The planes were banned from flying pending an investigation into their structural strength, following the crash of a European variant of the design (Rotax powered and 450 kg (992 lb) maximum take-off weight) that killed two people.

[12][13][14][15][16] Zenith Aircraft disputed the NTSB's conclusions and stated in a response on their website that "[w]e continue to believe wing flutter will not occur if the control cables are adjusted properly.

[18] The company hired an independent consultant, Dr. Uwe Weltin, an internationally recognized flutter and vibrations specialist and head of the Institut für Zuverlaessigkeitstechnik at the Technical University of Hamburg-Harburg who concluded that when the CH 601 XL is built and maintained to Zenair specifications, there is "no tendency to flutter or divergence found within the flight envelope of the CH 601 XL".

The company claimed that the report clears the Zodiac design of flutter-related concerns as long as CH 601 XL is built and maintained to Zenair specifications.

[21][22] The FAA issued a Special Airworthiness Information Bulletin on 7 November 2009 and strongly recommended that the aircraft not be flown until modifications detailed in an AMD Safety Alert are carried out.

What has prompted this "180-degree" shift, from insisting that the CH 601 XL design was fine "as is", to now mandating a list of upgrades requiring more than a dozen modifications?"

To this date, after thousands of man-hours of investigations, multiple design reviews and an unheard-of amount of testing, the accidents in question still do not share a common cause.

In offering this "Upgrade Package" I have had to set aside my own professional opinion (that the design is sound) as well as legal counsel's advice in order to provide builders, owners and pilots the "fix" that they have been asking me for.

With these upgrades (my "180° shift"), the safety margins of key airframe components have been dramatically increased...[28]On 12 November 2009, the FAA ceased issuing new Certificates of Airworthiness, requiring new registrants to prove that they have complied with the modifications before being permitted to fly the aircraft.

Hersman said on 13 November 2009: We are pleased that the FAA and the manufacturer have acted on the safety-of-flight issues that we identified with the Zodiac special light sport airplane.

The special review also identified issues with the airplane's flutter characteristics, stick force gradients, airspeed calibration, and operating limitations.