Instrument Procedures - Departures — Page 152, Lesson 229

Let’s start with the core idea of this section: obstacle clearance in instrument departure procedures. When we design a departure procedure, we are promising that the aeroplane will not be flown dangerously close to any obstacle at any point along the route. That is the implied guarantee. Now, how do we actually achieve that clearance? We do it in two ways: lateral clearance and vertical clearance. Lateral clearance means we keep the aircraft a safe horizontal distance away from obstacles. Vertical clearance means we keep the aircraft high enough above them. To make lateral clearance work, we require the pilot to fly the track accurately, within accepted tolerances. If the pilot stays within those tolerances, the aircraft is guided over a surveyed flight path. Within the bounds of that surveyed path, all obstacles can be determined and assessed. So the survey is what lets us know exactly what obstacles exist along the route. But here is the critical point: the surveyed area must have finite limits. It cannot be infinite. And it is not acceptable to survey, say, an area 5 nautical miles wide and then allow the aircraft to fly within guidance tolerance, 2.5 nautical miles either side of the desired track, with the same clearance everywhere. Why not? Because the extremities of the surveyed area must gradually permit higher obstacles. In other words, as you move further away from the desired track, the obstacles can be higher, because the aircraft is less likely to be there. At the limit of reasonable expectations of accuracy — that is, at the edge of the guidance tolerance, considering both equipment and flight technical accuracy — the guaranteed clearance is reduced to zero. So the clearance is not uniform across the whole surveyed area; it tapers off at the edges. This assessment is known as the creation of MOC, which stands for minimum obstacle clearance areas. MOC is discussed later in this chapter, so I will not go into it in detail now, but remember that term — it is the name for this whole process of defining those clearance areas. Now, there is another way to provide obstacle clearance: by assessing the highest obstacle to be flown over and applying a safety margin to the obstacle height. From that, we obtain an obstacle clearance altitude or height, abbreviated OCA/H. This is the method used to obtain MSA, which is minimum sector altitude, and with refinements, it is also the method used to obtain MDA/H, which is minimum descent altitude or height, for non-precision procedures. Let me clarify the difference between altitude and height here. Altitude is measured above mean sea level. Height is measured above the aerodrome elevation or the threshold elevation. So OCA/H is the minimum altitude or height at which the aircraft must be to ensure obstacle clearance. Now, here is where precision and non-precision procedures differ. Precision procedures provide height guidance, meaning the pilot gets continuous vertical guidance, typically from an instrument landing system. Because of that, an obstacle 1000 feet high at 10 nautical miles from the threshold is not as significant as an obstacle 150 feet high at 1 nautical mile from the threshold, assuming a 300 feet per mile glide slope. Let me unpack that. The glide slope is the descent path, and at 300 feet per mile, the aircraft descends 300 feet for every nautical mile it travels toward the threshold. So an obstacle close to the threshold, at 1 nautical mile, is much more critical because the aircraft is low there. An obstacle far away, at 10 nautical miles, is less critical because the aircraft is still high. That is why, for precision systems, OCA/H is range-from-threshold dependent. The clearance depends on how far you are from the threshold. Because of this, it should be obvious that OCA/H for precision procedures are less than OCA/H for non-precision procedures. Non-precision procedures do not provide the same height guidance, so they need larger clearance margins. Now, one very important operational point. The obstacle clearance applied in the development of each instrument approach procedure is considered the minimum required for an acceptable level of safety in operations. That is the absolute floor. If you have your own aeroplane and it is not used for commercial air transport, you may operate to the published OCA/H limits. But operators, meaning commercial air transport operators, apply higher criteria, which result in aerodrome operating minima. So the published OCA/H is the legal minimum, but commercial operators impose stricter limits for their operations. Let me also mention the figure. Figure 7.1 illustrates this concept, and you can see it on screen. It shows how the surveyed area and the clearance taper work together. So, to summarise what we have covered: obstacle clearance is achieved laterally and vertically. The surveyed area has finite limits, and clearance reduces to zero at the edge of guidance tolerance. MOC is the term for these minimum obstacle clearance areas. OCA/H is derived from the highest obstacle plus a safety margin, and it is used for MSA and MDA/H. Precision procedures have range-dependent OCA/H, which is lower than non-precision OCA/H. And finally, the published OCA/H is the minimum, but commercial operators use higher criteria for their operating minima. That is the foundation of obstacle clearance in departures.