Learn how to calculate instrument approach altitude, identify charted restrictions and minima, and cross-check a 3-degree descent accurately.
You normally do not calculate an instrument approach altitude from scratch. For Aviation & Real-World Flying, take each crossing altitude, glideslope-intercept altitude and DA or MDA from the current approach chart, apply the correct altimeter setting and required temperature corrections, then use a descent calculation only as a cross-check.
Where does the correct approach altitude come from?
The published instrument approach chart is the controlling source for approach altitudes. Its plan and profile views show the minimum, maximum or mandatory altitude at each relevant fix; our guide to decoding instrument approach charts explains the associated symbols and profile view in detail.
| Altitude | What it means | How to use it |
|---|---|---|
| Minimum sector altitude (MSA) | Obstacle-clearance altitude for a defined sector | Use it for situational awareness or emergencies, not as permission to descend along the approach. |
| IAF, intermediate or step-down altitude | A constraint at or between approach fixes | Remain at, above or below it as indicated by the chart. |
| Glideslope or glidepath intercept altitude | The published altitude for intercepting vertical guidance | Approach level at this altitude and intercept from below unless the procedure says otherwise. |
| DA or MDA | The applicable approach minimum | Use the line matching the procedure, equipment, aircraft category and operational authorisation. |
| Missed-approach altitude | The target altitude after commencing the missed approach | Set it for the missed approach, but do not climb early unless instructed or required by the procedure. |
Most charted procedure altitudes are barometric altitudes above mean sea level. A value in parentheses is often a height above a specified runway or aerodrome datum, but conventions vary. For example, a minimum printed as 720 (200) normally means fly to 720 feet on the altimeter; do not add 200 feet to it or set 200 feet as the barometric altitude.
A mistake we see constantly is treating the MSA as the next descent altitude. It is not part of the final descent profile unless the procedure explicitly uses that altitude elsewhere. Our practical simulator chart guide covers the difference between sector altitudes, crossing restrictions and approach minima.
How do I calculate or verify instrument approach altitude?
Calculate only a cross-check after identifying the published altitude, pressure datum and distance reference.
- Load the correct procedure. Match the airport, runway, approach identifier and transition in the chart and aircraft navigation system. Similar-looking procedures can have different fixes and altitude constraints.
- Set the correct pressure. Below the applicable transition level, procedure altitudes normally require the local QNH or altimeter setting. Leaving standard pressure set can move the indicated altitude by hundreds of feet; as a rough diagnostic, 10 hPa is about 270 feet and 0.10 inHg is about 100 feet.
- Identify the active segment. Use the altitude attached to the fix or segment you are actually flying. An at-or-above restriction is a floor, an at-or-below restriction is a ceiling, and a mandatory altitude must be crossed at that value.
- Apply required corrections. Cold-temperature or remote-altimeter corrections may apply when stated by the chart, regulations or operating procedure. Where required, the corrected altitude flown is normally higher than the published value. Check what any aircraft temperature-compensation function changes so that the correction is not applied twice.
- Cross-check the final path. Compare the published altitude with a three-degree descent calculation, but never replace a charted constraint with the calculated result.
- Select the correct minimum. Use DA for the appropriate vertically guided approach or MDA for the applicable non-precision minima line. Do not simply select the lowest number printed on the chart.
Can I use airport elevation plus 300 feet per mile?
Airport elevation plus roughly 300 feet per nautical mile gives a useful mental check for a three-degree final, but it is not precise enough to override the approach chart.
A more accurate geometric calculation is:
Altitude MSL ≈ threshold elevation + threshold crossing height + (distance to threshold in NM × 318)
For example, at 5 NM from a threshold elevated 620 feet, assuming a 50-foot threshold crossing height, the expected three-degree altitude is approximately 620 + 50 + (5 × 318) = 2,260 feet MSL. The 300-feet-per-mile shortcut gives 2,170 feet, which is adequate for detecting a large error but not for establishing a legal crossing altitude.
The calculation works only when the distance is measured to the runway threshold. DME may measure slant range to a ground station positioned elsewhere, while an RNAV fix can be offset from the threshold. Use the chart's published distance-to-altitude table when one is provided.
Which altitude applies to ILS, RNAV and non-precision approaches?
The applicable altitude depends on the type of vertical guidance and the minima being flown.
- ILS: Maintain the published intercept altitude and capture the glideslope from below. Once established, follow the transmitted glideslope to the applicable decision altitude.
- LPV or LNAV/VNAV: Follow the published constraints until established on the authorised glidepath, then use the applicable DA. Barometric VNAV can be sensitive to pressure and temperature, so observe any charted temperature limits.
- LNAV, localiser, VOR or similar non-precision approach: Honour every step-down altitude and do not descend below MDA without the required visual references and position. An advisory magenta descent path does not cancel charted restrictions.
For RNAV-specific equipment checks, vertical modes and minima selection, follow our RNP approach procedure for flight simulators.
Why do the chart, FMS and calculated altitude disagree?
Disagreement usually comes from pressure, datum, distance or procedure-version differences rather than faulty descent geometry.
- Wrong pressure or units: Check QNH against standard pressure and confirm whether the value is in hPa or inHg.
- MSL confused with AGL: Charted barometric altitude is not the same as radio-altimeter height above terrain.
- Wrong distance source: DME distance, along-track distance and distance to the threshold may all differ.
- Navigation-data mismatch: The FMS, simulator ATC and chart may represent different procedure revisions. Match the approach identifier, fixes and transition before relying on managed descent.
- Automatic compensation: Some aircraft models compensate a VNAV path or entered restrictions differently. Verify the model's behaviour before adding a manual cold-temperature correction.
- Incorrect mode: Capturing an advisory VNAV path is not the same as capturing an ILS glideslope or an authorised RNAV glidepath.
For real aircraft, current approved charts and operating rules take precedence over mental arithmetic or simulator behaviour. If the altitudes are correct but the aircraft reaches them in the wrong sequence, use our complete IFR simulator workflow to check procedure loading, briefing, descent planning and missed-approach preparation.