Mountain Flying – Downdrafts

Posted by duewest
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Flying in the mountains offers a whole set of challenges to pilots. Even more so in small, general aviation aircraft. Really “small” is not so much the issue as is “slow.” A first glance, it might seem that slow is actually an advantage for flying in the mountains. Some would argue that slow is better because it allows a pilot more time to react. Also, as speed is reduced, turn radius decreases, allowing the aircraft to be maneuvered in tighter confines. While these statements are true, they miss the very important fact that one of the biggest challenges faced by mountain pilots is the downdraft. Here speed, or the lack thereof, can be a real problem. Downdrafts are generally localized to small areas. For example, a single downdraft might only be a mile across on the vector by which an aircraft enters. The math is simple, an aircraft with a ground speed of 60 knots per hour (KPH) will loose twice the altitude during this mile course as compared to an aircraft with a ground speed of 120 KPH.

It is not at all uncommon to find downdrafts around the Rocky Mountains with a vertical velocity of 2000 to 3000 feet per minute (FPM). Less common, but still occasionally encountered, are downdrafts in excess of 10,000 FPM. Let’s consider a one mile vector through a downdraft of 3000 FPM. An aircraft traveling at 60 KPH ground speed will loose 3000 feet of altitude in this encounter. An aircraft traveling at 180 KPH ground speed would loose only 1000 feet of altitude. As you can see, the challenge for the slower aircraft is three times greater. Many pilots assume that turbo charged or turbine aircraft are better suited for mountain flying because of performance. Specifically, climb performance. Again, climb performance can certainly be of benefit to offset the downdraft but it is the wrong tool to use for the job. The reason most pilots of higher performance aircraft believe those aircraft are better suited for mountain flight is because, based on experience, they believe the performance was the tool best utilized to offset the downdraft. The reality is that the performance that generates the higher ground speed is what served them best, not climb performance. Again the math is simple, in order to negate the effect of the downdraft through climb performance, the climb performance must equal the downdraft. From our previous example, this means the aircraft must be capable of 3000 FPM climb. Few are the small (slow) aircraft flying about the mountains that are capable of sustaining this type of climb performance. (Even many turbine aircraft can not “out climb” these downdrafts.) For most aircraft, we must look elsewhere for the solution to flying in a downdraft.

First of all, when encountering a downdraft, increasing ground speed will reduce the amount of time (remember FPM) that an aircraft is effected. In other words, lower the nose. Second, and more important, is the position of the aircraft relative to descending terrain. Downdrafts, when encountering terrain, are forced to at least parallel the contour of the terrain if not “bounce” away from that terrain. So, if the aircraft is always in a position to fly over descending terrain, and does so, the downdraft has lost its ability to “blow” an aircraft into the ground. We teach a simple phrase to our mountain students: “Position maintained is better than altitude gained.” (We will blog about “altitude” later.)

In mountain flying accidents, many assume that aircraft were “blown” into the ground. More often than not, it was the failure of the pilot to take remedial action with regards to rising terrain. In other words, the aircraft was being flown uphill with a performance climb rate that was exceeded by the rate at which the terrain was climbing. Unfortunately, in many cases, the last effort of the pilot to avoid the rising terrain, was to raise the nose of the aircraft to the point of an aerodynamic stall. Usually, an uncoordinated stall, resulting in a low altitude spin. And a short spin at that.

The moral is this, technique is more important than power. (Glider pilots will agree!) Don’t read this blog and decide you are equipped and trained to fly in the mountains. Let us guide you through this discussion with hands on experience. You’ll find that flying in the mountains, safely,  is some of the most rewarding flying you will ever do.