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AAD: Theoretical Explanation

 


ciscobiz  (Student)

Nov 22, 2004, 2:45 PM
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AAD: Theoretical Explanation Can't Post

Can somebody please explain how these devices work. I wasn't informed about them on my AAF course. I just wore one (analog) or whatever that means.

Thanks,

Chris


mnealtx  (B 30496)

Nov 22, 2004, 3:08 PM
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Re: [ciscobiz] AAD: Theoretical Explanation [In reply to] Can't Post

Disclaimer: I am not an instructor, and any information received should be doublechecked with your instructor before acting upon it.

An AAD (Cypress, in this example) is a device that will cut the reserve closing loop if you pass through a specific altitude at or above a certain rate of speed. It does not guarantee a reserve deployment, merely opens the container.

I believe the FXC pulls the reserve pin, but I am not 100% sure on that. I'm sure that many wiser heads will be able to give you better information than I have.


larsrulz  (C 34603)

Nov 22, 2004, 3:13 PM
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Re: [ciscobiz] AAD: Theoretical Explanation [In reply to] Can't Post

I do not know what AADs they use in Switzerland, but the majority of US DZs use Cypres AAD. If this is the case, then all you need to know is that if you are below 750 feet (~230 m) and you are going faster than 78 mph (~125 km/hr) then the cypres will activate. There are some complexities to the student version if that is what your dropzone uses, but basically the device records the air pressure on the ground when it is turned on. It then knows what altitude it is at by detecting the air pressure in freefall. When you get too low, the cypres will activate and open your reserve canopy. If you wish to know more, you can either ask your instructor or check here and find the user manual.


(This post was edited by larsrulz on Nov 22, 2004, 3:15 PM)


Premier billvon  (D 16479)
Moderator
Nov 22, 2004, 3:15 PM
Post #4 of 19 (1260 views)
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Re: [ciscobiz] AAD: Theoretical Explanation [In reply to] Can't Post

> Can somebody please explain how these devices work.

They monitor air pressure, and see how quickly it changes. If the air pressure equals the pressure altitude that the device activates at (~750 ft for the cypres) and the air pressure is changing at a rate that means you are still in freefall (>78mph) then the device fires. Good AAD's also have algorithms that determine if something is impossible or not; if the device sees an acceleration towards the ground of greater than 32fps^2, for example, that's not possible and the reading is ignored.

AAD's, since they sense air pressure, can be fooled by things like falling on your back (will read a lower altitude) or by rotating to an upright position during opening (again, will read a lower altitude.) That's why opening below 1200 feet may cause an AAD to fire.

Also, changes in air pressure throughout the day may cause it to fire slightly high or low. Ground elevation is also an issue. Since an AAD does not know where the ground is, if you are freefalling over an 800 foot high hill, it will not fire before impact. The cypres will recalibrate itself if you descend, but will not always recalibrate itself if you climb (i.e. if you turn it on, walk up a 200 foot high hill to get to the plane, it will not realize that ground level is 200 feet higher.)


piisfish

Nov 22, 2004, 3:22 PM
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Re: [ciscobiz] AAD: Theoretical Explanation [In reply to] Can't Post

Hi Chris,

where did you do your AFF ?? You can PM me if you don't want to say publicly the name of the DZ.

If it was in Switzerland it was certainly a Cybernetic Parachute Reelease System,

the thread just above explains very well how it works.

I'm pretty sure that it was explained to you, but not 100% of what we are taught stays in the head...

If you want we can go have a drink and a talk in Lausanne anytime... I'm real close...


firstime  (B 28972)

Nov 22, 2004, 7:49 PM
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Re: [billvon] AAD: Theoretical Explanation [In reply to] Can't Post

Can a cypress recalibrate it self in freefall. If so this
is a much more soficticated device that I thought it was. HMMM


Premier billvon  (D 16479)
Moderator
Nov 22, 2004, 8:30 PM
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Re: [firstime] AAD: Theoretical Explanation [In reply to] Can't Post

>Can a cypress recalibrate it self in freefall.

If you're asking if it can reset where it thinks ground level is, then no. If you're asking if it can monitor itself and avoid firing under certain unusual conditions that might otherwise cause it to fire, then yes.


riggerrob  (D 14840)

Nov 24, 2004, 8:19 AM
Post #8 of 19 (984 views)
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Re: [firstime] AAD: Theoretical Explanation [In reply to] Can't Post

In reply to:
Can a cypress recalibrate it self in freefall. If so this
is a much more sophisticated device that I thought it was. HMMM

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

No.

Cypres is a "set and forget" device.
At the risk of over-simplification: turn your Cypres on - on the taxiway - and forget it until sunset.


speedy

Nov 24, 2004, 11:40 AM
Post #9 of 19 (953 views)
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Re: [riggerrob] AAD: Theoretical Explanation [In reply to] Can't Post

Quote:
At the risk of over-simplification: turn your Cypres on - on the taxiway - and forget it until sunset.

Yes is was risky to make that statement. I jumped at a DZ where we had to turn it on before every jump to adjust for the difference in height from the "taxiway" and the landing area.Tongue


Reginald  (D 28162)

Nov 24, 2004, 11:54 AM
Post #10 of 19 (949 views)
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Re: [billvon] AAD: Theoretical Explanation [In reply to] Can't Post

In reply to:
AAD's, since they sense air pressure, can be fooled by things like falling on your back (will read a lower altitude)...
In reply to:


Good post Bill. Just to add to this though. The algorithms are specifically designed to compensate for most of these items and it is VERY difficult to “fool” the AAD. It can be done, however.

More specifically, the air pressure difference created between “real” altitude and what is perceived in the burble the Cypress is in, in a person’s container when in a belly to earth position, is about 200 feet. The Cypress is set to fire at 750 AGL feet but given the burble it is firing at what it “perceives” is 950 AGL. In a test chamber a Cypress will fire at 950. Now, if you fall back to earth the entire skydive the Cypress will fire at 950 feet AGL, thinking it is at 750 AGL. However, it has sophisticated algorithms that will sense if a person is tumbling out of control. Obviously the air pressure readings will fluctuate as the Cypress goes in and out of a burble. The algorithms are capable of adjusting for a person in a non-stable body position and it will fire at 750 feet.

A Cypress will not be fooled by air pressure changes during the day as it constantly monitors the air pressure on the “ground”. However, if there are extreme changes between take off and jump run it will obviously not be able to detect these because it is not on the ground to do so.

Changing ground elevation is an important issue. For example, I jumped in Houston one morning and then drove to Dallas, which is a solid 500 higher. I turned off my AAD and turned it back on. This reset the device. This issue is addressed in the owner’s manual.

In short a modern AAD’s (Cypress) is a very sophisticated piece of equipment and is very reliable. There are some very narrow cases in which they will not do exactly what they are designed to do which is to open your reserve if you are still in free fall at 750 feet. It is remotely possible to trick one, however, but it is exceedingly difficult to do so and they are designed to compensate, whenever possible, for anything unusual that is going on.


Premier billvon  (D 16479)
Moderator
Nov 24, 2004, 12:16 PM
Post #11 of 19 (945 views)
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Re: [Reginald] AAD: Theoretical Explanation [In reply to] Can't Post

>A Cypress will not be fooled by air pressure changes during the day as it
>constantly monitors the air pressure on the “ground”.

It _can_ be fooled by a sudden drop in air pressure, such as occurs near a frontal passage. It assumes it's climbing; it has no "I'm on the ground" sensor. We know this because we had a specific case where we would drive back from the DZ to the airport and gain about 200 feet (although the airport and the DZ were at the same elevation.) Airtec assured us that it would NOT recalibrate in this case even if we were stuck in traffic.

Now, had the opposite been true, and we'd had to descend into a valley and then come out, it would have recalibrated since it senses a new 'lowest' point.


Reginald  (D 28162)

Nov 25, 2004, 5:49 AM
Post #12 of 19 (898 views)
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Re: [billvon] AAD: Theoretical Explanation [In reply to] Can't Post

Bill,

Agreed. There are some limitations of a Cypress. My point is that a lot of work went into mitigating all possible and that the end product is highly sophisticated and able to compensate for most issues in a real world setting. It will never be able to handle all of them though.


tdbuzz  (C 33928)

Nov 26, 2004, 4:58 AM
Post #13 of 19 (849 views)
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Re: [Reginald] AAD: Theoretical Explanation [In reply to] Can't Post

Hi
I noticed in the original question that you mention the words "having to wear" and "analog". Analog refers to the method by which data is displayed to you. An analog clock for example has two hands on it. A digital clock has only the numeric time displayed on it. An analog altimeter that a student wears on the wrist has a clock face with one hand on it. The altimeter works in the same way as an AAD by measuring the air preassure on a sealed chamber on the inside of it.


kallend  (D 23151)

Nov 26, 2004, 9:53 AM
Post #14 of 19 (823 views)
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Re: [tdbuzz] AAD: Theoretical Explanation [In reply to] Can't Post

In reply to:
Hi
I noticed in the original question that you mention the words "having to wear" and "analog". Analog refers to the method by which data is displayed to you. An analog clock for example has two hands on it. A digital clock has only the numeric time displayed on it. An analog altimeter that a student wears on the wrist has a clock face with one hand on it. The altimeter works in the same way as an AAD by measuring the air preassure on a sealed chamber on the inside of it.

Practically every measurement made originates as analog (exceptions being some quantum mechanical phenomena). I suspect the reference here is to whether the pressure data are processed as analog, or whether they are digitized and subsequent processed digitally.


mjosparky  (D 5476)

Nov 28, 2004, 5:37 PM
Post #15 of 19 (740 views)
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Re: [ciscobiz] AAD: Theoretical Explanation [In reply to] Can't Post

In reply to:
Can somebody please explain how these devices work. I wasn't informed about them on my AAF course. I just wore one (analog) or whatever that means.

Thanks,

Chris

The simple answer is if you are going too fast too low, this device will do what you should have already done.

Sparky


jumpinjackflash  (B 27757)

Nov 28, 2004, 7:49 PM
Post #16 of 19 (727 views)
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Re: [mjosparky] AAD: Theoretical Explanation [In reply to] Can't Post

Hey Michael,

I was just thinking, reading this post, I've heard alot about "saves" with Cypres, and they have a hell of a record, great device.

However, in the interest of pure numbers, have there been many go in with working Cypres that haven't fired???

Any numbers on these kinds of failures?

I'm assuming this hasn't happened very often. Typically, when I hear of someone going in, and no reserve is deployed, there seems to be no AAD installed either.


Anyway, just curious,




Jack


mjosparky  (D 5476)

Nov 29, 2004, 11:39 AM
Post #17 of 19 (691 views)
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Re: [jumpinjackflash] AAD: Theoretical Explanation [In reply to] Can't Post

In reply to:
Hey Michael,

I was just thinking, reading this post, I've heard alot about "saves" with Cypres, and they have a hell of a record, great device.

However, in the interest of pure numbers, have there been many go in with working Cypres that haven't fired???

Any numbers on these kinds of failures?

I'm assuming this hasn't happened very often. Typically, when I hear of someone going in, and no reserve is deployed, there seems to be no AAD installed either.


Anyway, just curious,




Jack

Good question Jack. I don't have an answer for you though. I haven't heard of one, maybe someone else can jump in with some information.

Sparky


Premier billvon  (D 16479)
Moderator
Nov 29, 2004, 11:43 AM
Post #18 of 19 (688 views)
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Re: [mjosparky] AAD: Theoretical Explanation [In reply to] Can't Post

>However, in the interest of pure numbers, have there been many go in
>with working Cypres that haven't fired???

>Any numbers on these kinds of failures?

Hard to say. Per Bryan Burke, a Cypres that does not fire when it should shuts itself down; thus, afterwards it appears that it was not turned on. I don't know if subsequent examination by Airtec would reveal if the cypres had actually been turned on or not during the freefall.


jumpinjackflash  (B 27757)

Nov 30, 2004, 1:11 PM
Post #19 of 19 (657 views)
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Re: [billvon] AAD: Theoretical Explanation [In reply to] Can't Post

Quote:
Per Bryan Burke, a Cypres that does not fire when it should shuts itself down;

Wow, that definitly makes it hard to determine.

It would be nice to hear from Airtec on how a postmortem is done on these things.

I mean, it would make sense for it to shutdown if it failed, and then boot back up in an auxillary mode, i.e. refuse to provide service, which would indicate it had failed.

However, if all one needs to do to reactivate the device is boot it back up, then, well, that's not only useless, but I would think dangerous as well...


Anyone on the board from Airtec that we could ask?


j



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