An IFR flight is cleared DIRECT to a published IAF for an approach at the destination airport. The cruising altitude is 10,000’, the IAF crossing altitude is 2,000’ and the T/D begins about 24nm prior to the IAF with standard rate of descent.
Since route has been previously cleared to that IAF waypoint, can the pilot commence his descent upon reaching the T/D, or wait to hear from controller or request descent instructions just prior to the T/D?
You are never allowed to simply vacate an assigned IFR altitude without clearance. When you were cleared to the IAF your clearance should have included "pilot's discretion down to ..." of "cross IAF at or above..." or similar words. If you don't get this, just ask. Never presume that you can just start down on your own though.
No, you can not descend from your assigned altitude until specifically cleared by ATC to descend. But yes, you can descend down to your Approach without further clearance given you meet a few parameters
First, make sure you are cleared for the Approach itself and not just cleared direct to the waypoint. Your assigned altitude is mandatory until you are established on your cleared Approach. Established means that you are on or immediately about to be on (less than a mile of) a published leg of the Approach. Once you are established on your published leg, you can descend to the at or above altitude published for the route.
If the altitude has a line above and below it, you must be exactly at the published altitude. If that is the case, you will be verbally stepped down to the altitude by ATC.
If the IAP includes a published Terminal Arrival Area, and you are cleared for he Approach, you are considered established on the approach when you are inside of the TAA’s lateral boundaries. The altitudes published for the TAA will dictate your altitude limits you have to remain above.
If ATC does not expect you to follow the IAP exactly to include its published altitudes, they will not clear you for the Approach. They will continue to vector you and give you altitudes.
The clearance given in the OP allows you to proceed direct to the IAF waypoint, but it does not clear the pilot to deviate from the currently assigned altitude, nor does it clear the pilot to fly the approach. These would require separate ATC clearances.
The route at your assigned altitude has been cleared to you - you do have that block of airspace - but not the airspace above or below it. Think in 3 dimensions. Now you may inquire about starting a descent at that point with ATC, but would need a specific clearance to do so.
