P6-2A Transfer Bus2 - 115v ac
Condition : Accomplish this NNC only when engine vibrations indicate above 4.0 units and airframe vibrations occur (to exclude a faulty indication). Consider this NNC also when vibration fluctuates by > one unit.
If vibrations continue after accomplishing the NNC, changing airspeed and/or altitude may aid in decreasing the vibration level. With the thrust lever in or near idle, the engine must be considered inoperative ! Use the NNC [Engine Failure or shutdown] (for fuel balance, etc) and the NNC [One engine Inoperative Landing] (flaps 15) as guidelines.
Engine vibrations may be caused by fan blade icing, ingestion of birds or FOD, fan blade unbalance or fracture, bearing failure or any other internal malfunction.
AVM indications are not valid at starter cutout and during large power changes, such as during engine spool-up at takeoff. And not all vibrations are indicated.
LIMITATIONS
L - There are no engine vibration limitations published by Boeing or by CFM.
Each operator should establish its own policy. If no specific SOP are published, the following values can be used as guideline :
On GND
If vibrations after engine start show above 1.0 units and the engine has no previous history (e.g. recent birdstrike), consider calling maintenance.
In FLT
All vibrations above 1.5 units should be reported in the ATL.
Vibrations above 2.0 units cause discomfort and can result in complaints from passengers.
CFM Industries recommends operators schedule a fan trim balance when N1 vibration levels are above 1.5 or 2.0 units, however without delaying the flight.
According to the Maintenance Manual, the operator should find the cause of the high vibration and take corrective action if engine vibration is 4.0 units or more on the Low Pressure rotor or 3.0 units or more on the High Pressure rotor.
Engine vibrations measured on the N1 bearing and on the fan frame compressor case are sent to the AVM signal conditioner. The AVM signal conditioner calculates vibration for these engine areas :
- Fan/low pressure compressor (FAN)
- High pressure compressor (HPC)
- High pressure turbine (HPT)
- Low pressure turbine (LPT)
The highest value of these calculations is displayed on the CDS. VIB indication on the flight deck is displayed in units, a table is used to convert to MILS.
(As installed) To find out which value is displayed on the CDS, refer to the ACMS Engine page. ACMS vibration values are also depicted in units.