Separation — Page 309, Lesson 405

We're now into the heart of wake turbulence separation. Let me set the scene first, because this builds directly on what we just covered about how a wing generates wake vortices. Remember, as an aircraft generates lift, air spills around the wingtips and forms two counter-rotating vortices. These trail behind and below the aircraft. Now, here's the critical operational fact: that vortex exists at the level of the generating aircraft and extends down to an altitude not exceeding 1000 feet below it. So if you're following another aircraft, and you're anywhere within that block of airspace—at their level or up to 1000 feet beneath them—you are in their wake turbulence zone, and separation must be applied. Now, the question becomes: how do we decide how much separation? That's where the wake turbulence categories come in. Aircraft are categorized by their maximum take-off mass, or MTOM, because that mass directly relates to the severity of the wake vortices they generate. Heavier aircraft produce stronger vortices. There are exactly three categories. First, Heavy: all aircraft types with an MTOM equal to 136,000 kilograms or more. Second, Medium: aircraft types with an MTOM less than 136,000 kilograms but more than 7,000 kilograms. And third, Light: aircraft types with an MTOM of 7,000 kilograms or less. One important note here: the MTOM is stated on the Certificate of Airworthiness for the aircraft—that's your authoritative source for which category an aircraft falls into. Now, the separation minima themselves. These are procedural, meaning non-radar separation. And there's a crucial qualifier: these criteria are only applicable where the following aircraft is lighter than the preceding aircraft. If the following aircraft is the same weight or heavier, these specific wake turbulence minima don't apply—the vortex from a lighter aircraft isn't a threat to a heavier one. Let's go through the scenarios. First, arriving aircraft, using timed approaches. If a Medium is arriving behind a Heavy, you need 2 minutes. If a Light is arriving behind a Medium or a Heavy, you need 3 minutes. So the lighter the following aircraft, the more time you give. Now, departing aircraft. For a Light or Medium taking off behind a Heavy, or a Light behind a Medium, you apply a minimum of 2 minutes. But this applies under specific runway conditions. It applies when they're using the same runway. It applies on parallel runways separated by less than 760 meters. It applies on crossing runways if the projected flight paths cross at the same altitude or within 1000 feet below the heavier aircraft. And it also applies on parallel runways separated by 760 meters or more—but only if the projected flight paths cross at the same altitude or within 1000 feet below the heavier. So you see, the wider parallel separation is fine unless the paths actually converge vertically. There's a note here that increases that separation. It's increased to 3 minutes where a Light or Medium is taking off behind a Heavy—or a Light behind a Medium—from an intermediate part of the same runway, or an intermediate point on parallel runways. So if you're not starting from the threshold but from an intermediate point, you need the extra minute. Next, the displaced landing threshold scenario. A separation of 2 minutes is applied between Light or Medium and Heavy, or between Light and Medium, when operating on a runway with a displaced threshold. This applies in two situations. First, when a departing Light or Medium follows a Heavy arriving, or a departing Light follows a Medium arriving. Second, when an arriving Light or Medium follows a Heavy departing, or an arriving Light follows a Medium departing—but only if the projected flight paths are expected to cross. Finally, the opposite direction case. A separation of 2 minutes is applied between a Light or Medium and a Heavy, or between a Light and a Medium, when the heavier aircraft is making a low or missed approach, and the lighter aircraft is either using an opposite direction runway for take-off, or is landing on the same runway in the opposite direction, or on a parallel opposite direction runway separated by less than 760 meters. So the pattern throughout is consistent: the lighter the following aircraft, the more vulnerable it is, and the more time we give. The key numbers to hold onto are 2 minutes and 3 minutes, and the key threshold is 760 meters for parallel runway separation.