Learn when to start descending for landing using the 3:1 rule, circuit timing, ILS cues and fixes for arriving too high or too low.
Start descending for landing at the point that lets you meet the published approach or circuit profile without steep dives or prolonged level flight. For a normal 3-degree path, allow roughly 3 nautical miles per 1,000 feet to lose, then add distance for slowing and configuring the aircraft.
In Aviation & Real-World Flying, there is no single mileage that suits every arrival. Aircraft performance, circuit procedures, terrain, wind, air traffic control and instrument restrictions all affect the decision. In a real aircraft, the flight manual, charts, instructor guidance, operator procedures and ATC clearance override planning rules of thumb.
Which point should trigger the descent?
The correct descent trigger depends on the type of approach being flown.
| Approach | Usual descent cue |
|---|---|
| VFR circuit | Typically abeam the intended touchdown point on downwind, after reducing power and configuring as required |
| Straight-in visual approach | A calculated point that intercepts a normal glide path while remaining clear of terrain and restrictions |
| ILS or approach with vertical guidance | Glideslope or glidepath capture from below, once authorised to descend |
| Non-precision instrument approach | The published final approach fix or descent point, subject to altitude restrictions |
| Airliner arrival | The planned top of descent, adjusted for restrictions, speed reduction, wind and configuration |
How do I calculate the top of descent?
Use the 3:1 rule for a quick estimate of where a normal descent should begin:
Distance in NM = altitude to lose in feet ÷ 1,000 × 3
If the aircraft is at 6,500 feet MSL and must reach 1,500 feet MSL, it has 5,000 feet to lose. The basic descent distance is therefore 15 NM. Begin slowing before that point or add enough distance for deceleration; a clean, fast aircraft may not descend and slow effectively at the same time.
For a roughly 3-degree path, required vertical speed can be estimated with:
Vertical speed in feet per minute = groundspeed in knots × 5
At 90 knots groundspeed, that gives about 450 feet per minute. At 140 knots, use about 700 feet per minute. Wind does not alter the geometry of a 3:1 path over the ground, but it changes the time available and therefore the required descent rate. Managed airliner systems may also move the calculated top of descent because they model wind, speed and drag.
Airliner planning must include crossing restrictions and the distance needed to reduce speed. Our worked 737 descent-planning method explains how those extra constraints affect the basic calculation.
When should a light aircraft descend in the circuit?
In a normal light-aircraft circuit, descent commonly begins on downwind when abeam the intended touchdown point.
- Maintain circuit altitude until reaching the aerodrome's normal descent position, unless local procedures say otherwise.
- Reduce power and allow the aircraft to slow while maintaining the recommended attitude and speed.
- Select configuration in accordance with the aircraft checklist and flight manual.
- Begin a controlled descent, then turn base according to runway position, wind and circuit spacing rather than using altitude alone.
A common visual reference is to turn base when the threshold is about 45 degrees behind the wing, but this is only a guide. A strong tailwind on downwind requires an earlier base turn; a headwind may require a later one. Published circuits, noise-abatement routes and ATC instructions take priority. Our circuit positioning walkthrough shows how downwind and base placement determine whether final is stable.
When does descent begin on an ILS approach?
On an ILS, final descent normally begins when the glideslope is captured from below and the aircraft is cleared or otherwise authorised to descend under the applicable procedure.
Localiser capture alone is not a signal to descend. Maintain the assigned or published altitude until the correct vertical guidance or charted descent point is reached. Intercepting an ILS from above risks false-glideslope capture and an unstable attempt to regain the profile; request more track miles or vectors instead of forcing the aircraft down.
The wording and authority of an approach clearance vary between jurisdictions, so the chart and ATC instruction remain controlling. Simmers practising transport-aircraft procedures can use our 737 ILS interception and configuration sequence to check that the aircraft has captured the glideslope and actually started descending.
What should I do if I start too late or too early?
If the aircraft is too high or fast, create more distance early rather than trying to rescue the approach with an excessive descent rate.
- Too high: reduce power, use approved drag devices within their limits and ask ATC for extra track miles when necessary. A forward slip may be appropriate in some light aircraft, but only when permitted by the flight manual and training.
- Too fast: level temporarily if altitude restrictions permit, configure earlier and avoid chasing the glide path while still accelerating downhill.
- Too low: add power and return to the correct profile without descending below a cleared, published or safe altitude.
- Unstable on final: go around rather than using steep banks, abrupt pitch changes or large late configuration changes.
A mistake we see constantly in simulator approaches is concentrating on altitude while ignoring energy. Being exactly on the calculated path is of little use if the aircraft is still too fast to extend the landing gear or select landing flap.
Use the stabilised-approach gate required by the operator or training organisation. A common benchmark is stabilised by 1,000 feet above the aerodrome in instrument conditions or 500 feet in visual conditions, but those figures are not universal. If the aircraft is not correctly configured, on speed, aligned and descending at an acceptable rate by the applicable gate, the safe decision is a go-around.