Altimeter Limitations

[tpdonlan 0]

This post is inspired by the front page article today regarding the use of analog or digital altimeters.

For some background - I only recently got my A license (hooray!) but I have been flying for years in private aviation and with the military. Many of our flight operations are conducted in the low level environment which has allowed me to enter the sport with an already modestly developed sense for altitude awareness and sight picture. Understanding this I'd like to tell you what happened to me on a recent jump and get some feedback.

It was my first jump off of the AFF and coach program from full altitude. While for the most part it was a beautiful day, the sky was pocked with an approaching storm front. We were jumping on what was to be the last load of the day. The aircraft climbed up to 12.5 over the DZ and entered the jump run. I was #8 to exit behind a 4 way belly and a 3 way coach and I was followed by solo. The exit went fine and I spent my jump practicing some turns since it was my first full altitude by myself in awhile and I knew I was rusty. At about 5000 feet I ran out of things to do and started checking my airspace for when I pulled at 3500. I scanned the horizon real quick and looked back at my altimeter, it read 4.5. I looked around again and looked back at the altimeter, this time it read 4.0. I looked down at the ground and the eerie feeling that something wasn't right started tingling. The ground was way too close and I decided that my altimeter was the limfac and did what was beat into my head by the instructors at the DZ when something isn't right. I waved and pulled. As I did so I saw that the guy who had gone out after me had managed to track into my airspace and deployed probably about 300 feet in front of me. I grabbed the riser and banked it hard towards the DZ and managed to get a glimpse of my altimeter pointing to 2500. I sincerely believe there is no way that I had descended 1500 feet between pulling and being fully open because that sucker had popped fast. Sine I had the guy that had pulled in front of me as a frame of reference for my deployment altitude I met up with him on the ground and asked what he had seen when he pitched. He said he threw it at 3500 but that was the time I was reading 2900. We were both using the same system and had a 600 foot split with the variables being our rate of fall which caused me to not suspect the device on my arm but the pressure change as a result of the approaching storm system.

My question then, is this something that any of you have seen? How do atmospheric conditions outside of meteorological phenomenon affected your equipment? Is there a digital system that posts your descent rate in real time that would have clued me in to crossing a pressure boundary like that?

[councilman24 37]

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I looked down at the ground and the eerie feeling that something wasn't right started tingling. The ground was way too close and I decided that my altimeter was the limfac and did what was beat into my head by the instructors at the DZ when something isn't right. I waved and pulled.

You did all of this AFTER you saw 4000'. You looked down, decided you were low, decided to pull, waved off, pulled. THEN you had canopy opening. You don't list a main probably since it's not your own yet but many/most mains take 500 to 1000' to open these days. Then you dodged the guy you saw AFTER opening using a riser and THEN glimpsed 2500'.

Sounds about right to me. You didn't descend 1500' feet between 'pulling' and being fully open. You did all this stuff. And opening fast to you and fast to an old timer like me are two different things. If each of the actions after 4000' took you 1 sec that gets you to 3000' before pulling. Experienced folks would do those things faster than 1 sec each but you may not.

He "threw" at 3500'. Did he see 3500', decide to throw, wave off, throw and was at your 2900'?

Nothing you've said sounds wrong to me. Altimeters can be off. But trying to judge against freefall actions after you've landed is not the best way to check.

If you think it's wrong send it to the manufacturer or have someone but it in a vacuum chamber locally. My bet is everything is fine.

There is no "pressure" boundary that will screw with your altimeter. The most normal ground pressure extremes (low storm to high clear) is about equivalent to 200'.

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

[tpdonlan 0]

I appreciate the feedback. There's a huge chance that the way I perceived things wasn't fully aligned with the way they happened. I was more or less wondering if there were idiosyncrasies that I wasn't yet aware of that I should be for the future.

[wstcstcmtr 0]

I'll let you borrow my sabre 1 if you want to know what a fast opening is like.

[stayhigh 2]

is this analog or digital alti??

Analog can get stuck, while going up or down.

Digital can also get funky, depending on how the wind hits the device. When I used to tandems, the altimeter will go 1000+- ft if I place my alti right behind the burble.

Bernie Sanders for President 2016

[tpdonlan 0]

It was analog. I borrowed one of the student altimeters, too, so I'm sure it's seen more than its fair share of abuse.

I'm torn wstcstcmtr, on the one hand you'd be saving me rental costs, on the other hand I'm not sure if I want to experience my balls stretching to my ankles.

[DHemer 0]

My analogue lags my audible by up to 400 ft in free fall

Meaning I get a break off warning at 4500ft in my ear but my analogue is reading 4900ft. After opening it is pretty much spot on though

This may be a reason for a the larger than expected altitude difference you saw. Get someone to test jump it for you who has a digital alti or audible

[3mpire 0]

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I'll let you borrow my sabre 1 if you want to know what a fast opening is like.

or a cobalt if you're, you know, serious about wanting to experience a hard opening in it's purest form....

[phoenixlpr 0]

3mpire

Quote

I'll let you borrow my sabre 1 if you want to know what a fast opening is like.

or a cobalt if you're, you know, serious about wanting to experience a hard opening in it's purest form....

What is your post has anything to do with the topic or subject?

[3mpire 0]

phoenixlpr

What is your post has anything to do with the topic or subject?

Councilman made a comment that a modern main can take up to 1000 feet to open, wstcstcmtr cracked a joke that a sabre1 opens fast (with less altitude loss), and I cracked a joke that a demon cobalt would open even faster.

lighten up francis.

[phoenixlpr 0]

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Councilman made a comment that a modern main can take up to 1000 feet to open, wstcstcmtr cracked a joke that a sabre1 opens fast (with less altitude loss), and I cracked a joke that a demon cobalt would open even faster.

I got about 800 jumps on my Cobalt, most of those with wing suit and pulling from flight. Somehow I did not get your joke.

[3mpire 0]

I had 13 jumps on a cobalt and in that time frame went through one set of risers, two break lines, and 1 cut away. What can I say.

[mattjw916 2]

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What can I say.

working as intended

NSCR-2376, SCR-15080

[stayhigh 2]

Have you ever jumped at Elsinore?? I remember talking to a dude who jumped I believe 170 ish sized cobalt, and his break line broke upon hard opening. I think the guy carried around extra breaks line just for that purpose.

I've jumped Cobalt as well, thought it was the worst opening canopy in the world, sold it for 100 bucks, and years later somehow I talk to him again, he said that he fell in love with it and wants another one, and he said he never had bad opening. WTF???

Bernie Sanders for President 2016

[Trafficdiver 8]

Occasionally my digital alti goes from 4.4 to 4.6 then back to 4.3. I use my eyes, my audible and my wrist mount to decide when to pull.

Sometimes I have a 1000' snivel on my sabre 2 as well. Sounds like what your experiencing is pretty normal.

[3mpire 0]

I have jumped there--that's actually where I chopped the cobalt. Got my money back shortly after getting home and got a different canopy and haven't looked back

[potatoman 0]

Please name the alti you are using.

Atmospheric pressure is minute, you won;t be out by 1000ft. The only thing that can get your alti way off, is burble or no burble, or on the analog just get stuck.

You had nothing more to do at 5000ft. Now you are watchcing paint dry. a second feels like forever, and you are doubting.

I am not saying what you are saying isn't true, but I have found that what I saw and what my video tells me are sometimes two different stories.

My setup, and just for safety sake, two different types of audibles, and an analog. This way I have two battery operated devices, and their calculations (I would hope) are a bit different to pick up variances, and the good old faithfull ananlog on the wrist.

Oh, you will realise it very quickly when you are going low. The earth will start to talk to you in a way you have never seen, and the horizon will dissapear.

You have the right to your opinion, and I have the right to tell you how Fu***** stupid it is.
Davelepka - "This isn't an x-box, or a Chevy truck forum"
Whatever you do, don't listen to ChrisD.

[rjblake 1]

I read this with great interest, as also recently off AFF and have my A-licence. All other jumping has been under a round from 700-2,000ft. I did a 2-way with my instructor, broke off at 5,000ft, tracked and pulled at what I swore was 3,500ft. My alti-track said my deployment was at 2,276ft. Instructor watched and filmed my track and asked me why I pulled so low. Going through it all, I believe the following happened:
5,000 - tracked away for too long and a bit steep (around 5secs)
3,500 - de-arch to slow fall, wave and pull (audible shouting at me probably another 2secs)
2,276 - under canopy with twists and slight dive
End result - off landing and good walk back to DZ.

All started with a bad spot (others jumpers off too), too low a pull, line twists with canopy heading away from DZ. Canopy was a Solo 230. During my AFF & A-licence, I'd been jumping a Navigator. All openings were soft but brisk, fully inflated within about 300ft. Not the case with this one. What did I learn - my transitions took longer, the canopy took longer to inflate and I lost more altitude - despite what I thought. 3-4 secs burns off 600ft easily and its a sequence of events that leads to things going wrong. My next few jumps, I paid much greater attention and found myself under a good canopy by 2,800ft. Nothing wrong with my alti or the canopy - all the human in control.

[Di0 2]

I've had a similar bad experience with a digital alti this week. It seemed like the screen "froze". I was tracking for a few seconds, since we broke off higher than I am used to, I though about looking at it halfway in the track, it said 3.6k.
It stayed that way for what I swear could have been a good 4-5 seconds, after/during which time I of course went ahead and pulled. After pulling, I went back to my alti and it was still set on 3.6k.
My 2.5k alarm on my audible went off shortly thereafter toward the end of the deployment process, which is more or less the altitude I roughly guessed to be at that point. I looked at the altimeter back and forth and I think I saw it de-freezing and going from 3.6 to 2.5 instantly.
It could have been just my impression, maybe. Maybe not.

I've had small problems with my N3 under canopy before, mainly it would revert back to airplane mode (maybe because of some ascension current) and thus it would make reading the altitude much harder, but never in freefall. I guess I'll wait and see if it does that again.

Bottom line, altimeter are for the most part expensive toys. Rely on them but not too much because the do break.

I'm standing on the edge
With a vision in my head
My body screams release me
My dreams they must be fed... You're in flight.

[Alti2_Staff 0]

Please send me an email with some more details about your strange N3 behavior. I would like to help you diagnose the source of the behavior and help you find a solution.

johnhawke@alti-2.com

Alti-2, Inc. Staff

[Di0 2]

Alti2_Staff

Please send me an email with some more details about your strange N3 behavior. I would like to help you diagnose the source of the behavior and help you find a solution.

johnhawke@alti-2.com

Oh. And I forgot to mention. Customer service goes out of your way to help you.
Just to clarify, I am very happy with my N3, it's an extreme complete unit that does, well, everything. Being an engineer myself, I know that this always mean extra complexity and so it's harder to diagnose. But the fact that you can theoretically use it in its basic form without ever pressing a button on it, it's quite priceless to me.
I'll shoot you an email tomorrow with more details, but in the meanwhile thank you for your offer to help! It is extremely appreciated and I love the fact that you stand by your (great) product.

I'm standing on the edge
With a vision in my head
My body screams release me
My dreams they must be fed... You're in flight.

[slotperfect 7]

You are quite welcome!

I look forward to hearing fro you...

Oops... Posted from the wrong account. Well, the message is the same

Arrive Safely

John

[GoHuskers 0]

Di0

I've had a similar bad experience with a digital alti this week. It seemed like the screen "froze".

I've had problems with my digital altimeter reading 1k high. I never realized that it might be the display lagging. Also from time to time it will be switch into canopy mode while in freefall. Minor nuisance but I've never lost altitude awareness from these little hiccups.

[Alti2_Staff 0]

Altimeter lag can be caused by a handful of things . . . usually when I help customers troubleshoot this problem it is something simple.

Recently we had two cases of N# lag; both were caused by the pressure sensor hole on the back of the unit being obstructed (sealed). The first user had taken the "STOP" cling label off the lens and put it on the back of his unit (he couldn't hear his alarms either). The second had adhesive Velcro (used to help mount the unit in his helmet) covering the pressure sensor hole.

Mounting location can also contribute to lag.

Analog altimeter lag usually indicates the device needs to be re-calibrated, or there may be debris in the mechanism.

Anyone experiencing altimeter lag with an Alti-2 device can contact me johnhawke@alti-2.com (386) 279-0017 and I will be glad to help them troubleshoot the problem.

Alti-2, Inc. Staff