Required vertical speed calculator
You have a crossing restriction ahead and an altitude to lose. This works out the rate of descent it would take, so you can compare it against what your aircraft will actually do.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The rate of descent is what you need to hold from now to meet the constraint. Compare it with a comfortable rate for your aircraft and configuration.
The gradient and angle are given as well, because a constraint that needs more than about 6 degrees is usually not achievable without speed brakes.
When this is useful
- Arriving high on an approach or an arrival procedure with a crossing restriction.
- After a late descent clearance, to decide whether the restriction is still achievable or whether to say so.
- Planning a step down through a series of altitude constraints.
How it is calculated
The distance divided by groundspeed gives the time available. Dividing the altitude to lose by that time gives the required rate of descent.
Because groundspeed sets the time available, the same geometry needs a much higher rate of descent with a tailwind than into a headwind.
rate of descent = altitude to lose x groundspeed / (distance x 60)
Worked example
- Altitude to lose
- 8,000 ft
- Distance available
- 25 NM
- Groundspeed
- 150 kt
You need 800 ft per minute, which is a 320 ft per NM gradient, or almost exactly 3 degrees.
Check: 25 NM at 150 kt takes 10 minutes. 8,000 ft in 10 minutes is 800 ft per minute. Separately, 8,000 ft over 25 NM is 320 ft per NM, and the angle whose tangent is 320 divided by 6,076 is 3.01 degrees, confirming this is essentially a standard 3 degree path.
Limitations
- It assumes a constant groundspeed and a straight path. Slowing down or being vectored changes both.
- It tells you what is required, not what is possible. Whether your aircraft can achieve it at the current speed and configuration is a separate judgement.
- A high rate of descent close to the ground is a recognised approach and landing risk, whatever the geometry says.
Common mistakes
- Using the distance to the airfield rather than the distance to the point where the altitude must be reached.
- Using true airspeed instead of groundspeed, which understates the rate needed with a tailwind.
- Accepting a very high required rate rather than telling ATC that the restriction cannot be met.
Sources
- Instrument Procedures Handbook (FAA-H-8083-16), arrivals , US Federal Aviation Administration