Microbursts & Gliding, Part II

Editors Note:  This week's newsletter is the second of a three-part series reporting in detail events leading up to a fatal accident in June 2022.  The first article, including Rick's first-hand account, is archived on the W&W blog page.  This week's article lays out environmental factors and the last seconds before the crash.  The article is longer than usual.  We hope you read through to the end.
 

As Rick pointed out (Microbursts & Gliding, Part I), Shmuel Dimentstein was one of the most experienced, competent, current, and safety-conscious pilots anywhere.  In the 2021 soaring season, he flew more than 35,000 cross-country kilometers, a distance almost equivalent to the circumference of the earth.  That year, according to OLC, only six pilots worldwide had done more cross-country flying than Shmulik.  Even in the current season, he had already flown more than 100 hours.

Rifle, the location of the accident, was Shmulik’s home airport. He was intimately familiar with it and the surrounding terrain.  He frequently hosted visiting pilots, providing them with detailed briefings of the area and the weather.  He had owned his HpH 304MS Shark for several years and was completely accustomed to the aircraft. Shmulik was also a very safety-minded pilot: if you examine his flight traces you will see many long cross-country flights but you will be hard pressed to find any signs of inappropriate risk-taking.

Soaring on June 9, 2022

By all indications and consistent with Rick’s report, June 9, the day of the accident, was a good and typical early summer soaring day in western Colorado and eastern Utah, the main task area around Rifle.

Skysight forecasted abundant cumulus clouds with bases rising from 16,000 to about 19,000 ft.  There was a modest chance for some overdevelopment and isolated showers in the afternoon but nothing that looked concerning.  The CAPE index, a measure of convective energy and instability, was below 100 joules, indicating a very low probability of severe weather or thunderstorms.

Boundary layer winds were moderate at 10-20 kt out of the WNW.  Surface winds were forecasted to be even lighter. The surface temperature in the afternoon was likely to reach 100 degrees F over the western desert generating strong thermals in the 6-10 kt range as is typical for the area at this time of year.  Moderate wind shear in some areas could make some thermals somewhat difficult to work but that, too, is typical.  Some passing high clouds were unlikely to be a factor.  500 km flights were easily doable with a good chance for even longer flights.

As Rick reported, four glider pilots launched from Rifle that morning.  The flight traces of Shmulik’s three visitors, including that of Rick Roelke, were uploaded to OLC here, here, and here.  These traces show good soaring conditions consistent with the forecast with pilots repeatedly climbing above 17,000 ft and achieving flight distances in the 400-600 km range.

Approaching Rifle

The last six minutes of Shmulik’s flight are plotted on the following map.  The data is from the publicly available ADSB tracklog. For each data point, you can see the time stamp, the altitude MSL, the Ground Speed in kt, and the Vertical Velocity in feet per minute (fpm).

The ground elevation at Rifle airport is 5536 ft MSL.  The first datapoint of the trace is at the top left.  At 5:31:34 pm, Shmulik had 9.4 miles to go.  He was at an elevation of 10,025 ft MSL, i.e. 4,489 ft above the airport.  He only needed a glide ratio of 14:1 to reach the airfield to arrive at a typical pattern altitude of 1,000 ft AGL.

The next few miles towards the airfield show nothing unusual.  There were some patches of moderate lift and sink as would be expected on a normal summer soaring day.  Shmulik flew quite fast at ground speeds between 100 and 130 kt, carrying a lot of extra energy. Unsurprisingly, the actual glide ratio of his 49:1 glider was much better than the required 14:1 and he approached the airport relatively high.

At 5:34:04 pm, Shmulik was over the town of Rifle, just 2.1 miles ENE from the center of the runway, getting ready to land.  At this point he still had an altitude of 8,275 ft MSL, i.e. 2,739 ft AGL.  This is much higher than what most pilots would consider an adequate safety margin.

Reported Winds on the Ground

Shmulik likely checked the winds on the ground by tuning to the frequency of the local AWOS (Automatic Weather Observing Service).  Between 5:10 and 5:34 PM, Rifle’s AWOS system reported light winds out of the west in the range of 4 to 9 kts (see chart below) with no wind gusts.  This is consistent with Rick’s report, which referenced 9 knots of wind.

With light westerly winds, Shmulik was likely planning to land on Runway 26, directly into the wind. He may have expected an easy and uneventful landing.

Pattern Entry and the Challenger Complication

Downwind Leg and Turn To FinalThe normal landing pattern for Runway 26 at Rifle is north of the airport with right turns to base and final.   At 5:35:22 Shmulik could have immediately entered the downwind leg of the pattern.  At this point he was just NW of the runway at an altitude of 8000 ft MSL (2464 AGL).

However, we see from his trace that he continued south past the west end of the runway to the southwest side of the airfield.  It is possible that he still considered himself to be too high for an immediate pattern entry. After all, a pattern entry altitude of approx. 1000 AGL is customary and Shmulik was still more than twice as high at this point.  He may have planned to remain on the south side until mid-field, cross the runway to the north, and then enter the normal right traffic pattern to runway 26. This would have slightly extended the flight path, helping him fly off the extra altitude.

However, Rick’s report suggests that there is likely a different – or at least an additional – explanation for why he continued to the south side of the airport.

“Shmulik and I discussed the landing order: as he was a bit lower we agreed he would go first. After we decided this, we heard a Challenger jet announce “Taxiing to 26 for takeoff”. That was the runway we would use to land. 

Rifle has a moderate amount of bizjet traffic; not constant but present. We always try to accommodate and be polite citizens. Shmulik called the Challenger and offered to delay but got no reply. I was still high so it was no problem for me. He tried again, with no reply. It’s worth noting that Shmulik had a close call in the past: a jet pulled onto the runway in front of him with no radio call. This near miss was avoided only by the jet taking off immediately in front of him. I am sure he did not want to repeat that. I speculate that the Challenger was on a different frequency temporarily, perhaps the ASOS.”

Based on this account it is likely that Shmulik continued to the south side of the airport to get a better look at the runway and observe the Challenger jet taking off – or at least to establish two-way radio contact to rule out the risk of a conflict.

Downwind Leg and Turn To Final

Under normal circumstances Shmulik would have had sufficient altitude to delay the landing by several minutes:  his glider’s minimum descent rate in still air was just 100 fpm.  Even a more typical descent rate of 200 fpm would have allowed Shmulik to hold for about 5-7 minutes before he would have had to proceed with the landing.

If Shmulik’s plan was to delay the landing this soon turned out to be impossible because he was not in still air at all.  As he continued to the south side of the runway he found himself in very strong sink of 700 – 1200 fpm and rapidly lost his altitude reserves.  Within one minute he lost a full 1000 feet.

However, at 5:36:23 he still had an altitude of 7025 ft MSL, i.e. a normally very “safe” pattern altitude of almost 1500 ft, and prudently began to head toward the east end of the runway.  At this point his ground speed was 81 kts, which – in still air – would reflect a normal pattern speed of approx. 65 kts IAS given the high density altitude.  16 seconds later, at 5:36:39 the sink rate diminished to 273 fpm.  Shmulik was now directly south of the west end of the runway.  The reduced sink rate must have been a relief.

However, 17 seconds later, at 5:36:56, Shmulik found himself once again in very strong sink of almost 1000 fpm.  Roughly at this time he must have decided to stay on the south side of the runway and fly a left hand pattern instead of crossing back to the north.  This would have shortened his approach, a seemingly prudent decision.  His altitude was 1339 AGL and his ground speed was 92 kts.  Had it not been for the strong sink he would have still been in a very conservative position for a normal landing.  Here is Rick’s report:

“As he descended, he called that he was in heavy sink and was going to make left traffic for Runway 26 (for which the normal traffic pattern is right).”

Another 18 seconds later, at 5:37:14, the sink rate doubled yet again, becoming extreme. Shmulik was directly south of midfield.  The ground came rushing closer at a rate of 1900 fpm.  Shmulik’s altitude had dropped by 625 ft in less than 20 seconds and he was now down at 714 ft AGL.  All of a sudden this had become an emergency situation.  His ground speed had dropped to 75 kt so he also had less kinetic energy reserve.  (Without knowing the horizontal wind direction and speed at this point it is impossible to say what his indicated airspeed was. It is quite likely that the air at that specific point was only streaming downwards with very little horizontal component.)

16 seconds later, at 5:37:30, he was still in very heavy sink of more than 1200 fpm and his altitude had dropped to only 264 ft AGL.  His ground speed was back up to 92 kts. Seconds thereafter he attempted to make a 180 degree turn to the left to line up with Runway 26.   Tragically, he only made it half-way through this final turn.  The last datapoint was recorded at 5:37:48 at an altitude of 14 ft, probably just a split second before impact.  Rick’s report describes it as follows:

“Shortly after this a call came from the Challenger that there was a glider crash.  

I was not sure I’d heard it correctly so I asked for clarification. “There has been a glider crash and we see no movement.” They truly had a front-row seat, as moments before they were hit by a gust so strong that they had rotated their jet to avoid a compressor stall. 

The last moments of the crash were recorded by an airport security camera. We were allowed to view the footage (but not record it). It showed Shmulik in a moderately steep turn, apparently carrying a lot of speed. In the background you can see dust and gravel being blown by the gust. Then at 90 deg to the runway and 150 to 200 ft you can see the inside wing start to drop and the nose go down. There was no opportunity to recover and it hit the ground hard, thankfully just out of camera view.

The Rifle ASOS recorded a gust of 43 kt from the south: a 100-degree shift in direction, putting it right on his tail.”

The wind gust could of course only be reported after it had been measured.  However, reporting it took longer than one might expect.  It wasn’t until 5:53 PM, 16 minutes after the crash, that AWOS reported that a 43 kt gust had occurred at 5:39 PM (one minute after the crash; 14 minutes earlier than it was reported).

Note that a 43-knot gust from a direction of 190 degrees was measured at 17:39, one minute after the accident (provided that the time stamp is accurate). However, the gust was not reported by AWOS until 17:53.  (Note: because the crash had occurred before the gust was even measured by AWOS, the reporting delay was not a contributing factor to the crash itself.  I speculate that the AWOS measurement unit may be located close to the ramp, perhaps a 3/4 mile away from the crash site. This would explain why the gust hit Shmulik before AWOS recorded it).

Editors Note; In two weeks we complete the series with an examination of microburst dynamics and how it proved to be an invisible trap.

It was a very sad day for all.  Shmulik was my friend.  I wish Elizebeth and his family the best through this tough time. ~Sean

Banner photo by Sean Franke

 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...