Now let's look at alternative procedures — Page 340, Lesson 454

Let me walk you through the holding and stacking procedures that govern how aircraft are sequenced when approaches are in progress. This is the heart of how air traffic control manages multiple arrivals safely and efficiently. First, we have the concept of clearing aircraft to hold at higher levels than their order in the approach sequence. What this means is that an aircraft can be assigned a holding level that is above other aircraft that are ahead of it in the approach sequence. This is permitted whenever the availability of discrete descent paths and/or radar makes it possible, subsequently, to clear the aircraft for descent through the levels occupied by other aircraft. So the key here is that we can stack an aircraft above others only if we have either separate descent paths — meaning the aircraft can descend without crossing the paths of others — or radar coverage that allows us to safely guide it down through the levels where other aircraft are holding. The word "discrete" is important: it means separate, distinct descent paths that don't conflict with the paths of other traffic. Now let's look at alternative procedures. If a pilot-in-command of an aircraft advises of an inability to comply with the approach control holding or communication procedures, then the alternative procedure or procedures requested by the pilot-in-command should be approved if known traffic conditions permit. So the pilot can request a deviation from the standard holding or communication procedures, and ATC should grant that request — but only if the traffic situation allows it. The phrase "known traffic conditions" means the controller evaluates what traffic is actually present and decides whether the alternative is safe. Now we move to paragraph 17.38, which is titled "Stacking (Approach Sequence)." Whenever approaches are in progress, the following procedures — called stacking — are applied. Let me explain what stacking is: it's the practice of holding aircraft at different levels, one above another, like a stack, while they wait for their turn to approach. The approach sequence itself is called "the stack." The first procedure is priority. The approach sequence, or the stack, is established to permit the arrival of the maximum number of aircraft with the least average delay. So the goal is efficiency: get as many aircraft in as possible while keeping the average delay as low as possible. But special priority may be given to two categories of aircraft. The first category is an aircraft which anticipates being compelled to land because of factors affecting the safe operation of the aircraft. Examples given are engine failure and fuel shortage. So if an aircraft is likely to be forced to land due to a safety issue, it gets priority. The second category is hospital aircraft, or aircraft carrying any sick or seriously injured person requiring urgent medical attention. So medical emergencies get priority in the stack. The next procedure is the procedural sequence. Except where timed approaches are in progress — and that's covered in paragraph 17.39, which we'll get to — succeeding aircraft will be cleared for approach, meaning they start the procedure and leave the stack, when the preceding aircraft meets one of two conditions. The first condition: the preceding aircraft has reported that it is able to complete its approach without encountering IMC. IMC stands for Instrument Meteorological Conditions — that means conditions where the pilot cannot see enough to fly visually, so they must rely on instruments. If the aircraft ahead reports it can complete the approach without entering IMC, then the next aircraft can be cleared. The second condition: the preceding aircraft is in communication with and has been sighted by the aerodrome controller, and reasonable assurance exists that a normal landing can be made. So the aerodrome controller — that's the controller responsible for traffic on the runway and in the immediate vicinity — must have both radio contact with the aircraft and visual sighting of it, and there must be reasonable confidence that the aircraft will land normally. So to summarize the key points: aircraft can be held at higher levels than their position in the sequence when discrete descent paths or radar allow safe descent through other aircraft's levels. Pilots can request alternative procedures if they can't comply with standard holding or communication procedures, and those should be approved if traffic permits. The stack is built for maximum throughput with minimum average delay, with special priority for aircraft with safety issues like engine failure or fuel shortage, and for hospital aircraft or those carrying seriously injured persons. And finally, the next aircraft is cleared for approach only when the preceding aircraft can complete its approach without IMC, or is in communication with and sighted by the aerodrome controller with reasonable assurance of a normal landing. Let me show you a diagram of how this stacking arrangement works in practice.