By BILL SCHROEDER, MCFI
The sky is clear, visibility is unlimited, and the early morning air is cool and calm. You decide to take that trip to the mountains that you have always wanted to do, but never seemed to have the time.
You will be taking off from your sea level airfield and flying to an airport located high in the Sierra Nevada mountain range. Earlier in the week you met with a flight instructor 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 as you have only flown in the mountains several times.
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. You obtain 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 25 knots. As you are planning a return well before afternoon you decide that the winds aloft will not be a factor. You file a flight plan and, after a thorough preflight, you and your passenger are ready to fly into the mountains.
You start the engine on your aircraft, a 160-hp Cessna 172, and receive clearance for taxi and takeoff. All is a go for what you believe will be a spectacular flight. After takeoff and climb to altitude necessary for crossing into the mountains, you notice a few “bumps” as you continue towards your destination. They are not severe, so you continue and land at the mountain airfield in just over two hours after starting the flight. You have performed well and both of you 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 knots. You contact the FBO for fuel and add enough to ensure that you will have ample fuel for the return flight, in addition to the required reserve as you know that the return flight will be headed west into the wind.
While obtaining fuel, you notice that the winds are increasing and that the windsock is showing a strong westerly wind well above 10 knots. It is time to go and you advise your passenger that you need to expedite the departure.
During the climb out, you notice that the turbulence has increased from just a few bumps to a steady pattern of up and down movements. Not wanting to alarm your passenger, you state that all will be better after crossing the summit of the Sierra Nevada and heading towards lower terrain.
As your 172 approaches the summit ridgeline, you feel much better as you can see over the ridge, indicating that the plane is higher than the ridge. It is still quite turbulent though, so you decide to climb even higher to ensure maximum clearance.
The plane is at full power and is properly leaned for the altitude as you pull 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. You pull back on the control wheel to stop the descent. The airspeed has decreased and the sink rate has increased to 800 fpm down. You continue to increase back pressure on the control wheel. You hear the stall warning horn come on as the airspeed continues to decrease and the altitude continues to decrease. It is now evident that you will not clear the ridgeline.
What should you do? How are you going to keep from descending into the mountain?
Does this story sound far fetched? Well, it isn’t, as accident reports reflect. What do you need to know about downdrafts and — more importantly — escaping downdrafts when flying in mountainous terrain?
There are some tried and true methods for crossing ridges or a series of ridges while heading into the wind. First of all, you should always approach a ridge at a 45° angle when within a quarter to a half mile from the ridge. It is dangerous to fly directly towards the lee side of a ridge when slightly above, level or below the ridgeline when the wind exceeds 10 knots.
Remember that a steady wind of 10 knots funneled through a mountain pass or any other type of uneven surface can greatly increase the speed of that wind. This is known as the “venturi effect.” Wind speeds can easily double and sometimes triple in that environment.
As you approach the ridge at a 45° angle, choose your crossing point and anticipated direction of turn, right or left, so that the turnaround path will be free of obstructions. This gives you the option of escaping toward lower terrain and reversing the course with only a 135° turn instead of a 180° turn.
Let’s assume that you have chosen a good escape route as you approach the ridge and, while attempting to cross the ridge at a 45° angle, you find yourself descending in a strong downdraft and it is doubtful that you can safely cross the ridge.
Immediately start the turn heading 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 your plane may still be descending, it is 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 will be encountered.
Maneuvering speed is used because it is the fastest speed possible that will prevent overstressing 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.
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.
Other downdrafts may 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. Be careful not to over-control, especially with elevator inputs. Fly an attitude and accept altitude loss. When the aircraft approaches lower terrain, the severe turbulence will usually subside, sometimes as suddenly as it began. At this point, the plane may still be unable to climb, as the air above is still moving downward, however, it may be possible to climb to a certain altitude where the rate of climb 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 and if the engine becomes stressed, let it be, as that is preferable to the stress caused by flying into a “cumulus granite cloud,” better defined as the side of the mountain.
Downwind of the downdraft you may find sloping terrain on the windward side that can be used to regain lost altitude. While there, 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, though, 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.
Please note that crossing ridges while flying in a downwind direction usually dictates flying directly at the ridge, rather than at the 45° 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.
Bill Schroeder was the first Master Certified Flight Instructor in Nevada and has been flying in the Lake Tahoe/Sierra Nevada region for more than 25 years. He is the chief check pilot for the Nevada Wing of the Civil Air Patrol and instructs the Nevada Wing “Mountain Fury” mountain flying training course. He also serves as the FAASTeam lead representative for the Western Pacific Region out of the Reno Flight Standards District Office and was awarded the FAASTeam Representative of the Year in 2011 for the Western Pacific Region. He gives private instruction in the Reno-Lake Tahoe area. For more information: FlightSafetyCounselor.com.