Globally the 601 XL suffered a number of catastrophic structural failures which in summary have been attributed to aeroelastic aileron flutter and a sub-optimal structural strength of the aircraft.
The NTSB/FAA in the States has grounded the entire US fleet and they have mandated and tested the upgrade. Prior to the release of the upgrade I was considering selling the kit due to the structural concerns however the upgrade talks to all the areas that I was concerned about.
See image 1 for an example of the change in thickness of the rear spar attach point before and after upgrade. The item installed on the aircraft is the upgraded version (obviously)!
Note: Almost a year on no Zodiac structural failures have occurred.
