How do I calculate an instrument approach altitude?

You normally do not calculate an instrument approach altitude from scratch. In Aviation & Real-World Flying, take every fix altitude, glideslope or glidepath intercept, DA/MDA and missed-approach altitude from the current chart, set the correct pressure datum, apply required corrections, and use descent geometry only as an independent cross-check.

Approach altitude is not one number

An instrument approach contains several different altitudes, each serving a particular segment or decision. Confusing them is one of the most common causes of premature descent in a simulator.

Most procedure altitudes are barometric altitudes above mean sea level, flown using QNH. A height in parentheses normally refers to a runway, touchdown-zone or aerodrome datum, but the chart legend controls. For example, minima shown as 720 (200) usually mean an altimeter altitude of 720 feet and a corresponding height of 200 feet; do not add the two numbers together.

The profile view is the controlling source for crossing restrictions, vertical-path intercepts and minima. Our explanation of approach-chart symbols and profile-view altitudes covers how to distinguish minimum, maximum and mandatory restrictions.

How do I calculate or verify an instrument approach altitude?

Start with the published altitude, then verify that its procedure, datum, pressure setting and distance reference match what the aircraft is using.

1. Confirm the exact procedure. Match the airport, runway, approach identifier, transition and chart revision with the procedure loaded in the navigation system. Similar approach names can contain different fixes and altitude constraints.
2. Identify the active segment. Use the restriction attached to the fix or leg being flown. An at-or-above altitude is a floor, an at-or-below altitude is a ceiling, and a mandatory altitude must be crossed at the published value.
3. Check the altitude datum. Determine whether the figure is an altitude above mean sea level, a height above a local datum or a flight level. Do not add airport elevation to an altitude already expressed as MSL.
4. Set the correct pressure. Below the applicable transition level, approach altitudes normally require local QNH unless the chart or operating procedure specifies another datum. Leaving standard pressure set can create an error of several hundred feet.
5. Apply required corrections once. Cold-temperature and remote-altimeter corrections may be required by the chart, regulations or operating procedure. Check whether the simulated aircraft has already compensated entered constraints or a barometric VNAV path.
6. Cross-check the vertical profile. Compare the charted altitude with the published distance-to-altitude table or a geometric three-degree calculation. Arithmetic can reveal a large error, but it cannot replace a charted restriction.
7. Select the applicable minimum and prepare the missed approach. Use the correct DA or MDA line, then verify the missed-approach point, initial track and target altitude before descending.

How do I convert QNH to inHg?

Convert hectopascals to inches of mercury with inHg = hPa × 0.02953. Hectopascals and millibars are numerically equal for altimeter-setting purposes.

• 1000 hPa ≈ 29.53 inHg
• 1013.25 hPa ≈ 29.92 inHg
• 1020 hPa ≈ 30.12 inHg

For the reverse conversion, use hPa = inHg × 33.8639. Enter the reported setting directly if the aircraft can display the same unit; that avoids unnecessary rounding.

As an approximate altimeter check, 1 hPa corresponds to about 27 feet and 0.10 inHg to about 100 feet near normal approach altitudes. Turning the pressure setting up makes the indicated altitude go up at the same static pressure. These figures estimate the effect of a setting error; they are not substitutes for the pressure-unit conversion formula.

How do I calculate MSA in aviation?

Pilots normally read MSA from the approach chart rather than calculating it themselves. Depending on the charting system, MSA can mean minimum sector altitude or minimum safe altitude; the chart legend defines the term and protected area.

A typical approach-chart MSA provides at least 1,000 feet of obstacle clearance within sectors extending about 25 NM from a specified point. Exact radii, sector boundaries, buffers, rounding and design allowances depend on the procedure standard. MSA also does not necessarily guarantee navigation-aid reception, communications coverage or controlled-airspace containment.

A simplified planning estimate is:

Estimated MSA = highest controlling terrain or obstacle elevation + required obstacle clearance

If the controlling obstacle were 3,430 feet MSL and the applicable clearance were 1,000 feet, the unrounded result would be 4,430 feet. If that procedure standard required rounding up to the next 100 feet, the estimate would be 4,500 feet. This is an illustration, not a method for creating an operational MSA from simulator scenery or incomplete obstacle data.

When the chart says MSA 4,500 feet, set QNH and interpret it as 4,500 feet MSL unless the chart states otherwise. Do not add airport elevation. More importantly, do not treat MSA as clearance to descend to 4,500 feet along any approach segment; published segment altitudes and ATC clearances still apply.

Can I use airport elevation plus 300 feet per nautical mile?

Airport elevation plus roughly 300 feet per nautical mile is a useful three-degree-path check, but 318 feet per nautical mile is more accurate. Neither value overrides a published approach altitude.

For a three-degree path:

Altitude MSL ≈ threshold elevation + threshold crossing height + (distance to threshold in NM × 318)

At 5 NM from a threshold elevated 620 feet, assuming a 50-foot threshold crossing height, the expected altitude is approximately 620 + 50 + (5 × 318) = 2,260 feet MSL. The 300-feet-per-mile shortcut gives 2,170 feet, which is close enough to expose a major setup error but not to establish a crossing restriction.

For a published angle other than three degrees, use:

Height above threshold = distance in NM × 6076 × tan(path angle) + threshold crossing height

The distance must be measured to the runway threshold or other datum used by the vertical path. DME may show slant range to a station located away from the threshold, while an RNAV fix can use along-track distance. Use a charted distance-to-altitude table when available.

Altitude alone does not determine the required descent rate because groundspeed matters. Our three-degree descent-rate cross-check explains the associated groundspeed calculation.

Which approach altitude applies to ILS, RNAV and non-precision approaches?

The relevant altitude depends on the vertical guidance and minima authorised for the approach.

• ILS: Maintain the published intercept altitude and capture the glideslope from below unless the procedure directs otherwise. Once established, follow it to the applicable DA; our focused guide to finding the ILS intercept altitude explains why guessing from distance can cause a false or premature capture.
• LPV: Observe every charted constraint until established on the authorised glidepath, then use the applicable DA.
• LNAV/VNAV: Use the published constraints and DA. Barometric VNAV may be subject to temperature limits and pressure-setting errors.
• LNAV, localiser, VOR or similar non-precision approach: Honour each step-down restriction and remain at or above MDA until the requirements for further descent are met. An advisory magenta path does not cancel a charted altitude.

An ATC vector or descent instruction does not turn MSA into an approach-segment altitude. When being vectored, maintain the assigned altitude until cleared and established as required, while checking that the resulting intercept complies with the procedure.

What is MDA, and is it the approach altitude?

MDA is the lowest barometric altitude permitted on the applicable non-precision approach segment without the required visual references and position for landing. It is one approach altitude, not a replacement for earlier crossing or step-down restrictions.

At a decision altitude, the aircraft is normally descending on continuous vertical guidance and the missed approach is initiated if the required conditions are absent. At MDA, the aircraft must not descend below the published altitude until those conditions are satisfied. Reaching MDA does not remove the need to identify the missed-approach point.

DA and MDA are altitudes above mean sea level. DH and MDH are corresponding heights above a specified datum. Choose the minima line matching the actual approach capability, aircraft category and equipment rather than selecting the lowest number on the page; our guide to selecting and using DA or MDA in a simulator covers that decision in detail.

What does “MISM-430” or “MISM 430” mean?

MISM-430 is not a universal formula or standard notation for approach altitude. Without the source, column heading or display context, the identifier and number cannot be decoded safely.

• Airport data: If the source explicitly says ELEV 430 ft, 430 is likely field elevation in feet MSL, not MSA or MDA.
• Radio-aid data: A value of 430 beside an NDB entry may be a frequency in kHz rather than an altitude.
• Aircraft display: A number attached to a fix can represent a coded altitude constraint, but its format depends on the avionics. FL430, for example, means flight level 430 and not 430 feet.
• Location or database identifier: MISM may identify a location or record while the hyphen merely separates another field.

Read the label, units and chart legend before using 430 in any calculation. If it is field elevation, do not add it to a published MSL altitude; use it only when converting an AGL height or performing a threshold-based geometric check.

Why do the chart, FMS and calculated approach altitude disagree?

Most disagreements come from mismatched procedures, pressure settings, altitude datums or distance references rather than faulty three-degree arithmetic.

• Different procedure revisions: The chart, simulator navigation database and built-in ATC may contain different fixes or constraints. Compare the full approach identifier and waypoint sequence.
• QNH left on standard: Verify both the pressure value and whether the instrument expects hPa or inHg.
• MSL confused with AGL: Barometric chart altitude is not radio-altimeter height above the terrain.
• Wrong distance source: DME distance, RNAV along-track distance and distance to the threshold are not automatically interchangeable.
• Wrong constraint selected: The altitude beside the next fix may belong to a different transition or segment.
• Automatic compensation: Some simulated avionics alter a VNAV path or entered restrictions for temperature. Applying the same correction manually can double it.
• Advisory path mistaken for approved guidance: A calculated vertical path does not provide permission to disregard step-down fixes or MDA.

A three-degree calculation should agree reasonably with a published altitude only when the angle, threshold crossing height and distance datum are the same. If a significant discrepancy remains, stop the descent and resolve it. In real aircraft, the applicable clearance, approved chart, operating rules and aircraft procedures take precedence over simulator behaviour or mental arithmetic.