Calculating Freefall Time

[bqmassey 0]

While I was jumping an analog altimeter, I would always calculate my "delay" by taking my exit altitude minus the altitude I waved off at, and using the little chart in my log book.

I bought an N3 and put five jumps on it. It regularly shows my deployment altitude to be 1000-1200 feet below my "wave-off" altitude.

I'm not exactly sure when the N3 logs the deployment. I'm usually busy with other things right about the time my canopy opens, not watching my alti.

So, what is the right way to log freefall time? From exit to wave-off? From exit to container opening? From exit to open canopy?

It'd be nice if I can just log the freefall time that the N3 shows in it's log, but if that's not the accepted method, I can do something else.

[NWFlyer 2]

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I bought an N3 and put five jumps on it. It regularly shows my deployment altitude to be 1000-1200 feet below my "wave-off" altitude.

That's about right once you count in time to deploy and fully open. Most loggers (I have a Protrack and an Altitrack) will represent the deployment altitude at about the point when you fully decelerate, which is some point after you deploy assuming all goes as planned.

Logging what your N3 shows is as reasonable an approach as any and it is what many skydivers do now that we've got such devices available. Approximations are just that and aren't going to take into account the difference in freefall time for skydives of different types.

"There is only one basic human right, the right to do as you damn well please. And with it comes the only basic human duty, the duty to take the consequences." -P.J. O'Rourke

[popsjumper 2]

Brandon,
I know of no altimeter of any sort that knows when you wave off.

It's generally accepted that the time shown is what you log.
It's generally accepted that that time runs from exit to in-the-saddle.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[Divalent 137]

Hmm, you mean you gotta be precise about this?

I was just writing in 60 sec for any jump from 14.5K (regardless actual pull altitude as long as it was 5K or less), nothing for a hop-n-pop, and ~30 sec for a very high pull. Should I be timing all this? (E.G., and add more for a long snivel?)

Am I gonna get busted by the log-book auditors? (If so, what's the penalty).

[nickfrey 0]

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Hmm, you mean you gotta be precise about this?

I was just writing in 60 sec for any jump from 14.5K (regardless actual pull altitude as long as it was 5K or less), nothing for a hop-n-pop, and ~30 sec for a very high pull. Should I be timing all this? (E.G., and add more for a long snivel?)

Am I gonna get busted by the log-book auditors? (If so, what's the penalty).

Just get a go-pro then you'll be able to know for sure!!!

[popsjumper 2]

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Am I gonna get busted by the log-book auditors? (If so, what's the penalty).

3 days in the box with Sandy.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[tkhayes 348]

In Canada a long long time ago, we were taught some numbers and had to manually calculate freefall times.

It takes 1483 feet to reach terminal of 120MPH, which takes 12 seconds.

120MPH is 176 ft/sec.

So if you jump from 13500', and you open at 2500', then you fell 11,000'.

11,000' -1483' = 9517'
9517/176 = 54 seconds
Add the 12 seconds to reach terminal and you get 66 total seconds.

If you jump at higher elevations and/or high altitudes, you would do an adjustment for about 10% of the freefall time.

We also used it to work backwards, i.e. the old student days had us doing a lot of shorter delays for progression, like 10's, 15, 20's, etc.

So as a student, who has to do a 20 second delay, opening at 3500', you would take the 12 seconds-to-termial out of the 20 total seconds, leaving 8 seconds

8 sec * 176ft/sec = 1408'

1408 + 1483 + 3500 = 6391

So a 20 second delay opening at 3500' would take 6500' altitude and that is what you would ask the Cessna pilot to give you.

I still use these numbers and I still teach them to (most of) my students. It is good to have something in your heard other than the piece of technology to tell you what is going on, and to have verification of what the 'little beepy device' is telling you.

[nigel99 507]

I was originally taught 10 seconds for the first 1000ft and 5 seconds for every 1000 ft after that. It makes the maths pretty easy to do.

Now like Divelent I just log 60 seconds

Experienced jumper - someone who has made mistakes more often than I have and lived.

[popsjumper 2]

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I was originally taught 10 seconds for the first 1000ft and 5 seconds for every 1000 ft after that. It makes the maths pretty easy to do.

I could have sworn I told you 11.5 and 5.5 but such is life.
There are anvils and there are feathers.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[nigel99 507]

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I was originally taught 10 seconds for the first 1000ft and 5 seconds for every 1000 ft after that. It makes the maths pretty easy to do.

I could have sworn I told you 11.5 and 5.5 but such is life.
There are anvils and there are feathers.

No I think people in Southern africa are more dense and fall faster. You said 11.5 & 5.5 but that is difficult to count...

Experienced jumper - someone who has made mistakes more often than I have and lived.

[pchapman 279]

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So, what is the right way to log freefall time? From exit to wave-off? From exit to container opening? From exit to open canopy?

Traditionally the convention was time from exit to pull. (In the US, the USPA [edit: not the FAR's as I first wrote, I think] may refer to pack opening altitude, a little after you actually start pulling, but people think of the altitude they pulled at.)

With electronic gadgets now, one just accepts whatever number they tell you -- to some point later in the opening when a large decrease in speed is apparent. What point that is, will depend on the gadget, the setting (eg, more sensitive for wingsuits), and the speed profile of your canopy opening.

Doing it without electronics was a massive guess or extreme pain in the ass if you tried to calculate accurately. (eg, "That was a 12.5 jump... except we got closer to 13... first half was sitfly, and then it was flatfly...let's split the difference...that gives X seconds... oh, no, I'm a liar and a cheat! ... I pulled a little higher than usual because it was a long spot... I have me knock off a couple seconds...but I don't recall exactly how high I pulled...")

It's not worth the time to calculate in detail. So if you still need freefall time logged for some license or something, you can work out an approximation for some typical jumps you do, and stick with that. (eg, 12.5ish, plus my typical opening alt, for mostly flatfly = X seconds)

[popsjumper 2]

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You said 11.5 & 5.5 but that is difficult to count...

Here's how you do it...
One potato, two potato, three potato, four....


Now, if you're chasing a bag of potatoes, it's easy!

Speaking of potatoes...next time you're here, get on a Mr. Potato Head jump.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[riggerpaul 1]

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Speaking of potatoes...next time you're here, get on a Mr. Potato Head jump.

Wow! You still do them? I haven't done one in probably 20 years. Great fun. Don't drop the potato!!!

[bqmassey 0]

Thanks for the replies y'all!

I'll go ahead and just use the times that my alti shows. I didn't know how particular you are supposed to be when you log that stuff, or what the accepted standard is for when the freefall stops.

I loaded the profile on to the computer. It's kind of interesting. It looks like it lags a little in starting the freefall timer on the exit, and then doesn't stop it until you're in the saddle. Sounds like it's close enough for our purposes!