Mountain Rotors

[talent]

Has anybody ever experienced turbulence from a mountain rotor? How does one distinguish it from "normal" turbulence in the lee of a mountain? Could it have possibly brought down United 585 in Colorado Springs in 1991? (Or was that just a Boeing smokescreen?)

[Prof2MDA]

Mountain rotor/wave turbulence's positive side is that usually you're VMC. On the bad side the turbulence is some of the worst you'll find anywhere, certainly at it's worst it will match any thunderstorm, although the IMC factor in thunderstorms makes control loss a higher pobability.

Research on the COS at the time was rotor streams, but all appearances now are a rudder hardover, perhaps coupled with some wave action. I don't know that we'll ever know for sure.

[WOK]

PROF2MDA says all things correct.

If you DO encounter rotor you will know it's rotor and not normal turbulence in lee of hills.

By definition it is normally only encountered in dep/arrival phase because they're the only times you tend to be tooling around near or below mountain top alts.

Can be evil, even if generated by quite small lumps in the ground.

[Jim lovell]

could well have bought down United 585. There were certainly rotors in the area at the time( they are 100% sure of this because of witness evidence, pilot reports, and rotor clouds visible)and if it was a large/powerful enough rotor a 737-200 wouldn't stand a chance! Although it has never been 100% confirmed it was due to either one of these 2 scenarios 1) rotors 2) rudder hardover, the facts obtained seem to point toward the the former.

[alosaurus]

The worst recorded incidents of U.K.standing rotor activity have been west the of Mid Wales coast in easterly winds.In 1987 a Hawk out of Valley recorded accelerations of one to seven 'g'.
The conditions for rotor streaming and standing rotors are mutually exclusive.
-Rotor streaming;stable air,stronger winds (typically more than 25 knots),semi organised type of mechanical turbulence in the friction layer (not above 2.5 times the ridge height)

-Standing rotors;winds as low as half that required for streaming,can extend much further downwind,worst turbulence occurs below ridge height.

If moisture is present roll clouds may form for both types.If not the only indication will be rapid fluctuations in surface wind speed when general flow is expected at around 20 knots or more.

[supermunk]

Rotor can be as low as 1000ft agl and can go from 2000fpm up to 2000fpm down in about 5 seconds, and thats in the UK. Imagine what it's like in the lee of real mountains. It's very sporting. Often all you see of it is a line of raggy Cu which doesn't look much at all, although I have seen a rotor cloud in which the rotation was clearly visible. I'm glad that I wasn't flying in the middle of it at the time

[Jim lovell]

Anyone remember that BOAC B707 that was destroyed by a rotor in 1966? The captain decided to change routing so the pax could get a good view of Mt. Fuji on such a nice clear day. As the aircraft approached the lee it got cought in powerful turbulence and virtually fell apart( wings snapped off, stabilators bent, engines wrenched off and even the fuselage snappen in half) not very pleasant to say the least!

[talent]

This is all terrifying stuff. I've two more questions.

1.What is the predominating factor for the creation of strong rotors - is it mainly terrain or wind strength?

2. Do they easily detach from the lee of a mountain and travel on their own, like a tornado or are they always attached to a specific location?

[Self Loading Freight]

There's an interesting piece about this at <A HREF="http://www.wco.com/~shp/waveforatc.html" TARGET="_blank">http://www.wco.com/~shp/waveforatc.html</A> which goes into details of how ATC can spot that an a/c is in downdraft/rotor and what can be done to help.

Quite a frightening read.

R

[alosaurus]

Talent-Nature of terrain is the more critical issue;any range which has a marked ridge to the lee can generate severe rotor turbulence.In the U.K. most incidents have involved the Welsh mountains,Scotish Highlands and the Pennines.
-Lenticular clouds can extend up to 100 miles downwind (high level).At low level standing rotors can form up to 20 miles downwind of a ridge.They do not break away but will form in different positions in different windspeed/stability conditions.Light winds and stable conditions produce short wavelengths,stronger winds and less stable conditions produce longer wavelengths.On a day such as this (strong easterly) I had my bumpiest ride ever coming into Manchester on 06.
In the case of rotor streaming the airflow does separate but the turbulent region remains close to its ridge (a few miles).
In both types incidents of severe turbulence have been recorded at very low level (not just at ridge height)

[FE Hoppy]

Often occure at Mount Pleasent Falklands.
40kts head to 40kts tail at 500'or down then up draufts. Best to stay clear...

[BEagle]

Preferably about 8000 miles clear!!

[AndrewE]

wasn't United 585 brought down because of that rudder problem they had/have on the 737's??

uuhhh ok i think i'll just keep my mouth shut....

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[Jim lovell]

AndrewE there is a full 6 page or so investigation of United 585 in AirDisaster Volume 3. The report does not entirely rule out the rudder hardover problem- but the evidence they obtained seems to point to mountain rotors.

[talent]

Jim,Andrew,
Boeing made a very convincing argument for a rotor in the wake of United 585 but the NTSB was never fully convinced and said that while a rotor may have struck the aircraft, triggering the deployment of the rudder, the fact that the latter jammed was the cause the crash, not the rotor. I'm no expert but from what I've read the NTSB claimed that the intensity of rotor likely to have been around that day would have been insufficient to twist a large jet transport out of the sky (eyewitnesses are reported to have seen it twist on its back, then dive stright into the ground)especially as nothing appeared to have broken off the plane in the air.

[The Hooded Claw]

With reference to your origonal question, yes I regularly experience rotor but only in my spare time!

At work it is unusual to encounter it as by the time we are close to the hills we are too high for 'proper' rotor (with the possible exeption of LBA!).

I am also a glider pilot and have encountered some pretty exciting conditions both in the glider, and flying the tug.

The worst rotor is experienced at airfields under or close to the primary wave. The steeper the downslope of the hill generating the wave, the worst it seems to be. Usually a roll cloud will mark its position, but if the air is dry, then a bit of local knowledge comes in handy.

It's unusual to find rotor much above the crest of the hill unless the lower unstable layer is deep when it can extend higher.

These days most people know when it is safe to fly, but sometimes a glider is thrown so far out of position behind the tug, that the only choice is to release. If you can hang on the strength of the wave above can be spectacular. I have experienced 2500 fpm at over 20000' in Scotland. The UK altitude record is over 38000' and I have also seen a tug aircraft at 5000', engine off and going up!

One word of caution; if you see 2 windsocks on the same field that are horizontal and pointing 180 degrees apart, then there is probably something pretty nasty going on above you. The solution is to forget it and open the bar!

[WOK]

All good advice.

The problems arise when those same two windsocks are pointing at each other......BELOW you.

Straps tight?

[Jim lovell]

Interesting to note the number of accidents due to this cause over the last 30 years.

-Jan 10 1964, a B-52 bomber, flying at 14,000 ft, 5nm east of Colorado's 13,500 ft Spanish peak, lost about 75% of it's fin and rudder- the gusts exceeded 140 ft per second
-March 5, 1966, a BOAC Boeing 707 broke up in flight at 16,000 ft , 10 nm SE of the summit of Mt. Fuji, Japan.
-August 6, 1966, a Braniff BAC-111 broke up in flight at night near Falls City, Nebraska at only 4,000 ft agl.
-December 2, 1968, a F-27B flying at 11,500 ft, broke up in flight after encountering extreme turbulence at Pedro Bay, Alaska.

In the case of United 585 and the proximity of 14,110 ft Pike's Peak- this could well be the cause(although rudder theory still disputed)