Learn to calculate jet top of descent, manage speed and altitude constraints, choose vertical modes and recover safely from a high approach.
Plan a jet descent backwards from the altitude and speed required at the approach fix, using roughly 3 NM per 1,000 ft as an initial path. In real-world aviation, then refine the top of descent for wind, restrictions, deceleration and aircraft guidance, and remain stable enough to continue or go around.
What information is needed before starting the descent?
A sound jet descent begins with a verified runway, arrival route, approach and clearance limit. Review the STAR and approach chart, noting every speed restriction and whether an altitude is mandatory, minimum, maximum or a window.
- Present cruise altitude and expected descent clearance
- Altitude and speed required at the first approach fix
- Runway elevation, terrain and minimum safe altitudes
- Forecast descent winds and expected anti-ice use
- STAR restrictions and applicable speed limits
- Distance needed to slow and extend flaps and landing gear
Set up and cross-check the flight-management system rather than assuming its calculated path is correct. A wrong runway, route discontinuity, omitted restriction or unrealistic wind forecast can move the calculated top of descent significantly.
For an actual flight, the aircraft flight manual, operator procedures, charts and ATC clearance always override rule-of-thumb arithmetic. Reaching the calculated top of descent is not itself permission to leave the cleared altitude.
How do you calculate a jet's top of descent?
For a normal three-degree descent, allow approximately 3 NM for every 1,000 ft of altitude to lose.
Descent distance in NM = altitude to lose in thousands of feet × 3
Suppose an aircraft must descend from FL350 to cross a fix at 3,000 ft. It has 32,000 ft to lose, giving a basic descent distance of about 96 NM. If that fix is 10 NM before the runway, the geometric starting point is roughly 106 NM from the threshold, before allowing for slowing, wind or level segments.
This is a planning estimate, not a guaranteed idle-descent profile. A strong tailwind increases groundspeed and leaves less time to lose altitude over each mile, while ATC restrictions, anti-ice operation and aircraft weight can also change the result. Our explanation of top-of-descent timing and common distance corrections covers those adjustments in more detail.
What vertical speed gives a three-degree descent?
A three-degree path requires a vertical speed of approximately five times groundspeed.
Vertical speed in ft/min ≈ groundspeed in knots × 5
- 300 kt groundspeed: about 1,500 ft/min
- 240 kt groundspeed: about 1,200 ft/min
- 180 kt groundspeed: about 900 ft/min
Use groundspeed, not indicated airspeed. The required vertical speed changes as the aeroplane slows or the wind changes, which is why a fixed vertical-speed setting rarely follows the whole descent accurately.
Which autopilot descent mode should you use?
The best descent mode depends on whether the priority is following a calculated path, complying with an immediate ATC clearance or making a small correction.
| Mode | Choose it when | Main risk |
|---|---|---|
| VNAV or managed descent | The programmed route and restrictions have been checked and you want to follow the computed vertical profile. | Incorrect route, winds or constraints can produce an unsuitable path. |
| Flight-level change or open descent | ATC assigns a lower altitude and speed control has priority over following the original profile. | Mode logic differs by aircraft, and intermediate restrictions may not be honoured automatically. |
| Vertical speed | You need a modest, closely monitored path correction or a specific rate. | The autopilot may sacrifice airspeed to maintain the selected rate. |
| Flight-path angle | The aircraft provides this mode and you want a defined geometric angle. | Speed and thrust still require supervision. |
No mode removes the need to monitor altitude, speed, thrust and the next restriction. Simulator pilots commonly select a large vertical speed, then watch the autopilot pitch towards an unsafe airspeed while trying to maintain it.
How do you fly the descent correctly?
Fly the descent by controlling both the vertical path and the aircraft's energy, checking progress against each published restriction rather than waiting for the final approach.
- Brief the arrival. Confirm the runway, STAR, approach, minimums, missed approach and expected configuration. For a type-specific example, see our Boeing 737 descent and approach set-up.
- Calculate and verify the top of descent. Compare the FMS prediction with the three-to-one estimate. Investigate a large disagreement instead of blindly accepting either figure.
- Set the cleared altitude. In real-world operations, descend only when authorised. Select the intended vertical mode and confirm its annunciation; pressing a button is not proof that the expected mode engaged.
- Monitor the path. Recalculate from the altitude remaining and track miles available. Check the next altitude and speed restriction after every route or clearance change.
- Manage speed early. Use thrust and permitted drag devices before the excess energy becomes unmanageable. Never extend flaps, landing gear or speed brakes outside the aircraft's published limitations.
- Update the altimeter correctly. Change between standard pressure and the local setting at the published transition level or altitude, which varies by country and airport.
- Join the approach in the correct state. Arrive at the initial or final approach segment at the charted altitude and a speed from which normal configuration is possible.
The final approach's vertical guidance is separate from the en-route descent profile. An ILS glideslope and an RNAV glidepath may look similar on the display but are generated differently; our comparison of ILS and RNAV guidance explains the operational distinction.
When should a jet slow down and configure?
A jet should start decelerating early enough to reach approach speed and landing configuration without steep manoeuvring, prolonged idle thrust or rushed checklist work near the runway.
There is no universal flap schedule. Aircraft weight, flap limits, wind, icing procedures, traffic spacing and the published approach all affect it. Follow the type's approved schedule; in a simulator, use the add-on's manual rather than copying speeds from another jet bearing a similar name.
Where a speed restriction applies below 10,000 ft, plan for it before crossing that altitude. Do not assume 250 kt is a universal worldwide rule: airspace regulations and ATC instructions differ.
What should you do if the jet is too high or too fast?
If the aircraft is high or fast, recognise it early, tell ATC where applicable and create more drag or more track distance within the aircraft's limitations.
- Slightly high: increase the descent rate or use approved speed-brake deployment while watching airspeed and passenger comfort.
- Fast but on path: reduce thrust and begin deceleration earlier; a short level segment can help if altitude permits.
- High and fast: request vectors, extended routing or a hold rather than diving towards the approach. Gear may provide substantial drag once below its extension limit.
- Below path: reduce the descent rate or level temporarily and add thrust as required. Do not remain at idle while becoming progressively lower and slower.
A mistake we see constantly in simulators is trying to cure excess height with a very steep descent, then attempting to lose excess speed in the last few miles. Jets do not shed altitude and speed efficiently at the same time. If normal corrections cannot produce a stabilised approach, discontinue it and go around.
When is the approach stable enough to continue?
The approach is stable when the jet is on the correct lateral and vertical path, in landing configuration, near target speed, using an appropriate thrust setting and descending at a controlled rate.
The exact stabilisation gate belongs to the operator or aircraft procedure; 1,000 ft above the runway is a common benchmark, but it is not universal. Required checklists should be complete, and only small corrections should remain. Once the descent and approach are stable, our guidance on airspeed control, flare and smoother simulator landings covers the next phase.