Are you looking for practical tips and expertise on managing the vibration levels of your business jet’s engines? Duncan Aviation’s Bill Walker shares insights on the common causes and preventative maintenance operators should undertake…
All aircraft engines experience vibration. That is normal and acceptable. However, because of the tight manufacturing tolerances of today, it’s important to balance the rotating engine groups as low as possible for reasons that include:
- Avoiding early wear and deterioration of parts;
- Avoiding damage to the aircraft; and
- Optimizing passenger comfort.
What Causes Jet Engine Vibration Issues?
The causes for engine vibration issues vary. Most of today’s turbofan engines have two rotating groups:
- The fan (Low Pressure) and
- The core (High Pressure).
The fan section spins much slower than the core but is larger in diameter and is usually the culprit for engine vibration.
Common Core Vibration Causes: Once balanced by a designated overhaul facility, the core won’t usually cause vibration problems. When vibration is experienced it tends to be that there is foreign object damage (FOD) or an oil-related issue.
Common Fan Vibration Causes: Fan vibration issues are more varied, and could be caused by dirt and contaminants from the atmosphere or lack of lubricant (dry lube) on the fan blade tang or fan disk bushing migration (depending on engine type), which prevents the fan blades from moving freely.
Typically, fan blades that are loosely installed find their proper load position with an engine at RPM. Fan blades that are tight, or locked in place may not find their load position and could prevent others from doing so, causing an unbalance which results in increased vibration.
How to Stay on Top of Jet Engine Vibration
It’s relatively easy to monitor your engine’s vibration levels. Operators who perform Engine Condition Trend Monitoring (ECTM) to track engine vibration will usually get an early detection of increased vibrations. This helps them investigate the cause and consider the preventative maintenance steps ahead of their next scheduled inspection interval.
Engine vibration or an unbalance can usually be corrected without removing the fan (Low Pressure) rotating group.
With the help of approved test equipment (including a vibration pick-up or accelerometer, a tachometer source, test harnesses and a vibration computer), the unbalance can be corrected while the engine remains on-wing.
How Long Does it Take to Fix Jet Engine Vibration Problems?
Some engines, such as the General Electric CF34, are easier to balance. In the case of the CF34 engine, the newer fan disk and spinner have screw-in type socket weights that help reduce the time it takes to balance the fan.
Honeywell’s HTF7000 and TFE731, and newer Pratt & Whitney and Rolls-Royce engines use different thickness washers as weights that are under spinner bolts, making the job just as easy.
However, some older model engines use riveted saddle weights or washer/nut/bolt weights mounted inside the spinner that are less easy to access, slowing the job down.
How to Prevent Jet Engine Vibration Issues Developing?
There are many preventative maintenance measures operators can take to address engine vibration. These include:
- Periodic fan blade inspections: With gloved hands, rotate the fan and wiggle each fan blade, making sure they all move freely.
- If a fan blade is removed, inspect the blade tang lubricant or bushing for migration. These are common causes for fan blades not moving freely.
- If the lubricant has deteriorated consider cleaning and re-lubing, or doing a touch-up of all the fan blades per the appropriate engine maintenance manual. (The engine maintenance manual will list the approved lubricants. These are usually readily available.)
- It’s a good idea to do a daily inspection of the fan for FOD.
- Always adhere to the engine maintenance manual for blending allowances.
Monitoring vibration levels and addressing them early will help to prevent lengthy, expensive maintenance events. And if you’re ever in doubt, discuss the necessary preventative maintenance measures with your MRO shop.
More information from www.duncanaviation.aero