Clearance content is included: callsign, flight path, clearance border(destination aerodrome, available report point, controlled airspace border), flight level along the route or for a specific part of the route, etc. and end time of clearance or clearance expiry time. Can you give an example of a clearance expiry time? How does it end, is there any specific time that is notified by air traffic controller to pilot to pass some specific point? How is it said? can you give me a clearance example?
The current version of the AIM says:
Clearance Void Times:
A pilot may receive a clearance, when operating from an airport without a control tower, which contains a provision for the clearance to be void if not airborne by a specific time.
A pilot who does not depart prior to the clearance void time must advise ATC as soon as possible of their intentions. ATC will normally advise the pilot of the time allotted to notify ATC that the aircraft did not depart prior to the clearance void time. This time cannot exceed 30 minutes.
Failure of an aircraft to contact ATC within 30 minutes after the clearance void time will result in the aircraft being considered overdue and search and rescue procedures initiated.
EXAMPLE− Clearance void if not off by (clearance void time) and, if required, if not off by (clearance void time) advise (facility) not later than (time) of intentions.
Let's say you're using a GCO or telephone to contact an ARTCC for your clearance out in a low-population area with little infrastructure making it impossible to contact ATC from the ground via standard radio equipment. After you receive your clearance, you will hang up the call or change frequencies and break contact with ATC.
Due to the break in communication, they will end your IFR clearance by assigning a void time in case you don't get airborne as soon as expected. If you delay beyond your void time, you would need to call and get a new clearance that you would expect to have issued with a new void time. The void time is short enough that the implied intention is for the aircraft to receive the clearance while stopped at the runway's hold short line already prepared for takeoff.
This is most commonly seen when starting your flight plan from a pilot controlled airfield. You can not start your IFR flight plan until you are in controlled airspace. At a controlled airfield, you would have to “hold/wait for release”. At an uncontrolled airfield, you would have from the time of activation until the end time of clearance or clearance expiry time to takeoff and have wheels up. You can not leave the ground one second past that time unless you call ATC for a new time. If you do not leave the ground, you must contact ATC ASAP.This is mainly an issue at fields where radio reception between ATC and aircraft at that field is spotty or nonexistent.
You are to then follow your clearance instructions into controlled airspace. Once you are within both radio reception and controlled airspace, you must either contact ATC, land as soon as practicable within Class G, E, or D (with clearance) airspace, or follow the Radio Inoperative procedures outlined in FAR 91.185. Contact ATC immediately upon landing. You can not deviate from this.
Remember, IFR can only be flown in controlled (A,B,C,D or E) airspace. Only there can ATC provide the required separation necessary for IFR flight. Although VFR Flight Following follows similar rules, VFR Flight Plans do not. Do not mistake this for the Expect Further Clearance time that you would get in flight.