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DOWN
AND OUT by Bill Schroeder, MCFI It is dangerous to fly directly towards the lee side of a ridge The sky is clear and the visibility is unlimited in the cool early morning calm. You've been asked to do a "mountain flying checkout" from your sea level airfield to an airport located high in the Sierra Nevada range. Earlier in the week you met with the student to go over the route of the flight and discuss the particulars of flying over mountainous terrain and into and out of high elevation airfields. You know that by leaving early in the morning, the flight will be smooth over the mountains and that the approach and landing at the mountain airfield will be uneventful. The student obtains a weather briefing that indicates that later in the day, the winds aloft at your flight altitude will become westerly and increase to just over 25kts. You should be home by then. During climb out in your 160 hp Cessna 172, you notice a few "bumps" as you continue towards your destination. They're not severe, so you continue and land at the mountain airfield in just over two hours. Your student has performed well, and you both decide to rent a car and go to a nearby restaurant for an early lunch. Upon returning to the airfield you notice that the winds have increased and appear to be variable at about 5 kts. While obtaining fuel, you notice that the winds are increasing and that the windsock is showing a strong westerly wind well above 10 kts. It is time to go, and you advise your student to expedite the departure. During the climb out, the turbulence has increased from just a few "bumps" to a steady pattern of up and down movements. As your aircraft approaches the summit ridge line, you feel much better as you can see over the ridge indicating that the aircraft is higher than the ridge. It's still quite turbulent though, so you tell the student to climb higher to insure maximum clearance. With the aircraft at full power and leaned for the altitude, the student pulls back on the control wheel to increase the climb rate. Suddenly, you notice that the aircraft is losing altitude and the VSI is indicating 500 fpm down. The student also notices and continues to pull back on the control wheel. The airspeed has decreased and the sink rate has increased to 800 fpm down. The student continues to increase back pressure on the control wheel. You hear the stall warning horn as the airspeed and altitude continues to decrease. It's now evident that you won't clear the ridgeline. What should you do? What should you instruct your student to do? This scenario isn't as far-fetched as you might think - just check the accident reports. Fortunately, there are some tried and true methods for crossing ridges or a series of ridges while heading into the wind. All in the Angles First of all, you should always approach a ridge on the leeward side at a 45-degree angle when within one-quarter to one-half mile to the ridge. It is dangerous to fly directly towards the lee side of a ridge when slightly above, level, or below the ridge line when the wind exceeds 10 kts. Remember that a steady 10-knot wind funneled through the venturi of a mountain pass can greatly increase in speed. It can easily double and sometimes triple in that environment. As you approach the ridge at your 45-degree angle, choose your crossing point and anticipated direction of your escape turn, right or left, so that it will be free of obstructions. This gives you the option of escaping toward lowering terrain and reversing the course with a 135-degree turn instead of a 180-degree turn. If you need to escape, immediately turn away from the ridge. If you were not at full power as you approached the ridge, add full power and fly in a climb attitude at maneuvering speed, Va not Vx or Vy. As the aircraft may still be descending, it's most important to get as far away from the ridge as possible as quickly as possible. The further away from the ridge, the less downdraft and the less turbulence you'll encounter. Maneuvering speed is used because it is the fastest speed possible that will prevent over-stressing the aircraft in severe turbulence. Although transitioning to maneuvering speed may increase the rate of descent, the overall time that the aircraft is in the downdraft is shortened, resulting in less altitude loss. Most downdrafts produce a sudden and severe jolt, followed by a lull and then another bump or series of bumps. These jolts may come from all directions, both vertical and horizontal. At times the turbulence is severe enough to tip the aircraft up on one wing. It may be difficult to maintain control, so be careful not to over-control, especially with elevator inputs. Fly an attitude and accept altitude loss. Keep in mind that some downdrafts are smooth and you may not even realize that you are in one unless you are watching your vertical speed indicator. For this reason, always monitor your vertical speed indicator in the mountains. Typical downdrafts are 1,000 to 1,500 fpm, and on occasion may be much higher. When the aircraft approaches lower terrain, the severe turbulence will usually subside, sometimes as suddenly as it began. At this point, the aircraft may still be unable to climb, as the air above is still moving downward, however, it may be possible to hold an altitude where the rate of descent goes to zero. It may be necessary to fly towards a windward slope or some distance downwind from the ridge that is causing the downdraft before the aircraft can establish a positive rate of climb. If the situation is critical, use all available power. Lean again for maximum power. If the engine becomes stressed, let it be -- that is preferable to the stress caused by flying into the "cumulo-granite cloud." Downwind of the downdraft you may find sloping terrain that can be used to regain lost altitude. Climb back up to an altitude that will give you 2,000 to 3,000 feet of clearance above the ridgeline while you are still several miles from the ridge and then proceed back towards the ridge you were originally attempting to cross. Remember that in high wind conditions, a downdraft may be encountered even if you are 2,000 to 3,000 feet above the ridge. Expect downdrafts and turbulence when the winds are strong. Note that crossing ridges while flying in a downwind direction usually dictates flying directly at the ridge, rather than at the 45-degree angle. This depends on the altitude as the ridge is approached. A straight line here is usually the quickest way to put distance between the aircraft and the ridge. Most of the accidents caused by a pilot encountering a downdraft are due to the pilot's concern about altitude loss, rather than an escape away from the ridge that is causing the downdraft. Flying at a higher airspeed will place the aircraft farther away from the slope in less time. Your altitude loss may be as little as 100 feet per minute more while flying at Va rather than Vy during the time required to get out of the area of the downdraft. Even though the rate of descent is greater when flying at maneuvering speed, it will be less overall. Always go towards lower terrain. Then as the downdraft dissipates, climb back up to an altitude 2,000 to 3,000 feet above the ridgeline and then proceed across it, still with an eye out for the downdraft. You'll stay safe, and you'll never wonder what to do next.
© Copyright 2000 Bill Schroeder |