why 750Ft firing altitude?

[phantomII 36]

To answer the original question.....

The 750ft/225m were chosen so that the unit really operates as a last chance. At terminal velocity we are talking 4 sec before impact.
When he invented the CYPRES Helmut wanted something reliable (Not like the FXC12000) and something that saves a skydiver from certain death.
At this altitude you can't argue that you still would have pulled the reserve if the CYPRES hadn't done it for you.

[councilman24 36]

I'll give you another simpler answer. It was designed after a friend if Helmut's went in unconscious. Unconscious you fall on your back. So it was designed for 1000'. As low as possible because major bias against AOD's, as known then, was fear of two out. The fact that some people forget to open a parachute falling on their belly gets it firing at 750'.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[pms07 3]

I think its also important to realize that the sport and our gear have changed significantly since Helmut first put the Cypres on the market. Canopies, generally, open much slower. We also open much higher than we used to. Pulling at 2000' (or lower) used to be routine. Now days pulling below 3000', for many, is a friggin emergency. Years ago few would have strapped on an AAD if it opened higher than 750' to 1000'....

[councilman24 36]

And in fact that was our (folks in PIA) argument to Helmut. He should change the firing altitude because the gear and skydiving had changed. He didn't believe he should change the cypres in order to fix someone else's problem. And he hasn't, but has allowed the user to choose a different value.

And to your point and for the youngsters it truely took more than 5 years for a cypres to become 'normal'. It was designed with the controller hid so no one had to know you had one. Early on people wouldn't get on the plane or the jump with you if you had an AAD, even a cypres.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[lyosha 46]

pchapman

***

Quote

It will fire at 750 feet, if the person falls belly to earth. At headdown, it will fire at 1000 feet.

Also incorrect. So much incorrect from people with so many jumps :(

This is a very interesting point. To which there is no quick answer. I believe you are incorrect lyosha, but for totally understandable reasons. You are totally right based on the Cypres manual alone, but there's more.

It comes from a basic understanding of how these devices function. It's not the burble, it's the pressure inside it. As you fall the air, your body compresses the air under you, and expands the air above you. This creates a high pressure zone underneath you (where an altitude reading will appear lower than it actually is) and a low pressure zone above you (where an altitude reading will appear higher than it actually is).

According to Vigil's tests the difference is ~260 ft between the two areas at ~120 mph.

When falling head down, the peak high pressure area is under your head, and peak low pressure area at your feet. The AAD is halfway between, in a "neutral area".

[mxk 1]

lyosha

******

Quote

It will fire at 750 feet, if the person falls belly to earth. At headdown, it will fire at 1000 feet.

Also incorrect. So much incorrect from people with so many jumps :(

This is a very interesting point. To which there is no quick answer. I believe you are incorrect lyosha, but for totally understandable reasons. You are totally right based on the Cypres manual alone, but there's more.

It comes from a basic understanding of how these devices function. It's not the burble, it's the pressure inside it. As you fall the air, your body compresses the air under you, and expands the air above you. This creates a high pressure zone underneath you (where an altitude reading will appear lower than it actually is) and a low pressure zone above you (where an altitude reading will appear higher than it actually is).

According to Vigil's tests the difference is ~260 ft between the two areas at ~120 mph.

When falling head down, the peak high pressure area is under your head, and peak low pressure area at your feet. The AAD is halfway between, in a "neutral area".

No one is disputing the pressure difference, but the AAD can make corrections. From the link above:

Quote

The CYPRES uses its microprocessor to study the situation and try to determine if the sudden increase in pressure is the result of an attitude change or decreasing altitude.

In other words, CYPRES may be trying to figure out whether it is currently in the burble or not, and apply an altitude correction so that it fires closer to 750 ft in body orientations other than belly to earth. I tried to write a simple algorithm that does this by calculating acceleration from pressure reading and setting some thresholds to distinguish realistic acceleration from that caused by the AAD entering/leaving the burble. Such algorithms are going to be very error-prone without additional sensors.

The AAD starts out knowing its altitude in the plane. The burble appears on exit or during climb-out, and increases in intensity on the hill. If the AAD knows the initial state (inside or outside of the burble), figuring out changes is a little bit easier. Figuring out the initial state is pretty difficult. Personally, I prefer the Vigil approach in this regard of having a much simpler algorithm that may fire anywhere within a 260 ft window.

[lyosha 46]

Yes, that is an additional point as to why it will probably fire closer to 750 than to 1000 ft.

[pchapman 261]

lyosha

When falling head down, the peak high pressure area is under your head, and peak low pressure area at your feet. The AAD is halfway between, in a "neutral area".

Ok of course no problem. I hadn't been clear on what you were objecting to. I was just using 750 and 1000 as simple approximations, rather than arguing numbers in between.

Clearly the pressure behind on object in freefall will be lower. And higher on front, eg highest at the stagnation point. For an AAD inside a skydiver's rig, with the skydiver on his back, I'm not sure just what the pressure will be -- due to the large size and volume of the pack, the pressure increase at the sensor might be a fair bit less than at the stagnation point itself.

In any case we don't know exactly what sort of pressure changes exist at the AAD sensor in different body positions.

[jumpsalot-2 2]

pchapman

***
When falling head down, the peak high pressure area is under your head, and peak low pressure area at your feet. The AAD is halfway between, in a "neutral area".

Ok of course no problem. I hadn't been clear on what you were objecting to. I was just using 750 and 1000 as simple approximations, rather than arguing numbers in between.

Clearly the pressure behind on object in freefall will be lower. And higher on front, eg highest at the stagnation point. For an AAD inside a skydiver's rig, with the skydiver on his back, I'm not sure just what the pressure will be -- due to the large size and volume of the pack, the pressure increase at the sensor might be a fair bit less than at the stagnation point itself.

In any case we don't know exactly what sort of pressure changes exist at the AAD sensor in different body positions.



This sounds like a case for Myth Busters .........

Life is short ... jump often.

[chuckakers 370]

jumpsalot-2

******
When falling head down, the peak high pressure area is under your head, and peak low pressure area at your feet. The AAD is halfway between, in a "neutral area".

Ok of course no problem. I hadn't been clear on what you were objecting to. I was just using 750 and 1000 as simple approximations, rather than arguing numbers in between.

Clearly the pressure behind on object in freefall will be lower. And higher on front, eg highest at the stagnation point. For an AAD inside a skydiver's rig, with the skydiver on his back, I'm not sure just what the pressure will be -- due to the large size and volume of the pack, the pressure increase at the sensor might be a fair bit less than at the stagnation point itself.

In any case we don't know exactly what sort of pressure changes exist at the AAD sensor in different body positions.



This sounds like a case for Myth Busters .........

+1

What a cool idea.

Chuck Akers
D-10855
Houston, TX

[rohicks 2]

jumpsalot-2

******
When falling head down, the peak high pressure area is under your head, and peak low pressure area at your feet. The AAD is halfway between, in a "neutral area".

Ok of course no problem. I hadn't been clear on what you were objecting to. I was just using 750 and 1000 as simple approximations, rather than arguing numbers in between.

Clearly the pressure behind on object in freefall will be lower. And higher on front, eg highest at the stagnation point. For an AAD inside a skydiver's rig, with the skydiver on his back, I'm not sure just what the pressure will be -- due to the large size and volume of the pack, the pressure increase at the sensor might be a fair bit less than at the stagnation point itself.

In any case we don't know exactly what sort of pressure changes exist at the AAD sensor in different body positions.



This sounds like a case for Myth Busters .........

http://www.discovery.com/tv-shows/mythbusters/about-this-show/mythbusters-submit-a-myth/

[jumpsalot-2 2]

Thanks for the link. I looked, to make sure they were still making new episodes. 2016's season will be the last one. They may already have it in the can, I don't know. I don't feel that I am computer savy enough to forward all the discussed forum info here. I would want to direct them to this forum. Is there anyone out there who would take this to Myth Busters ? Keeping in mind, this would be free research, and Myth Busters seems to go the extra mile in their research.

Life is short ... jump often.