MICROBURSTS & GLIDING, PART IV

How Does It Feel in the Cockpit When We Encounter A Microburst?

What we experience in the cockpit differs greatly depending on our altitude (and on the stage of the microburst’s development when we encounter it).

Microburst Encounters at Altitude

Imagine that you fly through the descending shaft of the microburst as illustrated below.

In this case, the only indication of a microburst may be very strong sink.  The onset of the sink could be quite sudden such that you bump your head on the canopy, or it can come about more gradually over a period of a few seconds.  In the western US, where we often fly 10,000 feet or more above the terrain we might not notice anything particularly unusual.  We have all have flown through patches of very strong sink lasting for about 30 to 90 seconds.  We might have been slightly annoyed that we just lost one or two thousand feet of altitude but that is likely all we noticed. Cruising at 80-100 kts we cover almost two miles per minute.  Normally, this is more than enough time to traverse through the confined area of most microbursts.

If we encounter the microburst a little earlier in its development, i.e. just when the air is beginning to drop past our flying altitude, we may also experience more turbulence and wind shear when entering and exiting the burst.  However, even then it seems rather unlikely that we would get in real trouble (provided that we are still at a safe altitude once we exit the sink).

Microburst Encounters Closer to the Ground

The encounter is quite different when we fly closer to the ground because we are now confronted with the horizontal outflows.  The lower we are, the more dangerous the situation. The above-referenced study by Wilson, Roberts, Kessinger, and McCarthy suggest that the greatest danger is at altitudes below 1000 ft AGL.  The reason is that we first encounter a headwind followed by a tailwind as we fly through the outflow area near the surface.  The difference in speed between the headwind and the tailwind tends to be greatest at an altitude of about 200 ft AGL.  Consider the illustration below.

In this case, we are likely to encounter a sudden headwind and therefore a surge in kinetic energy causing the glider to rise and accelerate, soon to be followed by a sudden tailwind and a rapid drop in airspeed that could force our glider to stall unless we were able to maintain a sufficient airspeed margin.

Avron Tal sent me this amazing picture of a microburst in Namibia. You can easily imagine that you’d hit very strong sink if you fly across the down shaft. You can also clearly see that the real danger would be near the surface – at the height of the outflow. Picture the headwind, followed by sink, and then a sudden and massive tailwind. It would be very difficult to escape once you get caught low.

The Greatest Danger is Below 1000 ft

The lefthand side of the following chart illustrates the differential in wind speed between the headwind and the tailwind at different altitudes for 12 different microbursts.  The solid line is the average.

You can readily see that the greatest wind speed differential, i.e. the greatest wind-shear is at altitudes below 0.2 km (i.e. ~600 ft) with the peak of the average at less than 0.1 km (about 200 ft). With increasing altitude the wind speed differential (and therefore the danger) decreases.  In some cases it can be measured up to about 0.6 km (~2000 ft).

Everything Fits

Now that we understand a lot more about microbursts we can readily see how all indicators fit together.

Shmulik was supremely unlucky because he flew directly through the center of the downdraft when he was on his downwind leg.  And then, just as he started to make the turn from base to final he was hit by the microburst outflow coming directly from behind.  At that point he had descended to an altitude of approx. 200 ft where the strength of the outflow is typically the strongest.

The following charts show radar images for 5:30 pm, 5:35 pm, and 5:40 pm. The purple circle shows the small cell from where the downburst most likely came from.

AWOS reported the strength of the gust on the ground at 43 kts.  Based on the data from the research study referenced earlier it is likely that at 200 ft AGL the outflow speed was about 10-20% greater than near the ground.  I.e., 50 kt or slightly higher.  Shmulik would have needed to fly at an IAS of around 100 kt to avert a stall and have a chance of maintaining control.

The time duration of the event was very limited, just as would be expected.  After the gust had come through, AWOS went back to reporting light winds out of the west.

That’s when the other three glider pilots returned to the airport, just minutes later. Based on their flight traces, all three landings appear completely normal and uneventful.

Rick Roelke followed Shmulik, touching down at 5:45:36, i.e. less than 8 minutes after Shmulik’s crash.  Bill Feiges was next, landing at 5:47:38, followed by Sean Franke who landed at 5:50:51.

It is worth noting that Shmulik had started his landing pattern significantly higher (!) than any of these three pilots: the altitudes of these three pilots on downwind at midfield were between 875 and 1148 ft AGL, i.e. typical and normal pattern altitudes.  Shmulik had been at 1339 ft AGL at an equivalent position in his pattern.  This means Shmulik had the greatest safety margin of all of them.  Also, none of these three pilots flew at a higher speed in the pattern than Shmulik did.

Could The Accident Have Been Averted?

This is very hard to say.  Perhaps the most important questions is whether the amount and location of the virga should have been so concerning as to prompt a reasonable pilot to delay their landing and wait at a safe distance for the virga to dissolve or move away.

Should the Landing Have Been Delayed?

Without being able to see the sky like Shmulik did, this is of course impossible to say.  However, by all accounts none of the pilots operating at Rifle at this time were overly concerned about the extent of the virga.  Everyone’s behavior suggests that many if not most pilots would have proceeded with the landing just like Shmulik did.

• Rick’s report stated, “There was virga over the airport (elevation 5537 ft) and to the north of the valley, and northeast as well. None of the wisps extended below 11,000 ft (cloud base was approximately 19,000). Cloud cover was scattered. The clouds producing virga were not towering – they were perhaps a bit bigger than non-producing clouds, but not much. It was a point of interest to me as we don’t see a lot of it in the eastern US – I was wondering what drove the difference. 
• Bill Feiges, one of the other pilots flying that day, wrote, “I did not think there was enough virga in the area to catch my immediate attention.” Bill is quite familiar with the weather in this area as he normally flies out of Steamboat Springs, CO, just 80 miles to the NW of Rifle.
• The pilots of the Challenger jet were clearly not overly concerned either, otherwise they would not have been taxiing to the runway for takeoff.
• Plus, none of the three glider pilots thought it necessary to delay their landing even after Shmulik had already crashed.

Bad Luck

Unfortunately, there was a tremendous amount of bad luck involved:

• The occurrence of a microburst with extreme sink in the pattern just as Shmulik returned from his flight;
• The delay caused by the intended Challenger launch, which likely prompted Shmulik to fly to the south side of the airport exposing him to more sink and the sudden tailwind (instead of a headwind) on the turn to final;
• The lack of a radio response from the Challenger which may have hampered Shmulik in his decision-making (e.g. preventing him from landing straight in on runway 08 when getting low); and
• Encountering the tremendous tailwind just as Shmulik was making his turn to final, i.e. at the worst possible moment, and at the worst possible altitude.

If only one of these factors would have been different it is quite possible – perhaps likely – that the outcome would have been different as well.

It is hard to argue that Shmulik did not have sufficient altitude when he returned to the airport at almost 3000 ft AGL with only 2 miles to go.  Or that he flew unusually slowly in the pattern.  The three pilots returning after Shmulik were aware that there had been an accident.  They would have been exceptionally careful.  And yet, none of them returned to the airport with more safety margin than Shmulik did.  None flew faster in the pattern.

I believe that any of us – if put in Shmulik’s position – may have done exactly the same thing he did.  Any of us could have suffered the same outcome.  Indeed, it is tempting to conclude that this was indeed Shmulik’s fate.  That nothing could have been done differently; that none of us can do anything different.  Even, that nothing can be learned from this.

I sincerely hope that this is not true.  I am the first to admit that based on what I knew before doing this detailed analysis I would likely have acted just like Shmulik did.  But that is not the same as to say that I won’t change anything in my flying going forward.  I believe that there has to be, and that there is, something that I and others can learn from this.

Editors None, the final part of the microburst series continues in two weeks with Lessons Learned.

Banner photo by Holger Weitzel, aufwind-luftbilder.de

 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...