AS350 B3e course · Astar hazards
Dynamic rollover
A pivot and a push
The essentials
When a skid is pinned — on a slope, stuck in mud or ice, caught by a tie-down or a long line — it becomes a pivot. Rotor thrust, tilted with the aircraft, rolls it about that skid.
Past a critical angle, full opposite cyclic can't stop it. The key control is the collective, lowered smoothly to put the aircraft back on the ground.
The Astar's tail-rotor thrust and translating tendency push left, which matters for which skid you watch.
A pivot and a push
If a skid is stuck, on a slope, or caught on something, it becomes a pivot. Rotor thrust, tilted with the aircraft, now has a sideways part that rolls the helicopter further about that skid.
Past a critical angle, full opposite cyclic can't stop it.
Source 1
Tip it
A pinned skid as a pivot, and the roll that gets away. This diagram needs JavaScript.
Roll the aircraft about its pinned skid and watch the thrust's sideways part grow.
Source 1
Static and dynamic
Static rollover is geometry: tilt any object far enough and its centre of gravity passes over the edge it is resting on. A helicopter sitting still on a steep enough slope will do it.
Dynamic rollover happens at a much smaller angle, because the aircraft is already rolling and the rotor is producing thrust. The roll has momentum, and tilted thrust adds a sideways push about the pivot. By the time it looks dangerous, the controls may no longer be able to stop it.
Source 1
What adds to the roll
With one skid pinned, several things can push the same way:
- rotor thrust, tilted with the aircraft
- tail-rotor thrust, acting high on the tail
- translating tendency — left in the Astar
- a crosswind from the opposite side
- lateral CG: fuel, a side-mounted load, a hoist, a passenger stepping out
- rate of roll, once it has started
None of them alone is unusual. The pivot is what turns them into a rollover.
Where it happens
Slope take-offs and landings, crosswinds, a skid caught on ice, mud, a tie-down or a light — and lifting with lateral cyclic while one skid is still on the ground.
Source 1
The mirror for the Astar
The handbook lists the worst case for American rotors: right skid down, right lateral CG, wind from the left, left yaw input — and says the opposite holds for clockwise rotors. For the Astar that's left skid down, left CG, wind from the right, right yaw input.
This is the mirror of the physics, not an Astar-specific study. The principle — never lift with a skid stuck — applies to both.
Slopes, in principle
On a slope the upslope skid is the pivot. Cyclic held into the slope keeps the aircraft from sliding and rolling downhill; as the collective comes down, the downslope skid follows.
The limit comes when the cyclic reaches its stop before the downslope skid is down. That slope is too steep for that aircraft, that day — lift off and land somewhere else.
Which side the slope is on matters too. The handbook says an American rotor has less lateral cyclic with the left skid upslope; mirrored, the Astar's is the right skid upslope. Your instructor and RFM set the technique and the limits.
High skids and slopes
High skids raise the centre of gravity, which lowers the critical angle. A slope adds its own angle before you've started.
The RFM's slope limits assume a particular configuration. On high skids, on soft ground, or with a crosswind from the upslope side, treat the limit as further away than it is.
Why the collective, not the cyclic
With a skid pinned, cyclic is working against the pivot and runs out of authority quickly. The collective attacks the cause: less thrust means less of the force doing the rolling.
Lowered smoothly, it puts the aircraft back on its skids. The same applies on lift-off — a smooth, vertical pull with both skids free, never lateral cyclic to unstick one.
Source 1
Key Lake, 2015
In January 2015 an AS350 B2 was pulling feeder cable through transmission towers near Key Lake, Saskatchewan, when the pulling tool stopped abruptly. The TSB found the abrupt stop, a skid catching the grapple line, extra ballast and a crosswind from the right combined into dynamic rollover. It rolled left, with a left lateral CG — the mirrored worst case. The pilot was seriously injured.
A line attached to something that can't move is a tether, and a tether is a pivot.
Source 4
HESLO rollover
In Alaska in 2018 an AS350 B3 was about to lift a load on a 100 ft long line. The pilot let the aircraft drift right; the right skid struck the ground and it rolled over onto its side. The pilot had minor injuries. The NTSB gave the cause as inadvertent dynamic rollover on take-off.
Eyes on the line, it's easy to miss drift. A skid that touches while drifting becomes a pivot.
Source 5
What turns an ordinary roll into dynamic rollover?
The pivot. Without one the aircraft simply translates; with one, thrust becomes a rolling moment about the skid.
Past the critical rollover angle…
Cyclic authority runs out. That's why the rollover has to be caught before it gets there.
What does a high-skid configuration do to rollover margin?
A higher centre of gravity lowers the critical angle.
On a slope, which skid is the pivot?
The upslope skid is on the ground first and last. The aircraft rolls about it.
Why does dynamic rollover happen at a smaller angle than static rollover?
Momentum and tilted thrust add to the geometry. Static rollover is the geometry alone.
Sources
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Clockwise-rotating helicopter differences · Helicopter Ground
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 10, Advanced Flight Maneuvers
- Aviation Investigation Report A15C0005 (AS350 B2, Key Lake) · Transportation Safety Board of Canada · 21 January 2015
- AS350 HESLO dynamic rollover · Aerossurance · Trans Aero AS350 B3 N354LA, Kobuk, Alaska, 25 July 2018
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.