For this topic, we'll start with controlled airports, those that have a control tower in operation. Quite often, the old "canned clearances" come into play at these airports. We've all heard them, whether applied to us or someone else. They go something like:"ATC Clears Air Canada one two three to the Toronto Pearson airport via the Moncton two departure, flight plan route, squawk 3101."
This is the basic clearance with a SID involved. What's the period of validity? How long can this clearance hang out there? This answer is quite simple. Indefinitely. Would ATC like to have this out there forever? No, certainly not. But as far as other IFR traffic is concerned, there is no danger. Since the airport is a controlled airport, the aircraft must receive taxi authorization and take-off clearance before he's allowed to become airborne. This means that he should never become an issue for other IFR traffic unexpectedly, so long as everyone on both sides of the radio does their job. Having said that, if you're going to be delayed for a significant time period, advise ATC and let them decide if they want to cancel it and have you call back. It's unlikely the clearance will sound any different in the end.
At uncontrolled airports, things are a little different. The clearance will not necessarily include a SID, even if a SID is published for that airport. Also, the clearance may contain phrases such as, "Do not depart until [time]," or, "clearance cancelled if not airborne by [time]." These are specified by ATC at the time the clearance is delivered to the aircraft (or to the relaying agency like FSS), since the IFR ATC unit has no direct control over the airport surface
. An IFR aircraft at an uncontrolled aerodrome may taxi out and take-off without intervention from ATC in the IFR unit, be it a TCU or an ACC. The catch is the surrounding airspace, however.
Control zones exist at many uncontrolled airports. They are meant as an extension of controlled airspace for two reasons. First, to provide IFR ATC with the authority to direct IFR traffic to enable them to provide separation. Secondly, to provide higher weather limits for VFR to improve the chances of "see and be seen" for both the VFR and the IFR aircraft operating within the control zone. If the airport lies within a control zone but has no operating control tower, an IFR aircraft may taxi out to the runway, but he may not get airborne without an IFR clearance. Hence the requirement to ensure an IFR clearance is received, and that it is valid at the time of take-off. If ATC included a phrase such as, "do not depart until 0105", the pilot may taxi out at any time (providing he considers other traffic), but he is not allowed to become airborne until 0105z. Other phrases might include reports from other aircraft or other such actions. For example, "do not depart until Golf Romeo Uniform Bravo reports leaving 4,000". This would mean that until GRUB, probably someone who taxied out and took off just before you, must make the call out of four thousand before you can take-off. And no, seeing a TCAS target with an appropriate altitude is not good enough. The radio report must occur. If you believe GRUB forgot to make the call, ask on the frequency where he is, or ask ATC for further instructions.
Cancellation times work the same way. A phrase like, "clearance cancelled if not airborne by 0110" may be included. This means that you must get airborne before the clock turns to 0110z. This does not, however, mean that you have until 0110 and 59 seconds to get in the air. And this is airborne
, not "on the roll". There has been some confusion over these two points in the past.
At an uncontrolled aerodrome that underlies controlled airspace, ATC may not direct the flow of traffic, and this includes even telling a departure which runway to depart, or a heading to turn to after departure. This normally means that IFR traffic doesn't flow very efficiently. If it were a busy airport with IFR traffic, the odds are a control zone would be established. In any case, an aircraft at these aerodromes would be perfectly within his rights to take-off without an IFR clearance, even in IMC. Just remember that you must receive a valid IFR clearance prior to entering
controlled airspace. This is where a familiarity with the airspace structure comes in handy, and it is often known well by the pilots who frequent such areas. For example, if controlled airspace begins "above 12,500," an IFR pilot could take-off and fly in IMC at 8,000 feet without an IFR clearance. In fact, if both point of departure and destination underlie controlled airspace, and no part of the flight plan route enters controlled airspace, the flight will never be in receipt of an IFR clearance, since ATC has no authority to direct the flow of traffic in Class G airspace. Note that a pilot must still file a departure message and an arrival message in a timely fashion for the sake of alerting services.