At what altitude must you descend to from FL210 within 25NM maintaining a GS of 300 kts?

To determine the altitude you must descend to from Flight Level 210 (FL210) while maintaining a ground speed (GS) of 300 knots within a distance of 25 nautical miles (NM), you first need to calculate the appropriate descent rate.

The basic formula to estimate the descent rate in feet per minute (fpm) is:

[

\text{Descent Rate (fpm)} = \frac{\text{Distance (NM)} \times 1000}{\text{Ground Speed (knots)}} \times 60

]

Here, by substituting the values:

• Distance = 25 NM

• Ground Speed = 300 knots

This gives:

[

\text{Descent Rate} = \frac{25 \times 1000}{300} \times 60 = \frac{25000}{300} \times 60 \approx 5000 \text{ fpm}

]

Next, let's determine how long it will take to descend from FL210 to an altitude. Since FL210 is equivalent to 21,000 feet, we will calculate how far you can descend within the 25 NM distance.

Using the same formula for descent time (since speed = distance/time):