I've been listening to a lot of live atc but mostly approach and departure, so I usually miss the point where clearances are given to descend from cruise. I know the phraseology is something like "[callsign], cleared to descend at your discretion" but my question is, does the crew need to wait for an explicit descent clearance before reaching t/d? If you reach t/d but haven't been explicitly cleared, are you expected to hold or is it expected that you follow your fmc profile?

 

 

does the crew need to wait for an explicit descent clearance before reaching t/d?

 

Absolutely -- you never, ever leave your cleared level without a clearance from ATC.

 

 

 

 

If you reach t/d but haven't been explicitly cleared, are you expected to hold or is it expected that you follow your fmc profile?

 

Neither -- you're expected to continue at your cleared level. Remember, ATC's objective is to keep aircraft separated. They have no idea what your FMC descent profile is: if everybody just started doing their own thing -- it would be chaos.

 

If you are approaching a point at which you'd like to start down, it's up to you to obtain a descent clearance from ATC -- usually a gentle nudge ("Bigjet 123, ready for descent") is enough. However, there may be a reason why ATC are keeping you high: there might be conflicting traffic, for example. If so, you'll normally be told the reason and to expect lower in X miles.

 

Most of the time, however, you'll be descended in good time (often a little earlier than ideal depending on the airspace restrictions), though some places are notorious for keeping you high.