Transport Canada CF-2023-33

Requires the installation of a monitoring circuit comprising relays, external to the air data monitoring system heater current monitoring units, that will detect the loss of the affected probe heating function and will result in appropriate crew alerting system messages. This AD also requires a revision of the normal and non-normal procedure sections of the airplane flight manual. Prompted by Bombardier's review of the air data system that found a single-point failure exists within the left- and right-hand heater current monitor units, since the same programmable logic device (PLD) is responsible for both the control and monitoring functions of each unit. The PLD could fail in a way that it would erroneously energize the heater control relay and switch the heaters off. This failure could lead to unannunciated loss of ice protection on the air data probes, resulting in the potential display of misleading airspeed and other indications to the crew.