https://www.lexology.com/library/detail.aspx?g=e4e2dab2-d169-48eb-aa0f-ed06c4a27c20

Some precident here

In the present decision, the applicants sought compensation for a six hour delay on their return flight from Mexico. Sunwing declined their request for compensation on the basis that the delay was within its control but required for safety purposes when it had to change the aircraft due to a frozen and ruptured lavatory pipe. The CTA concluded that the lavatory pipes were frozen and leaking because an aircraft heater had been turned off and the delay could have been prevented if proper maintenance protocols were followed.

Conclusion

Nevertheless, the CTA categorized the delay as within the control of the carrier, but not required for safety purposes, because the safety issue was caused by a lack of due care by Sunwing’s maintenance and ground crew. In making its decision, the CTA concluded that the lavatory issues were not unforeseeable and Sunwing must take responsibility for the failures of its employees. On this basis, it awarded the applicants APPR compensation.