Dangerous Winds in the Rockies

 

Rocky Mountains near Grand Teton National Park

A simple rule: Do not try to cross the high parts of the Rockies when surface winds exceed 25 kts. The air can be so turbulent that you cannot keep the plane upright. 

How about routes under these wind conditions where the mountains aren't quite so high, say 8,000 ft MSL or less? You can cross with suitable precautions that depend on whether you have tailwinds or headwinds.

In the tailwinds case, you simply stay at least 2,000 ft above ground level (AGL), and all will be well. 

This also works for the headwind case provided your engine is powerful enough to achieve sufficient ground speed. 

Lots of small planes with engines producing 100 hp or so cannot do this. Our Zenith 601HDS, with 80 hp Rotax engine, is in that category.

You then have two choices: You either don't fly, or if you have to fly for some reason, you fly low and exploit the terrain to overcome the adverse wind forces.

This post describes how you can do so, using two example scenarios of a recent trip.

We had to be in Rexburg, ID on a certain date in July to rent a car, drive to Grand Teton National Park, and camp there for a week of hiking.

Why were we committed to the schedule?

Getting a campground reservation in a national park has become difficult or even impossible. You can try getting a reservation half a year in advance using the recreation.gov website. With luck, you might succeed.

From then on you are committed: You either show up on the scheduled date, or you give up camping there for the current year since you cannot make any changes.

That was the case for us in July 2023.

On the first day of the trip, we flew from Dallas to Goodland, KS. It's a neat airport since they always make hangar space available and provide a courtesy car. 

We knew that the route of the second day from Goodland to Riverton, WY would be troublesome. At low level, westerly headwinds were forecast at 25 kts. Just a bit higher up, say beyond 2,000 ft above ground, the wind speeds increased to 35 kts or even 45 kts.

How could we get to Riverton? No doubt we had to fly low. But that choice by itself wasn't good enough. During much of the route, the terrain rises. Since winds at low levels follow the contour of the surface, the 25 kts headwind also produced a downdraft.

Why is a downdraft troublesome? The force needed to propel an airplane horizontally is much smaller that one lifting the plane to a higher altitude. As a result, overcoming just a modest downdraft, say of 300-500 ft per minute, requires significant engine power.

The combined effect: The ground speed of the plane declines badly to the point that one no longer can reach the destination.

How can we overcome the downdraft problem? 

The key is the behavior of the winds in valleys. Say, a valley runs north-south, and the winds are from the west. 

On the western slope of the valley, the winds tumble down and produce a downdraft of turbulent air. 

On the eastern slope of the valley, that turbulent air gets compressed and rises, thus creating a smooth updraft.

The solution to downdrafts in the mountains is then: Find valleys whose direction reasonably agrees with your route, and that at the same time are oriented somewhat differently from the wind direction.

Then fly on the updraft side of the valleys.

You not only avoid downdrafts but can convert the climbing induced by the rising air into additional speed.

Case 1: Use the Interstate as a Guide

The designers of the interstate system selected the routes so that both the construction effort and the resulting distances were reasonable. As a result, the interstate routes in the mountains often snake along in valleys. 

Unless such a valley is directly aligned with the headwind, you can always fly on one of the two sides of the interstate to catch an updraft. We did so after we had passed Laramie, WY. Here are the details.

Planned and actual routes northwest of Laramie, WY

The straight dark-green line is the planned route, going from the lower right corner toward the upper left corner. The wiggly light-green line indicates the actual flight.

It's not obvious from the picture, but the terrain rises substantially along the planned route starting about 20 miles northwest of Laramie. 

The downdraft produced by the strong westerly winds was substantial. Hence we veered to the left toward the interstate, here shown by double lines, then clung to the right hand side of that road to utilize the updraft. 

The result: We canceled the downdraft and only had to make up for the headwind occurring at a 45-degree angle.

Case 2: Use Any Road as a Guide, No Matter How Small

Engineers built all roads with the same principles as the interstate system, except that in the olden days the construction costs typically were more important than the resulting distances. Hence the roads of the Rockies almost always are embedded in valleys.

We used this fact near the Seminoe Reservoir, shown below in a photo of an earlier trip.

Seminoe Reservoir, WY

Here are the planned and actual routes.

Planned and actual routes near Seminoe Reservoir

Comparing the two pictures, you can see that the planned route leads to a dead end of the valley where tall mountains to the left of the route connect with lower mountains on the right. 

Surely at the connecting point the westerly winds produced turbulence and downdrafts that would have been difficult if not impossible to outclimb. 

See the little wiggly road to the left of the actual light-green route? It was our guide. We stayed to the right of it, over rising terrain, and got a strong updraft by the westerly winds. 

In fact, for several minutes, the plane climbed relentlessly, adding more than 3,000 ft in a few minutes to our altitude. A terrific result that carried us comfortably to Riverton.

The flight from Riverton to Rexburg, ID the next morning was easy. A slight headwind slowed us down just a bit, and we arrived at Rexburg midmorning before significant wind speeds could develop. 

Landings

There is more to mountain flying in strong wind conditions than just avoiding downdrafts. In particular, the landings can be very troublesome. 

Indeed, the orientation of the runways of an airport may be so different from the direction of strong and gusty winds that landing becomes impossible.

What to do? Several days before departure, you study the winds at the airports where you intend to land.

Focus on the planned arrival time since winds often change dramatically during the day. That preview gives you a clue when and where you can land.

Examine the NOTAMS (Notices to Airmen) for each airport since they warn of closed runways and other potential problems.

Prior to the Riverton trip, we had studied all that for several days. 

One runway was closed for repairs, but the main east-west runway was operational. Since we anticipated westerly winds, all seemed well. 

But when we arrived, the winds were 30 degrees off the runway direction, with speeds of 17 kts and gusts up to 27 kts. Not a simple situation since it translates to a crosswind component of about 9 kts, with cross-gusts, my own term, of 14 kts.

During the landing, we flew a crabbing approach to counter the drift of the crosswind, then used aileron and rudder to align the plane's axis with the direction of the runway just before touchdown. The jargon is, "We kicked the plane around."

Then the worst case occurred. 

Just as the upwind main wheel touched the runway, a strong gust hit the plane. We countered immediately with much more aileron, simultaneously using opposite rudder to maintain the direction. 

Then the second main wheel touched the runway, and down came the nose wheel. 

Whew, well done if I may say so.

This case may sound simple. But consider the following. 

On the flight to Riverton, we heard on the UNICOM frequency of another airport that winds were 27 kts gusting to 37 kts. 

If that wasn't just along the runway direction, it would have been very difficult or even impossible for a small plane to land there. 

Enroute Checks

We follow the rule of Chesley ("Sully") Sullenberger, the famous pilot of the "Miracle on the Hudson" flight who landed in the river, thus saving everyone's life. 

Every half hour of flight we scan the entire panel, check all switches and instruments, and make sure that the radio is tuned to the frequencies coming up next.

Then we examine the weather at all reporting airports along the route up to the final destination of the day.

In the mountains, this gives a picture how surface winds at airports are increasing and shifting as the day unfolds. 

We thus become aware of potential problems and can plan ahead for possible alternatives.

Staying in Grand Teton National Park

Camping and hiking in Grand Teton NP was wonderful, just like previous times. Here are some pictures.

On the flight home, we took a route new to us. FBO Lewis of Rexburg, ID had recommended it.

 

The flight departed from Rexburg in a northeasterly direction and went around the northern part of the Teton mountains.

Northern part of Teton mountains, early morning

Isn't it wonderful that a small airplane with tiny engine can provide such vistas?

Have any questions or feedback about flying across the Rockies? Please share your thoughts in the comments.