Aviation Rate of Descent Calculator
How to Calculate Rate of Descent in Aviation
Calculating the correct Rate of Descent (ROD) is a fundamental skill for pilots, whether flying a Cessna 172 or a Boeing 737. It ensures that an aircraft descends from its cruising altitude to a specific target altitude at a specific waypoint, maintaining a safe and stabilized approach profile. This calculation is critical for Top of Descent (TOD) planning, non-precision approaches, and complying with Air Traffic Control (ATC) crossing restrictions.
The Primary Formula
To determine the Vertical Speed (VS) required to reach a fix, you are essentially solving for rate over time. The basic physics involves three variables: the altitude you need to lose, the distance you have to lose it, and how fast you are traveling across the ground.
The calculation generally follows these steps:
- Determine Altitude to Lose: Subtract target altitude from current altitude.
- Calculate Time to Fix: Divide the distance (NM) by the Ground Speed (Knots), then multiply by 60 to get minutes.
- Calculate Required ROD: Divide the Altitude to Lose by the Time (minutes).
The 3-Degree Rule of Thumb
In many instrument approach scenarios (ILS or VNAV), a standard 3-degree descent angle is used. Pilots often use mental math to estimate the required ROD to maintain this slope:
- Formula:
Ground Speed ÷ 2 × 10ORGround Speed × 5. - Example: If your Ground Speed is 120 Knots, your ROD should be approximately 600 fpm (120 × 5).
This calculator provides both the exact geometric descent rate required to hit your specific fix and the standard 3-degree reference for comparison.
Descent Gradient (ft/NM)
Another useful metric is the descent gradient, which tells you how many feet you must descend for every nautical mile traveled forward. Standard instrument departures and arrivals often specify minimum gradients.
Formula: Altitude to Lose (ft) ÷ Distance (NM).
For example, to lose 1,000 feet in 3 NM, you need a gradient of 333 ft/NM. Knowing your ground speed allows you to convert this gradient into a vertical speed in feet per minute on your Variometer (VSI).
Practical Example
Imagine you are flying at 10,000 feet and ATC clears you to cross a waypoint 20 NM away at 4,000 feet. Your Ground Speed is 150 Knots.
- Altitude to Lose: 6,000 feet.
- Time to Waypoint: (20 / 150) * 60 = 8 minutes.
- Required ROD: 6,000 ft / 8 min = 750 fpm.
Using this calculator helps verify your Top of Descent point and ensures you don't arrive at the fix too high or too fast.