wind shear-

[YISkyDive 0]

Yesterday (Sat 28, 2005) I was at the DZ doing some Cessna loads to keep current because it was nice weather. The first load of the day was taking off into about 10 gusting to 15 MPH winds- but as the aircraft broke 150 – 300 feet it was a bumpy ride. We knew the uppers where hauling, too.

We got out- Unfortunately a little off the wind line and where landing on the West side of our DZ. The Spot was a little long too. I was the first to land- and basically went downwind along the taxi way and executed a “high” 180. I started my turn about 350 feet on a Sabre2 170. This is on the higher side for me- and I intentionally brought this around quick. A carving 180 is currently what I am working on. Well- As I got through the 140 of the turn- I realized that the canopy was diving Much more aggressively than normal and faster than normal. At about 60 feet I was on the toggles giving it light input to be sure that I wouldn’t have any problems- but actually I needed more when 30 feet came. I had no problems landing, or surfing- just that I was shocked that the canopy that normally doesn’t loose this much altitude lost a lot more at that time.

As I landed I watched a coworker fly over and start his turn into the wind- and also had an ULTRA aggressive AoA. 2 Seconds before impact I knew he needed to be on the toggles but I didn’t see him get on it till 15 – 20 feet. He impacted at about a 10 degree angle and hurt his knee. As far as I know everything else (femur / back / ribs) are alright- but I have not heard an update since he left the DZ.

The point of this post? Wind Shier. When I and the other jumper began our turns- I think we where turning with 15+ MPH to our backs. As we transitioned to our downward flight back we broke through a slower moving layer of air, which had an affect on the loading off the parachute. Our bodies continued to travel with the original amount of ‘tail’ wind- but the canopy wasn’t being pushed along with us- causing an increased wing loading- increase the length and severity of the dive. When I landed I think I landed with no more than 5 miles per hour of wind.

Since I got on my toggles fair soon( in my “roll out" picture- I knew something was wrong ) I was fine and never even found the “corner”- I just was using more of a forced recovery arc. The other jumper was either ‘turning into the wind’ or doing his standard approach as well and the increased AoA that he had caused his usual turn height not to work.

I never knew wind shier could do this- especially on a cold day you expect that the canopy will dive a lot less than on those hot muggy humid days. I just wanted to post this in the swooping and canopy control forum because its not in my place to start this as an incidents thread- being as I wont name the DZ, or jumper in question.

I just wanted the physics discussed amongst swoopers and to see what they think. I discussed this with someone very knowledgeable in physics and that person helped lead to this conclusion.

I know by my sight picture- that I started my turn high. I also know that I was amazed at how long the canopy was recovering. Since it looked wrong- I got on the toggles to fix the problem early- something the other jumper either missed, or was doing but I didn’t see.

I was far away- 500 feet? Maybe more. I couldn’t tell if it was a toggle whip ( doubt) or a riser turn. I believe I didn’t see him deep in toggles until 20 feet. But I am a ‘first hand witness’ so I was overwhelmed with the severity of the incident and started running to his aid and not remembering every detail.

I wish the jumper a speedy recovery! We all hope to see you at the D soon!

If anyone would like to discuss the physics with starting a turn in higher wind than ending- Id be very interested to hear what goes on. In this case we are talking about 180s- but obviously the importance of a 90- 270 would be very helpful for all who may find this very very weird situation at the DZ.

One other thing I wanted too add- 5 of us went up. I had ‘0’ bump the entire time under canopy. The jumper who had the hard landing- appeared to have 0 feedback in the canopy from what I could see, and another person doing a 180 approach I believe, had also no bump. Another person on the load reported a huge amount of turbulence on final. He felt huge rises and falls on final- he had a very long final. The last person, I didn’t talk too- so I am not sure about his approach.

The jumper also, I know, jumped the weekend before and there where a good amount of loads flown.

[diablopilot 2]

My experience suggests that the canopy probably reacted as it normaly would in relation to the air mass it was flying in, but your and the other jumpers perceptions of movement in relation to the ground is what caused the issues.

----------------------------------------------
You're not as good as you think you are. Seriously.

[ntacfreefly 0]

I agree with JP. Wind shear certainly does exist and it WILL affect your canopy, however 99% of the time that I hear jumpers complain about this is when it was really them sucumbing to a common illusion.

Quote

In this case we are talking about 180s- but obviously the importance of a 90- 270

90, 180, 270, etc doesn't matter. The effect of the wind direction on the canopy would remain the same.

Blues.
Ian

To the mind that is still, the whole universe surrenders. ~ Lao-Tzu

It's all good, they're my brothers ~ Mariann Kramer

[YISkyDive 0]

darn- thats how you spell it. Argh! I was looking for it too.


Im a little confused on the analytical thinking involved, so forgive me for being slow.

I understad the idea of airspeed versus ground speed. the canopy could careless what its doing in relativety to the ground- beacuse it will always act the same in the relative air.. but the relative air changed in this case from lets say 15mph WSW winds to 5MPH WSW winds.


from here I amat a loss- other than I understand why the canopy dives longer.. with the physics idea of the canopy not receiving the same amount of directional pull as my body is applying on it.

[bob.dino 1]

To the best of my understanding: if there was wind shear involved the canopy would not have continued on a smooth flight path; the abrupt change in wind velocity & associated turbulence would have felt just like flying through turbulence.

If that was absent, you likely succumbed to the illusion alluded to by ntac & JP. There's an excellent description of the effect in Brian Germain's The Parachute & Its Pilot, and I won't butcher it here. If you don't have a copy of the book, PM me, and I'll try to explain it for you...


edit to add: Wind Shear is the name for the boundary layer between two masses of air moving at different velocities. This boundary layer is usually characterised by turbulence, including downdrafts & other nasty beasties.

For anyone interested in the subject, I'd highly recommend picking up Understanding the Sky, by Dennis Pagen. It's not incredibly well-written, but it's the best reference available for the likes of us.

[YISkyDive 0]

Im stupid- I understand what you guys where saying now.

Ya- I thought of that too- but let me put it this way- On a 170 Sabre2 I was in a more aggressive dive, that was longer, and recovered longer, than on a 150Xfire 2 I jumped. (thats a complete side note- and I do not want that to enter into this conversation at all- merely an observation that I noticed)

So thats why I think something extremely weird was going on. Obviously the lack of feeling the turbulence indiactes what your saying- but also- if the turbulet layer between the two moving air masses was small- isnt it possible the canopy would have been less sensitive too it? Especially since it was moving faster?

Just wondering- I honestly do not know. The dive was just very aggressive, and very deep- deeper than on a smaller higher performance canopy.

[bob.dino 1]

If it was less sensitive to the turbulence, then it wouldn't have been in a markedly-steeper-than-normal dive, now would it?