Separation — Page 316, Lesson 414

Let’s talk about stacking — the way air traffic control manages a queue of arriving aircraft when the runway or the approach can’t take them all at once. First, the core principle: 16.52 Approach Sequence. When more than one aircraft is arriving, the normal rule is that the aircraft at the lowest altitude lands first. That makes sense — the one closest to the ground is closest to being on final, so it goes first. But when there are too many arrivals, we can’t just line them all up on the approach. So a holding pattern is established — that’s the racetrack-shaped orbit you fly while waiting. The collection of aircraft waiting in these holding patterns is called the stack. Aircraft are stacked vertically, one above the other, each at its own assigned altitude, waiting for their turn to start the approach. Now — and this is important — under certain circumstances, a later arrival will be given priority over earlier arrivals. There are four such circumstances, and I want you to know each one precisely. First: an aircraft that is compelled to land because of factors affecting the safety of the aeroplane or the occupants. In that case, the PIC — the Pilot in Command — of that aircraft is expected to declare an emergency using either “MAYDAY” or “PAN PAN” procedures. MAYDAY is the distress call for grave and imminent danger; PAN PAN is the urgency call for a situation that isn’t immediately life-threatening but still requires priority. Second: hospital aircraft, or aircraft carrying sick or seriously injured people. And here’s a specific phraseology point — international hospital flights will prefix their initial RTF — that’s radio telephony — with “PAN PAN MEDICAL”. So the pilot would say “PAN PAN MEDICAL” at the start of the radio call. Third: aircraft engaged in SAR operations — that’s Search and Rescue. Fourth: other aircraft as may be determined by the authority — meaning the competent aviation authority can designate additional priority cases as it sees fit. Now let’s move to the procedure itself — 16.53 Procedure. Stacks are normally established on the radio navigation beacons that serve as the IAF — the Initial Approach Fix — for the instrument procedures to be used. So the holding point is built around the beacon that starts the instrument approach. The vertical size of the stack — how many levels it can hold — may be limited by airspace considerations. And here’s the key consequence: when the stack is full, overspill stacks are established on remote beacons — that is, on beacons further away, so the extra aircraft have somewhere to hold. Arriving aircraft are cleared into the stack at the lowest available level. So you don’t join at the top and work down — you join at the lowest empty level. Normal holding pattern joining procedures are used — the standard entry and joining techniques you’d use for any hold. Then, aircraft are cleared to commence the instrument procedure from the lowest holding altitude — that’s the bottom of the stack — using timed arrival procedures. So the aircraft at the very bottom leaves the hold and starts the approach, and the timing is managed so each aircraft is sequenced properly. Now there’s a specific situation I want you to note. If a pilot states their intention to continue holding awaiting a weather improvement — they want to stay in the hold and wait for the weather to get better — while other pilots wish to make an instrument approach, then the holding pilot will be instructed to take up another holding pattern, or directed to rejoin the hold at the top. So that pilot doesn’t block the sequence — they get moved out of the way, either to a different hold or to the top of the stack, so the aircraft that want to approach can do so. And the excerpt cuts off mid-sentence at “If a pilot elects to attempt an instru…” — that’s where the passage ends, so we’ll pick up from there when we continue. Let me just recap the key points you need to hold onto: lowest aircraft lands first; the stack is built on the IAF beacon; vertical size limited by airspace; overspill stacks on remote beacons when full; you join at the lowest available level; you depart from the bottom using timed arrival procedures; and priority goes to safety-critical aircraft, hospital flights with “PAN PAN MEDICAL”, SAR aircraft, and authority-determined cases.