A pilot is conducting a nonprecision approach to an airport without an operating control tower. The final approach segment begins at the FAF, 5 nautical miles from the runway threshold. The MDA is 800 feet AGL. The aircraft's groundspeed is 100 knots. What is the approximate minimum time required from the FAF to the MDA, assuming a descent rate of 500 feet per minute?
- A1 minute 48 seconds
- B2 minutes 12 seconds
- C2 minutes 48 seconds
- D1 minute 36 seconds
Show answer & explanationAnswer & explanation
Correct answer: D. 1 minute 36 seconds
First, determine the altitude to lose. Assuming the FAF crossing altitude is at or above the MDA plus a buffer, and the question asks for time *to* the MDA, we'll use the descent from the FAF to the MDA. Let's assume a typical FAF altitude for a non-precision approach, for example, 2000 feet AGL for calculation purposes (since no FAF altitude is given, we'll calculate descent time to 800 feet AGL, assuming descent starts from above 800 feet AGL, and then calculate time to travel 5 NM at 100 knots). The question asks for minimum time *from the FAF to the MDA*. This implies it refers to the flight path from the FAF to the earliest point where MDA can be reached. The descent rate of 500 FPM is given. The distance from FAF to threshold is 5 NM. At 100 knots groundspeed, the time to cover 5 NM is (5 NM / 100 knots) * 60 minutes/hour = 3 minutes. The question is tricky because it asks for the *minimum time required from the FAF to the MDA*. This is usually limited by either the descent rate or the horizontal distance/speed. A common rule of thumb for non-precision approaches is to calculate a descent gradient that ensures reaching the MDA prior to the Visual Descent Point (VDP) or Missed Approach Point (MAP). A standard 3-degree glidepath is approximately 300 feet per nautical mile. To descend 800 feet at 500 FPM, it would take 800/500 = 1.6 minutes (1 minute 36 seconds). During this 1.6 minutes, the aircraft would travel (100 knots / 60 minutes) * 1.6 minutes = 2.67 nautical miles. Since 2.67 NM is less than 5 NM (distance from FAF to threshold), the aircraft will reach MDA before the threshold. Thus, the limiting factor is the time to descend 800 feet. So, 1 minute 36 seconds.
Why the other options are wrong
- A. Incorrect. This calculation would result from a different descent or altitude.
- B. Incorrect. This would imply a slower descent or much more altitude to lose.
- C. Incorrect. This would imply a significantly slower descent or more altitude to lose.
Nonprecision Approach Time to MDA
For a nonprecision approach, the minimum time required from the Final Approach Fix (FAF) to the Minimum Descent Altitude (MDA) is determined by the altitude to lose and the descent rate, provided the horizontal distance covered during this descent does not exceed the available segment length before the Missed Approach Point (MAP) or VDP.
- Calculate altitude to lose from FAF to MDA.
- Divide altitude loss by descent rate to get time.
- Ensure calculated time/distance allows reaching MDA safely before MAP/VDP.
Memory trick: Altitude Loss and Descent Rate Dictate Your Time.