Box Canyon Turn
Mountain flying, like Mother Nature, can be harsh and unforgiving for the novice, and especially the complacent experienced pilot, who fails to adhere to the two basic premises for mountain flying. It is a simple matter to flirt with the mountains if you always remain in a position to be able to turn toward lowering terrain and then never fly beyond the point of no return.
The first law, being able to turn away from the terrain while having some extra altitude to descend, encompasses the idea that you never enter into a canyon if there is not sufficient room to turn around.
The second law requires the pilot to establish a turn-around point whenever flying upslope terrain. The point-of-no-return is defined as a point on the ground of rising terrain where the terrain out climbs the aircraft.
The turn-around point is determined as the position where, if the throttle is reduced to idle, the aircraft could be turned around during a glide without impacting the terrain. Obviously, if you exercise this option, the power is not reduced to idle. This is merely a gauge to judge and establish the point over the ground where an escape turn must be made. For most light aircraft this will be approximately 500-feet AGL.
See figure: Steep turn course reversal – simultaneously being a bank, add power and extend partial flaps.
Even the unconcerned aviator flitting through the mountains at cruise power and curise airspeed must cultivate a conditioned reflex to maintain a position allowing a turn to lowering terrain and guarding against flight beyond the point-of-no-return.
These axioms must be a conditioned reflex rather than instinct, because instinct may be wrong in an airplane.
BOX CANYON OPTIONS
Released from the confines of the traffic pattern, you might find yourself heading for the mountains to swoop over ridges and soar beside majestic peaks. Caught up in the thrill, you lose track of your position and suddenly find yourself in a box canyon requiring course reversal maneuvering to escape what might become a precarious position.
Pilots have, in all seriousness, asked my advice about performing a hammerhead turn as an emergency procedure for getting out of a tight spot. The hammerhead turn is an aerobatic maneuver. Definitely it is best to avoid this maneuver in a "tight" situation.
The wing over, another aerobatic maneuver, is more often performed while playing in the mountains rather than being used as an emergency escape maneuver. The hammerhead turn and wing over requires the pilot to preplan, allowing sufficient airspeed to transition to a climbing pitch attitude. It is quite likely when you find yourself in an emergency situation your airspeed is likely to be close to a stall. Neither option is desirable or available.
Until you have practiced the box canyon turn and understand the mechanics of and the ramifications of an unintentional stall close to the terrain, the safest and most commonly used method of course reversal for escaping rapidly rising terrain or the narrowing confines of a canyon is to make a steep turn at a slow speed, using flaps if prudent.
The stall speed of an airplane increases as the square root of the wing load factor. In a level, coordinated 60-degree bank, regardless of airspeed, the airplane experiences a 2-G load factor. The square root of 2 is 1.41, so there is a 41-percent increase in stall speed.
Most pilots could care less about a formula for determining the radius of turn. Since this is "Math 101," we aren't concerned with computations either; but we should realize that the radius of a turn is equal to the velocity in true airspeed (knots) squared, divided by a constant of 11.26 time the tangent of the bank angle in degrees.
This teaches us the radius of turn can be shortened by either reducing the true airspeed, or by increasing the angle of bank. The greatest benefit is from a combination of both slowing and banking.
The ratio of turn radius to an increase in airspeed at a constant bank varies as the square of the true airspeed. If the airplane doubles its speed, it will quadruple the distance traveled. So even if the airplane is going faster (twice as fast in this case), it still takes twice the amount of time to complete the turn around (four time further traveled).
What about using flaps during this steep turn? Use them as appropriate to the flight conditions. Flaps were invented to allow an airplane to increase its approach angle to a runway without an increase in approach airspeed. They work because lift and drag are directly proportional. If the lift is increased (by applying flaps to increase the camber of the wing), the drag is increased (and hence, no increase in airspeed during a steep approach).
For most airplane the addition of flaps, up to half the total available, provides more lift benefits than drag because the drag can be "subdued" with excess power. At a high-density altitude it may not be possible to use full flaps without intentionally losing altitude to maintain a safe airspeed. If a trade-off between altitude and airspeed cannot be made because of rapidly rising terrain, limit the use of the flaps to the extent that the airplane will maintain a constant altitude during the turn.
Remember too that flaps reduce the structural strength of the airplane. The POH (pilots operating handbook) may say the airplane is certified for 3.8 Gs (gravity units) and with flaps extended 2.2 Gs (a 42-percent reduction). Many of the normal category airplanes are stressed for 3.8 Gs. This is the limit-load factor that should not be exceeded during maneuvering flight.
Okay, some say, what about the ultimate load factor. You know, that 50-percent safety factor required by regulation to be built into the airplane? Shouldn't the airplane be capable of flying at 5.7 Gs? Unless, like Peter Pan, you can fly without the airplane's wings, do not exceed the limit load factor. For certification the airplane must be able to withstand the ultimate load factor for a period of fewer than 2 seconds without permanent deformation of the structure. Otherwise the airplane may experience structural failure.
It is imperative that the pilot know his attitude in order to safely perform flight in a canyon, more especially the box canyon turn. Attitude is the relationship of the airplane to the horizon. The natural horizon is "hidden" in the mountains. The natural horizon is used to teach flying by outside visual reference. The instructor demonstrates the nose position in relation to the horizon for level flight, the wing position in relation to the horizon for straight flight, and the nose position in relation to the horizon during steep turns left and right. Next the instructor demonstrates a climb attitude at the best rate-of-climb airspeed. The student mimics this attitude. The airplane can be returned to level flight and the airspeed indicator can be covered and the student, by learning the pitch attitude reference in relation to the horizon is able to fly at the best rate-of-climb airspeed within plus or minus one knot.
The natural horizon is easy to use in the flatlands as a reference for basic attitude flying. In the mountains the natural horizon may disappear. By visualizing a horizon, basic attitude flying can still be maintained. The base of the mountains, at least six to eight miles away, will represent the natural horizon. What if the airplane is closer than the six to eight miles? Visualization is still used. Perhaps the mountains in the distance are visible out the side window. Project the same horizon visually to the front of the airplane.
The Box Canyon Turn is required when the canyon narrows or the terrain out climbs the airplane.
The necessity of a box canyon turn is an admission that you have violated the basic premises of mountain flight.
The "Natural Horizon" is the base of the
about six to eight miles away. Photo: Twin Mountain
Great Bear Wilderness, Montana
For the pilot who adheres to the basic premises of mountain flying there is no need for the box canyon turn. For the pilot who does, either intentionally or inadvertently, violate the basic premises it is best to consider making a steep turn at the slowest speed consistent with safety. So, rather than using the box canyon turn to extricate yourself from situation caused by bad judgment, consider practice of a box canyon turn a training maneuver used to enhance your understanding of flying and to improve your flying skills.
The box canyon turn varies from the steep turn in that it is either performed from level flight at such a slow airspeed than an unintentional stall is imminent, or with some excess airspeed at the beginning of the maneuver the nose may be raised prior to or coincident with initiating the bank so the airspeed is slowed for a smaller radius of turn.
To explain the box canyon turn it is necessary to consider two scenarios.
In the first, the pilot is flying along at cruise power setting and cruise airspeed.
In the second case, the pilot is flying at minimum controllable airspeed. This minimum controllable airspeed in probably not an intentional flight condition.
We have learned the airplane always stalls at the same critical angle of attack. When banking the airplane, the stall speed increase as the square of the wing load factor. Whenever the airplane is banked in a coordinated turn, it is balancing the centripetal force (the horizontal lift component that causes the turn) and the centrifugal force (the force of the turn that causes you to be pushed down in the seat). The turn takes place because the centripetal force pulls the airplane towards the inside of the turn. Without a compensating increase in the amount of total lift during the turn, the airplane will lose altitude. The total lift is divided between a vector force that sustains the weight of the airplane and its contents and the portion of lift that is directed sideward to cause the turn. The centrifugal force acts towards the outside of the turn. To maintain level flight while turning it is necessary to increase back pressure (more lift equals an increase in angle of attack). This increases the load factor and stall speed. Some pilots get into trouble with the box canyon turn without realizing it because they have been "conditioned" to maintain level flight when performing steep turns.
If the predicament that requires the box canyon turn is a result of flying into a narrow canyon, and the airplane is not too close to the terrain vertically, it is possible to increase the bank to reduce the radius of turn. It will be necessary to lower the nose to increase or maintain the airspeed required for the increased bank angle. The airplane will lose altitude but the radius of turn is smaller with the steeper bank.
There is an experience in the mountains that affects most pilots. If you have ever flown over water beyond the power-off gliding distance from the shore, you may have noticed the oil pressure gauge begins ticking ... and it hasn't done that before. Next the engine may appear to give a little shudder of roughness. This might happen several times before you again approach the safety of the shoreline. A similar phenomenon occurs when flying upslope terrain in the mountains. Your left arm becomes shorter. This is a normal self-preservation aspect of the flight. You unconsciously pull away from the rising terrain and often the deterioration of airspeed goes unnoticed.
CRUISE FLIGHT – BOX CANYON TURN
Assuming again that the basic premises have been violated and the airplane must be turned around because of increasing terrain or the narrowing of a canyon, when operating at or near the cruise airspeed the radius of turn may be too great for the box canyon turn. Proper procedure would be to increase the pitch attitude above the horizon, maybe in the range of 5 to 20 degrees above the horizon, depending on airspeed. This does two things. It trades airspeed for altitude and it slows the airspeed for a smaller radius of turn. At the same time, full power is added, the aircraft is banked and flaps are used as required (providing the airspeed has reduced to the flap operating range). When the airspeed slows to the desired speed, relax the back pressure and level the pitch attitude.
It is easy to become disorientated when making a steep turn in a canyon. Try to limit the bank to 30 degrees, if possible. If you find yourself in more serious trouble than a 30-degree bank will cure, increase the bank angle using both instrument and outside visual reference for pitch attitude.
I hesitate to use more than half the flaps during a box canyon turn for two reasons. First, full flaps denigrate the performance of the airplane and it will lose altitude. Second, the difference in stall speed at a 45-degree angle bank with full flaps and half flaps is something like two knots. The advantage of using full flaps isn't there. Check your owner's manual or POH for the exact speed to satisfy yourself.
SLOW FLIGHT – BOX CANYON TURN
Making a box canyon turn at slow airspeed is usually a critical flight situation because the airspeed will probably be slower than Vy or Vx due to the "short-arm" effect. While applying full power and half flaps, establish the bank at 30 degrees, unless a greater bank is required for a smaller radius of turn. Pay attention to the pitch attitude and airspeed. If there is any sign of a stall, either a warning horn or light or buffeting, relax the back pressure to prevent an unintentional stall.
The box canyon turn is an emergency procedure. The best advice is to avoid a situation that might require a box canyon turn course reversal.