Proficient Pilot

300 feet per mile

May 1, 2007

Aviation writer Barry Schiff retired from TWA in 1998.

When I was hired by Trans World Airlines in 1964, the airline's Jack Frye Training Center occupied all eight stories of an old brick building at 1307 Baltimore Avenue in Kansas City, Missouri.

Soakie's was a block away and a favorite watering hole for pilots in training to have lunch, most often a hot pastrami sandwich washed down with a mug of cold suds.

It was anticipated that when Elvin Gooch began teaching Electrical in monotone immediately after our return from Soakie's, eyes would glaze and heads would fall. (Gooch was even known by other ground instructors as the "Sandman.")

Those of us experienced in passing out knew to allow our heads to fall quietly onto a book instead of announcing our slumber with the distinctive sound made when a head hits the top of a wooden table. It amazed me that Gooch could continue lecturing about the Boeing 707's transformer-rectifiers throughout all of this without missing a beat (although I was not always sufficiently awake to observe). I recall once being awakened by a particularly loud crescendo of a snore that rose sharply above the sonorous ambiance created by others and broke through Gooch's level of tolerance.

"Wake up that man!" he once commanded to those who were still awake.

The anonymous reply came from the back of the class. "You put him to sleep; you wake him."

Thankfully, not all after-lunch classes were like that, but Electrical by Gooch was a great cure for insomnia. One of my favorite subjects was Flight Operations, and I had no trouble staying awake for that. It was there that I was introduced to the 300-foot-per-mile rule. The rule simply states that a conventional, 3-degree glideslope (normally the optimum vertical profile to use during a landing approach) descends 300 feet per nautical mile.

In other words, multiply your distance from touchdown by 300 feet to determine target altitudes while on final approach. For example, when three, seven, or 12 miles from touchdown, you should be 900 feet, 2,100 feet, and 3,600 feet height above touchdown, respectively. (A steeper 4-degree glideslope descends at 400 feet per mile, and so forth.)

This rule allows us to descend on an approximately 3-degree descent profile even when an electronic glideslope or visual approach slope indicator is unavailable. Although it can be used at any time, it is most valuable when executing a nonprecision instrument approach at night in low visibility and especially when over dark terrain. It helps to negate the effect of visual miscues and prevent premature descent from the minimum descent altitude (MDA), one of the riskiest aspects of such an approach.

As every instrument pilot knows, it is not permissible to vacate the MDA until other visual cues associated with the runway are in sight. The problem is determining the point at which it is safe to leave the MDA when such cues do become visible. For example, what if the pilot begins to see the approach lights when three miles from the runway while at an MDA of 400 feet? It would be hazardous to begin a descent at this point because the descent profile would be much too shallow (133 feet per mile), and the pilot would run the risk of premature ground contact (a euphemism for crashing).

The question then becomes, how does a pilot know when it is safe to leave the MDA? This is when the 300-foot-per-mile rule comes in handy. A normal descent from 400 feet should begin when one and one-third miles from the runway (determined by dividing 400 feet by 300 feet per mile), not when the lights might first become visible three miles from the airport.

Premature, shallow descents are unwise for two reasons. First, the regulations mandate that a pilot may not leave the MDA unless he is in a position from which to make a normal approach using a "normal rate of descent." A descent profile that is too steep or too shallow violates this regulation.

The second and most important reason for being patient about leaving the MDA is one not clearly understood by all pilots. Obstacle protection during an instrument approach is assured only while navigating a prescribed course at or above the minimum altitude shown. Once a pilot vacates the MDA, he assumes total responsibility for visually avoiding ground obstructions irrespective of how dark the night or how poor the visibility.

Many years ago, the beneficent FAA opted to show on instrument approach charts the exact location of the optimum point at which to leave the MDA, a point along the final approach course from which the descent profile would be about 3 degrees. Initiating descent before reaching such a visual descent point, or VDP, would obviously be risky, while doing so much after passing the VDP would require diving at the runway. We applauded the FAA for this. But then the program went aground almost as soon as it had begun. VDPs were established for some nonprecision approaches and not for others.

Now that virtually all airplanes used for instrument operations are equipped with GPS receivers, it is especially easy to establish your own VDP using the 300-foot-per-mile rule and the display of distance to the airport. You can even do this using a handheld receiver because it is being used for reference, not guidance, if you get my drift.

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