737 help with autopilot

[Guest JY415]

Can someone tell me why my 737 would vere to the left and start to decend sharply after engaging the autopilot.I haven't flown the 737 for a while and have obviously forgotten something silly.I am setting up the FMC and SID, inputing ALT, HDG of runway and SPD of 180knts and taking off manually, straight ahead, climbing to 1,000FT then hitting the CMD button. I notice that the speed button light then comes on (is that correct?) then for some reason the aircraft starts a turn to the left but the HDG isn't selected and even if it was it is displaying runway heading so shouldn'e need to alter course?!?!?! LNAV hasn't been selected so what is telling it to turn??? Also at the same time it goes from a nice steady manual climb to a sudden descent!??!?Just though someone kind person can give me some things to check and resolve this.Thanks guys for any help, this is driving me nuts!Graham

[Guest JY415]

Just as a follow on from my post...I am not sure if this is relevant or not but I was wondering if my 'Saved' flight may be corrupt so decided to start a fresh flight and just in case some setting in my FMC or MCP was causing the turn and descent I tried a new set up and just manually took off without any inputs into the FMC or MCP. I am nicely climbing at a good speed and so I then hit the CMD 1 button at 1,000ft and guess what.... the aircraft starts a left turn and begins to descend again!??!?!?!?!? Can someone try this and see if it happens to you too?Why is the aircraft changing course and making a turn and descending with absolutely no input commands other than pressing CMD 1 and turning on the autopilot.Guess this rules out any errors in my first flight when I set up everything in the FMC and MCP.Thanks for any help!Graham

[Guest Solex_nl]

I had the same thing happen to me this morning, after take off, setting CMD A, the aircraft descended fast and steered left.I have no clue why.

[Guest JY415]

Nice to know I am not the only one!Can anyone help with this?Graham

[ronzie 21]

What does the FD indicator in the PFD show. Be sure the FD is on before engaging the AP.The FD indication shows the pending output of the autopilot. If you engage AP then that correction goes to the controls.

[Guest JY415]

Interesting and thanks for your reply. What would happen to the aircraft if the autopilot was engaged without the FD turned on? I did have it switched on but when I did the quick test I mentioned in last post there with no input into the MCP or FMC so there wouldn't be any info for the FD to act on would there? I certainly didn't notice any bars to follow, but the aircraft still turned, how?I know this aircaft needs everything set up correctly but having tried that several times (and it takes ages!) and still have the turn/descent problem, I am just trying to test this one down to basic set up (if that is possible?) so I can find where this turn is coming from. CheersGraham