try connecting other autopilot (A/:( - if you break the FCC on one, other should still be working. TO be safe, always disconnect AP by pressing the disconnect button - map it on your yoke. Very convenient.

Or accidently break the FCCs. This can happen if you apply too much control force to the yoke (>110 lbf), read the intro doc for further information. Quite a cool "feature", but nasty when it happens to you. You might want to check the failure pages for broken FCCs. Just one of the possible reasons why an AP might not connect.

That's gotta be it...I'll try derating my takeoffs and hand flying on departure. I don't mind that at all. Or I suppose I could use a VS mode as well. Thanks for the tips! Maybe I'll even check out T2 :)

To the question of MCP speed for takeoff, set V2. The FD automatically directs you to MCP speed + 15 for TO, so if you set V2+15 it'll provide guidance for V2+30.