Microbursts & Gliding, Part III
What Caused the Crash?
I believe Rick’s analysis is spot on. The deadly trap was a microburst.
“The virga produced a microburst directly over Shmulik as he was waiting for the jet. He expedited his landing trying to fly out of what was likely epic sink. While his base leg was low it looked high enough to make the runway with plenty of energy to flare and roll out. But he then got hit from behind or descended into winds in excess of 40 kts and perhaps as much as 50, stalling the aircraft and removing any opportunity for control.”
As Shmulik began his final turn he faced two closely related problems that became impossible to overcome:
- Extreme sink of close to 2000 fpm, which had very quickly eroded his altitude reserves during the last part of his downwind leg.
- A sudden and very powerful wind gust from behind, which caused the airplane to stall and spin in just as he was in the midst of his final turn.
Just how quickly he lost altitude may be hard to imagine; especially for pilots from regions where 2000 fpm sink is very unusual. Some basic math illustrates the magnitude: a typical safe altitude at the end of the downwind leg (before turning base) is 500-600 ft AGL. The typical time that it takes to turn from downwind to final is about 20-40 seconds (depending on how close the pilot flew the downwind leg parallel to the runway). At a sink rate of 2000 fpm it only takes 15 seconds for the plane to lose 500 ft and reach the ground. In other words: if you’re at 500 ft AGL and 20-30 seconds away from reaching the runway and you are in 2000 fpm sink it is mathematically and physically impossible to get there.
Now, you might say that the sink rate is likely to diminish as you get close to the ground. This is of course true because the air cannot sink into the earth. But that is where the second problem arises: the sudden tailwind.
Near the ground the rapidly down-streaming air is necessarily diverted into a very strong horizontal flow along the surface. At the worst possible moment Shmulik descended into that strong horizontal outflow, which came directly from behind, at speeds exceeding 40 knots, maybe more. The stall speed of Shmulik’s glider was approx. 40-43 kts in straight flight and 44-52 kts in the turn (depending on his bank angle). A sudden gust of 50 knots would have caused a stall unless he had been flying at about 100 knots indicated.
The ADSB trace shows Shmulik’s ground speed of 92 kt as he began his final turn. At the high density altitude at Rifle a ground speed of 92 kt would have been equivalent to an indicated airspeed of less than 80 kt. If this included a wind component of 50 kt from behind, his true airspeed would have suddenly dropped to 30 kt, i.e. well below stall speed.
Once the glider stalled (at an altitude of only 100-200 feet) there was nothing that Shmulik could have done to avert the crash.
As Rick pointed out, the root cause of the sink and of the subsequent tailwind was almost certainly a (dry) microburst. To understand exactly what likely happened and what we may be able to do differently, we first have to learn more about microbursts.
What is a Microburst?
A microburst is defined as “a pattern of intense winds that descends from rain clouds, hits the ground, and fans out horizontally. Microbursts are short-lived, usually lasting from about 5 to 15 minutes, and they are relatively compact, usually affecting an area of 1 to 3 km (about 0.5 to 2 miles) in diameter. They are often but not always associated with thunderstorms or strong rains. By causing a sudden change in wind direction or speed—a condition known as wind shear—microbursts create a particular hazard for airplanes at takeoff and landing because the pilot is confronted with a rapid and unexpected shift from headwind to tailwind.”
Unlike tornadoes and other twisters, microbursts are straight-line winds. The air is streaming straight towards the earth. Near the ground, it is deflected sideways in all directions. The following streamline diagram is from the November 2020 edition of Soaring Magazine which describes the Mayhem at Minden, NV when a powerful 56 kt microburst destroyed several gliders on the ground.
Wet vs Dry Microbursts
Meteorologists distinguish between wet and dry microbursts depending on whether they are associated with precipitation hitting the ground. Wet microbursts can look very spectacular but this also makes them easy to see and avoid. Dry microbursts are much more insidious because they tend to be invisible until the downburst reaches the ground. And even then, the only visible sign may be blowing dust on the surface. This time-lapse video from the National Weather Service in Reno, NV captured a dry microburst with surface winds of 71 mph. Note that you can’t see the downburst itself. Only the blowing dust on the ground is visible.
Photo in Cross-Country Magazine from November 2015 depicting the outflow from a dry microburst in northern Nevada. The microburst would be invisible were it not for the dust getting kicked up on the ground. If the surface were less dusty the remnants of the virga directly above the dust would be the only indicator.
The atmospheric conditions favoring dry microbursts are illustrated in the Skew-T chart below from the University Corporation for Atmospheric Research. Note the very dry airmass near the surface and a more moist, sometimes saturated mid-level. Cloud bases are high and precipitation evaporates in the dryer layer below. This is visible as virga – streaks of rain or snow below the clouds. This evaporation causes evaporative cooling, which accelerates the downward motion of the falling air.
Once the downdraft reaches the surface it spreads horizontally in all directions. The downdraft itself is invisible. Only a ring of dust on the ground below the virga may signify the presence of a dry microburst.
The Skew-T chart at Rifle at 5:30 pm on June 9 greatly resembles the Skew-T above. Here, too, one can see the “inverted-V” shape at the bottom, signifying the very dry air near the surface and a more moist layer above. Such conditions are of course very common during the summer soaring season in the western United States.
Skew-T centered on Rifle on June 9, 2022 at 5:30pm. Source: Skysight
Evaporative Cooling
As mentioned, a key factor in the development of microbursts is evaporative cooling.
What is it and how does it contribute to a microburst? Everyone’s familiar with evaporative cooling: dip your hands into water on a hot dry day and feel how cool they become as the water evaporates. Evaporative cooling systems work according to the same principle.
As glider pilots we know that cool air is heavier than warmer air. So if falling rain evaporates (or falling snow sublimates), the air becomes cooler and heavier, thereby accelerating its downward momentum.
This is the exact opposite of the “cloud suck” effect that we enjoy when latent heat energy is released below cloud base, making air warmer, lighter, and more buoyant.
Virga Is a Warning Indicator
Evaporative cooling is happening by definition when virga can be observed: Virga is the visible indicator that rain evaporates (or snow sublimates).
From experience we know that sometimes there is massive sink below virga and sometimes there isn’t. Sometimes you fly through virga and you can even find yourself in lift. I cannot explain why this is the case; I can only speculate that sometimes the lifting motion is so strong that even rain and evaporative cooling cannot overcome it: in these cases the evaporative cooling may slow down the rate of ascent but it is not causing a downburst. However, if air is already sinking, evaporative cooling will accelerate the decent.
None of the pilots I asked about these phenomena claimed that they are able to reliably predict when there will be strong sink under virga and when there won’t be. And since we don’t know, I think we must take away from this accident that we have to be extremely careful when we fly below virga; especially so when we are relatively close to the ground.
Airflow Near the Surface
The following graph illustrates the airflow near the surface once the downdraft has reached the ground. You can see the air spreading out sideways in all directions.
Size of Affected Area and Duration
Microbursts are usually short-lived events, lasting for only a few minutes. They also tend to be confined to a relatively small area between 0.4 and 4 kilometers (2.5 miles) in diameter.
How Common Are Microbursts?
On summer days with strong convection, microbursts are a frequent phenomenon, especially in the dry climate of the Western United States.
In the summer of 1982, the JAWS (Joint Airport Weather Studies) program was set up to detect and observe microbursts near Denver’s Stapleton International Airport. Within 86 days a total of 186 microbursts were observed within a relatively small geographical area northeast of Denver. Microbursts were detected on more than half of these days. 83% of the microbursts were dry. (Source: Fujita/Wakimoto, JAWS Microbursts Revealed by Triple-Doppler Radar, Aircraft, and PAM Data)
99 of these microbursts were just within 10 nm of Stapleton International Airport. We can probably conclude from this that in the arid climate of the western United States microbursts are par for the course: They likely occur on almost every good summer soaring day.
Banner Photo by Mika Ganszauge
Clemens Ceipek first started to fly gliders in 1983, just after his 16th birthday. For a few years, Clemens flew fairly regularly and added certificates for winch launching and self-launch to the initial aero-tow license. Then came university, family, and a global business career. In 2017, Clemens finally decided to return to the sport that he once loved. His blog, SOARING - CHESS IN THE AIR, chronicles a second journey to become a better soaring pilot. It is, first and foremost, written for himself: to document what he learns, the mistakes, and the sense of wonder that is experienced. The blog can also serve to inspire other glider pilots and those who consider joining this wonderful, and incredibly challenging sport...