Glideslope / Glidepath Descent Calculator
Heights along a 3° (or any) glidepath: altitude at each distance, the 300-ft-per-mile check, and VSI for your approach ground speed.
The cross-check that catches glideslope failures and wrong-QNH days alike: 300 ft per mile plus the TCH. At 5 DME you should see ~1,640 ft on a 3° path — if the needle's centered and the altimeter argues, believe the disagreement and investigate.
Formula
⚠️ For flight planning and education only — verify with current charts, AFM and ATC clearances. Not for primary navigation.
Heights along a 3° (or any) glidepath: altitude at each distance, the 300-ft-per-mile check, and VSI for your approach ground speed.
About Glideslope / Glidepath Descent Calculator
A centered glideslope needle is a claim, not a fact — and the altitude-vs-distance cross-check is how instrument pilots audit it: roughly 300 feet per nautical mile plus threshold crossing height on a standard 3° path. This calculator generalizes the check to any glidepath angle and distance, and computes the ground-speed-matched VSI that holds the path — the three-number cross-check (distance, altitude, descent rate) that flies stabilized approaches and catches lying needles.
How to use Glideslope / Glidepath Descent Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula height = TCH + dist × tan(GPA) × 6076; at 3°: ≈ 318 ft/nm (the '300-per-mile' check) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Glideslope / Glidepath Descent Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula height = TCH + dist × tan(GPA) × 6076; at 3°: ≈ 318 ft/nm (the '300-per-mile' check) with sources cited on the page
- ✓The cross-check that catches glideslope failures and wrong-QNH days alike: 300 ft per mile plus the TCH. At 5 DME you should see ~1,640 ft on a 3° path — if the needle's centered and the altimeter argues, believe the disagreement and investigate.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How do I use the altitude-distance check on a real approach?+
At each charted or mental fix, compare: 'six DME, should be about 1,950 — I show 1,960, good.' Most plates print on-path crossing altitudes at the FAF and stepdowns; between them, 300×miles+TCH fills the gaps. A growing disagreement between needle and arithmetic means a misset altimeter, a sidelobe/false glideslope capture, or equipment failure — all discoverable while altitude remains to spend.
What VSI holds a 3° glideslope?+
Half your ground speed times ten: 105 kt → ~530 fpm... more precisely GS×5.3 (this tool computes exactly). The rate scales with GROUND speed, so a 20-kt headwind on final lowers the required rate noticeably — pre-computing it from the ATIS wind is what lets you fly the needle with trim and small corrections rather than chasing it.
What are false glideslopes and where do they live?+
Antenna geometry creates lobes at multiples of the path angle — the first false slope near 9° (and a reverse-sensing one near 6°). They're encountered arriving very high close-in, classically on slam-dunk vectors. The defense is exactly this page's arithmetic: a 9° path 'centered' at 4 DME would demand ~3,500 ft and an absurd 1,600 fpm — numbers that scream wrong if you've computed what right looks like.
Do RNAV glidepaths follow the same math?+
Identically — LPV and LNAV/VNAV glidepaths publish their angle (occasionally 3.5° or steeper at terrain-challenged fields) and TCH on the plate; substitute them in this tool. Baro-VNAV paths additionally shift with temperature (cold air lowers the true path — see the chart's temp limits), which makes the distance-altitude cross-check MORE valuable, not less, in the GPS era.
Related Field tools
Sunrise & Sunset Calculator
Exact rise, set, solar noon and day length for any place and date — the NOAA solar equations with the refraction fine print included.
● LiveGolden Hour & Blue Hour Calculator
Tonight's golden hour and blue hour, computed from sun elevation — the photographer's light windows with the angles that define them.
● LiveDay Length Calculator
Hours of daylight for any date and latitude, how fast it's changing, and the swing between your solstices.
● Live