Are Dry Microbursts Really An Invisible Trap? – Responding to Reactions

There’s been a lot of interest in my recent accident analysis “Invisible Trap Kills Glider Pilot – How To Avoid Microbursts.”  Within days it has been read by more than 5000 people, a significant number for our relatively small community.  I also received a lot of feedback and questions, some public, some private.  Most were quite insightful and thought provoking and I’d like to thank everyone for the engaging discussion.  It certainly helps to internalize the lessons we can learn from this.

Perfect capture of a microburst dust ring below wisps of virga in the Namib Desert. Many thanks to Avron Tal for sending me this picture.

In this post I would like to emphasize and respond to some of the comments and questions.

Be observant, patient, and wait!  Cold downdrafts created by virga displace warmer air near the surface creating updrafts glider pilots can, and should, use to avoid landing in downburst conditions. The very existence of virga indicates a soarable sky. Most western pilots have patiently worked such lift for an hour or more while waiting for conditions to improve near their destination airport. They made the decision to loiter long before descending to pattern altitude and sometimes miles away from the airport. The secret is to always be observant, patient and to take action to avoid dangerous conditions as early as possible.

Very well said!  If we can wait for the threat to pass this is clearly the best approach; especially when the virga is fairly isolated and the clouds are cycling.

The only caveat I would add is that waiting may not not always be the best strategy.  I have tried to wait out a storm only to watch a bigger and badder one to move in and the overall weather situation getting worse.  Through careful observation we must learn to anticipate what is likely to happen and make the best decision given the uncertainties involved.

Are dry microbursts really invisible? They occur below virga and generate dust rings.  Both of these are readily visible to an observant pilot. 

I completely agree that we must be observant and look for all possible warning signs.  However, I  would not count on microbursts always being readily visible ahead of time.

    • While microbursts probably occur mainly below virga there have been reported cases even when no virga was visible before the microburst occurred.  Also, while virga is visible, it is not readily apparent if there is a downdraft below.  In fact, having flown below virga many times, in most cases there was no major downdraft, or no downdraft at all.  Sometimes the air was even rising.  Unfortunately, such experiences can lead to complacency such that we underestimate the risk.
    • Dust rings only appear once the microburst has reached the ground.  Unfortunately, we can be unlucky with the timing and fly into a microburst in the landing pattern before any dust is visible.  This exact situation may have happened to Shmulik. The preliminary NTSB report states that at “about the time the glider [was] descending” [as filmed by the surveillance camera], “a dust cloud appears in the background travelling in the same direction as the glider.” The dust may not have been visible to Shmulik until he was on downwind and fully committed to landing.
    • A dust ring will also only be visible if the ground in the area is sufficiently dry.  That may not always be the case.

Dust isn’t the only indicator of microburst outflows.  We should also observe the ground for other markers such as the disturbed surface of lakes, wind rushing through crop fields, blowing smoke, etc.

Yes, great point!  We need to watch out for all markers of high surface winds.

We should make more pro-active use of our radios to warn other aviators of threatening weather.

Absolutely!  If you notice something, say something!

I don’t know when the pilots of the Challenger jet noticed the gust or whether they were even aware of the approaching glider (remember that Shmulik offered to delay his landing but did not get a response) but even if they were unaware it would have been prudent to immediately warn anyone who might be in the area of the gusting wind on the ground.

It’s impossible to say if such a warning would have alerted Shmulik in time to have the chance to take evasive action but we should all remember that we ought to warn other traffic immediately when we notice threatening conditions.

Considering the delay in the AWOS reporting,  it may also be helpful to proactively use the radio to ask anyone on the ground for the current winds (e.g. the local FBO).

Microbursts are not the only source of severe wind-shear close to the ground.

Yes!  This is another great point.  Sudden and very powerful surface winds can have various other causes.  They are also not limited to summer soaring weather.  Possible causes include:

    • Rotor turbulence, e.g. on wave days.  In Boulder, rotor, associated with wave aloft, is a frequent cause of severe ground level wind shear.  Sometimes the wind socks at both ends of the runway point in opposite directions!
    • Rapidly approaching cold fronts (or other fronts, e.g. sea breeze fronts). Here is an article and video of a pilot landing in cross winds set off by a cold front that arrived minutes earlier. In certain conditions blowing dust (a “haboob“) can make an approaching front easily visible.
    • Dust devils and other extreme lifting motions can also cause havoc near the surface.  Imagine being on final approach right when a  small-scale thermal breaks off the ground that can even send a 300 pound porta-potty flying high into the air.  (Btw – notice the blue sky in the video.) A more detailed assessment of such “rogue air currents” events can be found in this article in Soaring Magazine.

Shmulik was flying a motor-glider.  Why didn’t he start the engine?

There are wide-spread misconceptions about the capabilities of self-launching motor-gliders.  Once Shmulik was in the pattern the key thing that possibly could have helped him (besides a greater altitude) is a very high airspeed to get out of the sink and safeguard against the sudden tail wind.  This is not possible with an extended engine.  Extending the engine would have made the situation worse instead of better.

Here is why:

    • You actually have to slow down before you can extend the engine mast.  I don’t have a handbook for the Shark MS but similar gliders need to be flown well below 70 kt before the engine mast may be extended. (e.g. ASG 31Mi: 59 kts, Ventus 3M: 59kts)
    • The process of extending the mast and starting the engine is typically a multi-step process, not just the “flick of a switch”.  See the video below for an illustrative example.
    • With the engine running, the glider must be flown very slowly to generate a positive climb rate (usually around 55-65 kts).  Also, if you fly much faster, the engine will overspeed and may shut down.  If that happens, the propeller causes a lot of extra drag, comparable to half-extended airbrakes.
    • In still air, the climb rate under full power is likely in the range of 3-5 kts given the high density altitude environment at Rifle.  That does very little when you’re in 10-20 knot sink.

The engine could have been of help to sustain altitude at a safe distance from the airfield to wait until threatening weather has left the area.  However, had Shmulik wanted to wait 5-10 miles away he would not have needed the engine to do so because lift was readily available while he was on final glide.  But once he was in the pattern and experiencing the heavy sink it was already too late to try to deploy it.

The following video is a good illustration of a typical in-air engine start with a self-launching motor glider. (The procedure in Shmulik’s glider would probably have been a little (but not much) simpler than the one shown here given that his was a more modern design.)

As an aside, for anyone considering a motor glider, I highly recommend you review this article by Dave Nadler before you get carried away by your imagination. If you’d rather watch a YouTube video, here is one of Dave’s excellent presentations on this subject.

Why didn’t Shmulik fly straight ahead to a controlled-crash landing away from the airport instead of trying to make the runway?

It is definitely true that a controlled crash is statistically much more survivable than a “stall and spin” accident from about 200 ft.  That said, does anyone really think that this is the choice they would have made?  Here are some things to consider:

    • This option was only available before Shmulik attempted the turn to final, stalled, and spun in.  Once the glider stalled there was absolutely nothing he could have done to affect a different outcome.
    • Making such a radical decision would have required the foresight and conviction that reaching the runway is no longer possible and that a controlled crash is the only available option.
    • The possibility of a stall may not even have been on his mind: the ground speed of the glider before the stall was much higher than one normally experiences in the landing pattern – ADSB shows 92 knots.  This makes it unlikely that Shmulik even anticipated the possibility of a stall – let alone its imminent certainty – until it occurred.
    • Also consider the psychology: how do you rationally weigh – under extreme stress  and within very few seconds – the diminishing probability of a safe landing on a perfect runway against the probability of a certain crash with an uncertain outcome for your own survival?

Are you still confident that you would have instantly made the decision to fly a semi-controlled crash instead of trying to execute a safe landing on a 7000 ft runway?

Our energy is comprised of not just airspeed but airspeed and altitude together.  We need to manage both.

Yes, that is a critical insight.  That’s why I tried to lay out mitigation strategies for myself that account for both components if I must land in similar conditions (i.e., if I am unable to delay or divert).

    • Altitude:  I will enter the pattern high enough that I can be confident that I can complete the turn to final at about 1000 AGL even if I hit enormous sink. (In some situations this may require a pattern entry at 2000-3000 ft AGL).
    • Airspeed:  My baseline pattern speed in these situations will be 80 kt (20 kt above the yellow triangle speed) plus I will immediately add extra airspeed equivalent to any sink that I may encounter in the pattern.

 

10 Replies to “Are Dry Microbursts Really An Invisible Trap? – Responding to Reactions”

  1. When soaring near thunderstorms you are playing with fire to begin with. Anytime you are within 50 miles +/- of a storm you are your own weather test pilot. These are the conditions soaring pilots live for and they are the conditions soaring pilots die for, too. It makes sense to understand convection, and avoiding dangerous convection is one of those things. Soaring is supposed to be about fun, not funerals.

    1. Thanks for your comment, Mike. One key issue in this case is that there were absolutely no thunderstorms anywhere near that day (definitely not within 50+ miles). In fact, all reports say that there was not much vertical development at all. Just some regular cumulus clouds that produced some virga. This happens on almost every summer soaring day in the west. Microbursts do not require thunderstorms to form. So one of the lessons is that the absence of thunderstorms should not make us assume we’re safe.

  2. I experienced sever downdrafts while returning to Lone Pine airport in the Owens Valley a couple years ago. My Gopro was running throughout the landing and I was able to record the event. After reading the analysis, speculation, and reports following this accident, I decided to put a video together, along with the flight telemetry and comments to provide those who have never experienced severe sink a window into what it is like. The video link is below for those interested. Additionally, I have a plot of my 20 and 60 second integrated sink rate, but am unsure of how to share it here.

  3. Thanks Clemens for pointing out my lecture, but perhaps better would be: https://nadler.com/papers/Motorgliders_How_low_is_safe_for_an_air-start.html – will be in an upcoming soaring. The (first) OSTIV presentation video is also perhaps better than the slides linked above: https://www.youtube.com/watch?v=R–m0NDR0j8&t=11s

    On dry microbursts: Once returning to Minden I saw a dust ring form on a dry like (like the Nambia picture). It was shocking as the cloud above it looked like a normal cumulus – no towering or virga I could see.

    On dealing with virga: Decades back when I first flew out west, I asked Ray Gimmey how to approach Virga. Ray said: Through virga is usually OK, but never underneath. I understood because underneath all possible heat was sucked out of the air evaporating the virga, and the cold air has had some distance to accelerate – so the sink would be worst.

    Thanks again for your great articles,
    Best Regards, Dave

    PS: Now, back to repairing my motor-glider (RPM sensor problem)…

    1. Many thanks, Dave! I’ve replaced the link and also added one to your actual presentation. Thanks also for these excellent tips!

        1. Sorry Dave, not sure what happened. I tried to fix it but the link you sent went to a page that “not longer works.” I copied in a link I found on your website. Hope it’s the right one!

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