Let’s pick this up right where the obstacle clearance concept leaves off — Page 179, Lesson 258

Let’s pick this up right where the obstacle clearance concept leaves off. I want to walk you through the precise definitions of OCA/H and DA/H, because these are the numbers that actually govern your descent on an instrument approach. First, the core idea. OCA/H stands for Obstacle Clearance Altitude or Obstacle Clearance Height. It is the lowest altitude or height at which you can fly and still be guaranteed to stay above every obstacle in the approach path. The way it is built up is simple: it consists of the height of the obstacle itself, plus a safety margin called the Minimum Obstacle Clearance allowance, or MOC. So you take the tallest obstacle, add the MOC, and that gives you the OCA/H. This value is published on the instrument procedure plate, and it is aircraft category dependent — meaning different aircraft categories get different OCA/H values because they have different manoeuvring capabilities. Now, the critical relationship. OCA/H is the lowest that the MDA/H can be. MDA/H is the Minimum Descent Altitude or Minimum Descent Height. So the operator can set the MDA/H higher than the OCA/H, but never lower. The OCA/H is the floor. Let me now give you the three precise definitions, because each one has a different reference datum and a different purpose. First, DA/H for a Precision Approach Procedure. DA/H stands for Decision Altitude or Decision Height. It is defined as the lowest altitude or height at which a missed approach must be initiated to ensure compliance with the appropriate obstacle clearance criteria. In plain terms: on a precision approach, you are cleared to descend all the way down to this altitude, and if you have not established the required visual reference by the time you reach it, you must go around — you must initiate the missed approach. The reference datum for a precision approach is always the threshold of the landing runway. So the DA/H is measured from the runway threshold, not from the aerodrome elevation. Second, OCA/H for a Non-precision Approach. This is defined as the lowest altitude or height below which the aircraft cannot descend without infringing the appropriate obstacle clearance criteria. Notice the difference in wording — for a non-precision approach, you cannot descend below this value at all, because there is no decision point; you fly to a minimum and hold that altitude until you see the runway. The reference datum here is different. For non-precision procedures, the reference datum is the aerodrome elevation, or the elevation of the relevant runway threshold — but only if that threshold is more than 2 metres, which is 7 feet, below the aerodrome elevation. So if the threshold is significantly lower than the aerodrome, you use the threshold elevation instead. Third, OCA/H for Visual Manoeuvre, or Circling — abbreviated VM(C). This is defined as the lowest altitude or height above the aerodrome elevation, below which the aircraft cannot descend without infringing the appropriate obstacle clearance criteria. The key point here is that it is based on the highest obstacle in the VM(C) area, measured with respect to the aerodrome elevation. So when you are circling to land visually, you are protected against the highest obstacle in that circling area, and the datum is the aerodrome elevation itself. Now let me move to Operating Minima, because this is where the operator turns those published OCA/H values into the actual minima you will fly. In accordance with Annex 6 and JAR-OPS 1, the operator is required to ensure that aerodrome minima are specified for all aerodromes used in the operation. That is a regulatory requirement — you cannot just show up and fly an approach without published minima. Part of this process ensures that in all cases, DA/H or MDA/H is calculated taking into account the published OCA/H and something called the upper margin. The upper margin is specified by the operator, and it may be zero. If it is zero, then the DA/H or MDA/H would be exactly the same as the OCA/H. So the operator adds a margin on top of the obstacle clearance value, and that margin can be nothing. Why would the operator add a margin? Because the upper margin takes into account data that is variable in nature. Let me list what that variable data includes: the crew qualification, the OAT — that is Outside Air Temperature — anomalies in the configuration of the instrument system, the type, performance and handling characteristics of the aeroplane, the dimensions and characteristics of the runway, and the visual and non-visual aids. So if you have a less experienced crew, or unusual temperature conditions, or a particular aircraft type with specific handling characteristics, or a runway with unusual dimensions, the operator may add a margin on top of the OCA/H to give you extra protection. That margin is the upper margin, and it is the operator's decision. So to tie it all together: the published OCA/H is the obstacle protection floor, the operator adds an upper margin on top of it, and the result is the DA/H or MDA/H that you actually fly. The upper margin can be zero, making them equal, or it can be positive, giving you extra height above the obstacles.