OCA(H) stands for Obstacle Clearance Altitude or Obstacle Clearance Height — Page 175, Lesson 177

I want to walk you through the concept of OCA(H) and the key altitudes, heights, and points that define precision and non-precision approaches. Let's start with the core definition. OCA(H) stands for Obstacle Clearance Altitude or Obstacle Clearance Height. It is the lowest altitude or height above the elevation of the relevant runway threshold or aerodrome elevation, as applicable, used in establishing compliance with appropriate obstacle clearance criteria. For a precision approach procedure — that means when the ILS localizer and glide slope are both serviceable — OCA(H) is specifically the lowest altitude or height above the relevant runway threshold at which a missed approach must be initiated to ensure compliance with the appropriate obstacle clearance criteria. This definition comes from ICAO Doc 8168, 1993. Now, let's look at the specific values on the approach chart. We have a threshold crossing height, or TCH, of 56 feet. That is the ILS glide slope threshold crossing height — the height at which the glide slope passes over the runway threshold. Next is the Final Approach Fix, abbreviated FAF. It is located at 7.5 nautical miles DME range from the runway threshold, and also at 8.7 nautical miles DME range from LON VOR/DME. At that fix, 2500 feet — with 2423 feet in parentheses — is the minimum glide slope intercept altitude or height. The value in parentheses is likely the height above the aerodrome, while the larger number is the altitude above mean sea level. Moving inbound, at 4 nautical miles ILS ILL DME range from touchdown, the glide slope altitude is 1405 feet and the glide slope height is 1328 feet. Now, look at the symbols on the chart. The M symbol with an arrow signifies the non-precision approach Missed Approach Point, or MAPt/MAP. That applies when only the localizer is serviceable and there is no glide slope information. The arrow without the dashes means the applicable Decision Altitude or Height — DA/H — for the precision approach ILS procedure, where both localizer and glide slope are serviceable. Let me define those terms precisely. Decision Altitude or Height, DA/H, is a specified altitude or height in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. DA is referenced to mean sea level using QNH, and DH is referenced to the threshold elevation using QFE. The required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight-path. For this specific approach, the DA/H is 277 feet on QNH and 200 feet on QFE. The MDA/H for the localizer-only approach — when the glide slope is out — is 490 feet on QNH and 413 feet on QFE. MDA/H stands for Minimum Descent Altitude or Minimum Descent Height. It is a specified altitude or height in a non-precision approach or a circling approach below which descent may not be made without visual reference. The missed approach procedure is: "Climb straight ahead to 3000 feet — 2923 feet — then as directed by ATC, with a maximum indicated airspeed of 185 knots. In the event of complete radio failure, see 11-7." Now let's distinguish between the two types of approaches. A non-precision approach and landing operation is an instrument approach and landing which does not utilize electronic glide path guidance. Examples include localizer-only ILS, VHF Omni-range or VOR, Non-directional Beacon or NDB, VOR/DME, and similar approaches. A precision approach and landing operation is an instrument approach and landing using precision azimuth and glide path guidance, with minima determined by the category of operation. Examples include ILS, MLS — Microwave Landing System — and Precision Approach Radar, or PAR. Finally, let's define the Missed Approach Point, or MAPt. That is the point in an instrument approach procedure at or before which the prescribed missed approach procedure must be initiated to ensure minimum obstacle clearance. And one more term: Runway Visual Range, or RVR. It is the maximum distance a pilot 15 feet above the runway in the touchdown area can see marker boards by day or night when looking in the direction of take-off or landing. This is referenced from the Meteorology book.