WEAX 363 - Turbulence 3 PDF

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WEAX 363 – Turbulence 3 Now, a lot of you taking this class are probably familiar with the downburst phenomenon in the context of this issue right here specifically the issue of microbursts and what it can do to the operating environment for your aircraft. Obviously, this is going to present problems. I do have a video here of a fairly famous case involving a microburst that you know unfortunately led to a civilian aircraft accident with a number of fatalities. So let me play that for you. Tower, Delta 191 Heavy out here in the rain, feels good. 191 Heavy. We're not getting any warnings from the weather or from other pilots which we rely on as they come through it. As the pilots of Delta 191 prepare for landing, the rain begins to fall harder. At the foot of the runway, one of the most ferocious types of storm clouds stands in their way. Before landing check. Landing gear. Down, three green. At the time, the type of storm the Delta crew is approaching barely has a name. John McCarthy is one of the world's leading experts on these storms. It is a tiny thing, meteorologically speaking, compared to a big storm or a snow storm or hurricane. It's just like a needle in a haystack. The needle is a microburst. One of the deadliest, and at the time, most poorly understood weather phenomenon. They've taken down airliners before. But as Delta 191 makes its approach, there are no warning systems that can effectively alert the pilots of the danger they're in. Prior to 1985, the radars onboard the aircraft were built to detect thunderstorms, essentially heavy areas of precipitation. They were not effective, they were not even designed to detect a microburst. If you're at the kitchen sink and you turn on the water and it goes straight down and it splashes out in all directions, and that's kind of what a microburst is, except that it is extremely bad news if you're an airplane flying through it. When a plane hits a microburst, it encounters a complex and powerful set of conditions: downdrafts and tailwinds batter a plane. It's a deadly combination. Just short of the runway, Delta 191 flies into the microburst. You're gonna lose it all of a sudden, there it is. Push it up. Push way up. Way up! Way up. Way up. Hang on to the son of a- He's gonna crash! The pilots of Delta flight 191 did their very best to recover from this situation, and it didn't work out. I must have caught sight of them just at the last millisecond, and he cartwheeled into the tank in just an instant and then of course there was fire, not a ball of fire but a wall of fire. Just 27 people survived the crash of Delta 191. One hundred thirty seven people are killed. After the crash of Delta 191, the Federal Aviation Administration races to develop technology that can prevent microbursts from killing again. If there is one crash that we can look back on now and say, "this made things safer because we learned from it." It was Delta 191. So again obviously this is a you know potentially very serious situation that you have to be familiar with. So you know what what's going on here is you know that the downburst, the microburst what it's doing is it's significantly changing the wind structure in the lower part of the atmosphere. So if you look at this figure, look at what happens over a very short distance of time. The aircraft is coming in, they first experience abnormally strong headwinds, but then over a very short distance, those abnormally strong headwinds switch to abnormally strong tailwinds. Now that affects the lift on the aircraft. Right. I probably don't have to tell many of you in this class you know the the basic physics of flight. But you know as the air blows across the wing it create creates a pressure difference. High pressure on the bottom side. Low pressure on the top side and that leads then to the aircraft rising. OK. Well the lift force is increased by headwinds reduced by tailwinds. And if you mess with that structure over a short distance it's going to mess with the behavior of the aircraft. And then there are other issues too especially you know where this would be occurring in the lower part of the atmosphere where you're landing and takeoff speeds are close to stall speed, so not a good situation. Here, let's illustrate that in a bit more quantitative way how that works. OK. So we know that the airspeed, the wind blowing across the wing if you will, is going to be the sum of the ground speed. OK. Basically

how fast you're moving relative the ground plus the speed of the wind itself blowing across the wing. That gives you the airspeed which affects the lift on the aircraft. So here this aircraft is taking off. Right. I have a scenario where I've got a microburst, downburst sitting on the end of the runway. There's 40 mile per hour gusts with that. It's ground speed and column air would be 100 miles per hour. So the total speed of the air moving across the wing is 140 miles per hour. Real good lift in that situation. Well what happens in a very short distance? That 40 mile per hour headwind becomes a 40 mile per hour tailwind. And so now the airspeed across the wing has dropped to 60 miles per hour. The aircraft stalls and you know ends up crashing into the ground as depicted here. And then as I described to you earlier, an aircraft landing. Strong headwinds here increases the lift. The pilot may try to adjust to bring it back down to you know the glide slope here. But now they make that adjustment and then the headwind becomes a tailwind. You lose all that lift and you end up short of the runway. So very problematic. Now what I just showed you was sort of this situation where you go from headwind to tailwind flying through this general region. Depending on where the microburst is though you could also experience what's known as a crosswind burst. And of course that could be very problematic. I think again most of you know how significant of an impact a strong crosswind burst can have when you're trying to operate in the lower part of the atmosphere. So here is another example of a fairly famous microburst. I showed you the video earlier on the Dallas event. This is an event that didn't result in any fatalities or even an aircraft accident. Thank goodness. It just came very close to you know leading to a more significant event one involving Air Force One by the way, President Reagan back in nineteen eighty three. So what you're looking at on this graph is a wind trace. OK. And the time goes from right to left. So this was the situation one August, 1983 in the afternoon. This is 1402 in the afternoon. And during that time period the winds actually were quite strong at the surface blowing out of the northwest. You had winds you know somewhere between 15 and 20 knots blowing out of the northwest. Well at four minutes after the hour, Air Force One landed at the Andrews Air Force Base and literally about five or six minutes after that we experienced this strong microburst. Now remember the structure of the microburst where you had that Hershey Kiss strong gust of wind. Well the initial gust of wind here was upwards to 130 knots. Then that microburst moved across Andrews Air Force Base and you hit the stagnation cone, the eye of the microburst, and your speeds dropped next to zero. And then there was that secondary gust that I talked about earlier 84 knots here. Now if you look at the wind direction, prevailing winds were out of the northwest. So that microburst was moving from the northwest to the southeast. I give my initial gusts OK here out of the northwest. So the microburst is moving towards Andrews. Initial gust out of the northwest. It moves across Andrews here on the wind speeds drop. And then on the backside you get that secondary gust. If you look at the wind directions here, the winds were out of the southeast associated with that 84 knot gusts again it didn't lead to any air aircraft accident but it came very close, within about six minutes or so. The detection of these downbursts microbursts. What sort of technologies do we have? Well a certain type of radar can be used to detect these downbursts. In fact we have a network of what's referred to as terminal Doppler weather radars. These are weather radars that aren't at the airports themselves but they're typically located maybe 15 or 20 miles from the airport, scanning that region, scanning the lower part of the atmosphere looking for significant wind shear or microburst in the area. So significant low level wind shear or microburst activity in the area. Remember with Doppler radar, Doppler radars can see motion. And so the motion the high wind speeds associated with a downburst would be detected by these radars. The facilities that have them are indicated on the map here. So you see the major facilities across the country that would be susceptible to downburst activity currently have these radar. For example, you know there's one at Orlando, and one in Tampa, one down in the Miami area, several actually down in the Miami area. Now another detection system for downbursts are these low level wind shear alert systems. These are less high tech. Fairly simple right, because what's being done here is you're just laying out anemometers around the winds- around an airport rather. So you have

maybe four or five of these anemometers at different locations around the airport. And the idea is this, OK, let's have five anemometers around a particular airfield. I all of a sudden see a 50 knot wind at one of the anemometers. Well that is probably a microburst occurring at that location. Remember microbursts are fairly small scale. So it's very possible that would only be affecting maybe one or two of the anemometers at that location. We in fact have a low level wind shear alert system at Daytona Beach I believe there's four or five anemometers set up around the airport that provide this information. Now, there are also potentially onboard systems that can be used......