Multiple Approaches - Uncontrolled Airports
To answer a recently asked question, and hopefully help some folks understand why things are the way they are, we'll look at uncontrolled airports and how things with ATC work when more than one IFR aircraft are inbound at the same time.
For starters, the rule that ATC has to work with is that only one aircraft may be cleared for an approach at an airport at a time. This holds true for all airports in controlled airspace, with the exception of those in a terminal environment. A terminal environment is one where ATC has radar to the ground, a tower in operation, appropriate radar and radio backups, and an airspace structure internally which allows for closer monitoring than the average enroute sector can accomplish. Most airports in Canada are in ATC sectors which are fairly large, and may not allow the controller to "range down" enough to actually watch things as close as a terminal control unit is organized to do. Add to that the radar coverage issue, and you'll see that most airports, even if radar coverage is good, are below radar coverage to the point that even if ATC could monitor the airspace on a small enough range, they wouldn't be able to see well enough to provide a terminal service at most of them.
So back to the multiple approaches outside of a terminal operation. Without the above mentioned abilities, ATC has to go back to the all-covering rule of one on approach at a time. In VFR weather, ATC may only clear one IFR aircraft for an approach to an airport at a time. This means that until the first one lands, the second one must be kept at an altitude above that of the inbound aircraft until he lands. In fact, ATC must protect for the possibility of a missed approach, too. So if #1 is on approach out of 2,300 feet and the missed approach altitude is 2,500 feet, then the lowest available altitude for #2 is 3,500 until #1 lands or cancels IFR. If the weather is IFR, then it gets a little worse. ATC must now wait until #1 lands and taxis clear of the runway to be used by #2 before he can clear him for the approach. If we get the "on time", then we can issue a lower altitude to #2, but we still can't clear him for the approach until #1 has taxied off the runway.
The same statements mentioned above are true for an airport where a control tower is in operation but terminal control service is not provided. Only one on approach at a time at these controlled airports.
But wait. There are two more wrinkles. There is the possibility of successive visual approaches. #2 sees #1, ATC may be able to approve a visual approach, clearing #2 to follow #1. Another one that most pilots don't understand (and many aren't aware of) is the transfer of control of an IFR aircraft to the tower at a controlled airport. When a preceeding IFR aircraft, whether on a visual, contact or instrument approach procedure, enters the control zone, and weather permits, tower may be able to assume responsibility for control of the IFR aircraft, thereby allowing the IFR controller to issue an approach clearance to the #2 airplane. If you were following the thread earlier about the missed approach on a visual approach, you'll remember I said at a controlled airport ATC (in this case the tower) will issue instructions for the aircraft on a visual who must do a missed approach. That's what this statement was about.
So basically, outside of a terminal environment (only at larger airports), you'll normally only see one IFR aircraft on approach at a time.
For starters, the rule that ATC has to work with is that only one aircraft may be cleared for an approach at an airport at a time. This holds true for all airports in controlled airspace, with the exception of those in a terminal environment. A terminal environment is one where ATC has radar to the ground, a tower in operation, appropriate radar and radio backups, and an airspace structure internally which allows for closer monitoring than the average enroute sector can accomplish. Most airports in Canada are in ATC sectors which are fairly large, and may not allow the controller to "range down" enough to actually watch things as close as a terminal control unit is organized to do. Add to that the radar coverage issue, and you'll see that most airports, even if radar coverage is good, are below radar coverage to the point that even if ATC could monitor the airspace on a small enough range, they wouldn't be able to see well enough to provide a terminal service at most of them.
So back to the multiple approaches outside of a terminal operation. Without the above mentioned abilities, ATC has to go back to the all-covering rule of one on approach at a time. In VFR weather, ATC may only clear one IFR aircraft for an approach to an airport at a time. This means that until the first one lands, the second one must be kept at an altitude above that of the inbound aircraft until he lands. In fact, ATC must protect for the possibility of a missed approach, too. So if #1 is on approach out of 2,300 feet and the missed approach altitude is 2,500 feet, then the lowest available altitude for #2 is 3,500 until #1 lands or cancels IFR. If the weather is IFR, then it gets a little worse. ATC must now wait until #1 lands and taxis clear of the runway to be used by #2 before he can clear him for the approach. If we get the "on time", then we can issue a lower altitude to #2, but we still can't clear him for the approach until #1 has taxied off the runway.
The same statements mentioned above are true for an airport where a control tower is in operation but terminal control service is not provided. Only one on approach at a time at these controlled airports.
But wait. There are two more wrinkles. There is the possibility of successive visual approaches. #2 sees #1, ATC may be able to approve a visual approach, clearing #2 to follow #1. Another one that most pilots don't understand (and many aren't aware of) is the transfer of control of an IFR aircraft to the tower at a controlled airport. When a preceeding IFR aircraft, whether on a visual, contact or instrument approach procedure, enters the control zone, and weather permits, tower may be able to assume responsibility for control of the IFR aircraft, thereby allowing the IFR controller to issue an approach clearance to the #2 airplane. If you were following the thread earlier about the missed approach on a visual approach, you'll remember I said at a controlled airport ATC (in this case the tower) will issue instructions for the aircraft on a visual who must do a missed approach. That's what this statement was about.
So basically, outside of a terminal environment (only at larger airports), you'll normally only see one IFR aircraft on approach at a time.
posted by Michael at
06:31
