Colorado Mountain Wave in a Cessna 206 Turbo

I thought I’d document my experience in Colorado Mountain Wave in my 206 on a recent flight from Idaho to Centennial, CO. The details are as follows:

• Plane is a 206 Turbo with G1000 NXi and I imported the data from the data card into CloudAhoy which is how I made the video.
• Wind that day was 270 at 35 KTS at 15,500 MSL which was my crossing altitude.
• Crossing point was the East Portal which is a common crossing point for GA VFR aircraft. It’s kind of like a low wall with taller peaks north and south.
• I think there was a high altitude temperature inversion that day but unfortunately did not confirm that and I can’t find a historic source for winds aloft to confirm this. Let me know if you know of one.
• As I approached the rockies from the west, lenticular clouds were visible to the north and south with a cloud above me that may have been a lenticular cloud.

I normally cross the East Portal at around 14,000 but after seeing the conditions for mountain wave I climbed up to 15,500 to give myself a bit more room. As I crossed the ridge I had the autopilot enabled and I encountered a descending column of air – perhaps at 1000fpm. The autopilot did its job and worked to maintain 15,500 which meant the plane entered a climb within that descending column of air and I watched my airspeed slow down from about 123 indicated to 97 IAS.

I’d like to dwell on that for a moment. A friend of mine who is an accomplished instrument rated pilot was in a higher performance aircraft crossing the Sierras from west to east at night in IMC and encountered the same effect. A westerly wind was crossing the Sierras, which is also famous for mountain wave effect and creating a downward flow on the leeward side. The autopilot tried to maintain altitude and the plane slowed down enough that when he finally noticed it really got his attention.

I was told this story in person and reading between the lines this left a strong impression on him. I think part of the reason is that in the higher performance plane his stall speed was fairly high, so the potential to have the autopilot drop the airspeed enough to stall the plane was higher. I don’t know if his plane had Garmin ESP. Mine does, and it will nudge the nose down if the plane appears to be entering a power on stall. I plan to verify this.

Once the airspeed crossed 100 kts and continued to drop, I turned the AP off and pointed the nose down. At this point the airflow across the ridge and in the leeward side at 15,500 was laminar – very smooth. You can see this in the video below if you look at the G meter in the top left.

As I descended and moved further away from the ridge, I think what happened is I moved below that laminar flow. As I got out into the foothills and at around 13,700 MSL which was 5,200 AGL I was hit with moderate turbulence. In FAA terms, moderate is passengers screaming and crying and severe is the airframe at risk of taking damage. I’d say I was on the high side of moderate.

I experienced plus 1 G (or plus 2 if you want to nitpick and point out that 1G is at rest) and negative 0.5 to 0.7 although I haven’t verified this. You can see the G meter in the video. The aircraft can handle +3.8G and -1.52 flaps up.

I got one thing right and made two mistakes. Crossing at a much higher altitude and maintaining that higher altitude than normal give me plenty of room AGL to recover from a stall/spin scenario. The first mistake was to descend at an airspeed that was far too high given the potential for turbulence. I hit the turbulence at around 150kts indicated and within a second due to wind shear the ASI jumps to 163.

I pulled power at that point and gently nosed up to avoid increased G loading on the airframe, but was doing everything I could to rapidly reduce my speed. The turbulence was short and sharp. The G loading wasn’t particularly high in terms of what the airframe can stand at my weight and balance, and given that I’d burned most of my fuel, but the short sharp nature of it was of real concern.

I then made the second mistake which was to slow down too much. I think I got as slow as 83 indicated, which is way above the published clean stall speed of 62 KCAS but given the wind shear, I should have kept it at maneuvering speed which ranges from 106 KIAS to 125 KIAS with the lower speed being for lighter aircraft loading. I was probably around 110 KIAS given my fuel and load. 105 KIAS would have been a safe speed to do the entire descent.

I was talking to Denver Departure so called them up and gave them a moderate to severe turbulence PIREP which I revised a couple of minutes later to Moderate.

Right before I encountered the turbulence I saw my VSI indicate I was entering an ascending column of air. It’s hard to tell how fast it was ascending because I nosed up to reduce airspeed. My VSI peaked at 3100 fpm. But I then encountered a descending column of air which was also 3100 fpm with pitch level. That’s around 31 knots of descending air, to put it in perspective. My tail wind at that point was 40 kts based on the difference between TAS and ground speed via CloudAhoy. So 40 knots horizontal and 30 kts downward. Fun stuff.

This effect is due to a laminar mountain wave flowing over the rockies and extending in a repeating sine wave over the plains. But underneath that laminar flow you get a strong rotor effect which can do severe damage to aircraft.

Around the same time a TBM reported severe turbulence within KAPA’s (Centennial’s) Delta which is quite far east from the foothills. As I approached KAPA the turbulence became very intermittent but I can see how a rotor might have reached down into KAPAs Delta and knocked that TBM around given what I had experienced from 13,700 down to 11,000.

I’ve done a fair amount of reading about mountain wave, but that doesn’t prepare you for real world experience – and unfortunately in my case the knowledge didn’t bed down enough for me to slow down to maneuvering speed on my descent, which of course I’m really beating myself up over.

My takeaways are as follows:

• Avoid the leeward side of the Rockies in mountain wave conditions.
• If you absolutely have to be in the lee of the rockies in these conditions (I can’t imagine why – maybe research, maybe a mistake), give yourself plenty of altitude, slow down to whatever your POH says is turbulent air penetration speed, and lock everything down. You’re going to get beaten up. I was rolled a few times fairly aggressively too – to about 45 degrees before I caught it each time. Very quick rolls.
• Absolutely do not head westward towards the lee side of the slope with ascending terrain, even if you have a turbo. You’re liable to not notice your reducing AGL altitude and if you encounter a severe downdraft you could easily get smacked into the mountain. We recently lost a CAP piston single in the lee of one of the slopes. It looks like they entered the lee at only 500 AGL with 35 kts over the ridge.
• This time of year in fall/winter/spring we frequenty see mountain waves, so I’ll either avoid crossing the high Rockies, or try to time it when winds aloft are less than 20 kts which sometimes happens in the mornings.

Here’s a video of the telemetry imported into CloudAhoy which provides a simulation of the flight. I’ve added a narration. Clearly this is designed for interested aviators and not the YouTube crowd. 🙂

Further reading:

• Margaret Lamb’s Flying Colorado Mountain Weather (the bible)
• Exploring the Monster by Robert Whelan – a great book on the history of research into mountain save in Germany and the Sierras starting in the 1930s onward.