Glide Slope, Descent Rate & TOD Calculator
Compute required Rate of Descent (FPM) for any approach angle, 3:1 Top of Descent (TOD) distance, and wind effects on ground speed.
- GS × 5 Rule: On a standard 3° slope, required FPM ≈ Ground Speed × 5 (e.g. 120 kts × 5 = 600 FPM).
- 3:1 TOD Rule: Top of Descent distance in NM = (Altitude to Lose ÷ 1,000) × 3.
- 318 ft per NM: Aircraft loses ~318 feet of altitude per Nautical Mile flown on a 3° slope.
Descent & Glide Path Results
Why Airplanes Fly in Curved Arcs on Flat Maps (Great Circle Navigation)
When you look at flight tracking maps, long-haul routes look like curved parabolas or arcs rather than straight lines. This is not due to wind or weather avoidance—it is a fundamental mathematical property of traveling across a 3D spherical Earth.
Great Circle Route (Orthodromic)
A Great Circle is the intersection of the sphere with a plane passing directly through the Earth's center. It is the absolute shortest 3D distance between any two points on the planet. When projected onto a flat 2D Mercator map, this shortest 3D path appears curved as an arc.
Rhumb Line (Loxodrome)
A Rhumb Line crosses all meridians of longitude at the same constant compass angle. While it draws as a straight line on a flat Mercator map, it is actually a longer spiral curve across the spherical Earth. Flying a Rhumb line requires fewer heading changes, but wastes significant fuel and time on long flights.
Headwind & Tailwind Calculation Example (300 NM Flight)
Wind directly impacts Ground Speed (GS = TAS ± Wind). A headwind extends time in the air, compounding fuel burn:
| Wind Condition | Ground Speed | Time for 300 NM | Fuel Burn (10 GPH) |
|---|---|---|---|
| Zero Wind Baseline | 140 kts | 2h 9m | 21.4 gal |
| With 25 kts Headwind | 115 kts | 2h 37m (+28m) | 26.1 gal (+4.7 gal) |
Studying for RPL, PPL or CPL Exams?
Practice realistic CASA-style questions for Navigation, Performance, Meteorology, and Flight Rules with timed mock tests at AeroAcademic Australia.
Authoritative Standards & Verified Reference Citations
Calculations and mathematical logic on this page are grounded in published statutory rules, regulatory standards, and official government data published by the following bodies:
CASR Part 91 general operating and flight rules, Part 121 fuel reserves, and flight crew licensing standards.
Aviation weather services, standard atmosphere (ISA) profiles, METAR/TAF aerodrome weather reports.
Aeronautical Information Publication (AIP), En Route Supplement Australia (ERSA), and airspace procedures.
Interactive CASA exam practice question banks for RPL, PPL and CPL syllabus preparation, fixed-wing and helicopter.
Jandakot-based rotary and fixed-wing operations unit — real-world reference for helicopter performance and mission profiles.
Australia's largest aeromedical operator — 79 aircraft across 23 bases delivering emergency and primary healthcare to remote Australia.
NSW Police airborne support unit operating Bell 429 and Bell 412EPI helicopters for search, surveillance and rescue operations.
Victoria Police's aviation support unit, operating since 1975.
QPS Aviation Capability Group — police helicopter and fixed-wing operations across Queensland.
AW139 rescue helicopter fleet operating from Belmont, Tamworth and Lismore bases across Northern NSW.
Queensland-based rescue helicopter and aeromedical retrieval service, operating since 1979.
South Australia's specialist emergency medical retrieval and aeromedical service.
