June 1, 2001
By Barry Schiff
Barry Schiff holds seven flight instructor ratings and has been instructing since 1956.
The recent tragedy involving a Grumman Gulfstream III in Aspen, Colorado, has generated speculation about the circumstances surrounding this accident. Although the official cause won't be available for months, a general discussion about the challenges and hazards associated with night circling approaches is both appropriate and timely.
A circling approach begins with a conventional instrument approach procedure that leads a pilot to a point in space from which he can establish visual contact with the airport.
In the case of Aspen, the sole instrument approach is the "VOR/DME or GPS-C." It requires tracking outbound from the Red Table Vortac (DBL) on the 164-degree radial and descending from 14,000 feet to the minimum descent altitude (MDA) of 10,200 feet until either reaching visual conditions or the missed approach point, which is 11 nm south of DBL.
The pilot must establish visual contact with the runway while at or above the MDA and at or prior to the missed approach point. (If visual conditions are not so established, a missed approach is required.) After reaching visual conditions — the circling minimums are shown on the approach chart — the pilot must circle the airport at or above the MDA until the aircraft is in a position from which a normal descent to the runway can be made.
A circling approach (instead of a straight-in approach) is usually required when the final approach course makes an angle of more than 30 degrees with the runway in use. Circling also is required when the final approach course is unusually steep because of obstacles near the airport.
Obstacles rule at Aspen. The instrument approach ends only 1.4 nm from the airport and at an MDA that is 2,385 feet above the runway. Circling is required because there is no way to safely lose so much altitude during a straight-in approach to Runway 15. A landing in the opposite direction obviously requires circling even if there were no obstacles present.
After the pilot sees the airport and determines that the minimum visibility exists, he must circle the airport and keep it in sight at all times. One danger associated with this maneuver is the temptation to descend below the MDA prior to being in a position from which a normal descent to landing can begin. It is critically important for the pilot to understand that obstacle clearance is assured only when the aircraft is at or above the MDA. As soon as he descends below this haven, he assumes all responsibility for avoiding obstructions.
"Aha," you say. "How can a pilot avoid high-rise terrain at night when such obstacles cannot be seen?" Good question, and that, my friends, is precisely the point. It is what makes night circling approaches so hazardous. If a pilot cannot see that which he must visually avoid, it can be argued with little effort that he should not be there in the first place.
If the terrain surrounding the airport is griddle-flat when circling at an MDA of 500 feet, for example, no problem. There probably is nothing to hit below the MDA. But because an MDA only assures an obstacle clearance of 300 feet while circling, a relatively high MDA indicates the presence of some truly tall and threatening hazards.
The MDA at Aspen, which is 2,385 feet agl, indicates that there are lofty obstacles within the circling plane of the approach. This is born out by an ominous note on the back of the Jeppesen approach plate that applies to VFR flying into Aspen but serves as fair warning to those who are inclined to make circling approaches there at night. It says, in part, "Terrain will not allow for normal traffic patterns." In other words, if mountainous terrain interferes with flying a normal VFR traffic pattern at Aspen, imagine the treachery of making a circling approach to a landing there in reduced visibility at night.
Although the instrument approach to Aspen no longer is allowed at night, there are hundreds of other approaches with less dramatic but equally lethal obstructions lying in wait for the unsuspecting pilot.
A misunderstood aspect of the circling approach involves descending from the MDA. First, a pilot may not descend below the MDA to duck beneath a cloud that might get in his way while circling. Instead of beginning a premature descent and sacrificing the obstacle protection provided by an MDA, the pilot should execute a missed approach.
Second, a pilot may circle the airport at the MDA as many times as he would like as long as he remains clear of clouds, the visibility remains at or above minimums, and the runway is in sight at all times. He may not descend from the MDA, however, until the aircraft is in a position from which descent to landing can be made at a normal rate of descent using normal maneuvering. In other words, diving, flat descent angles, and steep turns are not allowed.
Although we can practice instrument approaches under the hood and in the simulator to our heart's content, it is impossible to practice the night circling approach without actually doing it for real with hilly terrain probing for the belly of your aircraft. Y'all be careful out there.
Visit the author's Web site ( www.barryschiff.com).
Safety and Education,
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