A mid-day sojourn in moderate winds went sour for one high-time
paramotorist resulting in a nearly disastrous impact. Surprisingly, the
pilot walked (probably limped) away.
At 20 feet or so while landing, a
quick downward gust started folding down the right half of his wing.
Momentum and relative wind did the rest, quickly collapsing 70% of his
paraglider. All that fabric, now presenting a vertical wall of drag,
caused an immediate turn that was aggravated by lift from the open
left side. The highly experienced pilot didn't know what happened until a turn began. Then it was way too late.
It took less than one second from the pilots first feel of
trouble to be in an unrecoverable situation. Lest
anyone think otherwise, realize that it takes the human brain at least
one second just to register a strange sensation. The only way out
of this was prevention.
Another 20 feet higher and it would have been much worse.
Another 100 feet higher and it would have been recoverable.
It appears he felt something because, although on landing approach, he
lifted his legs just before the collapse started. Also, it looks like the
wing surged forward somewhat. The most likely scenario is that he flew from
the rising air of a thermal into a downward swirl on the other side.
That jibes with reports from the pilot and witnesses. See sidebar on the
thermal theory. One witness said the pilot reported a blast of warm air
just before it happened. Wake turbulence (see sidebar)
is a possibility that appears far less likely.
Keep in mind, we're analyzing what took 3 seconds to finish.
It seems the wing surged forward, making it more susceptible to collapse
and this hit a bit of downward gust. Dampening that surge could have
certainly helped but it may not have been enough. When off power and in
turbulence, it's a good idea to try keeping brake pressure 2 or so
(about 1/4 brakes). I suspect this pilot, with his experience did damp
it slightly.
Thanks to Steve for sharing the video, James for helping with
information and Brent for his perspective.
Frame A through C show a normal, power off approach.
Frame D shows him bring his legs up but the wing hasn't felt anything
yet. Frame E reveals the leading edge just starting to be blown down
slightly right of center. In Frame F the slipstream is now pushing the
wing downward and causing a significant collapse. Notice the pilot is
still essentially level and in the same position he has been throughout.
His fate is sealed yet he is only aware that something is amiss. From
frame E to F is about a half-second. He has just started to turn right
in frame G and, at this point, is essentially a passenger. The only out was preventing entry.
An experienced paramotor pilot who saw it happen offered his thoughts:
It was around noon. And he was flying over
the field trying to get down a couple times before. He said he could
feel heat rising off the ground. Just before he crashed he was over
the black top taxi way.
So the hot sun could have contributed to it
for sure. I had just arrived at the field when it happened and I was
just off to the right of the camera. Paul's brother Steve shot the
video of the crash. I shot Ivan later and made the video.
Another flyer and eyewitness offers:
It was around 11:00 or so...He was flying on the downwind side of the runway.
After talking to him, he said that just before the wing gave way he
had a blast of warm air hit is face.
If you look at the
airport diagram, on the left of the page, he was east of runway
12/30. Also there are trees to the west and south at 30' to 60' feet.
With the wind coming over these trees, the heat of the runway, the
building to the east of that, (Look at the top picture of the
website), womething could happen.
He was the only one flying at the time, too.
Prevention
Flying within three hours of sunrise and sunset, and only during
forecast mellow conditions is the best prevention. Some pilots,
especially free flyers who are used to stronger conditions, will take on
the mid-day risk knowingly. Sometimes they get burned.
Winds appear to have been fairly light. If you walked outside and
checked, you have considered it nice.
Wunderground historical data showed it to be light that Sept 22
noontime. See the charts at right for a temperature graph. Thanks to
Terry Lutke for the weather information.
Skill and choice of equipment also has an effect on susceptibility. For
example, being heavy on a wing reduces the likelihood of a collapse at
some increase in severity. Your choice of a wing affects it, too. A
fast, high performance wing is more susceptible than a beginner wing
although it can happen to anything—if the air is going down faster than
the wing can handle, the leading edge will react this way. A
reflex wing, flown in its most resistant configuration, may handle it
better, too. Reflex wings don't fold under as easily although they
certainly can and you better know what configurations are susceptible.
Once aloft, a pilot can reduce his chances of this fate by those
suggestions covered under
Handling Wing Collapses. Essentially keep brake pressure 2 applied,
especially with the power off. Fly actively if you've already developed
that skill, otherwise, now is not the time to learn it. Using
just the right brake inputs to keep the wing overhead (active flying) is
a skill that takes many hours to master and pilots who don't fly
regularly in moderately bumpy air never acquire it. Strong turbulence is not
the place to learn since inappropriate inputs are worse than just
holding pressure 2 and letting your hands float with that pressure.
Video of Collapse and Crash
Some interesting points. Notice the video's time counter. It's basically over in
two seconds. Ivan is one tough bird! The windsock and other pilot input
suggests that winds at the time were relatively light (see sidebar on
Weather at the Time).
The accident happened at this airport although not to this pilot.
Photo taken by Phil Adkison of his wife Cynthia. Provided by James Zeman.
Downdraft Collapse
If you fly into a sharp, powerful downdraft it can fold the leading edge
down and cause a collapse.
The diagram below shows how a small, powerful thermal could cause it.
The exact same thing could happen when flying into the downward swirl of
rotor turbulence or a wake from another aircraft.
A paraglider can only fold downward so, as you hit the updraft portion,
you're lifted. It feels like when an elevator starts but lasts longer.
Sometimes the wing goes back, sometimes it moves forward depending on
airflow in the thermal. The base action is for the wing to go forward
slightly because of the momentary apparent weight increase while you
accelerate briefly upwards.
But when you fly out of the rising air and into the static or descending
air at the thermal's edge, the wing folds downward.
Weather at the Time
Below is a graph showing the winds recorded nearby from
Wunderground. As you can see, base line conditions were mellow.
Was it Wake?
I've flown through my own wake many times in spirals and other times and
have never had it do anything even close to this.
Of course it could have been wake turbulence. He's shown turning
back towards his flight path of 20 seconds prior. That would have
been close, but it should have been noticeably dissipated by the time he
got there. I could see it if he were on speedbar but
that doesn't look to be the case.
So, although wake is a possibility, it seems far more likely
that this was caused by thermal induced turbulence.
