AS350 B3e course · Rotors and flight controls
Ground resonance
When the rotor and the gear start to dance
The essentials
Ground resonance is a violent, self-feeding oscillation that can destroy a helicopter in seconds. It needs the aircraft in contact with the ground and the rotor turning.
It starts when the blades' lead-lag motion puts the rotor's centre of mass off the mast, and the airframe rocking on its gear feeds the motion instead of damping it.
The Starflex's frequency adapters are there to stop it. The handbook's response: if rotor rpm allows, lift off; if not, reduce to flat pitch and shut down — per your RFM.
How it builds
If the blades lead and lag out of step with each other, the rotor's centre of mass is no longer on the mast. It whirls round, shaking the hub.
On the ground, the aircraft can rock on its gear. If that rocking happens at a frequency that matches the whirl, each shake pushes the next one harder. Without enough damping, it grows until something breaks.
Source 1
Two springs and two dampers
Think of it as two systems coupled through the hub. The rotor has a lead-lag frequency, set by the frequency adapters' stiffness, and their damping. The airframe on its skids has a rocking frequency, set by the gear's flexibility, and whatever damping the gear provides.
Designers keep the two apart over the rotor-speed range used on the ground and add enough damping that any coupling dies away. Wear either side down — soft adapters, a cracked cross tube, a lost damper — and the margin shrinks.
The skid gear's part
The Astar's skid gear is part of the defence. The TSB describes two features: flexible steel strips extending down from the rear of the skids, which tune the gear's natural frequency away from ground resonance, and dampers on the front cross tube, between the gear and the fuselage, which absorb oscillation energy.
Both are worth a look on the walkaround — strips bent or missing, dampers leaking or loose — and both matter after a hard landing.
Source 3
What triggers it
Anything that knocks the blades out of step while the aircraft is on the ground:
- a hard landing, or one skid touching first
- landing or lifting with a lateral drift
- degraded frequency adapters — worn, cracked or debonded elastomer
- damaged or poorly maintained gear
It's partly a technique problem and partly a maintenance one.
Why the Astar is resistant, not immune
The handbook ties ground resonance to rotors with lead-lag freedom — typically articulated heads with three or more blades. The Starflex has that freedom, so it must have damping: that is the frequency adapters' job.
Good adapters and a healthy skid gear make ground resonance rare on the Astar. Neither makes it impossible, which is why their condition belongs on the walkaround.
Recognising it
Ground resonance is an oscillation that grows — a rocking or bucking on the skids that gets bigger with each cycle rather than settling. It can appear during start, run-up, landing or lift-off, whenever the rotor is turning and the skids are loaded.
An unbalanced rotor can produce a shake on the ground that feels similar. Either way the question is the same: is it getting worse? If it is, it needs an answer in seconds.
Tulita, 2018
An AS350 B2 was started on a hilltop helipad in −27 °C after three days of vibration on the ground and in flight. The main-rotor blades had been removed and refitted days earlier, without the track-and-balance checks the maintenance manual prescribes.
As the throttle was advanced, the helicopter began bucking fore and aft on its skids. The pilot lifted off to escape what he took for ground resonance — with the fuel flow control short of the flight gate and the rotor below governing speed. Rotor rpm drooped, control was lost, and the helicopter ended up 50 m downslope; the pilot was seriously injured.
Lift-off is an escape only with rotor rpm in the normal range. And a vibration after blade work is a maintenance question before it is a flying one.
What two things does ground resonance need?
Ground contact, so the airframe can rock on its gear, and a turning rotor whose blades can get out of step.
Which part of the Starflex head is the main defence against ground resonance?
The frequency adapters damp lead-lag motion, which is what keeps the blades from whirling out of step.
Ground resonance starts during run-up with rotor rpm well below normal. Is lifting off the answer?
No. Lift-off needs normal rotor rpm. In the 2018 Tulita accident a lift-off below governing rpm ended in loss of control.
Which parts of the skid gear help prevent ground resonance?
The strips tune the gear's frequency away from the rotor's; the dampers absorb the energy of any oscillation.
Sources
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Aérospatiale AS 350B Écureuil — in depth · Heli Archive
- Air Transportation Safety Investigation Report A18W0025 (AS350 B2 C-FWCR, Tulita) · Transportation Safety Board of Canada · 15 February 2018
- Starflex rotor hub — European Rotorcraft Forum paper · European Rotorcraft Forum
- AS350B2 accident after vibration from unrecorded maintenance · Aerossurance
Educational only. Not approved training material, and never a substitute for the Rotorcraft Flight Manual, an instructor, or your operator's procedures. Figures marked as manufacturer data or test results are not limitations. This content is a draft and has not yet been reviewed by an instructor.