Challenger 605
Maintenance Inspection Authority; Maintenance Powerplant; Maintenance Airframe; Maintenance Avionics
Maintenance Avionics 15; Maintenance Inspector 15; Maintenance Lead Technician 20; Maintenance Technician 25
Confusion; Troubleshooting; Communication Breakdown
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft; Chart Or Publication; Manuals; Human Factors
The crew reported a 'Stall Fail' Crew Alerting System (CAS) message in flight that was intermittent. The crew followed the Abnormal Checklist and disengaged one side of the Stall Warning System for the remainder of the flight. Upon inspection after landing; it was found that the Right-Hand (R/H) Angle Of Attack (AOA) transducer Voltage checks failed. These Voltage checks were provided by Bombardier Technical support and not listed in the Maintenance Manual (M/M). The unit in question passed the Linearity (Resistance Checks) which is a required check every 300 flight hours. It was interesting that the Resistance Checks passed but the Non-published Voltage Checks failed.There is an Airworthiness Directive (AD) (AD 2003-22-12) for this system for the Bombardier CL604 Challenger model; but not for the CL605 system. This AD requires that the resistance be checked every 300 hours. This unit would have passed that check. The Voltage Check showed a voltage error of approximately .8 volts low for every five degrees of AOA travel. This equaled approximately four degrees of error between the L/H and R/H AOA Vane which caused the stall fail message to illuminate. The R/H AOA transducer was replaced with a new unit and the aircraft was returned to service.It should be noted that the required AOA Transducer Resistance Check may not be enough on its own to check the serviceability of these units. Bombardier AOA transducer Part Number (P/N) 600 59154-3.
Reporter stated that icing was not an issue at the time the flight crew saw the 'Stall Fail' CAS light appear. There were multiple illuminations of the 'Stall Fail' light in flight that would go 'On'; then 'Out'; then come back 'On.' No indication is given to pilots whether the Left or Right Stall Protection System had failed. Per their checklist; pilots have to decide which yoke pusher actuator motor switch (Left or Right) to shut-off; leaving only one side with a pusher actuator operational. They will still get a Stick Shaker and Aural Warning if a stall occurs.Reporter stated they previously performed a repetitive 300-Hour Airworthiness Directive (AD) 2003-22-12 Linearity Resistance Check measurement of both AOA vane transducers. The Left AOA failed the Resistance Check and the AOA Assembly was replaced. The right AOA was OK. Shortly after; on a different flight; another 'Stall Fail' Warning CAS light appeared. But no faults were found in the aircraft's Maintenance Diagnostic Computer (MDC). He performed another Resistance Check and both AOAs passed. Since the Left AOA was new and the Right also passed the Resistance Check and Bombardier does not have a Fault Isolation Manual (FIM) to troubleshoot; they went to Bombardier's 'Smart Fix Plus' site to determine what to do next. No other information for troubleshooting 'Stall Fail' CAS warning involving the AOAs could be found.Reporter stated he performed a Voltage Check of the Right AOA using Non-published Voltage Checks. Resistance Checks only require three AOA vane positions to check. The Voltage Checks require verification of voltage readings at   15-different vane positions. The Right AOA assembly had failed the Voltage Check even though the same AOA had passed the Resistance Check that satisfies the AD.Reporter stated his concerns are that mechanics are signing-off AOA units as Serviceable after passing Resistance Checks performed per AD 2003-22-12. When in fact; the same units have failed the Voltage Check procedure which is not published in their Maintenance Manual (M/M); except under an Alignment Check section if the AOA Assembly is replaced for cause. Otherwise the Voltage Check is not applied. In their situation; the cause of the 'Stall Fail' CAS Warning light was not due to an AOA transducer Ohms Resistance failure; but a Voltage disparity. If the AOA Assembly fails a Voltage Check the unit must be replaced. That is why he believes the Voltage Check should be included with the Resistance Check to determine Serviceability and proper determination of the AOA vane positions.
An Aircraft Maintenance Technician (AMT) with Avionics experience reported the Linearity Resistance Check of AOA Vane Assembly transducers required by Airworthiness Directive (AD) on Bombardier aircraft should include a Voltage Check to properly determine the Serviceability of the AOA assemblies. Troubleshooting for an inflight 'Stall Fail' CAS warning revealed that the Right AOA vane had passed the Resistance Check; but failed the Voltage Check.