N1 vibration approaching exceedance faults after engine start

Effectivity: Global 7500 aircraft 

ATA 71 

By: Luc Beaudry – System Specialist (Powerplant & APU), Global 7500 Customer Service

This article reminds Global 7500 aircraft operators that an “N1 engine vibration approaching exceedance fault” (Fault Code: 7321214ENGRA, 7321214ENGRB, 7321214ENGLA or 7321214ENGLB) with no associated flight deck effects may have been reported. The fault occurs during engine start and remains posted for approximately two minutes after start and for approximately two minutes after the engine reaches idle. This article describes the cause of this fault and provides recommendations if it occurs. 

Background 

The “N1 engine vibration approaching exceedance fault” is set when the valid selected N1 vibration value is greater than the N1-vibration-approaching-exceedance limit (0.0001309 Inch per second peak (IPSpk)/RPM) for more than nine (9) seconds. Several engines reported this fault after the engines reached idle, with no effect on engine performance. 

Root cause 

The engine can experience rotor bow due to thermo-elastic effects that occur when the engine is hot and not rotating (i.e., following engine shutdown). Starting the engine or performing a max motor while the rotor bow exists can cause a temporary increase in core vibration. The residual vibration from engine start can manifest after the engine reaches idle, but once the speeds stabilize, this vibration returns to its nominal low value for the remainder of the flight. 

The mitigation for this condition is to motor the engine at reduced speed prior to engine start to help equalize the internal temperatures and relieve the rotor bow condition. Although the engine software includes this function, it is designed to monitor and address vibration produced on the engine core, not the fan. When starting the engine or performing a “wet” motor or “dry” motor of the engine, the FADEC system uses several parameters to determine if the rotor is still “bowed” or if it has reached a thermally stable condition.  

During a start or engine motoring (when the aircraft is on the ground) the FADEC system will modulate the starter air valve to keep the engine rotational speed in the correct speed zone.  

As the FADEC system controls this motoring, in normal operating conditions, the pilots do not need to take any additional action (i.e., when motoring is needed) as it will occur on its own. 

Interim corrective actions 

This fault scenario requires no maintenance action. 

Recommendations 

To minimize the likelihood of experiencing an N1 vibration exceedance related to this condition, Bombardier and GE recommend following the Idle Stabilization Time Requirements of five (5) minutes at idle after engine start before attempting to go to high power (Refer to the LMM PASSPORT20-A-72-00-00-00A-360A-D Engine operating limits - Design data/tolerances check). If an N1 vibration exceedance is detected during this timeframe, GE recommends monitoring vibration values on the Engine Indication and Crew Alerting System (EICAS) display to prevent exceeding the operation limit (PASSPORT20-A-72-00-00-00A-360A-D).  

Data review from previous events indicate this is a temporary spike that tends to go down within a few seconds. It is not recommended that an engine be shut down unless there is another indication of severe engine abnormality. In case that an exceedance is detected (Caution message “ENG N1 VIBRATION”) outside of the stabilization window, GE recommends following the Airplane Flight Manual (AFM) section 03-05 to clear this condition. 

Operators are reminded as well that under icing conditions, fan vibration may increase during the ice-shedding process (1300-1500 RPM or 21.55-24.86 % N1). 

The following information was also highlighted in the GE Fleet highlights (20-4Q-7200-04), which Bombardier strongly encourages operators to read for any updates on the Passport 20 engines.