Clear Air Turbulence

[bellevueace]

I remember a couple of years ago reading a report of a 747 from the far east to London suffering what was described as clear air turbulence. Apparently the aircraft just dropped like a stone suddenly for a couple of hundred feet, there was no prior warning of turbulence at all. There were a few injuries, one stewerdess hit her head on the cabin ceiling and a male passenger suffered a broken leg. The report stated that clear air turbulence cannot be detected and you only know about it when you fly into it. The pilot said there was never any danger to the aircraft or the passengers as he was always in total control. Apparently there is research ongoing to be able to detect this in the future. So what actually is clear air turbulence? Why cant it be detected? And given the suddeness of it obviously it caused panic among the passengers, but when things happen like this how come the pilots never appear affected? After all they are only human and to experiance something as severe without warning you would expect them to at least be a bit shook up by it. This experiance sounds shocking and ive always thought about it in the back of my mind. And if aircraft can glide safely how come in this case it just appeared to drop like a stone?

[EyesSkyward]

And if aircraft can glide safely how come in this case it just appeared to drop like a stone?

Because the air it was flying in was moving rapidly downward. Planes do glide just fine. But they're always gliding "inside" the air. If that air moves, they move with it. (And that's pretty much all clear air turbulence is--a change in the movement of the air you're flying in.)

Think of it this way: A person jumping off a building and another person taking a high-speed elevator from the Penthouse to the lobby might both look like they're "dropping like a stone". But there's obviously a big difference!

An airplane experiencing a quick drop due to turbulence is like the elevator person, not like the jumping-off-the-building person.

- Jeff

[MathFox]

First, what is turbulence: turbulence is fluid (air) moving in circles. While one would expect the air to move at near constant speed, with only gradual changes, turbulence are the whirls in moving air. There are several well-known sources of turbulence, the most important for aviation are:

thunderstorms
the borders of the jetstream
other areas with large changes in wind speed
wing-tip vorticesHow a plane is affected by those whirls is dependent on the orientation of the whirl and how a plane flies through it. The typical thunderstorm sucks air horizontally over the ground that rises in the cloud and is pushed outward at the top of the anvil. The air more or less follows the shape of a bagel.
When a plane flies horizontally through a thunderstorm, about half-way (where you would cut the bagel for cream cheese), it will notice the downdraft outside the cloud (and drop a bit), when it meets the cloud it will rise quickly, to find another downdraft at the other side of the cloud. Yes, the plane will fly (glide) through all of it, but as the plane gets lift from pushing at the surrounding air, the relative speed from the plane with respect to the surrounding air (air speed) is all important for flight, ground speed only matters for the arrival time at the destination.
Pilots know that the plane moves with the surrounding air, and if you are safely strapped in your seat with your four-point harness, the auto-pilot correcting for turbulence... you can complain about spilling your coffee.:D (They encounter turbulence more often than the average passenger)

Thunderstorms can be detected on radar and one can measure the speed of the droplets by taking advantage of the Doppler effect. Clean air turbulence is much harder to detect, scientists are working on Doppler radar that can work with the few specks of dust that are present in clean air. It certainly isn't "off the shelf" equipment yet that can be installed in all planes.

[Captain Hutch]

Hi Bellevueace,

Good questions! To start out I have found a website that gives a pretty good synopsis of the basics about turbulence:
http://www.casa.gov.au/airsafe/trip/turbulen.htm
Also we have a lot already in some other threads on this site, but I think you would like to get a pilot's perspective too.

Turbulence associated with clouds and thunderstorms can be displayed on the airplane's radar screen because the movement of water droplets can be detected by radar waves. And moving droplets mean moving air masses which mean turbulence. Clear Air Turbulence, by definition, has no visual phenomenom associated with it, nor are there any water droplets there either so the area is invisible to radar. Though, as MathFox has mentioned, the scientists are working on other methods of detection. There is always some turbulence associated with crossing a boundary of the jetstream, and if one flies too close to a thunderstorm then there will be turbulence too (airliners never, with almost no exceptions, will actually fly through a thunderstorm--the up and down drafts are severe).

Why, exactly is that area of turbulence there where we wouldn't expect it? It may be that the air well above has been disturbed by winds blowing over mountains below--"mountain waves", or it may be some left over wake turbulence from some jet that has passed through that area before. The latter reason is mostly unpredictable. On takeoffs or landings we can adjust our course slightly left or right or slightly higher than the aircraft ahead of us to avoid the wake of the aircraft in front of us. Or there may be small air masses moving and disturbing each other, like two or more water currents coming together, that we don't know about.

The main point is that we always expect some turbulence out there all the while relying on air traffic controllers and other aircraft to help us avoid the areas that we don't know about. Basically we expect the unexpected, but we know that the aircraft will keep flying--it is made to be stable. In the cabin the passengers may feel uneasy that they can't control the turbulence itself, but that doesn't mean that the situation is out of control. The plane WILL keep flying, and we control the airplane--that's the part that we know about; we know we can't control the turbulence, but we know that too--that is why it is not unnerving to experience it.

But you CAN control some things, like keeping your seat belt fastened while you are in your seat, even if the seatbelt sign is off, "just in case we run into any unexpected turbulence". Remember, when the seat belt sign is off that is not a command to unfasten your seat belt.

I hope that helps.

Hutch :tiphat:

[bellevueace]

Many thanks for taking the time to reply, the answers give me a greater understanding of what has been bothering me. Captain Hutch thanks for the pilots view, its reassuring to know this type of experiance dosent catch the crew completely unawares and the plane is always under control.