Proficient Pilot

Going around in circles

November 1, 2007

Aviation writer Barry Schiff lives in Los Angeles, California.

It has been 13 months since New York Yankee hurler Cory Lidle and his flight instructor, Tyler Stanger, inadvertently flew their Cirrus Design SR20 into a Manhattan high-rise.

Even though the NTSB has issued a probable cause for this tragedy, it remains a hot and frequent topic of conversation. Such discussions occasionally lead to this question: What is the best way to make a 180-degree turn in tight quarters?

Most pilots know that turn radius during coordinated flight at a constant altitude is determined by bank angle and true airspeed.

A classic example of how airspeed affects turn performance is provided by using as an example what was the world's fastest airplane, the Lockheed SR-71 Blackbird. If its pilot were to roll into a 30-degree-bank while sprinting at 2,000 knots, for example, turn rate would be only 0.3 degrees per second. A 180-degree turn would take 10 minutes and turn diameter would stretch from Dayton, Ohio, across Indiana to Chicago. That's what is meant by having to plan ahead. On the other hand, an Aeronca Champion at 50 knots in a 60-degree bank turns 38 degrees per second and has a turn diameter of only 256 feet.

Clearly, then, a minimum-radius turn results when an airplane is flown slowly and banked steeply. The trouble is that slow flight and large bank angles are incompatible because stall speed increases as bank angle steepens.

It can be shown that the minimum-radius turn occurs when the airplane is flown at its maneuvering speed and banked steeply enough to result in its limit load factor of (typically) 3.8 Gs. At such a time the airplane is on the verge of a stall.

A problem with this is that most pilots are reluctant to pull on the wheel as much as it takes to induce a 3.8-G load. We are uncomfortable with that much acceleration pressing us into our seats. Also, there is no way to determine in most airplanes when you have reached 3.8 Gs, although a pilot might consider that this load results when turning with a 75-degree bank angle.

A pet peeve of mine is that nonaerobatic airplanes do not have G meters, something that can be added inexpensively, requires very little panel space, and does not require a power source. How can a pilot be expected to abide by limit-load factors without such a gauge? He can't. (You will likely notice when observing a G meter that we usually overestimate given G loads, especially in turbulence.)

When executing a 75-degree banked turn, the airplane effectively weighs 3.8 times as much as it does in 1-G flight. The angle of attack must be quite large (to develop the needed lift), drag rises dramatically, and substantial power must be added to maintain airspeed. The trouble is that most lightplanes do not have sufficient power to prevent airspeed decay in such a turn, which results in a stall.

The typical engine propels an airplane rapidly at small angles of attack or slowly at large angles of attack. It rarely is powerful enough to do both, that is, to maintain relatively high speed at large angles of attack.

Lack of sufficient horsepower might have affected the Lidle flight. The SR20 has only 200 horsepower; the SR22 has 310. This additional 110 horsepower might have made a difference, although this is speculative.

What about slowing the aircraft to decrease turn radius and deploying the flaps to reduce stall speed? This is not a viable option. Most airplanes have a limit load factor of only 2 Gs with flaps extended, and flaps do not lower stall speed significantly.

A factor often not considered when turn diameter must be minimized is wind, which has more effect than is generally appreciated. Every knot of wind displaces an airplane 100 feet per minute in the direction toward which the wind is blowing.

In the case of the Lidle accident, it was estimated that the wind at the altitude of the SR20 was easterly at 13 knots. If correct, this means that the airplane was drifting 1,300 feet per minute to the west. If the airplane was being turned at the standard rate of 3 degrees per second, the airplane would have drifted 1,300 feet toward Manhattan during a 180-degree turn. During a double-rate standard turn at 6 degrees per second, the turn would take only 30 seconds and the aircraft would have drifted 650 feet to the west.

Turning downwind while attempting to minimize turn radius, therefore, is counterproductive. Turning into the wind, has the opposite effect and dramatically reduces turn radius. This is why pilots should fly on the downwind side of a valley or canyon and turn into the wind if a minimum-radius course reversal becomes necessary.

It also is a good idea to fly at least as fast as the maneuvering speed because you are bound to lose a bunch of speed during a steeply banked turn unless you have substantial horsepower under the cowling.

An aerobatic pilot with sufficient airspeed can execute an Immelman turn, which has the least horizontal turning radius (none!) of any course reversal. This is a half loop followed by a half roll. Some speculate that other maneuvers, such as a whifferdill, a hammerhead turn, or a wingover might have been more effective during Lidle's final turn, but nothing would have been as effective as a timely turn in the other direction.