Skyhook inadvertent disconnect.

[phantomII 36]

In an actual tandem fatality thread someone wrote:

"Even if properly rigged, estimat is that it disconnect prematurely in approx 15% of uses."

Since nobody commented or argued about this statement I have two questions.
1. Is this estimate correct in any way?
2. What do people think is the reason for this? (Esp. if properly rigged)

[Kenzdik96 26]

I don't have the numbers but the thing about skyhook is that even though it was designed to disconnect based on force (ie is the RPC or the cutaway main pulling stronger) it actually disconnects based on pulling angle (ie which direction is the cutaway main pulling). Even though skyhook was the first available MARD, it isn't really the best one around. Therr are systems out there that work based on force, and are agnostic of pulling direction. 

I am hoping UPT will update the skyhook some time in the future to address this.

[RiggerLee 61]

I've seen it fail a number of times during deployment. I would guess in the 10% to 15% range is about right. It is and I believe that it was designed to be a less aggressive system. It was only like the third system designed and the second to be marketed. I believe the thought process was more along the lines of it might help you but it probably would not kill you. Or so we thought till it did... But it's on the conservative side by design.

 

Lee

[Binary93 65]

6 hours ago, RiggerLee said:

Or so we thought till it did...

Not familiar with this, would you mind sharing please?

[dudeman17 267]

I'm not a rigger, but I am a TI. I've had at least one cutaway on a Sigma tandem rig where I believe the skyhook disconnected. That thing is supposed to stay connected during a cutaway, but it needs to disconnect if the reserve is pulled independently. It would be better to have it disconnect when it's not supposed to, than have it fail to disconnect when it needs to. I'm guessing that the design takes that into account.

[mark 102]

18 hours ago, phantomII said:

2. What do people think is the reason for this? (Esp. if properly rigged)

During a MARD deployment, just as the bag is lifting off, the MARD device is pulling on both the bag and the more-or-less inverted reserve pilot chute.  The bag/canopy has mass, the pilot chute has mass, and both legs of the inverted "V" of the bridle stretch and recoil repeatedly.  The masses are different, and the legs are different lengths, so the magnitude and period of the recoil oscillations are different.  If the pilot chute leg loads while the bag/canopy leg unloads, the MARD device disconnects.

This is the case for all MARDs in common use, except the Infinity MARD.

[phantomII 36]

8 hours ago, mark said:

.... and both legs of the inverted "V" of the bridle stretch and recoil repeatedly.  .....

Once you get to the "V", the skyhook is already working.
Disconnecting later on shouldn't be a big problem?

[SethInMI 145]

3 hours ago, phantomII said:

Disconnecting later on shouldn't be a big problem?

Disconnecting isn't a big problem. It does make the MARD ineffective though, so you are left with a normal RPC controlled deployment.

[mark 102]

6 hours ago, phantomII said:

Once you get to the "V", the skyhook is already working.
Disconnecting later on shouldn't be a big problem?

No.  You get to the "V" stage when the reserve bridle is first stretched from the bag to the MARD, as the bag is lifting out of the container.  The MARD needs to stay connected until the canopy is out of the bag.  After that it doesn't matter much if it remains connected or releases, except it's nice if it remains connected to the main -- easier to find everything.

Edited June 25, 2021 by mark

[mark 102]

9 minutes ago, SethInMI said:

Disconnecting isn't a big problem. It does make the MARD ineffective though, so you are left with a normal RPC controlled deployment.

Not exactly.  Depending on when the disconnection occurs, it's possible to launch the bag with enough momentum for the lines to pay out and for the bag to open without the help of the pilot chute.

[nwt 131]

On 6/24/2021 at 1:23 AM, Kenzdik96 said:

Even though skyhook was the first available MARD, it isn't really the best one around. Therr are systems out there that work based on force, and are agnostic of pulling direction. 

Which MARDs are better?

[Binary93 65]

4 hours ago, nwt said:

Which MARDs are better?

My current favorite is FORS (by Avalon). Extremely simple design, yet it isn't affected by the pull direction (only by force).

Infinity's MARD is also pretty nice design in my opinion. Trap by Mirage is also pretty great in my opinion as it provides zero force on total malfunction reserve deployment. You have to replace parts after several deployments IIRC, but that's what you pay for more security I guess.

 

[Deyan 31]

2 hours ago, Binary93 said:

My current favorite is FORS (by Avalon). Extremely simple design, yet it isn't affected by the pull direction (only by force).

Infinity's MARD is also pretty nice design in my opinion. Trap by Mirage is also pretty great in my opinion as it provides zero force on total malfunction reserve deployment. You have to replace parts after several deployments IIRC, but that's what you pay for more security I guess.

 

So I just checked that FORS you are talking about and it looks like they took the design of Eric Fradet and made it more complicated. (maybe to avoid paying royalties).