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Thread: Low Level Assault on Tejon Pass

  1. #1
    Senior Member turbopilot's Avatar
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    Default Low Level Assault on Tejon Pass

    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.

    Last edited by turbopilot; 10-29-2010 at 02:58 PM.
    Bob Anderson, CC11-00435, N94RA

  2. #2
    Senior Member couleeone's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    What an adventure! a scary experience! Thanks for sharing! So are you based at Bermuda Dunes now? I have cubdriving friends based there I can put you together with and I am in AZ at Falcon Field with my early model S1 for the winter. Maybe we can connect also.

    Send me an email at george@couleeairservices.com

    Geo
    www.couleeairservices.com 7AK -1CC skyharbor compressed.jpg

  3. #3
    Senior Member turbopilot's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    Quote Originally Posted by couleeone View Post
    What an adventure! a scary experience! Thanks for sharing! So are you based at Bermuda Dunes now? I have cubdriving friends based there I can put you together with and I am in AZ at Falcon Field with my early model S1 for the winter. Maybe we can connect also.

    Send me an email at george@couleeairservices.com

    Geo
    www.couleeairservices.com 7AK -1CC skyharbor compressed.jpg
    Actually given the excess horsepower in the CC I was pretty calm. Mostly I was just surprised. I have encountered outflow Santa Ana type local winds all over California but never off north facing canyons.

    My Carbon Cub is hangared over at Thermal (KTRM) at Thermal Aviation.
    Bob Anderson, CC11-00435, N94RA

  4. #4
    Senior Member John Hodges's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    Welcome to the exclusive "Steve Fossett Flying Adventures" fraternity, most of whose members we now refer to in past tense. Too bad Fossett wasn't flying a Carbon Cub!

  5. #5
    Senior Member randylervold's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    Wow Bob, great write-up, thanks for taking the time to write!
    Randy Lervold

  6. #6
    Senior Member turbopilot's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    Quote Originally Posted by randylervold View Post
    Wow Bob, great write-up, thanks for taking the time to write!
    Thanks for building an LSA that has enough power to deal with surprises from mother nature.
    Bob Anderson, CC11-00435, N94RA

  7. #7
    Senior Member turbopilot's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    I posted this experience over on the Cirrus Pilots forum. Fair amount of interest in the encounter there. Here is my reply to the suggestion that it would have been smarter to do a 180 degree turn rather than ride out the wave. Note I included a Google Earth video recreation of the ridge line crossing in the post.

    My guess is you are low time in type (in a cub). The weather knowledge required is different for slower & lower aircraft. So you were also low time in weather knowledge (and geography) for this airplane.
    Don't think this event has anything to do with the fact I was flying a Cub.

    Replay the entire scenario in an SR20 or SR22, neither of which could out climb the Carbon Cub in the same situation. With an SR20 or SR22 you would have to make a 180 even if that choice was just the lesser of two bad options.

    Here is a video I created in Google Earth. You can make the flight with me. This is exactly what I was seeing from the Carbon Cub. At the beginning of the movie you will see the California Aqueduct in the lower left hand corner as it enters a tunnel under the Tehachapi Mountains. I am aiming at the lowest point (4,500') in the Tehachapi range, a ridge you can see on the horizon. The movie is shot at 5,500 my altitude as I approached the ridge line.

    Watch the elevation at the bottom of the screen. Look for the second to the last ridge (the one with a road cut running across the mountain). That ridge is around 3,900'. Just over that ridge with the 4,500' highest ridge in view is the point where the 2000 fpm downdraft started and my speed quickly decayed to 20 mph as I pitched up to fight the wave. The wave hit its maximum about half way between the 3,900' ridge line and the last and highest ridge line at 4,500'.

    So as you can there was all ready a bunch of high terrain behind me by the time the full extent of the intensity of the wave was clear. Play out this scenario with an airplane that will only climb at 600 or 700 fpm at a 6,000' density altitude. You will be going down at 1,400 fpm as you execute a 180 degree turn.
    Bob Anderson, CC11-00435, N94RA

  8. #8
    Member TheCubWorks's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    Glad you made it.

    I learned to fly in a 7411' valley and had 10~14K mountains all around. I did not realize there was a difference between mountain flying and sea level flying until I was flying west of the Seattle aria. There is a world of difference that is unkind in/over obviously unfriendly terrain.

    I hope you will not not mind that I tagged this shameless commerce on to your post. Perhaps it could save someones life that is not as fortunate as you.

    If someone wants to learn the skills I have managed to develop over the past 18 years call
    The CubWorks 406 366 4836 We will go out and fly the wind in the mountains and land on some really neat back country strips.

  9. #9
    Junior Member johnmiller's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    Hi Bob, I have a PA18A-180 Super Cub and have flown that route a number of times though much higher, and have experienced turbulence and some waves. Thank you for the great article though. I will be much more cautious in that area. I am hangared during the winter at KUDD. give me a call or email. Would love to get together with you and do some flying. My plane was rebuilt by Cub Crafters 1.5 years ago. John Miller 253.732.1435

  10. #10
    Senior Member turbopilot's Avatar
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    Default Re: Low Level Assault on Tejon Pass

    Quote Originally Posted by johnmiller View Post
    Hi Bob, I have a PA18A-180 Super Cub and have flown that route a number of times though much higher, and have experienced turbulence and some waves. Thank you for the great article though. I will be much more cautious in that area. I am hangared during the winter at KUDD. give me a call or email. Would love to get together with you and do some flying. My plane was rebuilt by Cub Crafters 1.5 years ago. John Miller 253.732.1435
    John, left you a message. For anyone else in the area there is an Air Show this weekend at Thermal Airport (with associated TFR). My hangar will be open for anyone who wants to see the Carbon Cub. Hangar A5 at Thermal Aviation.
    Bob Anderson, CC11-00435, N94RA

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