Cypres "decision" altitude

[billvon 2,400]

Moved from a thread in Incidents:

> I was under the impression that the expert cypres would fire at
>750 feet give or take if you met the parameters.

Right; the "firing window" in freefall is 750 to 130 feet. If all parameters are met within that window it fires. Note also that it senses _pressure_ altitude, not actual altitude. So if you get stood up by your canopy at 900 feet, the cypres will sense a lower altitude and may fire.

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To explain this further -

There is a common misconception that a cypres 'decides to fire' at 1000 feet, then fires a few seconds later. That's not the case. When its firing parameters are met, it fires. When they are not met, it does not fire.

The reason there's this decide-to-fire thing going around is that the cypres reads pressure altitude, not actual altitude. When you are in freefall, there's a burble over your back, so your cypres reads 300 feet higher than normal. In other words, if you stuck your altimeter right where the cypres is in the rig, you'd see an altitude that was about 300 feet higher than it actually is.

The cypres takes this into account. It fires at a pressure altitude of around 1000 feet so that in freefall it will fire at about 700 feet when in someone's rig.

If you decide to pull at 1200 feet, your rig will rotate as your canopy opens, eliminating the burble. The cypres will go from reading 1200 feet (i.e. pressure altitude of 1500 feet - 300 foot correction) to reading 900 feet (i.e. pressure altitude of 1200 feet - 300 foot correction.) That's very close to firing altitude, and thus even a little bit of a snivel will put you under the "new" 1000 foot limit.

Another way to think about it is that it fires at 700 feet in freefall, 1000 feet while sniveling.

[speedy 0]

A rather simplified view of what's going on, but I guess it will do for most people that will read it here.

The misconception I hear most is more to do with the different firing parameters for the student version.

Dave

Fallschirmsport Marl

[LouDiamond 1]

A good description of this from the manufacturer can be found [url "http://cypres-usa.com/cyp13.htm"]here***

"It's just skydiving..additional drama is not required"
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[peckerhead 0]

My "decision" altitude begins before I get in the aircraft. At ground level, AGL.

I " decide" to not trust my life to gadgets. Even real expensive ones.