AC 90-87                                                         l/27/86

9. 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, semi-rigid, or fully articulated are affected to some extent. Tail rotor thrust and wind drag on the fuselage contribute to roll moment. In helicopters that are equipped with main rotor systems that turn clockwise (when viewed from above), tail rotor thrust would be in the opposite direction and right pedal instead of left pedal control would increase that thrust.

10. PILOT TECHNIQUE. when landing or taking off, with thrust (lift) approximately equal to the weight (light on the skids or wheels), the pilot should keep the helicopter cyclic trimmed (force trim/gradient) and prevent excessive helicopter pitch and 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 helicopters when avoiding conditions that would muse dynamic rollover. They are as follows:

b. Tailwind conditions should be avoided when conducting slope operations.

c. When the left skid/heel is upslope, less lateral cyclic control will be available due to the translating tendency of the tail rotor.

d. If passengers or cargo are loaded or unloaded, the lateral cyclic

requirement will change. If the helicopter utilizes interconnecting fuel lines that would allow fuel to automatically transfer from one side of the helicopter to the other, the gravitational flow of fuel to the down slope tank could change the center of gravity, resulting in a different amount of 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, further lowering of the collective may cause mast bumping. If this occurs, the pilot should return to a hover and select a landing point with a lesser degree of slope.

f. During a takeoff from a slope, if the upslope skid/wheel starts to

leave the ground before the down slope skid/wheel, the pilot should smoothly and gently lower the collective and check to see if the down slope skid/wheel is caught on something. Under these conditions vertical ascent is the only acceptable method of lift-off.

11. LEVEL, FLAT FIXED SURFACES. Dynamic rollover can occur on level surfaces as well. There are documented reports that indicate a skid/wheel has been caught on a fixed object of the ramp, or stuck with ice or in soft asphalt, and resulted in rollover. Failing to remove a tiedown or skid securing device has caused dynamic rollover.

12. FLOATING PLATFORMS. Reports have been submitted indicating the probable cause of accidents involved flight operations of helicopters on a floating platform. If the platform is pitching/rolling while attempting to land or takeoff, the result could be dynamic rollover.

William T. Brennan

Acting Director of Flight Standards

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