All Weather Operations — Page 65, Lesson 103

Let’s start with the definition of system minima, because it’s the foundation for everything in this section. System minima is a height — a specific altitude value — derived for the lowest permitted DH or MDH. DH stands for Decision Height, which is used in precision approaches; MDH stands for Minimum Descent Height, used in non-precision approaches. The system minima takes into account the characteristics of both the ground equipment and the airborne equipment — so it’s not a generic number; it depends on what equipment is installed on the ground at the airport and what equipment is fitted in the aircraft. System minima are related to the type of approach you’re flying. They are standard figures — fixed, published numbers — for both precision and non-precision approaches. So for each approach type, there is a system minimum that tells you the lowest you can go based purely on the equipment involved. Now, let’s focus on non-precision approaches specifically. The operator — that’s the airline or the flight department — must ensure that the system minima for non-precision approach procedures are not lower than the MDH values given in Figure 5.4. That figure lists the lowest MDH for each type of navigation aid. Let me walk you through that table. For a Localiser approach — that’s using the ILS without the glidepath, so just the localiser beam for lateral guidance — with or without DME, the lowest MDH is 250 feet. For an SRA — that’s a Surveillance Radar Approach — the value depends on how close the radar controller can guide you to the runway. If the SRA terminates at 0.5 nautical miles from the runway, the lowest MDH is 250 feet. If it terminates at 1 nautical mile, it’s 300 feet. If it terminates at 2 nautical miles, it’s 350 feet. For a VOR approach, the lowest MDH is 300 feet. If you have VOR with DME, that drops to 250 feet. For an NDB approach, the lowest MDH is 350 feet. If you have NDB with DME, it’s 300 feet. For a VDF approach — that’s VHF Direction Finding, where the controller gives you QDM or QGH bearings — the lowest MDH is 350 feet. And for RNAV/LNAV approaches — that’s area navigation using lateral navigation only — the lowest MDH is 300 feet. So that’s the table: each facility type has a published lowest MDH that the operator cannot go below. Now, there’s another important rule about Minimum Descent Height itself. The operator must ensure that the MDH for a non-precision approach is not lower than either of two values: the OCH/OCL for the category of aeroplane, or the system minimum. OCH stands for Obstacle Clearance Height, and OCL stands for Obstacle Clearance Limit — these are the heights or altitudes that ensure you clear obstacles in the approach path. So the MDH you actually fly must be the higher of those two values: the obstacle clearance number for your aircraft category, or the system minimum from the table. Finally, there’s a subtle but important point about how MDH and OCH are measured. They are based on the aerodrome elevation — that’s a specified point on the aerodrome, often just in front of the control tower. But runways often have a slight gradient — a slope — so the elevation of the runway thresholds could be considerably higher or lower than the stated elevation of the airfield. If the threshold elevation is 2 metres or more below the elevation of the airfield, then the MDH is measured from the threshold rather than from the airfield elevation. That’s a specific rule to ensure you have adequate clearance when the runway itself sits lower than the airport reference point. So to summarise: system minima give you the lowest MDH based on equipment; the actual MDH you use must also consider obstacle clearance; and the reference point for measuring that height can shift to the threshold if the runway is significantly lower than the airfield.