SERIOUS DOWNDRAFT AT THEEWATERS

[mikemat]

I thought I would share an unnerving experience which I and a fellow gyro pilot had on saturday. I think it would be appropriate here (rather than gyro talk) since it would have affected any light/microlight aircraft, I believe. We were en route to Morning Star and Diemerskraal respectively from Pearly Beach, approaching Theewaters from the South East. We were in the teeth of a strong North Westerly wind (ave ground speed about 28-31mph!) and decided to climb to 5500' to avoid the inevitable turbulence on the Eastern side of the Franschhoek pass. We duly climbed to 5500', but almost the minute we were over the dam we started descending at about 1000' a minute, this despite being full throttle (Subaru 165HP)and using best rate of climb. By the time we had crossed the dam, we had dropped to just under 4000'! Thereafter we started climbing again, and almost immediately hit a huuuuuge updraft which shot us back up to 5500' in no time. Crossing the Pass was thereafter was not a problem. Can anyone help to explain this significant downdraft? We were still about 3nm from the pass itself when this happened.Looking forward to comments.

[skybound®]

If it was not very turbulent sounds like a Mountain wave. If turbulent could have been the lee side rotor you went through initially followed by hitting lift created by the windward side of the mountain.

[Coyote]

Sounds like Wave - I heard of a pilot flying over the mountians in Cape Town hitting 2000 ft /m down 40 miles from the lee side of the mountain range. If wave sets up it can reach along way.

[Morph]

Yes definitely a wave on the leeward side of the mountain. I've heard more experienced guys say you need 1000ft clearance per 10knot wind. Sounds like 7500 and above would have been a better bet, but there is no guarantee of that. I certainly would not have tackled those mountains in a fixed wing with wind like that.

Glad you got back safely

[mikemat]

Thanks for the input. I was flying with a pilot who is significantly more experienced than I, and we both had a very comfortable passage over the pass. Happily, rotors handle wind like that very well and neither of us ever felt that we were in danger at any time.

[kloot piloot]

Ido not know the terrain that well, but wave is more likely to be found at higher altitudes and above the ridge line.

It sound to me like Mr. ROTOR.

Dennis Pagen, an American hang glider pilot has published very worthy books on micro-met.

[Big-D]

Dennis Pagen, an American hang glider pilot has published very worthy books on micro-met.

Understanding the sky - Very good book by Mr Pagen - I studied it, yet I am unsure what caused this downdraft I will go with Morph on this one

[skybound®]

This is all part of micro-meteorology and is often a topic I believe not covered in enough depth in the MPL/RPL/NPLs.

One accident report :

“A pilot flies with a tail wind in a wave system and intentionally stays under the maneuvering speed.  The air is wave-smooth and he could fly much faster but he is being careful!  He is about to fly over a mountain ridge and expects the next lee wave behind it and wants to do a 180 in it.  Suddenly the ASI jumps up well into the yellow arc.  The pilot pulls back carefully on the stick to reduce speed.  He doesn't want to pull the stick back too quickly at this high speed.  Then he hits hard turbulence and loses a wing.  At this high speed and not so great an altitude, the surprised pilot has only a few seconds to get out (without a rescue system such as the NOAH) or it is too late.
What happened?  As the pilot flew over the ridge, he fell out of the wave without knowing it.  Because of the strong tail wind, his ground speed was perhaps over 165 knots.  Because of the suddenly lost tail wind, the ASI jumped  then the glider hit the rotor turbulence that was stationary behind the ridge.  The aircraft came apart completely. “

Also around 15 years ago, a Microlight broke up in the air when flew into rotor. It was fatal.

[kloot piloot]

mikemat, I think this is your answer.

Refer figure 1 at bottom of article.

http://www.cgc.org.za/downloads/cloudba ... cester.pdf

[mikemat]

Kloot piloot, the article is spot on! It would seem that Villiersdorp (Theewaters) is a location where gliders experience this wave phenomenon. Thanks very much for the insightful article. Perhaps better to contemplate Sir Lowrey's Pass as a preferable alternative if possible

[FO Gyro]

I know of another gyro pilot that had a rough ride in the Tweewaterskloof Dam area recently with a North Westerly wind.

Maybe best to avoid when the North Wester blows.

Standing waves can go to over 20 000ft, so one can forget about outclimbing them.

[mikemat]

I agree that one cannot learn/know enough about weather phenomena as a pilot. I was speaking to a mate of mine, Steve Medcalf, at one of our geriatric waterpolo sessions last night, and told him about our mountain wave experience. For anyone that knows Steve, he was 'Mr Weather' in Cape Town before he retired a few years ago and he is a veritable fountain of knowledge on his subject, giving talks regularly. He also has a book on meteorology which I'm sure many of you will have read. He has offered to come and speak at Morning Star, and this would be a good opportunity to perhaps set up something for the Western Cape flying community in general (and beyond, I imagine, if you are prepared to cover his travel and subsistence!). Comments/suggestions?

[Morph]

Sure,

Tonight is the last meeting for the year. Perhaps the January meeting, last Wednesday of the month

[skybound®]

FAPE on AvCom is the PE Aviation Forecaster. He will be embarking on his MSc shortly and his chosen topic is mountain wave. He is also a man who loves to give back to the the fraternity - so look forward to what nuggets of info he will be able to share on the subject in time to come.