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SW Aviator Magazine
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3909 Central NE
Albuquerque, NM 87108
Phone: 505.256.7031
Fax: 505.256.3172 |
| publisher@swaviator.com |
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| Flying in the High Country |
| story by Fletcher Anderson, photos by Gerrit Paulsen |
They say bad luck runs in threes. Certainly it looked that way recently here when, in a three year period, we had three fatal crashes between my hometown of Telluride, Colorado, and the next closest airport in Montrose. The simple version of events was bad luck, all three aircraft were caught in downdrafts. They could not out-climb the deceptively easy looking terrain they were attempting to cross.
As a flight instructor at the second highest flight school in America, a local newspaper reporter asked me if bad luck like that had ever befallen me. "Oh yes," I replied, "it happened to me on four out of seven flights this very day." The reporter shook his head in disbelief and turned to someone with something more intelligent to say. No one's luck could be as bad as I claimed.
But, of course, mine is. Flying among the 14,000 foot peaks surrounding Telluride, I encounter a downdraft which exceeds my ability to climb about five or six days a week — which is about as often as I fly during the busy season. Everybody who flies around here shares that same experience. If it weren't for bad luck, as Woody Guthrie sang, we'd have no luck at all. But we seldom die from the experience.
To see why our luck is so bad, let’s start by getting out the operator's manual for a standard Cessna 172. (You can grab the manual for almost any small aircraft and get similar results.) Now let’s plan to load you, me, two relatively light friends, full tanks, and a credit card into our 172. This brings us up close to 2300 pounds, which is max gross weight. Turning to the Take Off Data page in the manual, we see that... Wow! The folks at Cessna only did the calculations for a maximum field elevation of 7500 feet MSL; 1600 feet lower than where I live! This means we are going to have to extrapolate. Here at Telluride (9080 feet MSL) the distance required to clear a 50 foot obstacle could be a mind-boggling 5,200 feet. Quite a change from the sea level ground roll of only 865 feet. This 50-foot obstacle clearance is particularly significant at Telluride because the end of the runway is in fact almost 100 feet higher than a dip at the center. Typically, takeoff distance at our airport is at least double, if not triple that of the same aircraft at sea level. But we can still get airborne, right?
Well, in winter we probably could, but remember that at 9,000 feet MSL standard temperature is only 28 degrees Fahrenheit. If the ice has melted off the runway, then density altitude is a bit higher. On an uncomfortably hot summer day of 80 degrees, the air density has thinned out to the equivalent of that at 12,200 feet. Could we get airborne now with a full load? I wouldn't bet on it. The data is way off the charts now, but my best extrapolation has us using a lot more than the 7000 feet of runway we actually have at Telluride Regional. It’s time to send two of the passengers home on the airlines, drain half the fuel out of the tanks, and wait until the cool of evening or early morning to depart.
Two factors are working against us at high altitude airports. First, because of the low air density, in order to get the same indicated airspeed we need to fly, the true airspeed must be about 20 percent higher than it is at sea level. Second, because there is less air for the engine to ingest, we are going to have to make do with only about 60 percent of the horsepower the same engine develops at sea level.
But this is an article about flying in the mountains, so let’s try a takeoff anyway…
…Well that takeoff was an experience I don't care to repeat, but here we are on climbout, doing so, we notice, at an excessively leisurely rate. Heading southeast out of Telluride towards northern New Mexico and the Rio Grande valley, we are immediately confronted with several 14,000 foot mountains right in our flight path. Head in nearly any direction and you are faced with similar circumstances — Colorado has 52 peaks over 14,000 feet. So, with just a hint of wind or everyday convective turbulence, we should climb to 15,000 feet to clear them safely. This is also a particularly convenient altitude for calculation purposes, because 15,000 feet is the top of the Cessna rate of climb table.
The rate of climb table confirms just how leisurely our climbout is. According to the table, at 15,000 feet with a full load we would gain a distinctly un-dramatic 22 — count them, twenty two — feet per minute. This scenario would generally only occur in winter, when the outside air temperature for a "standard temperature day" at 15,000 MSL is a cool five degrees Fahrenheit. When the sun heats things up to 20 degrees (still 12 degrees below freezing) our best rate of climb fully loaded is now DOWN at eight feet per minute. If you jettison the door, throw out all the luggage, kick out all your passengers for a final (but very scenic) free fall, and burn off half your fuel, the aircraft will climb better, but still at an anemic 280 feet per minute. On the hot days of midsummer, at any weight, even empty, the aircraft's best rate of climb just above these mountains is down.
For our flight, let’s assume we have only two people aboard and half a bag of gas, giving us the benefit of a theoretical rate of climb of 100 feet per minute on a warm day. Now it’s time to do a little math: there are 5280 feet in a mile. There are 60 minutes in an hour. That means one mile per hour is about 88 feet per minute. Now picture yourself standing on top of a mountain ridge, high up above the timberline. The air is calm, but not absolutely still; it takes a wet finger to be sure of the wind direction. That's close to a two knot wind, or 176 feet per minute. The almost-still morning air is wafting up the sunny side of the ridge at 176 feet per minute, and sinking off the shady back side at the same rate. On a steep enough hillside, the vertical component of the sinking air from a two knot wind has already exceeded our best rate of climb.
Of course a two-knot wind might not automatically exceed our ability to climb, but on a typical two-knot morning you can usually find somewhere where your best rate of climb is down. The Telluride flight school, Mountain Aviation Services, operates Cessna 172 trainers with the Air Plains 180 hp engine conversion, which will climb 300 to 400 feet per minute better than a stock 172. A Cessna 206 with 300 horses will do better yet. Here in the mountains, the better your climb performance, the greater your margin of safety.
Naturally, it would not be surprising to find wind over the tops of these 14,000 foot peaks blowing stronger than two knots. A call to Flight Service almost any day will reveal winds over 20 knots at that altitude. Considering that most of the earth's atmosphere and weather is under 18,000, that wind is going to have to squeeze over the tops of the peaks and through the passes. If the overall wind is 20 knots, terrain can make it twice that. This, in turn, could easily cause a localized downdraft greatly exceeding 1000 feet per minute.
Without even factoring in mountain wind phenomena such as convection, catabatic flow, mountain wave, transient departing rotor, Foehn, Chinook, etc., ordinary everyday winds flowing over the mountains regularly produce local downdrafts which exceed the ability of almost any single engine aircraft to climb. Does this mean you can't fly a standard Cessna 172 or any other light single engine aircraft in the mountains? Of course not. Lots of people do it every day with very acceptable safety margins. The trick is, you have to expect to loose altitude suddenly and unexpectedly, and you have to have an escape route ready.
How do we do plan an escape route? The single most important aspect is the constant, almost religious, assumption that you are about to get flushed. When you are constantly in the frame of mind that you are about to drop like a rock, you tend to focus more on your escape. The basic strategies are quite straightforward.
The most simple strategy is extra altitude. While the effects of mountain wave may be felt way above the altitudes we fly, the farther you are from a mountain, the less likely you are to hit it. A good rule of thumb is that the effects of rotor turbulence and downdrafts are generally not too strong above twice the ridge height (AGL). Another rule is to leave more than a thousand feet between you and the top of the ridge you are crossing.
The next strategy is to cross peaks, saddles, and ridges at an angle. A 45 degree angle to the crest of the ridge is a good rule of thumb. At, or maybe a bit before, the point where you would have to start turning if you were going to turn back, assume the worst and start the turn. Roll out on a new heading that will cross the ridge at a 45 degree angle. Then, if you can stay high enough to cross, great. If not, turning back is now only a 90 degree turn rather than a steep-banked 180 right in the midst of all the sink and turbulence.
Many mountain flying accidents occur when pilots elect to fly up a narrow valley hoping to cross the pass at the top. If you must fly up a narrow valley, always crowd up tight against one side. Not only do you need room to turn around, you need room to make that turn while flying through turbulence, while loosing altitude, and possibly while being blown way off your intended route. Narrow valleys are notorious collectors of sink and turbulence. How tight is tight against one side? Probably a great deal tighter than you think. The valley wall close beside you is not much of a hazard. If you lose altitude, you can just turn away from it. The tighter you glue yourself to the valley wall, the more room you have left on the other side to turn around.
Most of the time, one side of the valley is going to have relatively smooth lifting air; the other side turbulent sinking air. The sunny side of the valley may have convective lift while the shady side is sinking. When there is wind across the valley, the air can be flowing down one side into the valley and up and out the other. The biggest mistake you can make now is to attempt to read these conditions, pick the good side, and then rely on what you think is going to happen to get you over the pass. All to often you will be wrong. There are simply far too many factors at work for you to be aware of and analyze. Still, the wrong side of the valley is generally better than the middle.
What if you read the conditions wrong? If you are flying up the valley in sinking air, consider quickly crossing over to try the other side, if there is room. Better yet, as soon as there is the least hint of doubt in your mind, turn back!
Theories on how to accomplish a course reversal in a narrow valley abound. At least theoretically, the best of these is to do a chandelle, but a chandelle in turbulence is bringing you far too close to an unintended stall. Alternately, adding flaps will significantly tighten-up most aircraft's turning radius; I use this method regularly for air photo work in narrow valleys. However, I would not try it in an aircraft which has trouble holding altitude with the flaps down — and at high altitudes most of them do. Attempting either of these methods in turbulence can be much harder than in smooth air. Perhaps it is easiest and safest to just make a very steep banked climbing turn, keeping the airspeed around the best rate of climb. Turbulence can mean that you cannot use a steep bank angle, resulting in a much wider turn than anticipated. I would generally council against throttling back to turn tighter, preferring instead to keep the speed slow by climbing.
Lest anyone get the impression that all mountain flying is horrible downdrafts, I hasten to add that when the air is going down at prodigious rates in some spots, it is going up at the same rates nearby. You can't, I can't, and nobody else can correctly predict exactly what the air is doing everywhere every time in the mountains. There are simply far too many factors to analyze, and not all the facts are ever at your disposal. Couple these unpredictable up and downdrafts with the marginal high altitude climb performance of most light single engine aircraft, and you certainly have a recipe for bad luck.
The safe way to fly in the high mountains is to never count on the winds doing what you need, and to always leave yourself an escape route. If it looks like you might not clear the rocks in front of you, turn around. Now. Fly with the constant thought in your mind that you are just about to fly into serious sink and have to get out, fast. Always have an escape route, and always be on the verge of using it.
Fletcher Anderson is the operator of Mountain Aviation Services (970-728-1728), the second highest flight school in America, and Telluride Soaring, the world's highest glider operation. He has over 2000 hours experience flying small powered aircraft, 2000 hours flying paragliders, and a lot less times than he wants flying sailplanes, all in the mountains. He has given over 1000 hours mountain flying instruction and several hundred hours game spotting and photo flying very close to terrain.
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The material in this publication is for advisory information only and should not be relied upon for navigation, maintenance or flight techniques. SW Regional Publishing, Inc. and the staff neither assume any responsibilty for the accuracy of this publication's content nor any liability arising out of it. Fly safe.
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