Separation — Page 309, Lesson 403

Let’s start with radar separation, because that’s the foundation of everything we’re about to do. Radar gives the air traffic control officer — the ATCO — fairly accurate position information for an aircraft under their control. But it’s not perfect. There are three specific problems we have to deal with: slant range display, target discrimination, and loss of contact close to the radar overhead. Let me unpack each of those. Slant range display means the radar measures distance along the line of sight — the slant — between the antenna and the aircraft, not the horizontal distance over the ground. For an aircraft at altitude, that slant distance is longer than the true horizontal distance, so the display can be misleading. Target discrimination is the radar’s ability to show two close aircraft as two separate targets. If they’re too close together, they can merge into one blip, and the controller loses the ability to tell them apart. Loss of contact close to the radar overhead — that’s the classic “cone of silence.” Directly above the antenna, the radar beam can’t see the aircraft, so the target disappears from the display. Now, here’s the key idea. These errors must be handled the same way any other positional error is handled: by adding buffer allowances. The errors are worse for long-range radars used in area control, but they must still be considered for terminal radars covering a much smaller area. So what’s the basic standard? The basic radar separation standard is 5 NM. That means where two aircraft identified on radar are at the same level — same altitude — they are not permitted to approach closer than 5 NM to each other on the radar display. That’s the default. Now, that 5 NM can be reduced, but only when approved by the authority and only in specific circumstances. There are three specific occasions. First, radar capabilities. When radar capabilities so permit at a given location, the standard may be reduced to 3 NM. So if the radar is good enough, you can bring them down to 3 NM. Second, ILS localizer. Where two or more aircraft are established on the same ILS localizer course — that’s the instrument landing system’s lateral guidance beam — and they’re within 10 NM of the threshold of the landing runway, the separation standard may be reduced to 2.5 NM between contacts on the radar display. So on final approach, lined up on the same localizer, inside 10 miles from the threshold, you can compress to 2.5 NM. Third, simultaneous parallel approaches — Mode 2, dependent. During Mode 2 parallel runway operations, radar separation is applied. Between aircraft on adjacent localizer courses, the separation standard may be reduced to 2 NM between contacts on the radar display. So two aircraft on parallel runways, each on their own localizer, can be as close as 2 NM on the radar. Now let’s switch to procedural wake turbulence separation. This is a different kind of separation — it’s not about radar blips, it’s about the physical wake left behind an aircraft. Here’s the situation. When the wings are creating lift — from rotate to touchdown, meaning from the moment the aircraft rotates for takeoff all the way through to landing — wake vortices are created behind the aircraft. This is apparent in the form of turbulence. The severity of that turbulence is a function of aircraft mass, with the worst case being a heavy aircraft at low speed. So the heavier the aircraft and the slower it’s going, the more severe the wake turbulence. Think about a heavy jet just after rotation or just before touchdown — slow and heavy — that’s the worst case. Where an aircraft is following another aircraft, allowance must be made for this wake turbulence effect. Under certain circumstances it can be so severe as to cause structural damage — even catastrophic damage — to an airframe. So this isn’t just a comfort issue; it’s a structural safety issue. Now, the nature of the wake vortex. It emanates from the wing tip in the form of spiralling air, moving from the high-pressure area below the wing to the low-pressure area above the wing. That’s the mechanism — the wing generates lift by having higher pressure below and lower pressure above, and at the wing tip, that pressure difference causes the air to spiral around from below to above, creating the vortex that trails behind. So to tie it together: radar separation is about keeping aircraft apart on the display with buffer allowances for radar errors, starting at 5 NM and reducing to 3, 2.5, or 2 NM in specific approved circumstances. Wake turbulence separation is about the physical vortex trail behind an aircraft, worst for heavy aircraft at low speed, and it can be severe enough to cause structural damage. Both are separation standards, but they protect against very different hazards.