Let’s pick this up right where the separation rules change — Page 240, Lesson 314

Let’s pick this up right where the separation rules change. Once aircraft are established on the localizer centre line during ILS approach operations, the normal vertical and horizontal separation standards are overruled. That’s a big deal, so let’s make sure you understand why. Think of it this way: normally, air traffic control keeps aircraft apart by assigning different altitudes or different horizontal distances. But during these approach operations, once an aircraft is locked onto the localizer centre line — that’s the lateral guidance beam of the ILS pointing straight down the runway — those standard separation rules no longer apply. Instead, the separation is built into the procedure itself. Now, there are two modes of operation here, and I want you to keep them clearly separate. Mode 1 is called independent. The name tells you everything: what happens on one runway is totally independent of what is happening on the other. Each approach is handled as if the other runway doesn’t affect it at all. Mode 2 is called dependent. It does roughly the same job, but with a crucial addition: radar separation is applied longitudinally and laterally to the approaching aircraft. That means controllers actively space the aircraft along the approach path and sideways, so what happens on one runway becomes dependent upon what is happening on the other. The aircraft are no longer treated independently. Here’s the key mental picture for both modes: effectively, you have two arrival aerodromes, even though they are very close together. Two runways, two approach paths, but they’re so near each other that the airspace has to be carefully designed. Now let’s get into the specific airspace definitions, because these are the heart of the procedure. 12.12 — the Normal Operating Zone, or NOZ. This applies to Mode 1 operations. It’s airspace of defined dimensions extending either side of an ILS localizer course — or, if you’re using MLS, an MLS final approach track centre line. Within this zone, an aircraft operating normally on the ILS would be positioned. In plain terms, the NOZ is the corridor where a properly flying aircraft should be. Here’s a critical detail: the protected zone is encompassed by half-scale deviation of the CDI — that’s the Course Deviation Indicator, the instrument that shows you how far left or right you are from the localizer centre line. Half-scale deviation is the required accuracy of ILS navigation. So the NOZ is sized to contain an aircraft that’s flying within that half-scale tolerance. And note this carefully: only the inner half of the NOZ is taken into account in independent parallel approaches. So when controllers plan and protect the airspace for Mode 1, they only count the inner half of that zone. That’s a specific limitation you need to remember. 12.13 — the No Transgression Zone, or NTZ. In the context of Mode 1 independent parallel approaches, this is a corridor of airspace of defined dimensions located centrally between the two extended runway centre lines. So it sits right in the middle, between the two approach paths. Here’s what happens if an aircraft penetrates the NTZ: it requires controller intervention immediately. The controller must manoeuvre a threatened aircraft on the adjacent approach out of the way. Normally, that means a turn away from the other aircraft and an immediate climb to MSA — that’s Minimum Sector Altitude — or as per the Missed Approach Procedure. Once the threatened aircraft is safe, the controller will then attempt to assist the other aircraft — the one that caused the problem — to regain the approach, or to carry out the missed approach procedure. And there’s a hard requirement you must know: the NTZ must be at least 610 metres wide. That’s a minimum, not a suggestion. So let me tie this together. You have two parallel runways, two localizer centre lines. Between them sits the NTZ — a protected corridor at least 610 metres wide. On either side of each centre line you have the NOZ, where a normally flying aircraft should be, sized to half-scale CDI deviation, and only the inner half counts for Mode 1. If an aircraft drifts into the NTZ, that’s a transgression, and the controller immediately turns the threatened aircraft away and climbs it to MSA or the missed approach. That’s the structure of independent parallel approaches. You’ve got the two modes, the NOZ, the NTZ, the 610-metre minimum, and the controller’s response to a transgression. Take a moment to let that settle, and when you’re ready we’ll move on.