Instrument Procedures - Departures — Page 159, Lesson 233

Let's pick up with the departure procedure requirements. I want to walk you through what the regulations demand when we need an instrument departure. First, the fundamental requirement: where instrument departures are required, a departure procedure will be established for each runway to be used. So every runway that will be used for instrument departures gets its own procedure. That procedure will define the departure for the various categories of aircraft, and it's based on an all engines running PDG of 3.3%. Let me unpack that acronym — PDG stands for "procedure design gradient." It's the climb gradient the procedure is designed around, expressed as a percentage. The baseline is 3.3%, but the procedure may use an increased PDG if that's required to achieve minimum obstacle clearance. So the 3.3% is the default; if obstacles demand a steeper climb, the PDG goes up. Now, an important assumption baked into these procedures: they assume pilots will compensate for wind effects — known or estimated — when flying departure routes. And note the phrasing: the routes are expressed as tracks to be made good. That means the published track is the path over the ground you must achieve, so you correct for wind to stay on it. But here's the contrast: if radar vectoring is applied, pilots are required to fly the vector headings and not make allowance for the wind. So under radar vectors, you fly the heading you're given, no wind correction — the controller is handling the geometry. Now let's move to obstacle clearance, which I want to stress is a primary safety consideration in instrument departure procedures. Unless otherwise stated, a PDG of 3.3% is assumed. But here's how that 3.3% is actually made up. It consists of two components: 2.5% gradient of obstacle identification surfaces — or the gradient based on the most critical obstacle penetrating those surfaces, whichever is higher — plus 0.8% increasing obstacle clearance. So the 2.5% is the baseline obstacle identification surface gradient, but if a critical obstacle pokes through that surface, the gradient is raised to clear it. Then you add 0.8% on top as the increasing obstacle clearance margin. The published gradients will be specified to an altitude or height, after which the minimum gradient of 3.3% is considered to exist. So the published gradient applies up to a certain altitude or height; beyond that point, you can assume the standard 3.3% applies. The final PDG continues until obstacle clearance is ensured for the next phase of flight — that next phase being en route, holding, or approach. At that point, the departure procedure ends, and it's marked by a significant point. Now, the geometry of the obstacle clearance itself. The minimum obstacle clearance equals zero at the departure end of the runway — that's the DER, the departure end of the runway. From there, it increases by 0.8% of the horizontal distance in the direction of flight, assuming a maximum divergence of 15°. So as you fly away from the runway, your clearance floor rises at 0.8% of the distance you've covered, and the design assumes you won't diverge more than 15° from the track. For a turning departure, there's a specific provision: in the turn initiation area, a minimum obstacle clearance of 90 m — that's 295 ft — is provided. So where you begin the turn, you get that fixed 90-metre clearance. And increased obstacle clearance will be provided in mountainous terrain. If DME is available — that's distance measuring equipment — additional height and distance information is made available. Let me pause on mountainous terrain, because there's a specific definitional point here. What defines mountainous terrain is not specified — the regulation deliberately leaves it open. Instead, the designer takes notice of the prevailing wind conditions. The criterion is this: if the average wind speed of 37 kph or more produces significant down draughts, then increased obstacle clearance is applied. So the trigger is a combination — an average wind speed of at least 37 kilometres per hour, and that wind producing significant down draughts. When both conditions are met, the designer applies the increased clearance. Finally, aircraft category. The major consideration in planning a departure route is ensuring adequate obstacle clearance, as we've said. But in determining the track over which the aircraft will fly, speed is the determining factor. Aircraft are categorized by — and the excerpt cuts off there, but that's the principle: the category is driven by speed, because speed determines how tightly you can turn and how much room you need to stay clear of obstacles. Let me show you the geometry of that obstacle clearance build-up, because it's easier to see than to describe.