Vibration Diagnostics
Helicopters vibrate. The frequency tells you where the problem is — knowing this is critical both for preflight diagnostics and for troubleshooting in flight when something doesn't feel right. Three frequency bands map to three different rotating systems: low (main rotor), medium (tail rotor), high (engine/drive train). Where you feel the vibration in the airframe is also a diagnostic clue.
The three frequency bands
Low frequency (100–500 cpm)
Origin: Main rotor system.
Where you feel it: Through the controls, especially the cyclic. Sometimes a deep rumble through the airframe.
Common causes:
- Track and balance issues — blades not flying the same path
- Lead/lag damper wear (in fully-articulated systems)
- Blade damage — leading-edge erosion, dings, delamination
- Hub component wear (mast bearings, swashplate)
Low-frequency vibration that builds with airspeed is often a track issue. Vibration that's worst in a hover is more likely a balance issue. Either way, it's a "land and have it inspected" item.
Medium frequency (1,000–2,000 cpm)
Origin: Tail rotor system.
Where you feel it: Through the pedals.
Common causes:
- Tail rotor balance — most common
- Drive shaft balance
- Hanger bearing wear
- Tail rotor blade damage
- Coupling wear
Medium-frequency vibration through the pedals is often the first sign of an impending tail rotor failure. Don't dismiss building pedal vibration as "the helicopter just runs that way."
High frequency (2,000+ cpm)
Origin: Engine, drive shaft, transmission.
Where you feel it: Through the seat, the airframe structure.
Common causes:
- Engine accessories (alternator, magnetos, fuel pump)
- Gearbox bearings (main, tail, intermediate)
- Drive train alignment issues
- Engine mount wear or fracture
High-frequency vibration is often subtle at first and easy to dismiss. Unfortunately it can be a precursor to gearbox or bearing failure — both of which are catastrophic.
The Cabri G2 exception
One reason aircraft-specific knowledge matters: the Guimbal Cabri G2's tail rotor runs at a 10:1 ratio to the main rotor, much higher RPM than a typical light helicopter. Its tail rotor vibrations register as high frequency rather than medium.
If you fly the Cabri, a building vibration through the seat could be either the engine/drive train (typical interpretation) or the tail rotor (Cabri-specific). The diagnostic process changes. Read your POH; talk to your maintenance shop.
Diagnostic process — what to do when you feel something new
- Identify the frequency — low (rotor), medium (tail), or high (drivetrain).
- Identify where it's felt — controls (rotor), pedals (tail rotor), seat/airframe (engine/drive).
- Identify what makes it better or worse — airspeed? Power setting? Pedal input? Specific maneuvers?
- Decide: continue to home airport (if vibration is mild and consistent), divert to nearest airport (if vibration is building), or land immediately (if severe or accompanied by other warning signs).
- Document — write down the symptoms before you forget. Maintenance needs the data.
The discipline: never normalize a new vibration. Helicopters don't suddenly start vibrating without a reason. The reason might be benign (loose anti-collision beacon, slight track shift) or critical (incipient bearing failure). Don't guess — investigate.
Pre-flight vibration awareness
Before every flight: do you know what your aircraft normally feels like? If not, you can't recognize a deviation. Recurrent flight in the same aircraft builds the baseline reference. Pilots transitioning between aircraft often have to spend several flights re-learning what "normal" feels like.
Pay attention during run-up:
- How does the airframe shake at idle vs run-up RPM?
- Smooth on the cyclic? Or feeling something in the controls?
- Vibration changes with collective input?
- Anything new since last flight?
This is also why the same pilot/aircraft pairing produces better safety outcomes than rotation through a fleet — you know your aircraft.