SPD intv: While in VNAV the autopilot commands the airspeed and the speedwindow is closed. You cannot set an airspeed manually. Say you want to fly a higher or lower airspeed you can either go to the FMC VNAV page and enter the required speed there (as has to be done on a lot of older 737's). Or you can push SPD INTV. This wil open the speedwindow, allowing you to manually enter an airspeed you want the airplane to fly. When you want VNAV to take over speedcontrol again you just push spd intv again. The speedwindow will close and VNAV takes over again.Alt intv: On the 737 you use the ALT INTV button to cancel VNAV altitude restrictions. Say you got a SID out of amsterdam which requires you to climb to FL060. VNAV will have an altitude restriction installed that requires you to pass the endpoint of the SID at this flight level. If ATC tells you you can climb to a higher FL, say FL200 and you want to keep using VNAV, you turn FL200 in the MCP and you push altitude intervention. This will cancel the FMC altitude restrictions between FL060 and FL200, allowing VNAV to climb. On the LEGS page you can find the altitude restrictions for each waypoint, you can cancel them here too. The ALT INTV logic works the same way for a descent. On the VNAV page there is also a CLB DIR button I believe (or is this only 747). Same function as alt intv. By the way, when you are in a climb and you set your MCP altitude higher than your VNAV cruise altitude, and push alt intv, this will change the VNAV cruise altitude to the altitude that is set in the MCP.Tom