FAA Instrument Flight Instructor (FII)IFR Procedures and ApproachesMedium
A pilot is conducting an instrument approach to an airport without an operating control tower. The approach plate specifies a final approach fix (FAF) 5 NM from the runway threshold and a published glide path angle of 3.0 degrees. If the aircraft's groundspeed is 120 knots, what descent rate (FPM) should the pilot maintain to stay on the glide path?
- A600 FPM
- B480 FPM
- C720 FPM
- D800 FPM
Show answer & explanationAnswer & explanation
Correct answer: A. 600 FPM
The rule of thumb for calculating approximate descent rate in feet per minute is to multiply the groundspeed (in knots) by 5 for a 3-degree glide path. In this case, 120 knots * 5 = 600 FPM. More precisely, the formula is (Groundspeed * Glidepath Angle * 100) / 60, or (Groundspeed * 5.3) for 3 degrees. Using 120 * 5.3 = 636 FPM. The rule of thumb (GS * 5) is commonly taught and sufficient for multiple choice questions where 600 FPM is an option.
Why the other options are wrong
- B. This would be for a lower groundspeed or shallower glide path.
- C. This would be for a higher groundspeed or steeper glide path.
- D. This value is too high for the given groundspeed and glide path.
Descent Rate Calculation (3° Glidepath)
To calculate the approximate descent rate (FPM) for a 3-degree glide path, multiply the groundspeed (knots) by 5.
- Formula: Groundspeed (knots) x 5 = FPM descent rate.
- Applies to standard 3-degree glide paths.
- Provides a quick mental calculation in the cockpit.
- More precise formula: (Groundspeed * Glidepath Angle * 100) / 60.
Memory trick: Groundspeed x 5, to descend just right.