Advertisement
Key Terms
DA
Decision Altitude
Altitude (above MSL) at which a missed approach must be initiated if required visual reference is not established. Used for precision and APV approaches: ILS, LPV, LNAV/VNAV. At DA — instant decision, no delay permitted.
DH
Decision Height
Same decision point as DA, but expressed as a height above runway threshold elevation (AGL) rather than above MSL. ICAO preferred term for precision approach operations. DA and DH refer to the same moment in the approach.
MDA
Minimum Descent Altitude
Lowest altitude to which the aircraft may descend on a non-precision approach (NPA). Aircraft levels off at MDA and continues in level flight until the Missed Approach Point (MAP), visual contact is established, or time limit is reached.
MDH
Minimum Descent Height
Same as MDA but expressed as a height above aerodrome elevation (AGL). Some approach plates publish both the altitude (AMSL) and height (AGL) value — confirm which datum your altimeter is set to (QNH vs QFE).
MAP
Missed Approach Point
The point at which a missed approach must be initiated if the required visual reference has not been established. For non-precision approaches: defined by time from FAF, a fix, or a DME distance. For precision approaches: coincides with DA/DH.
OCH / OCA
Obstacle Clearance Height / Altitude
The lowest height/altitude above the relevant runway threshold elevation at which a missed approach provides adequate obstacle clearance. Published DA/DH values are derived from OCH plus required margins.
DA vs MDA — The Critical Difference
Operational Safety — Know Which AppliesConfusing DA and MDA procedures is a serious safety risk. The go-around trigger and flight technique differ fundamentally between precision and non-precision approaches.
Precision Approach (DA/DH)
ILS — LPV — LNAV/VNAV
- Both lateral AND vertical guidance provided throughout approach.
- Fly the glideslope continuously down to DA — never level off.
- At DA: instant decision. Visual references in sight — continue. No visual — go around immediately.
- Any delay below DA without visual is a violation and a safety hazard.
Non-Precision Approach (MDA)
VOR — NDB — LNAV
- Lateral guidance only. No published glideslope to fly.
- Descend to MDA, then level off. Do not descend below MDA.
- Continue level flight toward MAP while looking for visual references.
- If visual contact established before MAP — land. At MAP without visual — go around.
Precision Approach (ILS / LPV):
──────────────────────────────────────────────────────
Glideslope
FAF ──────────────────────────────────► DA/DH
│
If visual: Land
If no visual: GO AROUND ►
Non-Precision Approach (VOR / NDB / LNAV):
──────────────────────────────────────────────────────
FAF ──────────── Descend to MDA ──────────► MAP
│ │
LEVEL OFF If no visual:
Visual? → Land GO AROUND ►
Required Visual References at DA/MDA
Per ICAO Doc 8168 (PANS-OPS), at least one of the following visual references must be distinctly visible and identifiable before the approach may be continued below DA, or before initiating a landing from MDA:
Approach lighting system (ALS)
Threshold markings
Threshold lights
Runway end identifier lights (REIL)
Touchdown zone / markings / lights
Runway or runway markings
Runway edge lights
Visual approach slope indicator (PAPI/VASI)
Identifiable — Not Just GlimpsedThe visual references must be sufficient to allow the pilot to assess the aircraft's position and rate of change of position relative to the desired flight path. A fleeting glimpse does not constitute adequate visual contact. If in doubt — go around.
Advertisement
RVR Requirements by Approach Type
| Approach Type | Min. RVR | Notes |
|---|---|---|
| Visual approaches (VFR) | No RVR specified | Minimum flight visibility applies per local rules |
| Non-Precision (NPA) — no ALS | 1500 m | Without approach lighting system |
| Non-Precision (NPA) — with ALS | 800 m | Basic approach lighting present |
| NPA with full approach lights | 550 m | Full ALS (CAT I type); depends on ALS classification |
| Cat I ILS | 550 m | With full approach lighting; 800 m if no RVR equipment at airport |
| Cat II ILS | 350 m | Autoland or HUD capability; enhanced crew training required |
| Cat IIIA ILS | 200 m | Full autoland; fail-operational system; DH below 100 ft or no DH |
| Cat IIIB ILS | 75–200 m | Autoland + rollout guidance; DH below 50 ft or no DH |
Operator-Specific MinimumsActual minima for a specific operator may be higher than the published plate minima due to airline operating specifications (OpSpec), aircraft certification limits, or crew recency requirements. Always apply the more restrictive value.
Runway Visual Range (RVR) Explained
- Definition: The distance over which a pilot of an aircraft on the runway centreline can see the runway surface markings or the lights delineating the runway or identifying its centreline.
- Measurement: Transmissometers (forward scatter meters) measure the extinction coefficient of the atmosphere at ground level. Converted to visual range automatically.
- Reporting zones: Three values reported — Touchdown Zone (TDZ), Mid-point (MID), and Rollout (END). For example: R28L/0600/0400/0350
- Critical value: For approach, the TDZ RVR is the operationally critical value. For rollout/taxi, the MID and END values govern.
- When RVR unavailable: Reported meteorological visibility (MET VIS) converted by ATC using standard conversion tables.
- TREND indicator: RVR reports include a trend: U (upward), D (downward), N (no change). Vital for planning when visibility is fluctuating near minimums.
Missed Approach Procedures
Pre-brief Before Every ApproachThe missed approach procedure must be fully briefed — track, initial altitude, routing, and relevant ATC instructions — before commencing the approach. In a real missed approach there is no time to look it up.
- Reach DA/DH (precision) or MAP (non-precision) without required visual references — decision is immediate. No hesitation, no extra seconds below DA.
- Apply full go-around power / TOGA thrust. Call "Go Around" clearly on intercom and to ATC.
- Pitch up to go-around attitude. Arrest the rate of descent — positive rate of climb first priority.
- Retract gear (if down) once positive climb is confirmed. Retract flaps in stages per aircraft procedure.
- Accelerate to go-around / initial climb speed. Maintain wings level until obstacle clearance altitude is reached.
- Follow the published missed approach track exactly. Deviating from the published procedure removes terrain/obstacle clearance assurance.
- Advise ATC of the missed approach and intentions. Request further clearance for holding, diversion, or another approach as appropriate.
Mental Model — "Aviate, Navigate, Communicate"During a missed approach: fly the aircraft first, follow the published track second, talk to ATC third. Never allow communication to distract from flying the go-around profile.
Circling Approach Missed Approach
- If visual contact is lost during circling, immediately turn toward the landing runway and climb to the circling minimums altitude.
- Once established climbing, execute the published missed approach for the instrument approach that was originally flown.
- Circling minimums are significantly higher than straight-in minimums — circling in poor visibility requires careful judgment.