# VFR Top of descent calculation^{[PDF]}

## Contents

## Introduction

The top of descent or TOD is the point for an aircraft to initiate a descent to a lower lever for arrival at the destination airfield. The aircraft is leaving the cruise phase of the flight and start a descent phase to the first approach altitude expected over an aerodrome circuit, a landmark or entry VFR points.

## Angle of descent rate

- Descent angle (°) ≈ descent rate (%) x 0.6
- Rate (ft/NM) ≈ 60 x Rate (%)
**->**1% descent rate = 60ft/NM - Rate (ft/NM) ≈ 100 x Rate (°)
**->**1° descent angle = 100ft/NM

### Rule of 3

**In aviation, pilots adopted a formula to assure a slow, based on a 3° descent rate, steady and comfortable descent: the rule of three or "3:1 rule of descent".**

The "rule of 3" is used by pilots flying small aircraft as well as by those flying airliners.

## Top of descent (TOD) calculation

### Top of descent calculation (for light VFR)

Using this rate in feet/min, you can easily calculate the top of descent time.

__Example:__

An aircraft is at 3500ft and I need to join the aerodrome circuit at 1500ft. Aircraft needs to loose 2000ft and will use -500ft/min vertical speed. Time before arrival = 2000/500 = 4 min. The pilot now knows that he must start his descent 4min before joining the circuit.

(In order to make a quick calculation, round your altitude to the nearest 500ft. (example: for 3300ft, you will take 3500ft))If the pilot wants the distance calculation, we can use this basic calculation

**->**Factor_inv (NM/min) = TAS/60 Distance = Factor_inv x Time_before_arrival

__Example:__

The pilot knows that he must start his descent 4min before joining the circuit, his altitude is 3500ft and speed 120kt Distance = (120/60) x 4 = 8 NM

The pilot shall start about 8NM from the circuit or destination airfield.### Distance based top of descent calculation

A more complex calculation can be used for VFR flight using aircraft which can fly at an altitude over FL100 and/or the speed is greater than 150kt.

__Example:__

We are flying at 5500ft
We need to descent to next point at 1500ft at standard rate **3°**.

**D (NM) = (5500-1500)/100 / 3° = 40/3 = 13.3 NM**

## Descent rate calculation

If you want to descent using a **3°** descent path and you expect a vertical speed in feet per minute to set the value on your autopilot panel, you must use the calculation below using the ground speed.

__Example:__

Ground speed is 110kt, and descent rate is standard 5%. Note that the descent rate value can be approximated using only 5% instead of 5.2% to ease the calculation. Vertical speed = 5 x 110 = 550ft/min,

(if we use 5.2% instead, vertical speed will be 572 ft/min**->**less than 5% error).

But this formula has some restrictions:

- Ground speed in some aircraft is not known because the wind is not known
- Ground speed can change during descent if this speed is not maintained constant.

## See also

- None

## Reference

- None

## Author

- VID 150259 - Creation
- VID 450012 - Wiki Integration

## DATE OF SUBMISSION

- 00:52, 14 May 2021

## COPYRIGHT

- This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.

## DISCLAIMER

- The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.