A pilot is conducting an instrument approach to an airport without a control tower. The approach plate indicates a glidepath angle of 3.2 degrees. The aircraft's groundspeed is 120 knots. What is the approximate required descent rate (in feet per minute) to maintain this glidepath?
- A800 FPM
- B720 FPM
- C640 FPM
- D540 FPM
Show answer & explanationAnswer & explanation
Correct answer: C. 640 FPM
The formula for calculating the approximate descent rate to maintain a glidepath is: Groundspeed (knots) x 5 = Descent Rate for a 3-degree glidepath. For other glidepath angles, you can adjust this. A more precise formula is: Descent Rate = Groundspeed (knots) * 100 * tan(glidepath angle). Or, a common rule of thumb is 'Groundspeed x 5' for a 3-degree glidepath, then adjust proportionally. For 3.2 degrees: (120 knots / 60 minutes) * (3.2 degrees * 100 feet/NM) ≈ 640 FPM. More simply, for every degree of glidepath, it's roughly (Groundspeed * 100) / 60. So, for 3 degrees, 120 * 5 = 600 FPM. For 3.2 degrees, (120 knots * 3.2) * (100/60) = 640 FPM.
Why the other options are wrong
- A. Incorrect. This would be a descent rate for a significantly steeper glidepath or faster groundspeed.
- B. Incorrect. This would be a descent rate for a steeper glidepath or faster groundspeed.
- D. Incorrect. This would be a descent rate for a slightly shallower glidepath or slower groundspeed.
Descent Rate Calculation (Glidepath)
To maintain a specific glidepath angle during an instrument approach, the required descent rate (in feet per minute) can be calculated using the aircraft's groundspeed. A common rule of thumb for a 3-degree glidepath is Groundspeed x 5. For other angles, adjust proportionally or use a more precise formula.
- Descent Rate = Groundspeed (knots) x 5 (for 3-degree glidepath).
- Adjust proportionally for other glidepath angles.
- Formula: Groundspeed (knots) * 100 * tan(glidepath angle).
Memory trick: Groundspeed Times Five, Then Adjust for Your Dive.