Plane & Pilot
Monday, January 1, 2007

Box Canyon Hazards


Beyond mountains, airspace restrictions & tall buildings can also define tight spots


The visibility isn’t the best going up the mountain pass. On the far side lies better weather and home. Behind are a tent, camp, cold and wet weather, and insufficient gas to go elsewhere. The pilot continues deeper into the pass, hoping conditions will improve. The ceiling is steady, but the terrain is rising. They’re headed south, and winds are westerly at 20 knots, with gusts. The pilot hugs the right side of the pass for traffic.

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Box Canyon Hazards

The visibility isn’t the best going up the mountain pass. On the far side lies better weather and home. Behind are a tent, camp, cold and wet weather, and insufficient gas to go elsewhere. The pilot continues deeper into the pass, hoping conditions will improve. The ceiling is steady, but the terrain is rising. They’re headed south, and winds are westerly at 20 knots, with gusts. The pilot hugs the right side of the pass for traffic.

Suddenly, clouds obscure the rising terrain, and it’s obvious he isn’t going to make it through the pass. It’s time to turn around, but the opposite canyon wall looks awfully close. The aircraft’s vertical fin is already in the clouds, and the surrounding terrain is much higher—climbing isn’t an option. Neither is a descent. From cruise configuration, the pilot initiates a hard left turn, banking 45 degrees in an imitation of a check ride aced years ago. Unfortunately, the aircraft has just turned into a tailwind.

Two days later, searchers find the remains of the aircraft near the top of the pass. The wreckage pattern leads downhill, on a northerly heading. The NTSB accident database is littered with stories of pilots who failed to turn around in the space available to them.

Years ago, I was introduced to the de Havilland Beaver by Jack Corey. I remember most of the information conveyed to me during the checkout, but two topics stand out. The first is a flight regime that has destroyed many de Havilland aircraft: operation in the region of reverse command, or flying on the back side of the power curve. The other lesson, repeated until it was second nature, involved turning the Beaver around in a tight spot. With Corey growling at me from the right seat, I turned again and again in airspace I would have thought only a helicopter could work in. Years later, the lessons learned that day likely saved my life and the life of my passenger.

It’s important to be aware that not all box canyons are found in the mountains. High-rise buildings and metropolitan areas may rise above a VFR flight corridor, such as in New York City’s East River. Airspace restrictions may also create a virtual box as well—in those cases, I’d rather maneuver safely and risk facing an entire team of FAA lawyers.

The weather doesn’t have to be bad for things to go awry—many incidents occur on sight-seeing flights in VFR conditions. Either way, know any canyon very well before venturing into it. You can fly above the canyon to discern whether there are any new obstructions, such as wires or towers that you’re unfamiliar with.

No matter what the scenario or aircraft, there are several key factors that will help you turn around in minimum airspace.

Before The Turn
First and foremost, slow down before you get into a tight spot. Because airspeed and bank angle dictate the radius of a turn, slower speeds and/or steeper bank angles will result in a tighter turn. Many pilots wait until they’re actually starting the turn to slow down—that’s too late. What speed should you target? I use 1.3 Vso initially. Practicing turns with slight variations in speed helps find the best speed for your airplane. Don’t forget that stall speed varies with weight, and adjust accordingly.

Configure the airplane for the turn before you initiate the turn. This will vary from aircraft to aircraft, but look for the configuration that offers the best tradeoff between lift and drag. Most airplanes will warrant a flap setting at about half deflection, but some aircraft turn tighter with full flaps, so practice at altitude until you find the best configuration for your aircraft.

Wind direction is the most important consideration in determining which side of the canyon to hug while proceeding up canyon. If you’re flying south with a westerly wind, as described in the scenario above, starting the turn from the east side of the canyon provides a headwind as you turn across the canyon. If there’s a lot of wind, there may be downdrafts on the west side of the canyon. But remember, the radius of the turn is a function of speed over the ground. If you cross the canyon with a tailwind, your best effort may not be good enough.

Practice the procedure for minimum radius turns repeatedly at altitude so that the maneuver becomes second nature. When you’re looking at sheer rock walls through the windshield, you need to have confidence and competence in your technique. A GPS will help evaluate your turn radius during practice.

Everything described to this point must be done before you initiate that lifesaving turn. Slow down, configure, move to the wall that offers the best starting point, and practice. Preparation is the key to success.

During The Turn
Let’s revisit our scenario: Clouds immediately above—can’t go up. Rocks below—can’t go down. What’s the best strategy to get turned around?

Pose this question to a dozen pilots, and you’ll hear as many answers. Some advocate a chandelle—a climbing turn at the conclusion of which you should be within a couple knots of stall speed. In our scenario, we can’t climb and we don’t want to be so close to stall speed in the mountains and turbulence.

Others suggest a diving turn. But we’ve continued to descend as we’ve gotten deeper into this deal—to the point where we can no longer descend. Furthermore, a descent suggests more speed, and speed equates to a larger turn radius.

The technique I use has worked in the light aircraft I’ve flown, including that harridan of canyon turns—the Beaver.

Here’s the technique, as I’d perform it in a Cessna 172:

Slow down and configure the airplane before you get to the tight spot: 70 mph and flaps set to 20 degrees. Depending on the operating weight, 70 knots is a little over 1.3 Vso.

When the airplane is trimmed, roll smoothly into a steep, coordinated turn. This doesn’t have to be a maximum-rate roll—steady and smooth works here.

As you pass 30 degrees of bank, apply full power, and up-elevator to initiate the turn. Continue the roll to 50 degrees of bank. With practice, you’ll find a pitch attitude (generally a little higher than cruise attitude) that will maintain altitude. The idea here is to turn with minimum radius, while holding altitude. Keep pulling hard as the airplane turns, and at the 180-degree point perform a smooth rollout and power reduction.

The airplane should come around as if on rails. If it buffets a little in the turn, back off the pull just a tad. With full power, the airplane will tolerate a lot before it stalls. Practice at altitude to perfect the technique and to determine how much pull it takes. And remember, in actual practice, this is a last-ditch lifesaving maneuver. Done well, the airplane will finish at the same altitude that you entered the turn. Practice the maneuver until you nail the altitude every time.

All aircraft—from basic trainers to taildraggers to high-performance models—can get into trouble with box canyons. With each aircraft, the flap setting and target airspeed may be different, but the basic technique is the same.

So, remember, practice turning around till you have the technique down pat. Should you anticipate a tight spot ahead, slow the airplane down and configure it for a turn. Most times, you’ll get through the pass just fine. Flying slow with flaps will take a little longer, but should the space close in on you, reduced airspeed and bank will get you out of there in one piece.

Virtual Box Canyons

Mountain walls aren’t required to form a box canyon, as was evidenced by the October 2006 accident involving New York Yankees pitcher Cory Lidle and his flight instructor, Tyler Stanger. Sightseeing over New York’s East River, the two pilots flew within a narrow VFR corridor, surrounded by LaGuardia Airport’s Class B airspace. These limitations defined an invisible, but potentially hazardous, box canyon.

Within confined areas, it’s always preferable to make any necessary turns into the wind. At the time of Lidle and Stanger’s flight, winds were recorded at 095 degrees at 13 knots. As reported by the NTSB, radar data showed their aircraft entering a 180-degree turn to the west—downwind—to avoid bravo airspace. The easterly tailwind would have reduced the airspace available for turning by 400 feet. Prior to the turn, the aircraft had been positioned approximately mid-river, and as such couldn’t take advantage of the entire width of the corridor, further reducing maneuvering space.

NTSB reports suggest that if the aircraft hadn’t banked steeply at 53 degrees upon commencing the turn, even greater bank angles would have been required throughout the turn, making a stall a possibility. All of these factors may have contributed to the aircraft’s crash into a Manhattan high-rise building.

Radar tracks of the aircraft’s path over New York’s East River can be viewed at http://ntsb.gov/Pressrel/2006/ N929CD_final_turn_3radars.pdf.

 





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