Control of Aircraft — Page 340, Lesson 453

I want to walk you through holding procedures — this is the part of air traffic control where aircraft stack up while waiting to make an instrument approach. Let's start with the basics. Holding is what happens when an aircraft can't proceed directly to its approach — it must fly a racetrack pattern over a fix, waiting for its turn. When holding is required as part of an arrival procedure leading to an instrument approach, the approach controller controls the holding procedure. That's the stack — the vertical column of aircraft circling at different levels. Control may be delegated to a radar controller, who is also called a director. Now, the key rule: holding and holding pattern entry shall be accomplished in accordance with procedures established by the appropriate ATS authority — that's the Air Traffic Services authority — and published in Aeronautical Information Publications, which we call AIPs. Here's the important fallback: if entry and holding procedures have not been published, or if the procedures are not known to the pilot-in-command, then the appropriate air traffic control unit shall describe the procedures to be followed. So the pilot is never left guessing — either it's published, or ATC tells you exactly what to do. Let me give you three specific requirements. First, the holding point. Aircraft shall be held at a designated holding point. The required minimum vertical, lateral, or longitudinal separation from other aircraft — according to the system in use at that holding point — shall be provided. So separation is not a fixed number here; it depends on which separation system is in use at that particular holding point. Second, separation while holding. When aircraft are being held in flight, the appropriate vertical separation minima shall continue to be provided between holding aircraft and en route aircraft — but only while those en route aircraft are within five minutes flying time of the holding area, unless lateral separation exists. So picture this: a holding aircraft circling, and an en route aircraft passing nearby. As long as that en route aircraft is within five minutes flying time of the holding area, you must keep vertical separation between them — unless they're already laterally separated. Once it's beyond five minutes, that requirement no longer applies. Third, holding levels. Levels at holding points shall be assigned in a manner that will facilitate clearing each aircraft to approach in its proper priority. Normally, the first aircraft to arrive over a holding point should be at the lowest level, with following aircraft at successively higher levels. That's the classic stack — first in, lowest; next, one level up; and so on. But there's an important exception: aircraft particularly sensitive to high fuel consumption at low levels — such as supersonic aircraft — should be permitted to hold at higher levels than their order in the approach sequence, whenever the availability of discrete descent paths and/or radar make that possible. So a supersonic jet that burns fuel heavily at low altitude can be placed higher in the stack than its turn in the sequence would normally dictate, as long as the descent paths or radar allow it to still get down in time. Let me show you what this looks like visually. That figure shows the holding pattern geometry. The key idea to carry with you: holding is a controlled waiting procedure, the approach controller runs the stack, separation rules protect both holding and en route traffic, and the level assignment balances priority with fuel efficiency.