FAA Instrument Rating Airplane (IRA)IFR Procedures and ChartsMedium
A pilot is flying an RNAV (GPS) approach to Runway 27 at an airport. The approach chart shows a Final Approach Fix (FAF) at 5.0 DME from the airport reference point (ARP) and a Missed Approach Point (MAP) at 0.5 DME. The approach is designed for a descent gradient of 3.00 degrees. What is the approximate altitude lost per nautical mile for this approach?
- A425 feet per NM
- B250 feet per NM
- C380 feet per NM
- D318 feet per NM
Show answer & explanationAnswer & explanation
Correct answer: D. 318 feet per NM
To calculate the altitude lost per nautical mile for a 3-degree glideslope, multiply the angle by 100. So, 3 degrees * 100 = 300 feet per nautical mile. However, a more precise method involves multiplying the groundspeed (in knots) by 5 and dividing by 60, or using the rule of thumb that for every 1 NM, you descend approximately 318 feet per 3-degree glideslope. The answer option closest to this rule of thumb is 318 feet per NM.
Why the other options are wrong
- A. This value is excessively high for a standard 3-degree glideslope.
- B. This is a common miscalculation, often confusing descent rate with descent gradient.
- C. This value is significantly higher than the actual descent rate for a 3-degree glideslope.
3-Degree Glideslope Descent Rate
A standard glideslope for instrument approaches, typically 3 degrees, requires a specific rate of descent to maintain the proper vertical path to the runway.
- Standard glideslope angle is 3 degrees.
- Approximate descent rate is 300-320 feet per nautical mile.
- Descent rate in feet per minute is groundspeed (knots) * 5.
Memory trick: Three degrees means three hundred feet per mile, roughly.