Box canyon (aviation)
In aviation, a box canyon, also referred to as a blind canyon, is a narrowing canyon with steep walls on either side of a floor that gradually increases in elevation. Box canyons often have narrow turning distances for aircraft and end in abrupt changes in elevations. Box canyons can also refer to limitations created by airspace restrictions or obstacles created by tall buildings in metropolitan areas. Flights into box canyons by pilots are often not intentional and due to pilot error. Box canyons can represent a risk of collision with terrain or stall.
Definition
A box canyon is generally characterized as a valley with a rising canyon floor and a narrowing canyon width. In mountainous regions, box canyons tend to end with abrupt changes in elevation or dead ends exceeding in elevation. The walls of box canyons can gradually narrow as the canyon approaches its end.In areas with airspace restrictions, such as New York City, a box canyon-equivalent can be formed by airspace restrictions. Airspace over the East River is available to flights under visual flight rules, but airspace to the north of Roosevelt Island is under class B airspace relating to LaGuardia International Airport. The corridor of VFR airspace functions as a box canyon for pilots that do not have the proper air traffic control clearances to enter the class B airspace.
Hazards
Lighting conditions below the canyon rim in box canyons can obscure surrounding terrain and cause pilots to believe that they are further away from terrain compared to their true separation. When flying inside box canyons, pilots can focus too much attention on high ridges ahead, causing them to neglect the slowing rising terrain of the canyon floor. This can cause a pilot to attempt a turn back at a point where both vertical and horizontal distance have decreased due to the narrowing canyon geography. During turns, the stall speed increases due to increased load factor. A turn back attempt may result in a stall if the aircraft's airspeed is not properly kept track of by the pilot.Box canyons can be present in mountainous regions at high altitude. Risks with flying at high altitudes such as hazardous winds, mountain waves, and poor visibility apply to flights within box canyons. Weather conditions in mountainous areas can result in pilots flying in conditions exceeding what they are capable of handling. Due to their presence in high elevations above mean sea level, density altitude can negatively affect aircraft performance when flying in box canyons. High density altitudes necessitates higher speeds to create the same lift experienced at lower altitudes, results in slower rates of climb, and reduced efficiency and thrust on propeller aircraft. Aircraft at high density altitudes have higher ground speeds, making turning out of a box canyon require a larger turning radius compared to an equivalent turn at sea level. Inexperience with mountain flying can lead to surprise in pilots regarding different aircraft performance.
Prevention
When flying in mountainous conditions, maintaining proper situational awareness can help avoid pilots accidentally flying into box canyons and risking a controlled flight into terrain. Flight training manuals from Transport Canada note that the need to turn out of a canyon is "likely due to poor decision-making." If a pilot needs to escape a box canyon, a canyon escape turn may be executed. A canyon escape turn involves a potentially descending 180° turn at low velocity with high g-forces. Despite the belief of many pilots, a chandelle is not the proper technique for reversing course out of a box canyon. The Federal Aviation Administration's Airplane Flying Handbook and Pilot's Handbook of Aeronautical Knowledge do not list a chandelle as the proper escape method and mountain flying experts do not recommend usage of such a maneuver. A chandelle does not give proper turning radius or altitude to be a maximum performance maneuver to reverse course.To execute a canyon escape turn, pilots maneuver the aircraft to minimize the turn radius by flying at a slower airspeed while increasing the bank angle and g-forces. Different values for airspeed and bank angle have been recommended. The Aircraft Owners and Pilots Association suggests that pilots should slow down to the flap extension speed, deploy the flaps, and turn with a bank angle of 45°. Transport Canada recommends to pilots in their flight training manual that the airspeed should be reduced to no less than maximum endurance speed and the turn should be conducted with a bank angle of 30°. Other sources recommend bank angles up to 60°, an airspeed 1.83 times the stall speed, or an airspeed for the best rate-of-climb. Civil Air Patrol recommends to pilots in their mountain flying training to keep airspeed at the designed maneuvering speed, increase the bank angle to 60°, extend the flaps to between 10–20°, and add power when necessary to keep the airspeed above the stall speed.
The equation for the turn radius in feet, R, in respect to airspeed in knots, V, and the bank angle is as follows:
While flying in canyons or valleys, pilots are instructed to not fly down the center of a box canyon and instead fly on either side. This is to ensure that pilots have space to conduct a 180° turn or a canyon escape turn. Civil Air Patrol recommends that pilots should fly on the upwind side of the canyon so turns will be into the wind. Federal Aviation Administration documents instruct pilots to not fly up box canyons without first flying down them to ensure there is proper space to turn around if necessary. They also instruct pilots to never attempt to out climb terrain due to reduced aircraft performance and to plan to fly above mountain passes. Recommended techniques for flying in mountains have pilots climb to an altitude suitable for crossing planned ridges before starting the turn to the ridge.