Repelling Terrain

Rising terrain is a problem in the mountains, but it can cause a mountain of trouble in unlikely places, too


There are large parts of this country where the terrain can impede your climb-out after takeoff. Sometimes it can even be a factor during en route climbs.

It takes just a quick glance through the NTSB database to find dozens of accidents in which the aircraft was unable to outclimb rising terrain. Those accidents reveal a number of common factors involved with this type of accident, in addition to the rising terrain. High density altitude carries much of the blame, to be sure, but so does restricted maneuvering area, adverse winds, heavy aircraft weights and relatively low power-to-weight ratios.

When most people think of rising terrain, they picture mountain peaks that rise nearly 13,000 feet above sea level. Its obvious that such terrain rises at near-vertical angles and exceeds the climb angles of most aircraft. Whats not so obvious is that the terrain can rise at a fairly shallow angle and still exceed the climb capability of light aircraft.

One pilot looked westward from Rawlins Municipal Airport in Wyoming several years ago and observed only relatively flat sagebrush-covered terrain. He planned to depart straight out from runway 28 to continue his trip west.

Much to the pilots surprise, the climb rate was anemic as he climbed out from the 6,813-foot msl airport due to the density altitude. While many parts of Wyoming appear to be flat, the average elevations of the valleys are well over 7,000 feet and, with warm temperatures in the summer, density altitudes will exceed 10,000 feet in the lowest areas of this region.

As the pilot continued flying westward, the terrain seemed to be coming up to meet him. The aircrafts climb angle could not keep him above the terrain, even though he had been at Vy since takeoff. He raised the nose and tried climbing at Vx, and then pulled back some more. Soon the stall warning horn started whining and the pilot was forced to land with the stall warning horn blaring away.

How Can This Happen?
Take an average four seat trainer such as an older Piper Cherokee. Toss away the POH numbers and look at what flight testing can tell you about this airplanes real performance. One study I conducted found that this particular aircraft has a 150 fpm climb rate at 8,500 feet. At a forward speed of 70 knots, the aircraft is able to climb at barely a 1.2 degree angle. At that rate, after roughly three miles the aircraft will climb to approximately 360 feet agl.

Imagine taking off and being only 360 feet above the ground after youve flown three miles. If the ground rises just 100 feet in your direction of flight, you can be lower than the tops of cell phone towers before you know it. Even though the terrain doesnt rise as dramatically as the Teton Mountains, there are vast areas of the country where light aircraft cant outclimb the gently rising terrain.

High density altitudes are a factor common to many of these accidents. In October 1999, a Cessna 172R took off from Aspen, Colo., presumably en route to the Front Range area. October can be a gorgeous time of year in the high Rockies, with the very crisp mornings, golden aspen covered hillsides and crystal blue skies.

The pilot took off at 12:30 p.m. and flew eastbound toward Independence Pass. The pass has a road that is open only during a few months of the summer because it looms above 12,000 feet. The terrain climbs very steeply, almost vertically, in that area.

Witnesses observed the Cessna flying slow, with a nose high attitude as it impacted terrain at 11,948 feet msl. Density altitude was calculated to be 14,110 feet msl.

If youve never had a normally aspirated aircraft at a density altitude of 14,000 feet, it is quite an eye-opener to see how little power the engine is able to produce, even though the throttle is fully opened. There simply is no excess power to accelerate or climb in the average light aircraft.

I Think I Can, I Think I Can
Aspen is a rather glaring example where the density altitude problem seems obvious, but less dramatic examples can be found all over. Consider the flatter terrain in Wyoming. Evanston,Wyo., in the southwest corner of the state, sits amid the rolling hills, buttes and wind swept terrain at a comfortable 7,163 feet. The flat terrain can lead unsuspecting pilots to believe that density altitude isnt going to be a factor when taking off from Evanston.

But wait. On a normal 80 degree F summer afternoon, a spin of an E6B shows the density altitude is a whopping 10,000 feet. If youve never taken off from a runway during a density altitude of 10,000 feet, heres what you can expect. The acceleration rate is less than impressive. In fact, its downright anemic. Oh, and by the way, there is a 0.5% upslope to the west if youre taking off into the prevailing summer winds.

Lets say that your aircraft has a 100 fpm climb rate at Vy at such a density altitude. Youll be lucky to achieve a 0.8 degree climb angle. You may like your chances with a 7,300-foot runway, but dont be surprised if youre only 30 feet above the ground a mile off the end of the runway.

Sure, compared with the Tetons this terrain looks much flatter, but its still high and those gentle upslopes still have gravitational forces that seem to pull aircraft closer.

So maybe you think youll outsmart the ground by climbing at Vx. Remember that Vy decreases with density altitude and Vx increases with density altitude. They intersect at the aircrafts theoretical maximum operating altitude.

So Vx may be 55 knots and render about a 100 fpm climb rate at 10,000 feet. That will give you about a – degree climb angle, but not for long. The oil temperatures will be going through red line within a few minutes because the engine is working hard in thin air. What little air is flowing over your air-cooled engine isnt resulting in much engine cooling.

Flying at any speed other than Vy for that density altitude will result in a slower sustained climb rate, and flying at any speed other than Vx for that density altitude will result in a shallower climb angle. It just doesnt get any better.

There is a visual illusion associated with pitching the nose up that tricks the pilot. When you pull the nose up, your eyes are directed along that elevated sight line and you assume the aircraft will proceed more or less along that line. However, at high angles of attack the aircrafts flight path angle is not nearly the same as its pitch attitude.

Pulling up the nose of the aircraft may result in a temporary increase in the climb rate and angle, but it wont be sustained for more than a few seconds.

Up, Up and Away. Please
Consider also the pilot who departed in a Cessna 206 from a remote lake after a week of camping. The pilot flew south along a river drainage while attempting to cross a mountain range. He was using a sectional map and a topographic map to plot his route but mistakenly turned into a valley that was about five miles north of where he intended to turn.

The terrain was rising and he began climbing through 4,300 feet msl. The valley became too narrow to turn around. The terrain continued to rise and the pilot noticed his airspeed was decreasing until the stall horn sounded. The airplane collided with terrain.

This is a common problem, especially when flying toward mountain passes. Its easy to fly up a valley that gets narrower near the mountain pass and find that the options for turning around get smaller. Its really important to know for certain that you can outclimb the terrain prior to taking off.

These maneuvers are complicated because uneven terrain creates its own microweather. The difference in the suns rays striking the slanted ground creates thermals on the sunny slopes and downdrafts on the shady slopes. Additionally, the prevailing winds flow around the terrain and create a variety of updrafts and downdrafts.

While the updrafts can be useful in gaining altitude, the downdrafts can easily overpower a light aircraft.

This pilots experience is not unusual. During the return leg of a cross-country flight through Utah, the pilot attempted to fly through a mountain pass that was more than 8,500 feet high. The density altitude was greater than 10,000 feet, which the airplanes POH showed would yield a climb rate of less than 100 fpm.

While attempting to get through the pass, the aircraft encountered downdrafts that overpowered the airplanes meager ability to climb and the aircraft descended into the terrain. Ah, the Rockies, you say? It doesnt have to be.

A Helio 295 departed from the grass strip for a local flight around the ranch in Llano, Texas. The pilot took off with 30 degrees of flaps and the airplane became airborne less than halfway down the 2,600-foot runway.

The airplane climbed to approximately 100 feet agl and then the climb rate vanished as the airplane was headed toward rising terrain. The pilot retracted the flaps to 20 degrees to improve the climb performance. He maintained controlled flight and managed to avoid some trees while he was looking for an open area.

He lost control as he slowed the airplane prior to striking some other trees. A 10- to 20-knot headwind prevailed and the pilot blamed the crash on a downdraft created by the orographic effect of the wind blowing over the ridge line.

In uneven terrain, downdrafts exceeding the climb capability of most general aviation aircraft are common. Adding power and pitching up to Vy usually wont get you out of the mess.

Aircraft weight is another important factor that will affect your climb rate. On one warm October day, a recently licensed private pilot ordered the Cessna 172RG be refueled to capacity, which resulted in the aircraft being about 90 pounds overweight for takeoff. The aircraft departed a mountain airport with an 8,100-foot density altitude and climbed toward gradually rising terrain.

The pilot radioed that he was unable to climb. The pilot of another aircraft saw the 172 fly approximately five miles at treetop level, with the wings rocking and the nose high. Then it finally ran out of room, struck the trees and crashed, killing all four aboard.

You have control over the weight of the aircraft. Remember your first solo flight in that underpowered trainer? Without the weight of the instructor, the aircraft accelerated faster, took off in less distance and climbed much quicker to pattern altitude.

Limiting the weight of the aircraft is vital to improving its climb performance. Many large aircraft have the ability to dump fuel, which gives the pilots options should there be engine failure(s) or other problems that limit the airplanes ability to climb. In fact, in the 727, most captains brief that in the event of a second engine failure the flight engineer should immediately commence the fuel dump without waiting for the captains instruction.

The Cessna 172RG may be a full-tanks-plus-four-people airplane in some conditions, but not with an 8,000-foot density altitude and rising terrain. I live at 7,000 feet msl and wouldnt begin to think about putting four people into the average four-seat general aviation aircraft around here. Consider downloading the aircraft to the necessary fuel, minimum baggage and minimum passengers for the flight. Wait to depart when its cooler. Pick a different route.

Its important for you to know the climb capabilities of your aircraft under the environmental conditions present at the time.

The winds in uneven terrain tend to be complicated and not as simple as the drawings in textbooks, so it can be difficult trying to precisely predict the effect of winds. However, plan for downdrafts. As a rule of thumb, a 10-knot wind at the top of a ridge can produce a 1,000 fpm downdraft.

Know the terrain you are flying into. Make sure you have enough maneuvering room to turn around if the terrain is rising faster than you are. Approach mountain canyons with extreme caution.

Plan to cross high terrain at its lowest points. Consider flying around the higher terrain.

Limit the airplanes weight as much as possible. Wait for cooler weather and lower winds. Use an aircraft with enough power for the density altitude.

These are the most effective measures you can take to avoid being caught with the stall warning horn blaring, the ground just feet below the wing tips and nowhere to go. Out of altitude, out of airspeed and out of room is not a good place to be.

Also With This Article
Click here to view “Terrain Trumps Pilots.”
Click here to view “How Fast, How Far.”

-by Pat Veillette

Pat Veillette is an aviation researcher who works in the training department of an air carrier and flies whatever he can get his hands on.


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