How to remedy multiple electronic engine control VDT faults

Effectivity: Global 7500 aircraft 

ATA: 73 

By: Luc Beaudry – Global 7500 Customer Services 

Global 7500 aircraft operators are advised of a potential electronic engine control (EEC) fault condition that can occur during engine shutdown (both on-ground and in-flight). This article explains the fault condition and how to resolve it.

The fault can be triggered if the EEC loses variable differential transformer (VDT) feedback after a master lever reset (MLR) within the EEC.The MLR occurs when the engine run switch is set to the “OFF” position at the end of a mission. All EEC part numbers in the field are susceptible to this condition.

Background

Bombardier has received reports from in-service aircraft where the EEC posted faults on multiple VDTs at the same time. All the events reported occurred during an on-ground shutdown.

Investigation has identified a susceptibility in the digital-to-analog converter (DAC) that could cause the EEC to lock up after an MLR. The root cause has been identified as latching of the clock timing to the DAC after an MLR during engine shutdown. The fault clears after a power cycle of the EEC channel circuit breakers (CB) associated with the reported faults. The power cycle of the EEC channel resets the clock timing and re-enables DAC functionality.

Each DAC drives the excitation signals for the VDT sensor interfaces. Therefore, at engine shutdown, a loss of the DAC signal results in the loss of feedback from the VDTs. 

On the GE Passport 20 EECs, there are three DACs used to power the various VDTs. If the fault should occur, the INFO message L (or) R FADEC FAULT 3 will appear (Code 732N35/732N36). Caution CAS (Crew Alerting System) message L (or) R REVERSER FAIL (Code 732C10/732C12) could also appear.  

The Caution CAS messages are caused by a failed thrust reverser door VDT signal. The thrust reverser VDT signal is located on the DAC #3 (Group-3) along with HPTACC, LPTACC and HPV. 

Depending which of the three DACs experiences the fault, the following fault message groupings will be posted on the Onboard Maintenance System (OMS): 

Group-1 

• RIGHT/LEFT ENGINE CH.A/B FMV VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321155ENG) 

• RIGHT/LEFT ENGINE CH.A/B VBV VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321297ENG) 

• RIGHT/LEFT ENGINE CH.A/B VSV VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321306ENG) 

• RIGHT/LEFT ENGINE CH.A/B TLA RVDT OUT OF RANGE LOCAL CHANNEL FAULT (Fault Code 7321289ENG) 

Group-2 

• RIGHT/LEFT ENGINE CH.A/B SBV/NAI VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321344ENG) 

• RIGHT/LEFT ENGINE CH.A/B JPSOV VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321390ENG) 

Group-3 

• RIGHT/LEFT ENGINE CH.A/B HPTACC VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321187ENG) 

• RIGHT/LEFT ENGINE CH.A/B LPTACC VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321209ENG) 

• RIGHT/LEFT ENGINE CH.A/B HPV VDT SENSOR OUT OF RANGE AMBIGUOUS FAULT (Fault Code 7321380ENG) 

• RIGHT/LEFT ENGINE CH.A/B TR LOWER DOOR LVDT SENSOR FAULT (Fault Code 7321420ENG) 

• RIGHT/LEFT ENGINE CH.A/B TR UPPER DOOR LVDT SENSOR FAULT (Fault Code 7321416ENG) 

Recommendation 

Should an EEC encounter multiple VDT faults as listed above due to a DAC lock-up during an on-ground shutdown, the EEC power can be reset via the listed circuit breakers. The EEC will then self-correct, and the faults will disappear. There is no need to replace the EEC.  

If the issue persists after performing this reset, or if the issue repeats three times on the same EEC, GE recommends replacing the EEC. 

If this issue occurs following an in-flight shutdown, the L (or) R FADEC FAULT 3 will be posted in the info message. In this situation, continue with normal operation and reset the EEC power after landing, per the instructions above. If additionally, to the L (or) R FADEC FAULT 3 Info message a L (or) R REVERSER FAIL is posted, follow the guidance provided in the flight manual. After landing, perform the EEC reset. 

This information was also published in GE Fleet Highlites (21-3Q-7321-09), which Bombardier strongly encourages operators to read for Passport 20 engine updates. 

The SmartFix Plus Fault Isolation procedures related to the faults listed in this article were updated to include the Fleet Highlites article with this information.  

GE is working on a permanent solution to address this issue.