January 1, 2012
By Rod Machado
Reasonable people can disagree about how to fly an instrument approach, with both sides of the argument having some merit. This applies to a discussion I had with an experienced instrument flight instructor about how to descend to the minimum descent altitude (MDA) on a nonprecision instrument approach. We disagreed over two approaches to approaches. The constant airspeed technique (my recommendation) has a pilot making a descent to the MDA, leveling off, and flying to the missed approach point (MAP). If the pilot has the required visibility and identifiable runway environment, he descends and lands, but only after reaching the visual descent point (VDP).
The constant rate technique (the alternate recommendation) has a pilot flying a constant-rate descent in hopes of reaching the MDA at the precise point where it intersects the VDP. At that time and place, the pilot decides whether or not he has the required visibility and identifiable runway environment sufficient to land the airplane. If not, he executes a missed approach.
When I ask instructors why they support the constant-rate technique, they typically say that a quick, rapid descent directly to the MDA increases the chance of controlled flight into terrain (CFIT). They assume that a pilot shouldn’t be trusted to quickly descend to an altitude that’s relatively close to the ground.
So here’s the truth about CFIT accidents. They are rarely caused by pilots descending through the MDA and into the ground after leaving the final approach fix. Instead, most CFIT accidents are caused by pilots leaving the MDA prematurely after sighting the runway environment (often way before reaching the VDP). Perhaps influenced by approach illusions or attempting to duck under a cloud layer, some pilots end up flying an abnormally shallow glidepath, which can turn a perfectly good airplane into a dirt bike. CFIT accidents have dramatically decreased over the years, primarily because of the increased use of VDPs found on many instrument approach charts. If a pilot waits to descend from the MDA until reaching the VDP (as he should), he can expect to make a normal, obstruction-free descent to the runway’s touchdown zone.
The problem with using the constant-rate technique for nonprecision approaches, however, is that it doesn’t give you the best chance of landing under low-visibility conditions. Proponents of the constant-rate technique want you to treat your simultaneous arrival at the MDA and the VDP (which is seldom simultaneous) as a decision altitude, as if you’re flying an ILS approach. Since no electronic glideslope is involved, your ability to reach the MDA and VDP at the same time is only as accurate as the least predictable variable influencing your descent (which is typically the wind, thus your groundspeed). This assumes, of course, that you precisely control your airspeed and descent rate, too. In most instances, you’re as likely to arrive at the MDA beyond the VDP as you are to arrive at it prior to reaching the VDP. The former might place you at an excessively high altitude for landing. The latter might place you at an excessive distance from the threshold, making it more difficult to assess the in-flight visibility and runway environment.
Standalone nonprecision approaches seldom have an approach lighting system (ALS)—the ALS is associated with ILS approaches. These lighting systems extend 1,400 to 3,000 feet from the runway environment, making it easier for a pilot to evaluate his landing requirements on an ILS approach. The lighting typically associated with nonprecision approaches is runway lighting (typically MIRL, REIL, and some type of VASI), which is located on the landing side of the threshold. When making your decision about whether or not to land, the farther you are from the runway and its lighting, the more difficult that decision will be in low-visibility conditions.
The constant airspeed technique also keeps you above the MDA until reaching the VDP. Certain VASIs, however, are designed to be seen better at lower altitudes. For instance, with one mile visibility during the day in fog, the PAPI can be seen at a distance of 1.5 miles from the threshold at 480 feet agl. Under the same conditions, at 700 feet agl, you’ll need to be 0.9 mile from the runway threshold to identify the PAPI.
There’s little or no advantage to remaining higher on a nonprecision approach during low-visibility conditions. The advantage lies in getting down to the MDA quickly where you’ll have more time to watch for the runway lighting to appear. Of course, you should always wait until reaching the VDP before beginning your landing descent. Descending at 800 fpm at 90 to 100 knots should get you down to the MDA in plenty of time to help you identify the runway environment. Strangely, some instructors refer to this descent rate as “diving and driving” to the MDA, but it’s hard for me to imagine why, especially when you consider that the FAA’s maximum descent rate for an approach is 1,000 fpm when less than 1,000 feet agl.
Ultimately, nonprecision instrument approaches were originally designed to be flown by descending and tracking, which isn’t diving and driving. This is why the Air Force Instrument Flying Manual says that you should “arrive at the MDA with enough time and distance remaining to identify [the] runway environment and depart [the] MDA from a normal visual descent point...”
Rod Machado is a flight instructor who owns a Cessna 150. Visit the author’s blog. Rod Machado’s training books are available in iPad format.
Safety and Education,
Pilot Training and Certification
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