Note: Checking and retrimming main canopies is a common requirement for riggers. All canopies
and all kinds of suspension lines get out of trim in time with use. The rigger that knows how to
quickly check the trim on a customer’s canopy, and then determine how much work it will take to
return it to the original configuration, is performing a valuable service. In most cases, as long as the
suspension lines are in good condition, retrimming a canopy will add hundreds of jumps to the life
of the canopy and return the performance to almost as good as when new.
This chart and procedure are tailored to apply to normal ram-air canopies. Certain designs, such as
elliptical canopies, may have nonstandard trim measurements that require more detailed measurements.
The rigger should consult the manufacturer’s manuals or technical data for these canopies.
1.1 The first thing the rigger must do is to complete a measurement of the lines of the canopy.
Figure A shows a matrix for measuring the lines of the canopy. Simply fill in the boxes for
each dimension measured.
1.2 Start by laying the canopy on the left side, anchor the connector links, and flake it out as if for packing. Most canopy lines are measured from the inside of the connector links, which is called the “zero” point.
1.3 Anchor the end of the tape measure even with the zero
mark at the end of the lines using the pony clamp [Figure
1.4 Start with the top outside right front corner line of the
canopy. This line will be line 1A. Depending on the number
of cells to the canopy, on a 7-cell canopy, the left corner
line will be line 8A. On a 9-cell canopy, this line will
be line 10A. Make sure that the canopy is oriented correctly
so that the measurement sequence follows the
boxes on the chart.
1.5 Measure all the A lines first, then proceed to the B, C and D line groups. The lines should be measured under approximately 10 pounds of tension. As the rigger moves through the lines and changes riser groups, the end of the tape measure should be moved to the corresponding connector link and re-clamped.
1.6 Measure the control line groups and fill in the boxes.
2.1 Compare the measurements to the original line lengths as in the manual.
2.2 Depending on the type of canopy and type of line used, most canopies tend to have the center
A lines stretch due to the load on opening. The outside lines that attach to the stabilizers
and the control lines, tend to shrink due to the friction generated by the slider on opening.
The key concept to remember is that the length of the lines is not the critical dimension.
What is most important is the trim differential, which determines the angle of attack of the
canopy. The trim differential is the difference in the line length between the A,B, C, and D
lines. The most accurate method of measuring this is to use the A lines as the base dimension
and then measure A-B, A-C, and A-D. This method takes into consideration the tolerance
allowed. This dimension is what makes the canopy open and fly correctly. If the overall
length of the lines is 2-3" longer or shorter, but the trim is correct, there will probably be no
appreciable effect on the canopy. With this in mind, it may be desirable to in effect “shortline”
the canopy during the trim process in order to not have to replace any main lines.
2.3 Because of the fact that the cascaded main lines may stretch at different rates, the adjustment
to be made to the lines that have stretched (i.e., gotten longer) will need to be done at the
canopy end and not at the connector link.
2.4 Remove the bar tacks or zigzag stitching at the canopy end of the lines.
2.5 Adjust the line length as needed to return to the original dimensions. Re-tie the knots or finger-
trap as necessary and resew as per the original.
2.6 For the lines that may have shrunk in length, it may be possible to gain as much as 2-3" of line
adjustment by utilizing the extra line that is finger-trapped into each line attachment point.
Depending on the type of line used on the canopy, the amount of excess line finger-trapped may
vary. Dacron lines can have as little as 2-3" inserted into the finger-trap and hold securely. The
smaller and more slippery Spectra® line is recommended to have at least 6" of line inserted into
the finger-trap. So, depending on the amount of line that is needed to be gained for the trim adjustment,
the rigger may be able to adjust each end of the line to gain the necessary adjustment.
2.7 If this needed line can be gained, remove the bar tacks or zigzag at each end of the line and
reattach after adjusting the length. Before permanently attaching, the rigger should simply tie
the lines in place and check the trim. If everything is within limits, then re-tie or finger-trap
and restitch with bar tacks or zigzag as per the original.
3.1 Check final trim dimensions against original.
3.2 Check continuity so there are no twists or crossed lines.
3.3 Verify all junctions are either knotted or finger-trapped and sewn as per the original.