Altitudes and minima

Every number on a chart is referenced to something. Altitudes are given above mean sea level ( MSL ) — your altimeter reads them when set to the local QNH . Heights in parentheses, like (200), are above ground level ( AGL ) — that is what the runway "sees" beneath you.

• MSL (ft) — altitude above sea level, read on the altimeter with QNH set.
• AGL (ft, in parentheses) — height above the airport or terrain.
• Flight Level (FL) — used above the transition altitude with standard pressure 1013 hPa / 29.92 inHg.

On a precision or APV approach ( ILS , LPV ) you descend on a continuous glide path to a Decision Altitude ( DA ): at that single point you decide — land or go around. On a non-precision approach ( VOR , NDB , LNAV ) you descend to a Minimum Descent Altitude ( MDA ) and level off until you either see the runway or reach the missed approach point .

European AIP plates do not print a DA — they print OCA (OCH) , the Obstacle Clearance Altitude (Height) . That is the lowest altitude/height at which the procedure still guarantees clearance from terrain and obstacles — the number the state surveyor computed, nothing more.

Your DA or MDA is derived from it : the operator takes the published OCA(H) and adds margins — aircraft and crew requirements, regulatory minimums (a CAT I ILS can never go below 200 ft above the threshold, even if the OCA is lower), company policy. Jeppesen -style plates do that work for you and print DA(H) directly; AIP plates print the raw OCA(H) and leave the margins to the operator. Either way: your minima can never be lower than the OCA . In the sim, treat the printed OCA as your floor.

Below the OCA(OCH) columns, ICAO minima tables carry a row labelled VM(C) OCA (OCH AAL) — Visual Manoeuvring (Circling) . That is the minimum for flying the instrument approach to one runway, then circling visually to land on another. The height is referenced AAL — Above Aerodrome Level , because while circling you care about the ground under the airport, not a single threshold. The VM(C) row exists even on straight-in approach plates — it is there for the day you break off and circle.

Some minima tables do not give you one OCA but several — each tied to a missed-approach climb gradient . Procedure design assumes the missed-approach obstacle surface rises at a nominal 2.5% from the missed approach point; where terrain sticks up into that surface, the OCA goes up with it. If your aircraft can outclimb the surface, the state may publish alternative OCA rows for steeper gradients — the 2.5% figure is always published, the steeper options are extras.

Innsbruck is the extreme case: at the standard 2.5% the OCA is 4,900 ft; a 5.0% go-around buys 3,300 ft — 1,600 ft lower. Note that the missed approach point itself moves (D-7.5, D-4.5, D-3.5 OEV): the steeper your climb, the further down the valley you may continue. Lido-style minima tables print the same idea as a column header like " GA 4.5%" — GA meaning go-around gradient. The FAA system assumes 200 ft/NM (about 3.3%) unless the chart notes a higher required climb.

Two conversions worth memorizing:

So 2.5% at 140 kt groundspeed is roughly 350 ft/min. And one caveat: a published gradient above 2.5% is a requirement, not a suggestion — real-world rules make the operator verify the aircraft can actually deliver it engine-out at the expected landing weight (a twin only has to demonstrate 2.1% single-engine in approach configuration). In the sim nothing checks this for you — it is a briefing item, not something the FMC will flag.

Minimums always come in pairs: an altitude AND a visibility. RVR (Runway Visual Range) is measured along the runway — in metres on ICAO/Jeppesen charts, in feet on FAA charts (e.g. "RVR 2400"). A plain number like "800" or "1/2" is statute visibility or metres depending on the chart publisher.