I was CRZ at FL256,FMA says MCP SPD and ALT HOLD.Then I was asked to desend to FL148.SO ,I set FL148 on MCP (while FL256 in FMC).Then I push VNAV butten ,nothing happend. why?
But then i put ALT INVT butten and VNAV,it worked,stared VNAV PTH descend.
I had passed T/D
In what situaitions,VANV dosen't work?