Dynamic Rollover

Definitions

Rollover caused when the helicopter rotates around a fixed pivot point other than its center of gravity.

Objective(s)

Understand what 3 conditions are necessary for dynamic rollover, list operations that carry higher risk of dynamic rollover, and describe the proper recovery.

Description

• List 3 conditions that must be present for dynamic rollover to occur.
• List operations and loading conditions that can increase the risk of dynamic rollover.
• Describe how to reduce the risk of dynamic rollover.
• Describe the proper recovery procedure.

Instructional aids

None specified.

Recognition

• Any rollover requires a rolling moment and a pivot point. A static rollover occurs when the center of gravity (CG) moves past the pivot point. This is the type of rollover that occurs if you were to try and push the helicopter over.
• Dynamic rollover occurs before the CG passes over the pivot point at what is called the critical angle.
• The critical angle is the angle at which dynamic rollover cannot be prevented; it is also the greatest slope angle that you can land the helicopter on while maintaining a horizontal main rotor attitude.
• For dynamic rollover to occur, there must be a pivot point, a rolling moment, and thrust.
  • The pivot point comes from the skid catching or contacting an object on the surface;
  • The rolling moment is rotation along the longitudinal axis (usually);
  • The thrust is provided by the main rotor.

Avoidance and recovery

• Dynamic rollover can occur at any phase of flight that places you in proximity to a fixed object
  • During pick-up or set-down, if a skid is stuck to the surface by snow/ice, mud, or tarmac, or if the pilot fails to correct for lateral drift
  • While hovering or hover-taxiing near the surface, if a skid contacts an object on the ground (eg, a fence post, uneven pavement, surface debris)
  • When making a hover autorotation, if the pilot does not correct for sideward drift
  • During slope operations, when insufficient cyclic authority is available
• Avoidance during instructional maneuvers
  • Practice hover autos while facing into the wind
  • Avoid hovering close to ground obstructions that could catch a skid
  • Use a smooth, controlled, 2-step lift-off technique wherein all yaw and lateral movement is neutralized and collective is applied slowly and smoothly
  • Maintain at least a 5-foot skid height when hovering
• Any operation that limits the available cyclic authority
  • Crosswind conditions (especially left crosswind)
  • Lateral CG right of center (eg, during student solo flights)
  • Leftward yaw (TR can contribute to rolling moment)
  • Up-slope roll when landing on a slope
• Recover by lowering the collective smoothly if an excessive roll that cannot be arrested by cyclic inputs develops
  • During slope operations, lower the collective if the upslope skid leaves the surface before the downslope skid
  • Especially during slope operations, rapidly lowering the collective to prevent upslope dynamic rollover can induce a static rollover downslope

Teaching Points

• Student solo flights can end in a dynamic rollover if the student focuses on the unusual attitude of the helicopter rather than flying the pick-ups and set-downs to minimize drift and yaw.
  • Have the student calculate the weight and balance prior to the flight. Not only will this help them anticipate the difference in attitude, seeing the change in the calculated CG once they are in the cockpit is correlation.
  • Conduct the first solo when the aircraft has less fuel on board. Full fuel shifts the CG forward, increasing the risk of the skid heels digging into the ground.
  • Because of the tank cross feed, the distribution of fuel to the main and auxiliary tanks has no effect on lateral CG.
  • Add ballast to the storage compartments. Although this won’t help much with lateral CG, it can move the longitudinal CG forward some.
  • Discuss how far aft the CG will be when the student has full fuel for his solo cross-country flight.
• Hover autorotations are high risk maneuvers because of dynamic rollover
  • Teach the student to slowly and smoothly roll off the throttle, rather than snapping it off
• Running landings are high risk maneuvers, in part because of the risk of dynamic rollover
  • Choose the surface for running landings carefully; runway asphalt can differ from smooth and relatively low friction, to coarse and high friction
  • Do not accept any deviation in yaw
  • Avoid centerline and other runway markings, since these can have more or less friction than the runway surface
  • Lower the collective smoothly to avoid inducing a rolling moment

Real-life advice and experience

• An engine failure in  a hover or at a low altitude carries the risk of dynamic rollover. Left cyclic input (to offset translating tendency) and left pedal input (to cancel rightward torque) cause leftward drift and yaw when engine power is lost. This drift and yaw can cause dynamic rollover on ground contact.
• Skids can freeze to icy surfaces, or can adhere to tarmac/asphalt in hot weather

Additional resources

AC 90-87 (Helicopter Dynamic Rollover)
Flight Safety Foundation article [link]

httpv://www.youtube.com/watch?v=aMxTTZAHcFc

httpv://www.youtube.com/watch?v=ye5dRqS7Pm4