Microbursts have the power to knock any aircraft out of the sky, even jets. And since we're still in the middle of summer's convective weather, there's an increased risk everywhere in the US.
Microbursts took down Eastern Airlines Flight 66 in 1975 (a 727), and Delta Airlines Flight 191 in 1985 (an L-1011).
Since these two accidents, microburst detection has come a long way, and microburst hazards are a standard part of training. But, they're no less of a factor now, and most small airports used by light aircraft have no way to detect a microburst. So, how do they form, and where should you expect them?
All It Takes Is A Cumulonimbus
Microbursts are still the subject of considerable research, but we do know that nearly any cumulonimbus cloud can form one. In fact, thunder and lightning aren't even necessary. And while microbursts can erupt from airmass storms, squall lines and severe supercell storms; isolated, single-cell storms may pose the greatest danger. Pilots often (and mistakenly) don't expect severe weather from these common storms. But if one sprouts a microburst, it can cause you major problems.
Heavy Precipitation Forms The Downdraft
A microburst's signature trait is a column of rapidly descending air, which can fall at 6,000 feet per minute. Heavy precipitation is the culprit here, forming a downdraft through drag and cooling.
As rain falls, friction between the rain and air begins to pull the air downward. At the same time, dry air begins to evaporate the rain. It takes energy to turn liquid water into a gas, which comes from the heat in the surrounding air. Essentially, the rain sucks the heat out of the air as it evaporates, cooling the air column.
The column of rain and air is now cooler than the surrounding atmosphere. Since cold air is more dense than warm air, it weighs more and begins to descend. As the rain and air fall, the rain continues to evaporate, cooling the air even more. The descent rate speeds up, and you have a microburst.
Wet Or Dry
In humid climates, precipitation may not completely evaporate in a microburst before it reaches the ground. These are called wet microbursts.
However, in dry climates like Colorado or Utah, all of the precipitation often evaporates before it reaches the ground, leaving an invisible column of air speeding down at 6,000 feet per minute. These are called dry microbursts. You'll see virga in the air, followed by a swirling cloud of dust spreading out from the base of the microburst column.
It doesn't matter if a microburst is wet or dry, however. Both are dangerous.
Spreading Out And Dying Out
As the column of air in a microburst reaches the ground, it begins to spread out in a circle. It also begins to curl up and in, creating a vortex ring around the outside of the microburst.
Microbursts are small, and are officially less than 2.5 nautical miles wide (about 4 kilometers) with peak winds lasting less than 5 minutes. The more generic "downburst" covers wider events, but both are commonly called a "microburst."
Microbursts usually last only a few minutes, usually intensifying in the first five minutes and then dying out. Multiple microbursts can happen in a row, and they can create a sequence that lasts for 30 minutes or more.
The Low Level Wind Shear Experience
If you take away one thing from this article, it's that all microbursts are dangerous. Besides the obvious downdraft that could push you into the ground, the massive wind shear across the microburst can cause you to stall, and adds to your descent rate.
Here's an example flight through a microburst, step-by-step.
1) Imagine you're approaching a microburst at 80 knots. As you enter the vortex ring, you experience turbulence and a rapid increase in airspeed as you pick up a headwind. You start to climb as your performance increases.
2) As you enter the strongest part of the horizontal shaft, your airspeed peaks at 100 knots, an increase of 20 knots. You're still climbing as you enter the downdraft.
3) Inside the downdraft, your headwind starts to switch to a tailwind. You're caught in the downdraft, sinking quickly toward the ground. Your airspeed begins to decrease.
4) As you exit the downdraft, your tailwind increases rapidly. The shear drops your airspeed to 60 knots, which is 20 knots below your original speed. You're at a high angle-of-attack, and still descending.
5) You encounter more turbulence as you enter the final vortex. If you haven't already hit the ground, you may begin to climb again as you exit the vortex.
Total Shear = Double The Peak Wind Speed
While the downdraft in a microburst is dangerous on its own, wind shear makes the situation significantly worse. If the peak gust speeds in the outflow are 25 knots, you'll experience 50 knots of shear as you cross through the microburst. That means your airspeed will increase around 25 knots, then quickly drop 50 knots before you exit.
However, gust speeds in the outflow can reach 45 knots. If that were the case, you'd experience 90 knots of shear, which is more than any aircraft can safely handle.
Avoiding A Microburst
How do you stay out of a microburst? Some airports have Low Level Wind Shear Alert Systems (LLWAS), Terminal Doppler Weather Radar (TDWR), or Weather Systems Processor (WSP), that can detect a microburst as it occurs. However, these are usually only found a large Class C or B airports that serve airline traffic. If these services are available, ATC will alert you to wind shear conditions.
At smaller airports, pilot reports and your eyes are the best way to avoid a microburst. Simply put, don't fly underneath a thunderstorm. And, if you see a rain or a virga shaft descending from a cloud with dust blowing up from the ground, file a pilot report for a microburst and stay clear.
Also, pay attention to other traffic. Prior to Eastern Airlines Flight 66's crash, a DC-8 reported severe wind shear and another Eastern L-1011 nearly crashed. Flight 66's crew was aware of the conditions and decided to continue the approach.
Wait It Out
Microbursts are short lived, with peak winds lasting only 5 minutes or less. If they're caused by a single-cell storm, you can often divert or delay your arrival until the storm is clear. If you see one, wait it out. Flying through or landing in the face of a microburst is simply the worst option.
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