Every instrument approach ends at a number: the lowest you may go before you must either see the runway or go around. But that number comes in two flavours that behave in opposite ways, and confusing them is one of the more dangerous mistakes on an approach. One is a gate you pass through; the other is a floor you must not break.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Two minima for two kinds of approach
The minimum you fly to depends on whether the approach gives you vertical guidance.
A precision approach or an approach with vertical guidance brings you down a glidepath to a decision altitude (DA). Because you are descending continuously on a glidepath, you cannot stop in mid-air at the DA; you arrive at it and decide, in that instant, to land or to go around.
A non-precision approach, with no glidepath, brings you to a minimum descent altitude (MDA). Because you manage the descent yourself, you can, and traditionally did, level off at the MDA and fly along it, looking for the runway.
That structural difference, a continuous glidepath versus a managed descent, is why the two minima are defined and used so differently. It is not pedantry; it changes what you are allowed to do with the number.
Decision altitude: a gate
A decision altitude is the height on the glidepath at which a missed approach must be initiated if the required visual reference has not been established. The key feature, set out in ICAO Doc 8168, is that the aircraft is expected to descend slightly below the DA as the go-around is begun, because an aeroplane cannot reverse a descent instantly. That small dip is built into the obstacle clearance for the procedure, so it is safe and anticipated. You decide at the DA; you do not have to have stopped descending by the DA.
Minimum descent altitude: a floor
A minimum descent altitude is the altitude below which you must not descend unless you have the required visual reference to continue to a landing. There is no built-in allowance for dipping below it, so the discipline is different: you must arrive at the MDA and not go below it until you either see the runway and can land, or reach the missed approach point and go around. Where the DA tolerates a momentary descent below, the MDA does not. Busting an MDA, descending below it without the runway in sight, is exactly the error the floor exists to prevent.
DA, DH, MDA and MDH: the datums
Two pairs of terms describe the same points against different datums, and the difference matters for the lower categories.
- A decision altitude (DA) and a minimum descent altitude (MDA) are read on the barometric altimeter, referenced to mean sea level.
- A decision height (DH) and a minimum descent height (MDH) are referenced to height above the ground or the threshold, and the DH for CAT II and CAT III is read on the radio altimeter, because at those very low minima the aircraft needs to know its height above the surface precisely, not just its altitude above the sea.
So an ILS CAT I uses a DA or DH; a CAT II or III uses a radio-altimeter DH; and a non-precision approach uses an MDA or MDH. The letters tell you both the kind of minimum (decision versus floor) and the datum it is measured against.
The required visual reference
Neither minimum lets you continue on faith. To descend below a DA, or below an MDA, you must have the required visual reference in sight and be in a position to make a normal landing. The rules list what counts, the approach lighting, the runway threshold and its markings or lights, the touchdown zone, the runway identification, and require enough of it to continue safely. If, at the DA or by the missed approach point, you do not have the required references, the decision is made for you: go around. The minimum and the visual reference work as a pair; reaching the height is not permission to land unless you can also see what the rule demands.
Where the numbers come from, and the CDFA twist
The published minima are not arbitrary. They are built up from the obstacle clearance altitude or height (OCA/OCH), the lowest altitude that guarantees obstacle clearance for the procedure, with margins added for the aircraft category and the operation. That is why the same runway can show different minima for different approaches and different aircraft.
There is one practical wrinkle when a non-precision approach is flown as a continuous descent final approach (CDFA). Because a CDFA flies a continuous descent and goes around at the minimum rather than levelling off, operators often add a margin to the MDA to create a derived decision altitude/height, so that the go-around, with its small dip, does not actually bust the MDA. It is a neat reconciliation: the MDA stays a floor, but you fly to a slightly higher derived figure as if it were a decision gate.
Why the difference exists: obstacle protection
The gate-versus-floor distinction is not an accounting quirk; it comes straight from how the obstacle protection is built. On a precision or vertically-guided approach, the procedure designer knows the aircraft will be on a defined glidepath and can model the go-around exactly, including the brief descent below the DA as the aircraft transitions from descent to climb. That dip is inside the protected surface, so it is safe by design, which is precisely why you are allowed to reach the DA still descending and decide there.
On a non-precision approach there is no glidepath to anchor that model, so the protection is built around a level minimum with no assumption that the aircraft will sink below it. The MDA is therefore drawn as a hard floor: descend below it without the runway in sight and you are outside the surface that was protected for you. The same logic explains the missed-approach design. From a DA or the MAPt, the published missed approach assumes a climb at a defined gradient along a protected path, and the minima and the go-around are designed together. Understanding that the numbers come from protected surfaces, not from habit, is what makes the rule about the dip, allowed at a DA, never assumed at an MDA, click into place.
A worked example
You fly an ILS down the glidepath to a decision altitude of 550 ft. At 550 ft you look up and decide. You have the approach lights and the runway in sight and are positioned to land, so you continue, and it is entirely normal that the aircraft passed a few feet below 550 ft in the instant you committed. The DA was a gate, and you went through it with the required visual reference.
Now fly the same runway as a VOR approach to a minimum descent altitude of 820 ft. This time you descend to 820 ft and level off, because the MDA is a floor. You fly along at 820 ft toward the missed approach point, looking for the runway. If it appears and you can land, you leave the MDA and descend visually; if you reach the missed approach point still at 820 ft with nothing in sight, you go around, never having dropped below the floor. Same runway, two minima, two completely different rules about what the number permits.
Common pitfalls
- Treating an MDA like a DA. You may dip below a DA during the go-around; you must not descend below an MDA at all without the runway in sight.
- Continuing below the minimum without the required reference. Reaching the height is not enough; you need the specified visual references and a position to land.
- Confusing DA and DH datums. A DA is altitude above the sea on the baro altimeter; a DH for CAT II and III is height above the surface on the radio altimeter.
- Forgetting the go-around dip on a CDFA. Without a derived margin above the MDA, a go-around at the MDA can briefly bust it.
- Assuming the minima are the same for every aircraft. They are built from the OCA/OCH plus margins that vary by aircraft category and operation.
In Pilot EFB
Pilot EFB is a study and planning companion for instrument approaches, helping you keep the gate-versus-floor distinction straight and read what each chart's minima box is telling you, alongside the approach plate and the rest of your offline-first briefing. It does not set your minima, read your altimeter, or make the decision, and the figures you fly come from the current chart and your approved minima. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and fly the approach from your official source of record.