Circling With Precision

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This article originally appeared in IFR magazine, Oct. 2005.

Airmanship

Circling at your home airport isn't that tough. Familiar landmarks help establish proper lateral spacing from the runway. Because you know the landmarks well, you can pick them out from circling altitudes and in low visibility. Circling to an unfamiliar field in challenging conditions is another story. I was flying the C-130 to a remote radar station known as Tin City on the Alaskan Seward Peninsula. Visibility was two miles in blowing snow with winds gusting to 20 knots off the Bering Strait. After a tight procedure turn, to avoid Russian airspace, we descended on the NDB signal. At MDA we could see the cliff with the orange panel reminding us not to land short, but little else. We couldn't distinguish any local landmarks to circle to the opposite runway. The sky was white with blowing snow. The runway was white except for the panels that marked the touchdown zone. Rooftops were white and, worse yet, the hills lining the runway on three sides were white and blended into the sky. Without a system to get in the correct position, landing would have required a lot of luck or a crash response team.

That's Not Right

Low circling altitudes and odd arrival angles make for deceptive distances. Poor visibility compounds the problem. A calculated turn puts you in the right spot without the guesswork.

Once the airport comes into view, the TLAR method -- "That Looks About Right" -- is a tempting way to establish a downwind. The hazard of TLAR is that low visibility or darkness makes the runway appear farther away. As a result, it's common to choose a downwind too close for a normal base turn. This is made worse by the typical lower-than-pattern circling altitude. We're used to the view from a 1000-foot AGL, VFR traffic pattern. Placing the runway in the same relative position in the window at circling altitudes can cut lateral spacing from the runway in half. The setup is now tight spacing and overshooting the turn to final. The trick is finding an accurate means of placing the airplane one turn diameter away from the runway and then compensating for any crosswind lurking at MDA.

The Overfly Method

Consider an approach course perpendicular to the runway, such as the GPS-A at Waseca, Minn., (KACQ). The approach is a 223 heading, almost 90-degrees off for a landing on Runway 15. The simple spacing solution is to turn and overfly the runway in the landing direction, turn 180 degrees to downwind, and then turn 180 degrees to final. The beauty of this maneuver is its simplicity. The first 180-degree turn puts you exactly one turn diameter away from the runway on downwind. A base turn at the same bank angle and airspeed as the first turn aligns you with the runway for landing. That is, if there is calm wind. When winds blow you toward the runway, evaluate the drift when first heading upwind and then correct accordingly on downwind. If more than 30 degrees of bank are required to avoid overshooting final, a go-around or continued circling at/above MDA for another try may be wise. For winds that blow you away from the runway, adjust your spacing on the downwind or use a shallower bank turning to final. Adjustments to roll out on the runway centerline should be made as early as possible in the turn to final. If necessary, fly a base leg for a few seconds and then complete the turn. Circling to a landing in the opposite direction -- such as I was faced with at Tin City -- can be done with 90-degree turn to crosswind, followed by a 90-degree turn back to the downwind. This s-turn puts you on downwind one turn diameter from the runway, but such maneuvers, unless well-practiced, can be disorienting.

Known-Radius Circles

A better way of establishing one-turn-diameter spacing from the runway is by calculating turn radius for your typical circling true airspeed and bank angle. When your radius of turn is known, you can skip the step of overflying the runway by just displacing yourself the correct distance from the runway, flying downwind to just past the numbers, and then making a 180-degree turn to final. You can calculate your no-wind radius of turn ...
turn radius = (KTAS ^2) / (11.26 * TAN (angle of bank))
... or approximate it from a chart. For a preferred bank angle, I like 20 degrees. It gives leeway to safely steepen or lessen the bank during the last 90 degrees of turn to final. A turn at 20 degrees of bank and 100 knots has a radius of 2440 feet or a diameter of 4880 feet. To use this number when circling to an opposite runway, arrive at the MAP and turn either 30 degrees or 45 degrees from runway heading. Fly until you are one-turn-diameter laterally -- in this case 4880 feet -- from the runway. Now fly downwind until abeam the numbers and make your 20-degree-bank turn to final.
For approaches perpendicular to the runway (the aircraft at right), cross the runway and fly one turn radius away. Then turn 90 degrees. For approaches from the opposite runway (the aircraft at bottom), turn 45 or 30 degrees and fly 1.5 or two times the turn diameter. Fly a short downwind if needed to get past abeam the numbers and make a 180-degree turn to final. Adjust your bank angle in the latter part of the turn to roll out on course.
How do you know when you are 4880 feet laterally from the runway? Use GPS (see "Lateral Displacement and GPS" above right) or timing. For 30 degrees, fly a distance twice your turn diameter; in this case, 9760 feet (1.6 miles). At 100 knots, that's 58 seconds. With runways less than 5000 feet, a better technique is to use a 45-degree turn and fly 1.5 times the turn diameter. Here that's 7320 feet or 44 seconds. (Technically the multiplier is 1.4, but the difference is usually less than two seconds.) If you misplace your calculator, there's a rule of thumb that says to turn 30-degrees for 60 seconds or 45-degrees for 45 seconds. That's pretty close for 100 knots and 20 degrees of bank. For approaches perpendicular to the landing runway, cross the runway and fly straight for the distance of one turn radius -- 2440 feet, or roughly 15 seconds in our example -- and then turn 90 degrees to the downwind to add another turn radius. You're now one turn diameter from the runway and in a good position to turn and land. The key note with this procedure is that it uses true airspeed, not indicated. In Florida, flying 100 knots on the airspeed indicator may be a diameter of 4880 feet, but in Telluride, Colo., (9078 MSL), your true airspeed is 118 knots and the diameter jumps to 6800 feet -- close to the limits of protected airspace for a Cat. A aircraft (1.3 nm). Practice these techniques in visual conditions and customize them for your airplane. Find an airspeed and bank-angle combination that gives you the turn diameter you want for your destination.

Making it Simpler

It would be great if you could easily fly a given radius every time, but variation with true airspeed makes that challenging. Estimating your true airspeed is simple and useful. You PFD fliers can just read it off the display. Another option is to use the radius chart in reverse to find a bank angle for a desired radius. If you want a 3000 foot radius at 100 knots (TAS) you must fly a bank angle of 16.5 degrees. In practice, holding just over 15 degrees through the turn should get you close enough. You'll find that a given airspeed, bank angle, and timing works well for most approaches. Again, watch out for larger radii due to higher true airspeeds at high-altitude airports.
Radius of turn varies with both bank angle and true airspeed. To find your radius of turn, follow the line up from your true airspeed to your desired bank angle and then move right to find the radius. Double this figure for a 180-degree turn diameter. A standard-rate turn can also be used. At 95 knots (true), a standard-rate turn has a radius of just over 3000 feet.
These calculations may seem like a lot of trouble, but their purpose is limiting the TLAR guesswork for the turn to final. Even with a perfect turn to final, remember that low visibility also causes the illusion that you're high and could lead to a low, dragged-in, approach. In Tin City's snowstorm, I cleared the cliff with the runway directly below. At 127 knots circling speed, I turned 45 degrees and timed for 51 seconds. With the tailwind, that put me slightly beyond the threshold, but well within the 1.7-mile, Category-C limit. At the end of 51 seconds, I began a level turn toward final, varying my bank slightly to roll out on final. According to procedure, descent was initiated when landing was assured. The headwind on final provided a short, but manageable final, with attention to the VVI (VSI) and radar altimeter.

Try it Yourself

My approach to Tin City wasn't the first time that I tried the 45-degree turn and timing technique. Circling in low-visibility, white-out conditions to a short-field landing isn't the time to try something new. Practice these techniques at a familiar field in good VFR. Try some math and cross-check your performance using the length of intersecting runways or other known ground references. Refine your techniques for your preferred bank angles and airspeeds. Watch how close you come to the limits of protected airspace. Practice in a variety of wind conditions and learn to compensate appropriately. Augmenting your visual circling with instrument techniques improves safety and confidence when faced with circling in low visibility conditions at an unfamiliar field.
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