Rough Air on Approach...
Posted July 30, 2007
You may have seen the story on our Friday evening or Saturday morning news, or here on WRAL.com (http://www.wral.com/news/local/story/1644020/), about the American Airlines flight from London that encountered a strong burst of turbulence or wind shear on approach to RDU. Several flight attendants were apparently treated for minor injuries and the passengers received a pretty significant scare, one noting that it felt like the plane suddenly dropped 50 to 75 feet. Although this is a difficult number to estimate, the feeleing of having the bottom drop out is by no means a good one, especially when you're already at a fairly low altitude.
Although we don't have information on the exact flight path and height above ground of the aircraft when it encountered the turbulence, we do know it occurred Friday afternoon when there were several thunderstorm cells in the vicinity. Greg Fishel did a little hunting through Doppler radar imagery Friday evening, and retrieved the images above. The first is a reflectivity image (basically how much radio energy is being scattered back to the radar, with the greatest amaounts indicating locations of heavy precipitation and/or hail) that shows a moderately intense thunderstorm cell over southeastern Franklin County between Wake Forest and New Hope. The second and third images are Doppler Velocity displays, which show wind components going away from the radar in shades of red and those approaching the radar in shades of green. In these images, we see a divergence signature just northwest of New Hope (winds toward the radar as high as 46 mph, with winds away from the radar as high as 20 mph just beyond the strong inbound winds, meaning air in the middle is rapidly spreading outward). Since we are sweeping the storm at a rather low altitude (around 1500 feet) in these images, the divergence may indicate a strong downbust is underway, with the air impinging the ground being forced to spread rapidly outward. These outflows can be rather shallow, and there can be significant turbulence or strong vertical wind shear along the upper edge of the outflow.
This is the kind of system that aircraft typically attempt to route around in order to avoid being caught in strong updrafts, intense downdrafts and the intense turbulent shear at the boundary between those. However, even outside the storm there can be outflows that can affect flight performance and that can create a lot of discomfort for passengers on occasion. While there's no way to be certain exactly what the situation was with the incoming flight on Friday, it seems like a situation where the descending aircraft could have settle into a turbulent shear zone at the top of a strong outflow - this could create a lot of bouncing and shaking, for example - and then crossed into the pool of outlfow air below. with a sudden change in ambient wind direction and speed. If this change in direction is such that the plane is suddenly subjected to a strong tailwind, or equivalently a sudden decrease in headwind, then it's effective airspeed (and the lift associated with that airspeed) will rapidly decrease. If this occurs fast enough, the plane can drop in a hurry, until the pilot can increase power settings enough to offset the lost airspeed.
In any of its forms, strong or severe turbulence can be a scary experience, especially for passengers who don't have any sense of being in control of the aircraft. I've been in a couple of those situations, once at Washington National when very strong wind shear near the surface had our plane going up, down and rolling hard left and right, to the point that it was unlandable and the pilot did a go-around after descending to about 1-200 feet off the ground. Luggage bins were popping open and one person had to be treated for heart trouble, and I definitelty had some white knuckles myself that day. My other big turbulence encounter was on a smaller aircraft at flight altitude one evening in the vicinity of strong shear aloft, this time in clear air turbulence. Looking out the window a bit earlier, I had seen Kelvin-Helmoholtz wave clouds (last image above) not far away, and apparently we flew into and through a breaking wave just a little later. The plane was jostled around pretty hard, then whooshed upward, followed by a very hard and sustained descent (the flight attendant flew up off the floor) then just as suddenly we were back in fairly calm air and the rest of the trip was uneventful.
If you have a turbulence encounter you'd like to share below, feel free.
On a much lighter note (lighter than air, in fact!) I wanted to pass along this link that Chris Thompson sent today - it's a time lapse of hot air balloons that is a real different way of seeing them - be sure to keep an eye on the poor eagle toward the end of the video... see the video at