Separation — Page 309, Lesson 409

Right, let’s get into the separation rules for departing aircraft. This is where the controller’s job gets really precise, so I’ll walk you through it step by step. First, a definition you need to know cold: essential local traffic. This is any aircraft, vehicle, or personnel on or near the runway, or traffic in the take-off and climb-out areas or the final approach area, which may constitute a collision hazard to a departing or arriving aircraft. The key phrase there is "collision hazard." If something is sitting in those zones and could get in the way of an aircraft taking off or landing, it’s essential local traffic. And here’s the rule: where essential local traffic is known to the controller, the information must be transmitted to the flight crews without delay. No waiting, no "I’ll mention it later." It goes out immediately. Now, how do we separate departing aircraft from each other? Under IFR — that’s Instrument Flight Rules, flying on instruments — departing aircraft will normally be separated by requiring each aircraft to follow a SID, a Standard Instrument Departure. That’s a published route out of the airport. The ATCUs — Air Traffic Control Units — co-ordinate the issuing of clearances, and where possible, standard clearances are used. So the idea is, everyone gets the same predictable clearance. These clearances will normally be specified by the approach controller and passed to the aircraft by the aerodrome control tower. So you have two controllers working together: the approach controller decides the clearance, the tower actually gives it to the pilot. Now, what about departures in VMC — Visual Meteorological Conditions, where the pilot can see and fly visually? Here the aerodrome controller will clear an aircraft for take-off once the preceding aircraft has either: passed the upwind end of the runway, or made a turn away from the runway. So the controller waits until the aircraft ahead is either off the far end of the runway or has turned off to the side, clearing the path. Now let’s talk numbers, because this is where it gets specific. Where wake turbulence separation is applied — that’s separation to avoid the swirling air left behind by an aircraft’s wings — departures are sequenced to minimize delays and maximize runway utilization. So the goal is efficiency, but safety comes first. For IFR traffic, the standard separation between departures is 5 minutes. That works out to 12 movements per hour. Now, that 5 minutes can be reduced to 2 minutes between aircraft following the same departure track, providing the preceding aircraft has filed a FP — a Flight Plan — speed 40 knots greater than the succeeding aircraft. So the aircraft ahead must be flying 40 knots faster than the one behind, on the same track. That way, even with less time, they won’t catch up. And it can be reduced even further, down to 1 minute — which is 60 movements per hour — providing the track of the succeeding aircraft diverges from that of the preceding aircraft by 45° or more. So if the aircraft behind is turning off at least 45 degrees from the path of the one ahead, you can squeeze them to just one minute apart. And finally, the obvious point: where parallel runways are used for simultaneous take-offs, or diverging runways are used, you can achieve higher utilization rates — meaning lower separation. Because the aircraft are physically going in different directions, you don’t need as much time between them. So to sum up the ladder: 5 minutes standard, down to 2 minutes if the aircraft ahead is 40 knots faster on the same track, down to 1 minute if the tracks diverge by 45° or more, and even tighter if you’re using parallel or diverging runways. That’s the whole picture for separating departures.