As many people probably know, there has recently been a 4-way CYPRES fire due to what appears to be a loss of altitude awareness. Many of the current responses in that thread amount to: use your eyes instead of your gadgets. I've been thinking about this and I want to float the following thoughts I have on this topic to hopefully get some meaningful critique.
Before I say anything else I will say this: if I am in freefall and I look down and see something that looks below my usual deployment altitudes (3000'-4000'), especially if I am beginning to get substantial ground rush, and I do not have a very strong reason to believe my altimeter is not malfunctioning, I will deploy (hopefully, although this is a different discussion) the correct parachute. Please do not bash me on the head about this. I am not planning to go in staring at my altimeter with my hands on my handles, or in any other way for that matter.
Having said that, I am unconvinced that it is wise to not rely on a mechanical altimeter to measure altitude. The human brain or any of its ancestors as far as I know never had to measure altitude, only distance. We use our brain to navigate our environment on the ground and it does not matter much exactly how far away something is if it's more than a few hundred feet. I do not know the exact numbers but, from my experience, depth perception becomes more and more limited with distance. By depth perception here I mean primary cues that measure the exact distance, such as stereopsis or parallax, and not secondary cues that infer distance from other things.
As far as I know, and again I can be wrong, there are no primary cues that our brain can use to measure altitudes like the ones in skydiving (see http://en.wikipedia.org/wiki/Depth_perception). The only primary cue that I can believe could work in skydiving on this list is kinetic depth perception - measuring distance from the speed with which object size changes. This is, presumably, one of the causes of ground rush. Arguably though, this still requires the brain to assume something about the size of the object to begin with and so may actually be somewhere between a primary and a secondary cue. It is always possible that there is some major undiscovered mechanism that is not on this list but I don't know if I'm willing to bet my life on that.
As a result, when we train our brains to recognize altitude we train our brains to recognize a myriad of visual cues that are only symptomatic of altitude in our current location and environment: the area of the fields, the diameter of the treetops, the widths of roads. Here's my punchline. It seems to me that this is just as likely, if not more likely, to "malfunction" than a mechanical altimeter. What if you unconsciously train yourself to recognize altitude by the "visible length" of (angle subtended by) the runway, as one poster on this forum did? If you start at a DZ where the runway is long and then move to a DZ where it is short, your eyes will start lying to you. What if all the farmers around the DZ decided to plant corn one season and wheat the next?
Perhaps even more importantly, mechanical altimeters malfunction randomly and those malfunctions can often be easily detected. On the other hand, if your eyes ever do malfunction it will be at the worst possible time: you are at a new DZ, you're jumping at dusk or dawn, you're on a solo and have less chance of realizing your mistake, etc. Don't forget that you can always wear more than one mechanical altimeter but you've only got one set of eyes. Of course, all of this has little to do with altitude awareness on the whole since usually the problem seems to be that whatever method people use to measure altitude, they forget to check it.
To summarize, we wouldn't rely on just our bodies to take the role of a parachute. Is it really that wise to rely on our eyes to take the role of an altimeter? Or is this, like many other things, more part of our tradition? As of right now, while I spend most plane rides training myself to recognize break-off and deployment altitudes visually and I can do it quite well in the plane, I do not expect myself to be able to do it accurately. In other words, unless I am in serious danger, I will trust my altimeter over my eyes. This is doubly true if I am at an unfamiliar DZ or in unfamiliar lighting conditions.
False confidence and complacency are grave dangers to skydivers. With this in mind, please try to limit replies that mostly consist of slogans or arguments of the form "it works for me and I'm a ninja." http://icanhascheezburger.com/2008/02/28/funny-pictures-i-come-with-sarcasm/
Proudly uncool since 1982.
