Wake Turbulence — Page 192, Lesson 286

I want to walk you through the wake turbulence separation standards that ICAO has laid out for landing and take-off. This is critical knowledge for operating safely behind larger aircraft, and I’ll explain every term and number as we go. Let’s start with the conditions that trigger these separation rules. The standards apply when both aircraft are using the same runway, or when they are using parallel runways separated by less than 760 metres. They also apply when an aircraft is crossing behind another aircraft at the same altitude or less than 300 metres (1000 feet) below. So if you’re following another aircraft and your flight path crosses behind theirs within that vertical window, the separation minima kick in. Now, look at the table for landing separation. We have three aircraft weight categories: Heavy, Medium, and Light. These are the standard ICAO wake turbulence categories based on maximum take-off mass. The table tells you, for each combination of leading and following aircraft, the required distance in nautical miles or the time equivalent. If a Heavy aircraft is leading and a Heavy is following, the required separation is 4 nautical miles. No time equivalent is given for that pair — the dash means it’s distance-based only. If a Heavy leads a Medium, you need 5 nautical miles or 2 minutes. If a Heavy leads a Light, you need 6 nautical miles or 3 minutes. If a Medium leads a Heavy, the separation is 3 nautical miles — no time given. If a Medium leads another Medium, also 3 nautical miles. If a Medium leads a Light, you need 5 nautical miles or 3 minutes. Notice that the time equivalents only appear for the pairs where the following aircraft is lighter than the leading aircraft — Heavy/Medium, Heavy/Light, and Medium/Light. That’s because the wake turbulence hazard is greatest when a smaller aircraft follows a larger one. Now let’s move to the departure separation standard. The table for take-off has a slightly different layout. For departure, if a Heavy aircraft is leading and a Medium or Light is following, and they are departing from the same position, the spacing is 2 minutes. If a Medium leads a Light from the same position, also 2 minutes. But if the leading aircraft is Heavy and the following is Medium or Light, and they are departing from an intermediate point on the runway — meaning the following aircraft starts its take-off roll from further down the runway, not from the threshold — then the spacing increases to 3 minutes. The same 3 minutes applies if a Medium leads a Light from an intermediate point. These separation minima apply specifically when take-off and landing operations are being conducted on parallel runways less than 760 metres apart, or where the projected flight path of the following aircraft crosses that of the leading aircraft at the same level or within 1000 feet lower. So it’s not just about being on the same runway — crossing flight paths within that vertical band also triggers these rules. There’s an additional rule for arrivals and departures where flight paths cross at a runway with a displaced landing threshold. A separation of 2 minutes is to be applied between arrivals and departures when a medium or light aircraft arriving is following a heavy departing, or when a light arriving is following a medium departing, where flight paths cross. The reverse situation — a medium or light aircraft departing following a heavy arriving, or a light aircraft departing following a medium arriving — also requires that 2 minute separation. A similar 2 minute separation applies when a light or medium aircraft is departing or arriving after a heavy — or a light departing or arriving after a medium — has made a low or missed approach in the opposite direction. So if a heavy aircraft goes around and you’re about to depart or arrive behind it, you need that 2-minute buffer. The same separation criteria apply for parallel runways less than 760 metres apart. Now, the excerpt also includes two practice questions. The first asks about the behaviour of vortices on the ground. The correct answer describes that in the absence of crosswind, the vortices move downwards and outwards from the runway centre line at a speed of approximately 5 knots. With a crosswind, the downwind vortex is stationary and the upwind vortex moves away from the centre line at approximately 5 knots. The other options describe incorrect behaviours — for example, the upwind vortex being stationary, or both moving away in the upwind direction. The second question asks when the strongest vortices are generated. The correct answer is heavy aeroplanes, travelling slow, in a ‘dirty’ configuration. ‘Dirty’ means the aircraft is in a high-drag configuration — flaps and landing gear extended — which produces the strongest wake turbulence. Heavy and fast in a clean configuration, or heavy and slow in a clean configuration, do not generate the strongest vortices. These are the book’s practice questions — let’s try them one at a time.