A pilot is conducting an instrument approach to an airport without a control tower. The approach plate indicates that the final approach fix (FAF) for the RNAV (GPS) approach is 5.0 NM from the runway threshold. The published minimums for this approach are LNAV MDA 600 feet MSL. The airport elevation is 100 feet MSL. What is the approximate required descent rate (feet per minute) from the FAF to the MDA, assuming a ground speed of 90 knots?

Correct answer: C. 450 FPM

To calculate the required descent rate, first find the total descent required (MDA - airport elevation = 600 - 100 = 500 feet). Then, calculate the time from the FAF to the MDA at 90 knots (5 NM / 90 knots = 0.0556 hours = 3.33 minutes). Finally, divide the total descent by the time (500 feet / 3.33 minutes = ~150 FPM). This is incorrect. The calculation should be: (MDA - FAF alt) / (FAF to MAP time). Or, using the common formula: (Ground speed / 60) * (FAF to MDA altitude difference / FAF to MDA distance). Let's re-evaluate. A common rule of thumb is 300 feet per minute per 100 knots ground speed for a 3-degree glideslope. However, this is a non-precision approach. The problem asks for the descent rate from the FAF to the MDA. The distance from the FAF to the runway threshold is 5.0 NM. The altitude difference is 600 MSL (MDA) - 100 MSL (airport elevation) = 500 feet. This isn't quite right. The question is asking for the descent rate from the FAF to the MDA. It doesn't state the FAF altitude, but rather the MDA. So the descent is from the FAF to the MDA, over the distance from the FAF to the MDA (which is not necessarily 5 NM if the MDA is reached prior to the threshold). Given the options, and assuming a standard 3-degree glidepath for planning purposes, a common rule of thumb is 'ground speed in knots x 5 = descent rate in FPM'. So, 90 knots x 5 = 450 FPM. This rule of thumb is used to achieve a 3-degree descent angle. While the approach is non-precision, pilots often aim for a constant descent angle from the FAF to the MDA. The altitude difference from FAF to MDA is not given, but rather the MDA itself. The question implies a constant descent from FAF to MDA. If we assume the FAF is at an altitude that allows for a 3-degree glide slope to the MDA, then the rule of thumb (GS * 5) is the most appropriate method. This yields 450 FPM for 90 knots.