Canyon Turns: Between a rock and a hard place

This article originally appeared in Pilot Getaways magazine, Winter 2001/2002

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By adhering to a few basic procedures, canyon flying can be exciting, challenging, fun, and above all, safe.

Aviat Husky in the Sawtooth Mountains - photo copyright George Kounis

photo copyright George Kounis

Airspeed and Bank:

Slow down and use medium bank turns. Flying slowly in a canyon is critical to safe operations because it gives the pilot more time to react and is the best way to decrease turn radius while maneuvering. Pilots need to keep their heads out of the cockpit when flying in canyons and confined spaces, thus it is critical to know the correct power settings needed to achieve a desired airspeed, such as maneuvering speed (VA), or maximum flap extended speed (VFE). Flying in a narrow canyon is not the time to be distracted by hunting for power settings, so pilots should prepare by determining the correct "target" power settings and airspeeds ahead of time. The power required to maintain a given airspeed in level flight depends on weight and position of the center of gravity (CG); more power is required as the airplane gets heavier and the CG moves forward.

To determine the correct power settings, load the aircraft to the desired weight and CG position and go flying out in the open (not in a canyon). Maintain altitude while reducing power until the desired airspeed is obtained, and record the power required. Make some medium bank turns at your chosen airspeeds while holding altitude to determine whether additional power is needed in the turns. Keep in mind that the pitch attitude might also have to decrease slightly in a turn to maintain airspeed. By determining target power settings in advance, pilots can slow down to a desired airspeed smoothly and with confidence while maintaining situational awareness in a canyon.

At a given airspeed, the steeper the bank angle, the tighter the turn. Tighter turns are needed in confined areas, but steep turns in canyons can lead to many problems, including visual disorientation. In a canyon there may be no horizon to reference airplane attitude, so shallow to medium bank angles greatly decrease the risk of becoming disoriented. Another risk factor of steep turns is that bank angles beyond 30° rapidly increase the load factor and stall speed. For example, the load factor in a 30° bank is only 1.15 G with a 7.5% increase in stall speed. However, a 60° bank imparts a 2G load factor and a 41% increase in stall speed.

Stall speed vs bank angle

The best way to avoid disorientation and pulling G's that could stress the airframe and increase your stall speed is to limit bank angles to 30°.

By slowing down and using medium turns a pilot can maneuver or turn around in a relatively narrow space. But how much should the airplane be slowed down? For example, suppose a pilot is flying in a canyon and needs to turn around safely to avoid weather, hazards, or to circle over an airstrip to check out the conditions. For operations in canyons more than 3/4 mile wide, VA is generally a good target airspeed in most general aviation aircraft. However, in narrow canyons a good technique is to slow to VFE and extend flaps to stabilize the airplane. Reference the Pilots Operating Handbook to determine the flap setting recommended for a maximum performance takeoff. That flap setting generally gives the maximum lift to drag ratio for the wing, and is the best flap setting to use for slow flight in a canyon. For most general aviation airplanes, a power setting somewhere around the bottom of the green arc on the manifold pressure gauge will yield a speed between 70 and 90 knots at takeoff flap setting. Use of full flaps is not recommended as it creates a lot of drag and requires more power (power that may not be available at higher density altitudes).

Here's an example:
At a given bank angle, turn radius varies directly with the square of the airspeed. For an airplane with a cruise speed of 140 kts, in a 30° bank, turn radius will be

Turn radius.....=.._______(velocity in kts)2_______
............................11.26 x tangent (bank angle)

.....................=.._________ (140)2________
....................................11.26 tan 30°

.....................= 3000 feet
That is equal to a turn radius greater than half a mile, so it would require twice that distance, more than a mile, to execute a 180° turn at a cruise speed of 140 kts in a 30° bank.

In a narrow canyon, the pilot would have to make a steeper bank turn to execute the maneuver and be subjected to the aforementioned hazards of steep banks. However, at a little over half that speed, or 75kts, the new turn radius would be:

Turn radius
.....=.._____ (75)2______
............................11.26 tan 30°

.....................= 865 feet
Now the 180° turn can be executed comfortably and safely, while still maintaining a 30° medium bank, in about 1700 feet, or 1/3 of a mile.

Turn radius calculations are mathematically correct, but will vary depending on pilot technique (maintaining a constant bank angle and airspeed). Wind and varying groundspeed will affect turn radius with reference to the ground (or canyon wall), leading to the next rule:

Winds and Turbulence:

Fly in smooth air. When there is a lot of wind or turbulence in canyons, professional pilots are usually back at the hangar. Canyon winds can be unpredictable, and it is prudent to avoid flying in high winds or turbulence. Another, possibly not so intuitive, reason is that when flaps are lowered as suggested to slow and stabilize the airplane, the limit load factor of the wing may be drastically decreased. For example, an airplane that is normally certified for 3.8 positive G's may only be certificated for 2.4 G's with flaps down. Thus, slowing down and using flaps to maximize safety could compromise the structural integrity of the airframe in turbulence.

Position Relative to Terrain:

Remain in a position to turn toward lower terrain. This means flying along the side, not in the middle, of a canyon and being aware of the escape route at all times. In general, try and fly downstream (downhill) in canyons and follow drainages that have large rivers or streams in them; the canyons will be wider and gradients shallower, and the terrain less likely to rise more steeply than the climb performance of the aircraft. The "old and not so bold" bush pilots say, "always have a back door". By increasing your safety margin and options, these simple rules will help you to keep your hand on the doorknob.

Aviatt Husky near Redfish Lake - photo copyright George Kounis, 2000

photo copyright George Kounis

Author: Amy L. Hoover