By Ted Lee
A common problem we see with students practicing non-precision instrument approaches is difficulty leveling off at the MDA. Here is a simple technique to stabilize this critical step in the approach.
Both LNAV and LP (the GPS equivalent to a localizer-only approach) have an advisory glide path that, when captured, will take you right to the ground. However, unlike precision approaches, where you immediately initiate the missed when not visual at DA, with a non-precision approach, you have to level off and continue to the MAP (or MAWP) at MDA. This is where you can easily blow the approach on a flight test by not being able to hold MDA accurately. More importantly, if you were doing the approach in bad weather, it would be extremely valuable to have the aircraft hold MDA accurately while you try to pick up the visual cues needed to land.
The solution is to use VS mode for vertical guidance in the last 200 ft before MDA. In the past, there was no need to set the selected altitude to anything because you just rode the glide path and disengaged the AP at DA. With this new technique, you must set the MDA as the selected altitude to be used for level off. Then all you do is intercept and capture the GP using the APR mode at about 200 ft above MDA select VS mode. At that point, the AP will have stabilized at a particular rate of descent, and when you select VS mode, this rate will be continued, and in less than a minute, the AP will level you off at MDA. All you have to do is increase power to maintain airspeed, and then you can look forward for the runway. Even if your flight test examiner fails the AP, you will still have the flight director to help you.
About the Author
Ted Lee is a retired air force officer and flew the CF-104 and CF-18. He provides advanced instruction to multi-engine and instrument students at Genesis Flight College.