Could you please confirm that if I run the Autopilot, set altitude with the big alitude knob and press AltSel on the Flight Control Computer nothing will happen if I fly level.Could you confirm I need to first set the plane to go up or down if I want the plane to reach the altitude I select ?Just not sure about that.Also there is no possibility to select Vertical speed with a knob or something I have to set VS manualy and then press VS on the Flight Control Computer.Am I right or do I miss something ?

ALTSEL just arms the new altitude, it does not cause the airplane to climb or descend. Press VS on the flight director panel, then use the UP/DN dial on the pedestal (or press Shift-3) to set the climb/descent rate.Or, if you want to descend, press IAS on the flight director, then come back on the power levers until you get the descent rate you want. You adjust the IAS the same way that you change the VS rate (same dial on pedestal).

Thank you, I missed the UP/DN dial on the pedestral.Arnaud de Saint hilaire

Yeah, the basic idea is this: planes like the 744 and MD-11 automatically capture the altitude set in the MCP when you engage a vertical mode. On the J41, hitting the ALT SEL button is the extra step needed to arm the capture. So you need to engage a vertical mode (IAS or VS) and then press ALT SEL to ensure that it actually captures the new altitude when you reach it.