What altitude and I really deploying at?

[Westerly 61]

I have a rather strange corundum that I cant figure out. I have my audible set to 5.2k to track and 4k to deploy. I typically track for about 5 seconds and deploy on time. Well for the last 40 jumps or so my Visio2+ has constantly registered deployment altitudes in the 2.8k – 3.2k range, which is really low for a 5.2k track start. So for the last dozen jumps I decided to watch my altimeter through the whole deployment sequence to see what is going on. All the jumps are similar and here is what I got for the last one:

Physically threw PC handle: 4k
Noticeable tension on the risers, feet under me, starting to slow: 3.6k
Canopy fully open, slider down: 2.75k.

Well for that jump my Visio2+ registered a deployment altitude of 2.92k and my AAD registered 2979’. So I decided to go in the second-by-second playback menu where I can see my airspeed and elevation for any given second within the jump. The Visio shows me going 120 MPH all the way through 3k. It doesent register me as deployed until there is a major change in airspeed from one sample to the next (probably 30 MPH+). So looking at the log, it does in fact show me as going terminal all the way down through 3k. Also, I know for a fact that the sensors in my AAD and Visio are very accurate and if they say a specific elevation, it’s right.

So that leaves the corundum. I KNOW my Visio read 4k when I threw my handle, I watched it count down as I had my hand on the handle 4.20, 4.15, 4.10, 4.05 (okay, throw!). I have watched the Visio count down to 4k right as I pitched for the last dozen jumps or so. I KNOW that I had something overhead starting to slow me down by the time I was passing through 3.5k on most jumps. I couldent see the main because I was staring at my Visio, but I sure as heck could feel something overhead and my feet were under me. Yet, I also know that the sensors on my AAD and the Visio are very accurate and reliable. I also know both the Visio and AAD considers "deployed" as the canopy is starting to snivel enough to slow you down, and NOT the canopy is fully open and you're not descending fast anymore.

Thus we have two things that conflict each other, yet both seem to be correct.

[ajay 1]

I guess there is some lag in the sensor detecting and processing a speed change large enough, which IMO isn't a bad thing. If all it does is make you sure to pitch on time as opposed to run it low, no harm, no foul.

Also, from PC to open canopy, 1250ft seems... excessive?

PS - it's Viso :)

blank

[pchapman 261]

I didn't analyze your numbers in detail, but I'll note that the burble, even behind one's wrist, can affect altimeters.

Hand position can make a difference of some hundreds of feet!

My example:
One time I was doing a terminal solo jump, and like you watched my wrist alti intently to check opening altitudes and distances. Now I had a mechanical alti, but the principle remains the same.

I was ready to pull, left wrist low down infront of me, and yanked out the pilot chute at exactly 2500' on the alti.

I changed my body position as I did so, with the alti angled more vertically rather than flat to the earth. Suddenly the alti dropped about 500', as best I could tell on the mechanical alti. Hey, where did my altitude go?!

AAD companies mention how the burble (behind one's whole body) can have an effect of about 300', although that's not the exact number Vigil and Cypres use. Maybe a wrist is enough to give the same effect. Between that and a second of time at terminal, that does get into the 500' range.

So when I was fully under canopy my alti read 1300', despite thinking I had pulled at a reasonable 2500' (when that was above the US/Canadian minimums), with a canopy with a not too long snivel.

[wmw999 2,114]

Yes, a burble and position make a difference. As does the use of different instruments.

I have an analog altimeter on my chest, and an audible in my helmet. The analog (one of two identical ones I own, one of them has been checked twice in separate altitude chambers) always reads 3-400 feet below where the audible thinks I am, as evidenced by when the analog goes off.

Before audibles, the analog WAS the altimeter that everyone used, and so the fact that it said 4500 when your audible won't go off for 2 more seconds was irrelevant. But each one is just a reference point, and best compared with itself, kind of like when you weigh yourself with a household scale, it's best to use the same one -- the gym might weigh 5 lbs heavier, the doctor's 2 lbs lighter, etc.

Clocks used to be the same; 100 years ago, the railroad conductor got to coordinate the time for the towns he went through. He set his watch in the morning at whatever authorized source, and then people in each town who needed accurate time would set their clocks by his. Clocks weren't as accurate as they are now.

Until altimeters use faster processing and GPS (or some other centralized standard, that isn't impacted by body position and/or weather), it's going to be a similar case for them. You'll go nuts trying to synch them exactly. Just use them as a tool to separate and initiate deployment (or whatever else) at a safe altitude.

Wendy P.

There is nothing more dangerous than breaking a basic safety rule and getting away with it. It removes fear of the consequences and builds false confidence. (tbrown)

[councilman24 36]

wmw999

Yes, a burble and position make a difference. As does the use of different instruments.

I have an analog altimeter on my chest, and an audible in my helmet. The analog (one of two identical ones I own, one of them has been checked twice in separate altitude chambers) always reads 3-400 feet below where the analog thinks I am, as evidenced by when the analog goes off.

Before audibles, the analog WAS the altimeter that everyone used, and so the fact that it said 4500 when your audible won't go off for 2 more seconds was irrelevant. But each one is just a reference point, and best compared with itself, kind of like when you weigh yourself with a household scale, it's best to use the same one -- the gym might weigh 5 lbs heavier, the doctor's 2 lbs lighter, etc.

Clocks used to be the same; 100 years ago, the railroad conductor got to coordinate the time for the towns he went through. He set his watch in the morning at whatever authorized source, and then people in each town who needed accurate time would set their clocks by his. Clocks weren't as accurate as they are now.

Until altimeters use faster processing and GPS (or some other centralized standard, that isn't impacted by body position and/or weather), it's going to be a similar case for them. You'll go nuts trying to synch them exactly. Just use them as a tool to separate and initiate deployment (or whatever else) at a safe altitude.

Wendy P.

Hey Wendy, Can you fix the second paragraph so I know which one is which?

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

[wmw999 2,114]

Wuss!
The analog reads low, the audible just tells me where it thinks it is when it gets there

Wendy P.

There is nothing more dangerous than breaking a basic safety rule and getting away with it. It removes fear of the consequences and builds false confidence. (tbrown)

[councilman24 36]

Sorry I can't tell analog from analog. And I still like my alitimaster II on my chest strap also.

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

[Bob_Church 7]

councilman24

Sorry I can't tell analog from analog. And I still like my alitimaster II on my chest strap also.

I consider the digital on my wrist, I'm up to a Neptune 3 now, more of a toy than anything. I like the info for my log book and it's handy for water jumps. But yes, it's my chest mounted MA2-30 that I consider my "real" altimeter.

[Westerly 61]

I dont think it is an issue of the burble. Both my AAD and my Viso read the same altitude even though they are in two completely different places. Also, the altimeter is not on the back of my hand, it's above my head when I am pulling and it's in clean air (you compensate with your left hand when reaching back with your right. Basic AFF stuff). I added a 3rd altimeter to my chest today. It read the same as the other two. Further, even AAD manufacturers say that the effect of the burble is minimal and always less than 250' difference between registered and actual. This has to be the case otherwise AADs would not work. AADs are in the burble as they are right on your back.

I had someone film my deployment today. From the time my PC was out of the BOC to the time the parachute was completely out of the bag was about 0.86 seconds. From the time I threw the PC handle to the point that I was orientated vertically with my feet toward the ground was about 1.36 seconds.

Granted I only filmed one of my deployments, but it felt the same as any of the other previous ones. So I just dont get it. Three different devices are all telling me that it is taking 1000' just for me to start slowing down but the video clearly shows the canopy out of the bag in about 1 second after throwing the PC.

Also another fact in support of the altimeters not being incorrect is the fact that I look at my altimeter after the canopy is open. It is in fact 1200'+ between when I see 4k on my altimeter and I pull to what I see on the screen once under canopy.

[DBCOOPER 1]

"typically track for about 5 seconds"

Each "about" second your off by is 166 feet. Do you track for 5 seconds and then wave off?

"I also know both the Visio and AAD considers "deployed" as the canopy is starting to snivel enough to slow you down"

How do you know that? What exact speed does the Visio determine your deployed?

What canopy do you jump?
How many feet of altitude do you think you should lose between deployment and fully open?

Replying to: Re: Stall On Jump Run Emergency Procedure? by billvon

If the plane is unrecoverable then exiting is a very very good idea.

[Westerly 61]

DBCOOPER

What canopy do you jump?
How many feet of altitude do you think you should lose between deployment and fully open?

I jump a Lotus 170. It has a smaller-than-OEM slider on it and it does not open that soft. Not sure, but certainly not 1400'. I checked the log on my Viso. It is saying that my canopy slows me form 120 MPH to 20 MPH in about 400'. As such, the rest of that 1200 - 1400' that I am supposedly falling should be freefall, but I also know that to be incorrect since I filmed my deployments. In the video my canopy was out of the bag in less than 1 second after pitching my PC. As such, I am really at wits end trying to figure out why it takes so long for the canopy to stop me dispite the fact that I am not really running a soft opening canopy. It actually opens hard sometimes. Here is what I know:

- In my tests I consistently pulled at 4k. I mean I am watching the altimeter and pull the PC when it says 4k, not I wave off and then pull at 4k.
- Film shows that my canopy is out of the bag in less than a second after pulling.
- Despite pulling right at 4k, I am usually not under canopy with the slider down until about 2.75k.
- Even though I pull at 4k, my Visio and AAD are not registering deployments until around 3k and there is no way they are off by that much because if they were then AADs wouldent work in the first place. An AAD that is off by 1000' would not be a functional product for skydiving.

So it's rather strange. All the instrumentation is saying that I am in freefall for nearly 1000' after pulling my PC, but a video of my deployment clearly says otherwise.

[pchapman 261]

I can't give you a solution to your puzzle, but can comment.

If there are errors, it is most likely at the start of the whole thing in freefall, rather than at the end at low speed.

If your wrist alti says 2750' after opening, and your AAD and Viso claim just around 3000' for the "opening" in their data, that sounds non unreasonable.

Every device calculates "opening" a bit differently but it tends to notice the fast slowdown near the end of the opening sequence.
(Example of variation: My Protrack detects openings at different altitudes, relative to what I see on my wrist after opening, depending on whether it is in regular mode or in SLO mode that catches wingsuit and hop and pop openings better.)

How data is averaged internally matters too. My Protrack data downloaded to the computer didn't seem to match up well with events in the sky, until I realized that the smoothed curve for vertical speed is the average of the last 6 seconds of raw data. The raw data bounces around 10-20mph every quarter second so needs some smoothing.

That delayed averaging results in most events starting to really show up 3 seconds after they actually happened, even if they technically start to show up within the first second. So when doing detailed analysis, I send the data to Excel from L&B's Jumptrack software, and change the averaging to using 3 sec previous and 3 sec after.

So again we have a way in which altitude gadgets may differ in ways we are not sure about, and may lag in reporting events.

How it all works together in your case, I don't know, but there are different sources for errors.

[Westerly 61]

pchapman

That delayed averaging results in most events starting to really show up 3 seconds after they actually happened, even if they technically start to show up within the first second. So when doing detailed analysis, I send the data to Excel from L&B's Jumptrack software, and change the averaging to using 3 sec previous and 3 sec after.

That is interesting and would explain things. I noticed that when looking sample-by-sample in my Viso, my Viso is showing me going terminal velocity all the way through 3300' even though I am pulling at 4k. This never really made sense to me as video shows the parachute out of the bag 1 second after deployment and so there is no way I could be in freefall for 750' after I pulled.

[LeeroyJenkins 2]

The viso and AAD count deployment when your parachute is open. Generally when they detect a vertical speed of les than ~25 MPH. What you need to do is get a flysight, it will give you much better readings. You can also put a camera on your head facing backwards and count frames.

[RockSkyGirl 10]

I have the same thing going on, though I use an Atlas instead of a Viso. My Atlas consistently reads lower than my OptimaII audible indicates, both on the ride up (by about 60 feet) and during freefall descent (by 200-600 feet depending on if I'm solo or in formation; for some reason - maybe the formation's burble? - it reads much lower when I'm part of a large formation than it does a few seconds after I've gotten back on my belly at the end of a solo freefly jump).

There have also been times, not uncommon, where it is still displaying descent in freefall mode when I am stowing my slider - and it's another few seconds before it clicks over to canopy descent mode. So it's definitely lagging, and when I log my pull altitude I base it off how many seconds it was after my audible started beeping rather than what the Atlas says, because the Atlas always tells me I'm pulling lower than I think I am.

I have a FlySight coming in the next week, so I'll be able to start comparing the GPS logs from that with my audible and my Atlas. That'll be the really telling thing, and I'll be able to say "know" instead of "think."
Never thought it might be the burble affecting the Atlas, though; it's on my forearm angled in, not on the back of my hand, so there's air both above and below (my arm isn't blocking the wind from hitting the altimeter due to the mounting angle). Though maybe that's different when the burble is huge, as it is in a bigway formation.

[benlangfeld 1]

Quote

I have a FlySight coming in the next week, so I'll be able to start comparing the GPS logs from that with my audible and my Atlas. That'll be the really telling thing, and I'll be able to say "know" instead of "think."

Is the FlySight known to be much more accurate than typical altimeters in the vertical plane? I had always assumed direct pressure measurements would be more accurate and GPS was only necessary for the horizontal plane.

[Westerly 61]

RockSkyGirl

I have the same thing going on, though I use an Atlas instead of a Viso. My Atlas consistently reads lower than my OptimaII audible indicates, both on the ride up (by about 60 feet) and during freefall descent (by 200-600 feet depending on if I'm solo or in formation; for some reason - maybe the formation's burble? - it reads much lower when I'm part of a large formation than it does a few seconds after I've gotten back on my belly at the end of a solo freefly jump).

There have also been times, not uncommon, where it is still displaying descent in freefall mode when I am stowing my slider - and it's another few seconds before it clicks over to canopy descent mode. So it's definitely lagging, and when I log my pull altitude I base it off how many seconds it was after my audible started beeping rather than what the Atlas says, because the Atlas always tells me I'm pulling lower than I think I am.

I have a FlySight coming in the next week, so I'll be able to start comparing the GPS logs from that with my audible and my Atlas. That'll be the really telling thing, and I'll be able to say "know" instead of "think."
Never thought it might be the burble affecting the Atlas, though; it's on my forearm angled in, not on the back of my hand, so there's air both above and below (my arm isn't blocking the wind from hitting the altimeter due to the mounting angle). Though maybe that's different when the burble is huge, as it is in a bigway formation.

Well I cant speak to the Atlas but I believe the Viso is accurate. I've compared my Viso to several other Visos on the same load and every unit read the same within 10 feet. In freefall I've looked over at other jumper's Visos and they read the exact same as mine. Further, my Viso consistently reads the same deployment altitude as my AAD in every jump within 30 feet or so.

For the most part my Viso reads the same as my audible. On the way up, it's within 20 feet or so. On the way down on occasion I have noticed the beeps did not coincide with the Viso accurately, but generaly speaking it's accurate.

There is one key point here. My Viso reads the same deployment altitude as my AAD and the AAD MUST be accurate. An AAD absolutely must be accurate or else the entire premise behind how an AAD works would be invalid.

What does your AAD say you deploy at and how does that compare to your Atlas?

[Blis 1]

benlangfeld

Quote

I have a FlySight coming in the next week, so I'll be able to start comparing the GPS logs from that with my audible and my Atlas. That'll be the really telling thing, and I'll be able to say "know" instead of "think."

Is the FlySight known to be much more accurate than typical altimeters in the vertical plane? I had always assumed direct pressure measurements would be more accurate and GPS was only necessary for the horizontal plane.

Direct pressure readings are rather unreliable in our sport becouse the airflow around us quite turbulent and depending on the orientation you have low and high pressure zones that interfere with the measurement...

[kleggo 138]

What do your eyes (looking at the ground) tell you?
I trust them more than I do devices.

cheers

Craig

[DougH 270]

Westerly

There is one key point here. My Viso reads the same deployment altitude as my AAD and the AAD MUST be accurate. An AAD absolutely must be accurate or else the entire premise behind how an AAD works would be invalid.

I get what you are trying to say, but read the manual. The AAD reads differently depending on your orientation.

The pressure readings must be accurate, but translation of those pressure readings compared to your true AGL altitude varies based on body position.

"The restraining order says you're only allowed to touch me in freefall"
=P