The pilot said her airspeed indicator was erratic and she wanted to land at the nearest suitable airport. Dutifully, the controller vectored the Arrow’s pilot to a nearby airport but didn’t suggest what might be the cause. I was getting mountain wave but only about 10 knots above and below my Beechcraft Baron’s normal cruise airspeed. I keyed the mic and said, “mountain wave.” The controller took the bait and mentioned this to the Arrow’s pilot, who didn’t understand nor did she care. She stopped short and likely went on a potentially expensive path of finding a “problem” with a functioning airspeed indicator.
There was a high likelihood the Arrow’s pilot was flying on autopilot equipped with automatic trim. While the waves increased and decreased airspeed, the autopilot would have dutifully done its job holding altitude and trimming away control pressure, masking the wave conditions. My Baron’s S-Tec 30 autopilot doesn’t have auto trim, but it does have altitude hold, and occasionally on this day it would squeal to prompt me to nudge the trim up or down. Without an autopilot, it would be clear to any pilot that maintaining altitude would require a lot of trim changes as he or she flew through the up- and downdrafts.
What the Arrow pilot must have failed to notice was the concurrent increase and decrease in angle of attack as she rode the ups and downs of the wave. If it was affecting my heavier Baron by plus or minus 10 knots there’s a good chance that Arrow was getting 15- to 20-knot airspeed excursions, which would lead to considerable changes in pitch between the tops and bottoms of the wave. Likewise, a good noise-canceling headset and nicely sealed door(s) may have masked the noise of increasing and decreasing airspeed. Other clues such as concurrent GPS groundspeed changes and engine temperature indications would be apparent.
We configured for maximum rate of climb: cowl flaps open, full throttle, climb rpm, right rudder trim, all while losing about 500 feet per minute.Flying a Cessna P210 with a friend many years ago brought the most extreme example of mountain waves I’ve ever experienced in my nearly 17,000 hours of flying. Again, I was on the lee side of the Appalachians flying from Winchester, Virginia, to Asheville, North Carolina, at 18,000 feet. A cold front had passed and the winds were blowing out of the northwest across the peaks and valleys of the mountains. The incoming cold air scooped the warm air out of the valleys and created massive waves that overcame the P210’s abilities. We received a block altitude clearance of 17,000 feet to FL190 because we could not maintain altitude.
Boy, did I wish that P210 had auto trim because on the downside of the wave, we configured for maximum rate of climb: cowl flaps open, full throttle, climb rpm, a fair amount of right rudder trim, all while pathetically losing about 500 feet per minute. On the other side of the wave it was the opposite: cowl flaps closed, low power, cruise rpm, some left rudder trim, and keep the airspeed out of the yellow arc, all while climbing 500 feet per minute. Despite the waves and exhausting piloting duties, the ride was perfectly smooth.
At the airline, waves aren’t too much of an issue when flying around the Appalachians. We are usually above them. The Rocky Mountains, however, are another story. The Rockies create waves that reach well to and above our cruising altitudes. Since we have autothrottles, the first sign of entering wave conditions is movement of the power levers in the Boeing 737 that I fly. Airbus pilots don’t have the luxury of moving levers as they are left in the Climb detent. But engine indications would tell the story.
When the waves really get going they can overcome airliners as well, which can lead to high-altitude upsets. At high altitudes, jets live in a very narrow airspeed envelope. You could be 10 knots shy of the redline and only 20 knots above stall speed. When the waves get going you can see how this can be a problem. To solve it, when waves are forecast, we cruise at a lower altitude, which expands our airspeed operating window.
During the downdraft portion of the wave, power may be set to the maximum yet airspeed still decreases. In the updrafts, power comes way back to maintain airspeed, but be careful not to pull off too much. At high altitudes, jet engines are extremely slow to spool up and pulling off too much power could lead to disaster when you fly into the next downdraft and the power hasn’t come back. In these cases, it’s more appropriate to use the speedbrake (spoilers) during the updrafts to control airspeed rather than pulling off a lot of power. It may sound counter-intuitive but it works by keeping the engines spooled and ready for the next downslope.
Savvy mountain fliers are well aware that waves are a serious threat to safety and know that when the winds are strong at altitude, there’s a near-100-percent chance of wave activity that could prevent safe flight of light airplanes. Jets are better equipped to handle the waves, but they still create serious operational challenges that command respect.