AOPA Pilot Magazine - January 2012 - 30

LICENSETOLEARN

BY ROD MACHADO

Getting down

easonable people can disagree about how to fly an instrument approach, with both sides of the argument having some merit. This applies to a

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 discussion I had with an experienced instrument the threshold, making it more difficult to assess the flight instructor about how to descend to the in-flight visibility and runway environment. Rod Machado’s minimum descent altitude (MDA) on a nonpreciStandalone nonprecision approaches seldom training books are sion instrument approach. We disagreed over two have an approach lighting system (ALS)—the ALS available in iPad approaches to approaches. The constant airspeed is associated with ILS approaches. These lighting format. technique (my recommendation) has a pilot making systems extend 1,400 to 3,000 feet from the runway a descent to the MDA, leveling off, and flying to the environment, making it easier for a pilot to evalumissed approach point (MAP). If the pilot has the required vis- ate his landing requirements on an ILS approach. The lighting ibility and identifiable runway environment, he descends and typically associated with nonprecision approaches is runway lands, but only after reaching the visual descent point (VDP). lighting (typically MIRL, REIL, and some type of VASI), which is The constant rate technique (the alternate recommendation) located on the landing side of the threshold. When making your has a pilot flying a constant-rate descent in hopes of reaching decision about whether or not to land, the farther you are from the MDA at the precise point where it intersects the VDP At the runway and its lighting, the more difficult that decision will . that time and place, the pilot decides whether or not he has the be in low-visibility conditions. required visibility and identifiable runway environment suffiThe constant airspeed technique also keeps you above . cient to land the airplane. If not, he executes a missed approach. the MDA until reaching the VDP Certain VASIs, however, are When I ask instructors why they support the constant-rate designed to be seen better at lower altitudes. For instance, with technique, they typically say that a quick, rapid descent directly one mile visibility during the day in fog, the PAPI can be seen at to the MDA increases the chance of controlled flight into terrain a distance of 1.5 miles from the threshold at 480 feet agl. Under (CFIT). They assume that a pilot shouldn’t be trusted to quickly the same conditions, at 700 feet agl, you’ll need to be 0.9 mile descend to an altitude that’s relatively close to the ground. from the runway threshold to identify the PAPI. So here’s the truth about CFIT accidents. They are rarely There’s little or no advantage to remaining higher on a caused by pilots descending through the MDA and into the nonprecision approach during low-visibility conditions. The ground after leaving the final approach fix. Instead, most CFIT advantage lies in getting down to the MDA quickly where you’ll accidents are caused by pilots leaving the MDA prematurely have more time to watch for the runway lighting to appear. Of after sighting the runway environment (often way before reach- course, you should always wait until reaching the VDP before ing the VDP). Perhaps influenced by approach illusions or beginning your landing descent. Descending at 800 fpm at attempting to duck under a cloud layer, some pilots end up fly- 90 to 100 knots should get you down to the MDA in plenty of ing an abnormally shallow glidepath, which can turn a perfectly time to help you identify the runway environment. Strangely, good airplane into a dirt bike. CFIT accidents have dramatically some instructors refer to this descent rate as “diving and drivdecreased over the years, primarily because of the increased ing” to the MDA, but it’s hard for me to imagine why, especially use of VDPs found on many instrument approach charts. If a when you consider that the FAA’s maximum descent rate for an pilot waits to descend from the MDA until reaching the VDP (as approach is 1,000 fpm when less than 1,000 feet agl. he should), he can expect to make a normal, obstruction-free Ultimately, nonprecision instrument approaches were origidescent to the runway’s touchdown zone. nally designed to be flown by descending and tracking, which The problem with using the constant-rate technique for isn’t diving and driving. This is why the Air Force Instrument nonprecision approaches, however, is that it doesn’t give you Flying Manual says that you should “arrive at the MDA with the best chance of landing under low-visibility conditions. Pro- enough time and distance remaining to identify [the] runponents of the constant-rate technique want you to treat your way environment and depart [the] MDA from a normal visual simultaneous arrival at the MDA and the VDP (which is seldom descent point….” simultaneous) as a decision altitude, as if you’re flying an ILS approach. Since no electronic glideslope is involved, your ability Visit the author’s blog (www.rodmachado.com). 30 • JANUARY 2012

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AOPA Pilot Magazine - January 2012

Table of Contents for the Digital Edition of AOPA Pilot Magazine - January 2012