I started my flying career in 1960's flying the skies of southern California in a Cherokee 140. I flew my way through the killing zone mostly in the mountains of the west in normally aspirated airplanes. Thought I had the mountains all figured out but they always have a surprise even for experienced pilots.
Over the years I have had many battles with Tejon Pass in Southern California. Tejon Pass is a mixmaster of weather intrigue. From the north there is an abrupt rise from the floor of San Joaquin Valley with a vertical rise of nearly 9,000' to the mountains surrounding the pass. The upper desert of the hot Mojave Desert between 2000 and 3000 elevation points like an arrow right at the pass. And from the south the marine flow is almost always trying to push up and over the pass. Unless you fly the coast of California the ridge line just east of the Tejon Pass at 4,500' is the lowest point you can cross to fly from Northern to Southern California. So for normally aspirated pilots a trip north to south in California is all about the that pass.
I have been flying turbocharged airplanes for decades and crossed the pass hundreds of times. But all of the flights were up in the teens. Even with strong winds oriented just right most you would get was moderate turbulence and about 50 miles of smooth mountain waves.
For the last week I have been planning to move the Carbon Cub down to Palm Springs from San Luis Obispo. The best route for most weather, scenery, restricted airspace and traffic is to fly KSBP KPMD KTRM. That route takes you right across Tejon Pass. The weather and winds for the last week have been a no go to fly the Carbon Cub on any route to the south. Very high winds aloft.
Yesterday the weather improved with winds aloft forecast to be easterly then southerly at 15 knots at 6,000' and 9,000'. No problem for the Cub since all I needed to do is cross the ridge line at Tejon Pass then fly the rest of the route at 500' or below AGL. Tejon Pass has always been a problem for aviators. Matter of fact the NWS started a weather station in the pass manned by four employees at Sandberg, CA to report the weather for aviation in 1933. The weather station still exists today as an automated station at KSDB. Most of the experienced aviators who do battle with the pass low level watch that station very closely.
So yesterday morning before taking off I checked the Sandberg station. The winds had calmed down from previous days were they had been running 40 kts with gusts to 65 kts. Yesterday morning the winds were down to southerly winds 20 kts with gusts to 25 kts. I thought it strange that the winds at 4,500' at the station were higher than the winds aloft forecast. But I decided to launch and see if the winds aloft forecast were right.
Driving up to the pass at 4,500 the winds aloft forecast were pretty close: 15 kts varying between easterly and southerly. As I approached the pass over Interstate 5 below I made a right turn (second arrow below) in order to commence my assault of the ridge line at 90 degrees. I climbed to 5,500' to give myself 1,000' clearance above the ridge line which seemed about right for a 15 knot wind across the ridge line.
As I got within 5 miles of the ridge line the Carbon Cub began to move around in unusual way. I had light to occasion moderate turbulence approaching the ridge line. About what I would expect for leigh side winds of the ridge with 15 knots over the ridge line. All of a sudden the little Cub began an elevator ride straight down. I added full power and pitched up to 70 mph. Thank goodness the Carbon Cub is so overpowered. A full power climb in the Carbon Cub at 70 mph will easily get 2,500' fpm or more. As the Cub fought the wave off the ridge line we were able to get around 500 fpm up so I knew we were in a wave driving us down by over 2,000 fpm. I watched the winds aloft meter on the Dynon unwind while looking at my options to exercise the 180. The winds aloft hit 60 mph right on the nose on the Dynon and the Cubs ground speed fell to 20 mph. I was hovering, nose up at full power.
The turbulence was not too bad so I decided to watch the situation. I could see the ridge line right over my nose and I was very, very slowly moving toward it. The biggest problem I had was that the CHT's were going very high as the little Cub hung on the prop fighting the wave. Hottest cylinder was up to 420 dF. I was going to call it a day at 450 dF if I had not flown out of the wave yet.
I finally hit the ridge line and just as abruptly as the wave had started the winds aloft settled back to around 20 mph and we were on our way to Palm Springs.
So what happened? After I landed I looked at the weather again. The winds aloft forecast was right, but I did not pay close enough attention to the pressure patterns around Southern California. I noticed right away that high pressure was south of Tejon Pass while low pressure was north of the pass. There was a 7mb difference in pressure between Sandberg and Bakersfield 50 miles to the north. So the high pressure air was funneling off the high pass dropping down to the San Joaquin Valley valley floor 5,000' below.
So the take away is that there is more to mountain waves than just winds aloft. Local pressure differentials can create very large waves with relatively low winds aloft over the high country.
The result would have been very different if I did not have the excess power found in the Carbon Cub. Most LSA's have less than 100 hp. Many smaller non aspirated airplanes have similar or worse power to weight ratios. Given the local geography of the Tejon Pass ridge line approaching from the north (sort of a box canyon to get to the lowest ridge line) I doubt very much that a lower powered airplane could have performed a 180 without impacting the terrain under similar circumstances.