| PURPOSE. To familiarize the helicopter pilot/instructor and pilot school personnelof the hazards associated with dynamic rollover. BACKGROUND. An increasing percentage of helicopter accidents are being attributedto dynamic rollover, a phenomenon that will, without immediate correctiveaction, result in destruction of the helicopter and possible serious injury.This advisory circular informs helicopter flightcrews of the cause of helicopterdynamic rollover and measures to take to prevent such occurrences. |  |
DISCUSSION.
Helicopter pilots in general are required to be skillful in operationson both improved or unimproved surfaces. During normal or slope takeoffsand landings with same degree of bank angle or side drift with one skid/wheelon the ground, the bank angle or side drift can place the helicopter ina situation where it is pivoting (rolling) about a skid/wheel which isstill in contact with the ground. When this happens, lateral cyclic controlresponse becomes more sluggish and less effective than for a free hoveringhelicopter. Consequently, if a roll rate is permitted to develop, a criticalbank angle (the angle between the helicopter and the horizon) may be reachedwhere roll cannot be corrected, even with full lateral cyclic, and thehelicopter will roll over onto its side. As the roll rate increases, theangle at which recovery is still possible is significantly reduced. Thecritical rollover angle is also reduced. The critical rollover angle isfurther reduced under the following conditions:
a. Right side skid down condition;
b. Crosswinds;
c. Lateral center of gravity offset;
d. Main rotor thrust almost equal to helicopter weight; and
e. Left yaw inputs.
| CRITICAL CONDITIONS. When certain elements of helicopter operations are at or near theirmost critical condition, such as high gross weight, right lateral centerof gravity, crosswind from the left, hovering with only the right skid/wheelin contact with the surface and with thrust (lift) approximately equalto the weight, very little right roll rate is correctable for any givenbank angle. (See Figure 1.) |  |
CYCLIC TRIM.
When maneuvering with one skid/wheel on the ground, care must be takento keep the helicopter cyclic control properly trimmed (if equipped withforce trim/gradient), especially laterally. For example, if a slow takeoffis attempted and the tail rotor thrust contribution to rolling moment isnot trimmed out with the cyclic, the critical recovery angle may be exceededin less than 2 seconds. Control can be maintained if the pilot maintainsproper cyclic trim and by not allowing helicopter roll and pitch ratesto become too great. The pilot should fly the helicopter into the air smoothlykeeping excursions in pitch, roll, and yaw small and should not allow anyuntrimmed cyclic (force trim/gradient) pressures.
NORMAL TAKEOFFS AND LANDINGS.
When performing normal takeoffs and landings on relatively level groundwith one skid/wheel on the ground with thrust (lift) approximately equalto the weight, the pilot should carefully maintain the helicopter positionrelative to the ground with the flight controls. Maneuvers should be performedsmoothly and the cyclic should be trimmed (force trim/gradient) so thatno pitch or roll movement rates build up, especially roll rate. If thebank angle starts to increase to an angle of approximately 5° to 8°and full corrective cyclic does not reduce the angle, the collective shouldbe reduced to diminish the unstable rolling condition.
SLOPE TAKEOFF AND LANDINGS.
 | When performing slope takeoff and landing maneuvers, the publishedprocedures should be followed and care should be used to keep roll ratessmall. The pilot should slowly raise the downslope skid/wheel to bringthe helicopter level and then lift off. If landing, the pilot should landon one skid/wheel and slowly lower the downslope skid/wheel using combinedmovements of cyclic and collective. If the helicopter rolls to the upslopeside (approximately 5° to 8°), the pilot should decrease collectiveto correct the bank angle |
USE OF COLLECTIVE.
Collective is more effective in controlling the rolling motion thanlateral cyclic because it reduces the main rotor thrust (lift). A smooth,moderate collective reduction (at a rate less than approximately full upto full down in 2 seconds) is adequate to stop the rolling motion. Careshould be taken, however, not to dump collective at too high a rate thuscausing fuselage - rotor blade contact. Additionally, if the helicopteris on a slope and the roll starts to the upslope side, reducing collectivetoo fast may create a high roll rate in the opposite direction. When theuphill slope skid/wheel hits the ground, the dynamics of the motion cancause the helicopter to bounce off the upslope skid/wheel and the inertiacan cause the helicopter to roll about the downslope ground contact pointand over on its side. The collective should not be pulled suddenly to getairborne, as a large and abrupt rolling moment in the opposite directionwill result. This movement may be uncontrollable. If the helicopter developsa roll rate with one skid/wheel on the ground, the helicopter can rollover on its side. (See Figure 3.)
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HELICOPTERS AFFECTED.
While this advisory circular primarily addresses the skid type helicopter,dynamic rollover can occur in either the skid or wheel equipped helicopter.All types of rotor systems, rigid, semirigid, or fully articulated areaffected to some extent. Tail rotor thrust and wind drag on the fuselagecontribute to roll moment. In helicopters that are equipped with main rotorsystems that turn clockwise (when viewed from above), tail rotor thrustwould be in the opposite direction and right pedal instead of left pedalcontrol would increase that thrust.
PILOT TECHNIQUE.
When landing or taking off, with thrust (lift) approximately equal tothe weight (light on the skids or wheels), the pilot should keep the helicoptercyclic trimmed (force trim/gradient) and prevent excessive helicopter pitchand roll movement rates. The pilot should fly the helicopter smoothly off(or onto) the ground, vertically, carefully maintaining proper cyclic trim.Techniques for takeoff and landing are basically the same for all helicopterswhen avoiding conditions that would cause dynamic rollover. They are asfollows:
a. Less lateral cyclic control will be available during crosswind operationswhen the wind is coming from the upslope direction.
b. Tailwind conditions should be avoided when conducting slope operations.
c. When the left skid/wheel is upslope, less lateral cyclic controlwill be available due to the translating tendency of the tail rotor.
d. If passengers or cargo are loaded or unloaded, the lateral cyclicrequirement will change. If the helicopter utilizes interconnecting fuellines that would allow fuel to automatically transfer from one side ofthe helicopter to the other, the gravitational flow of fuel to the downslopetank could change the center of gravity, resulting in a different amountof cyclic control application to obtain the same lateral result.
e. Care should be exercised so that the cyclic limits are not reached,resulting in mast bumping. If the cyclic control limit is reached, furtherlowering of the collective may cause mast bumping. If this occurs, thepilot should return to a hover and select a landing point with a lesserdegree of slope.
f. During a takeoff from a slope, if the upslope skid/wheel starts toleave the ground before the downslope skid/wheel, the pilot should smoothlyand gently lower the collective and check to see if the downslope skid/wheelis caught on something. Under these conditions vertical ascent is the onlyacceptable method of liftoff.
LEVEL, FLAT FIXED SURFACES.
Dynamic rollover can occur on level surfaces as well. There are documentedreports that indicate a skid/wheel has been caught on a fixed object ofthe ramp, or stuck with ice or in soft asphalt, and resulted in rollover.Failing to remove a tiedown or skid securing device has caused dynamicrollover.
FLOATING PLATFORMS.
Reports have been submitted indicating the probable cause of accidentsinvolved flight operations of helicopters on a floating platform. If theplatform is pitching/rolling while attempting to land or takeoff, the resultcould be dynamic rollover.