Class D boundary crossing and a VFR aircraft without a clearance
El Alto's control zone is Class D from the surface to 12,000 ft AMSL, with a 5 NM radius. A locally based Piper Cherokee is flying VFR at 12,500 ft, above the zone ceiling, and requests to descend into the zone to land at LPB. The controller is working a Boeing 737 on an ILS approach at 10 NM final, descending through 11,000 ft. The Cherokee is 3 NM northeast of the zone boundary, squawking 1200, and has not yet been identified. Terrain northeast of the field rises to 13,000 ft. The controller has primary radar only, no secondary radar coverage at that altitude.
1. Is the Cherokee currently in controlled airspace, and what must happen before it can descend into the zone?
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At 12,500 ft, 3 NM northeast of the boundary, the Cherokee is outside the Class D control zone both laterally and vertically: the zone tops at 12,000 ft and extends 5 NM. So it is not yet subject to a clearance, but it is about to enter airspace where it will be.
Before it can descend into the zone:
- Identify it. With no secondary radar at that altitude, identification must be by position report or by primary radar correlation. The controller should ask for position, altitude, and heading and confirm the report against the primary return if one is available.
- Issue a clearance into the Class D airspace with a specific level and routing, and obtain read-back. A VFR aircraft entering Class D needs a clearance; 'requesting to descend' is not a clearance.
- Separate it from the 737. The 737 is at 11,000 ft descending on final, 10 NM out. If the Cherokee descends into the zone, it must be kept clear of the 737's approach path and given a level that provides at least the VFR/IFR separation the controller can assure without surveillance. At minimum, keep the Cherokee at or above a level that does not conflict, and route it to a downwind or base leg that does not cross the final approach course.
- Terrain. Terrain northeast rises to 13,000 ft; the Cherokee at 12,500 ft is already close to terrain. The controller should not clear it to descend until it is clear of the high ground, i.e., inside the zone where terrain is lower.
Judgement: the controller should not clear the Cherokee to descend through the 737's level. Sequence it behind the 737 or route it clear of the final.
2. The Cherokee's pilot says 'I'm VFR, I don't need a clearance.' How should the controller respond, and what are the controller's responsibilities if the aircraft enters the zone without one?
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The pilot is mistaken. Class D airspace requires an ATC clearance for all aircraft, VFR included. The correct response is a clear, calm instruction: 'Cherokee, El Alto, negative. Class D control zone requires a clearance. Remain outside the zone at 12,500 ft, report position.'
The controller should not argue about regulations on frequency; state the requirement, give an alternative (hold outside, orbit, or remain clear), and keep the frequency usable for the 737.
If the aircraft enters without a clearance:
- Protect the 737 first. Issue traffic information to the 737 and, if necessary, break off the approach or instruct a go-around. The 737 is the aircraft under control and on an instrument approach; the unknown VFR is the hazard.
- Attempt contact with the Cherokee, issue a clearance or an instruction to leave the zone, and record the details.
- Do not assume the pilot will comply. Without secondary radar, the controller may not see the Cherokee at all; the primary return may be weak or absent.
- Report the incursion to the supervisor and file the occurrence report. The controller's duty is to protect controlled traffic and to document, not to enforce penalties.
The key judgement is that the controller's obligation to the 737 does not depend on whether the Cherokee is legally in the zone. Separation is owed to the aircraft under control regardless of the other aircraft's compliance.