Earlier today I had trouble engaging VNAV, The FMC gave me an error "check mpc altitude". I had both the FMC/MCP altitude set at FL360. Does anyone know why I couldn't engage VNAV?Dave FisherCYYZ

It depends if you're in crz mode of if the plane has started to decend. Once you're going down the mcp altitude has to be less than the next restriction or vnav will disengage. In your example, if you set the mcp alt to 20000 say, than you could engage vnav and start a vnav path or spd descent.cheersantps. that is why you must set the mcp altitude to your first altitude constraint before t/d if you want to use vnav path descent.

I've figured out what I did wrong. The're was a discrepancy between the FMC cruise alt. and the mcp alt.Thanks Dave Fisher