sábado, 5 de noviembre de 2016


Approach plates can be deceptive

Most instrument-rated pilots--and I'll bet a few instructors--have no idea that the missed approach point (MAP) shown on some approach charts can be distressingly far away from the real missed approach point. Further, the visibility requirements noted in the minima section may be wildly optimistic, as demonstrated the other day by one of my IFR students. "Whoa!" I exclaimed as my student began a sudden descent to the runway following a nonprecision approach to minimums. We were simulating a GPS approach where the MAP was at the runway end. I let him fly to within one-half mile of the approach end before raising his hood and waiting for what would happen next. As expected, the power came to idle, the flaps deployed, and the airplane pitched sharply downward as he attempted to land on the rapidly diminishing length of concrete below. After a go-around and landing, we retired to the classroom to chew on this.

Let's look at what is required to descend below MDA. According to FAR 91.175, we must have:

The required flight visibility; 
At least one item associated with the runway environment, as defined in FAR 91.175(c)(3); and 
A position from which we can make a normal descent and landing 

It is this last one that causes the problem. Not every instrument instructor teaches his or her students how to calculate the required descent and the need to establish a practical MAP based on the realities of the approach to be flown, rather than just blindly accepting the published MAP location on the chart. Both VFR and IFR pilots are guilty of not figuring out when they need to begin descent in order to avoid arriving in the terminal area too high and too fast. The problem is exacerbated by the fact that many high-performance airplanes will not easily go down and slow down simultaneously.

But when it comes to the most critical part of the approach, many IFR pilots just accept the location of the MAP as shown on the approach chart, especially when it is at the approach end of the runway. The pilot arrives at the minimum descent altitude (MDA), levels off, and begins to search for the airport. In low visibility, the runway often appears suddenly out of the murk and the pilot tries to descend and land without realizing he or she is too high to complete the landing without some sort of dramatic maneuver involving lots of flaps, chopping of power, pushing over the nose, and increasing speed. None of this is in keeping with the idea of a stabilized approach, and such antics certainly don't meet the "normal descent and landing" requirements of FAR 91.175. Resulting difficulties include hurried descents, rushed decisions, runway overruns, and missed approaches.

My student really never had a plan for the vertical part of the approach procedure, and I'll bet that the majority of other pilots don't, either. Shouldn't we be teaching that the MAP isn't always practical as printed, and that the visibility requirements may be just a wishful dream? This problem can exist on any nonprecision approach, but especially when the MAP is at the runway end. Factors to consider include the altitude between the final approach fix (FAF) and the MDA; the descent required to reach the runway; and the visibility, wind, and ceiling. In many GA aircraft, pilots fly approaches at 120 knots, which is two miles per minute. With a descent rate of 500 feet per minute, they lose approximately 250 feet per mile. If the approach requires a descent of 500 feet from the MDA to the runway, the pilot must see the landing environment at around two miles out to have any hope of completing the approach safely. Therefore, the practical MAP is two miles from the threshold, not at the runway end. This also means you need at least two miles of visibility to land, regardless of what the approach minimums call for.

The FAA is slowly improving many of the instrument approach procedures by specifying a visual descent point (VDP) at which the pilot should expect to see the runway and continue along a 3-degree glideslope to the threshold. (This point is marked with a bold "V" on the profile view on Jeppesen charts, and it must be observed if you have the capability of identifying the VDP.) By noting the distance remaining at the VDP, you can establish a realistic minimum visibility that should result in a safe and stable approach.

For a pilot making 120 knots on final, it's easy to figure out the real MAP by dividing the required altitude loss from the MDA to the touchdown zone by 300. The result is the distance in miles that you need to see the runway. This value may be somewhat conservative, but even if you adjust it for your particular rate of descent in feet per mile, it will still give you a way to avoid an ugly surprise and possible dangerous attempt to salvage a blown approach.

Ken Wittekiend, a CFII, co-owns Eagle Training Solutions in Burnet, Texas. He owns a Beech Bonanza and a Piper Super Cub, and with more than 8,100 hours in his logbook is still enthusiastically instructing full-time. He specializes in advanced IFR training and tailwheel proficiency.

CFI Roundtable slated for Expo

If you plan to attend AOPA Expo 2007 in Hartford, Connecticut, this October, please join us for AOPA Flight Training's annual CFI Roundtable. The meeting will be held Saturday, October 6, from 11 a.m. to noon in the Capital 1 room of the Marriott Hartford, which adjoins the convention center. Topics of discussion will include the sport pilot certificate, technically advanced aircraft, and AOPA's Project Pilot. For more information, e-mail Mike Collins.

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