A problem with UPT’s Sigma Tandem Parachute System (and a solution)

Why so many baglocks on Sigma tandems?

As a student of parachute design and as a certified Rigger, I have learned that a staged deployment of the main canopy is the safest way to deploy a parachute. All sport rigs do it this way, but one Tandem system does not: The UPT Sigma Tandem System.

For all tandem systems, at pull time, the drogue becomes the system’s pilot chute, pulling the main bag out of the container and getting the main parachute to full line-stretch.  This should make for positive extraction of the main bag, just like a regular pilot chute on every other sport system on the market today.

Unfortunately, the UPT Sigma Tandem System’s drogue is designed to collapse as soon as a drogue release handle is pulled. This design “feature” is marketed as making for a softer opening of the main parachute, but in reality, it is a band-aid to address a problem with its drogue design: the drogue has too much drag. In freefall, this is good for taking really heavy loads (so the video guy can keep up), but at opening time, when the drogue is released, it produces so much drag that it makes for some “spirited” openings without collapsing it first, and collapsing is required to save people’s necks and to prevent exploding main parachutes. UPT’s solution is to collapse the drogue at pull time. That is like throwing out an un-cocked pilot chute on a sport system.

The problem with this design feature is this: once collapsed, how much drag does that collapsed drogue produce? And secondly, is it enough drag? Keep in mind a bagged tandem main weighs about 15 lbs.

What I do know is that after looking at hundreds of videos of Sigmas – this one is just an example – the main bag is not always positively extracted at pull time. The tandem pair falls through the “trap door” as they accelerate because the drogue has just collapsed, and the main bag is lazily lifted off the tandem instructors’ back, with the main parachute lines lazily zig-zagging from side to side, not “stretched” straight to line-stretch. It works most of the time. However, it also increases the incidences of off-heading openings, linetwists and baglock malfunctions (the heavy bag spinning around a tight line-stow due to insufficient drogue drag).

The problem worsens with age, as the drogue kill-line shortens with repetitive usage.

There is a solution to this problem: lengthening the drogue’s kill-line length to ensure the drogue always has a bit of drag, and never “fully” collapses. This is my recommended modification for all Sigma jumpers and DZs.

Here’s is UPT’s July 2012 instructions on ensuring your kill-line is to their specifications:

Drogue-Kill-Line-Calibration

Don’t adjust to factory specifications! Instead, follow the optional instructions at the end the document to lengthen your kill-line. This will make for a more positive extraction at pull-time, reducing the incidences of off-heading openings, linetwists and baglocks malfunctions.