Riding the Wave

Recently I’ve been flying around the Lake Tahoe area of California/Nevada. I had a student performing the GPS A approach into Minden Airport (KMEV). Minden is known as a haven for glider pilots because of the “wave” that forms when conditions are right. A standing mountain wave forms as the wind drives in from the west, rises over the mountains, and then plummets down to the desert valley floor. It’s actually a little more complicated than that; you can learn more here.

We were flying in from the southwest to intercept HOYJO intersection. The winds in the Minden valley were out of the southwest at 16 gusting to 28 but the ride at 11,000 was not bad. This flight was in a rather well equipped helicopter complete with a 4-axis autopilot. The pilot was new to autopilots in general and we were coupled in 3-axis mode. The ability to hold heading and/or the navigation course line as well as altitude is a 2-axis autopilot. The ability to descend/climb and capture an altitude is the 3rd axis. The ability to capture and hold speed (think power) would be the 4th axis. Our indicated airspeed was 100 knots, but we weren’t in a hurry and the VNE of helicopters is limited at higher altitudes (another discussion). Our groundspeed was about equal to our airspeed. At HUYJO we turned east on course to OZEDU and descended to 10,300 feet. We had just leveled at 10,300 when the airspeed jumped to 140 knots! The power (torque in a helicopter) was still set to maintain 100 knots. A quick look at the groundspeed indicated 168 knots. What’s happening here? The initial reaction from my student was that the wind was “pushing” us so we were “faster”. But we know that ground speed (read tail wind) and indicated airspeed really have nothing to do with each other. The answer was the wave. We were caught in the up flow of the wave and the autopilot was doing exactly what it was being told to do, maintain altitude. How does the autopilot maintain altitude? By pitching down and because we had the power set the airspeed increased dramatically.

The autopilot in a Cirrus works exactly the same way, it pitches to altitude. The pilot could reduce power to fix the sudden increase in airspeed. But avoid excessive power changes with the autopilot engaged. In this case (and many others) the best solution may be to disconnect the autopilot, level the wings, and ride the wave out. Contact approach and inform them of your predicament. We were in the “up flow” of the wave; the “down flow” of a mountain wave can be extreme. In the case of down flow a TAWS/CFIT escape could be warranted. In the Cirrus a TAWS escape would be to level the wings, apply full power, and pitch to 15 degrees; then flaps up (check airspeed) and adjust to Vx. To let “George the autopilot” handle the situation is to ask a little much of old George. One other item of note before we end this post. The GPS A at KMEV is a circling approach. That’s what the “A” denotes. But a perusal of the approach plate shows a straight in approach. The approach heading is 160 degrees with a runway heading of 16. What gives? Look at the MDA. This approach only gets you down to 1502 feet above the runway. So, the only way to land from that altitude at the missed approach point would be to circle. Unless, of course, you have a helicopter;)

First Post

Well, this is my first foray into the world of blogging. My goal is to provide antidotes and information about flying. Some posts will be about operating TAA equipment. Basically I’ll attempt to post unusual stuff that you might not encounter everyday. Other posts will cover procedures and also just some of my flying experiences around the country. Since I fly both airplanes and helicopters you can expect some of both. So, I hope any viewers will be patient as I learn how to do this blogging thing.