# AC 90-87: Helicopter Dynamic Rollover - Type: Advisory circular (guidance, not mandatory unless a rule or AD requires it) - Status: Historical - Approved: January 27, 1986 - Family: AC 90-87 - Pages: 6 - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/DAFC815ADE62A03886256A03006E1583.0001 - Page: https://openairdocs.org/ac/90-87 ## Versions - AC 90-87 (Historical), approved Jan 27, 1986 ## Text ### Page 1 U.S. Department of Transportation Federal Aviation Administtutian Advisory Circular Subject: HELIO3FE3R DYNRMIC ROUOVER Date: l/27/86 AC No: 90-87 Initiated by: MS-840 ChaIbgt% 1 l PURPOSE. To familiarize the helicopter pilot/instructor and nilot school personnel of the hazards associated with dynamic rollover. ' 2 0 BxKGRouND. 9n increasing permtage of helicopter accidents are being attributed to dynamic rollover, a phenannon that will, without i&iate corrective action, result in destruction of the helicopter and possible serious injury. This advisory circular informs helicopter flightcrews of the cause of helicopter dynamic rollover and measures to take to prevent such occurrences. 3 l DISCUSSIOV. Helicopter pilots in general are required to be skillful in operations on both improved or unimproved surfaces. During normal or slope takeoffs and landings with same degree of bank angle or side-drift with tie skid/wheel an the ground, the bank angle or side drift can place the helicopter in a situation where it is pivoting (rolling) about a skid/wheel which is still in cosltact with the ground. When this happens, lateral cyclic control response becanes more sluggish and less effective than for a free hovering helicopter. Consequently, if a roll rate is permitted to develop, a critical bank angle Ub angle between the helicopter and- the horizm) my be reached where roll cannot be corrected, even with full lateral cyclic, and the helicopter will roll over onto its side. As the roll rate increases, the angle at which recovery is still possible is significantly reduced. The critical rollover angle is also reduced. The critical rollover angle is further 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. 4 l CRITIC9L CONDITIONS. When certain elmmts of helicopter operations are at or near their most critical condition, such as high gross weight, right lateral center of gravity, crosswind from the left, hovering with only the right skid/wheel in contact with the surface and with thrust (lift) approximately equal to the weight, very little right roll rate is correctable for any given bank angle. (See Figure 1.) ### Page 2 AC 90- 87 l/27/86 MAIN . -. CROSSWIND f WEIGHT During normal t&offs to a hover and landings from a hover, cross slope takeoffs and landings, and takeoffs from the ground with bank angle or side drift? b a situation can exist where the helicopter will pivot about the skid/wheel which remains on the ground and enter a rolling motion that cannot be corrected with full lateral cyclic input. 5 l CYCLICTRIM. When maneuvering with one skid/wheel on the ground,-,ore.must - , i' be taken to keep the helicopter cyclic control properly trimned (if equipped with force trim/gradient), especially laterally. For example, if a slow takeoff is attempted and the tail rotor thrust contributicn to rolling manent is not trimned out with the cyclic8 the critical recovery angle may be exceeded in less than 2 seconds. Control can be maintained if the pilot maintains proper cyclic trim and by not allowing helicopter roll and pitch rates to becaw too great. The pilot should fly the helicopter into the air smoothly keeping excursions in pitch, roll, and yaw small and should not allow any untrimmed cyclic (force trim/gradient) pressures. 6 . NORMAL-FFS ANDLWDINGS. when performing normal takeoffs and landings on relatively level ground with one skid/wheel on the ground with thrust (lift) approximately.equal to the weight, the pilot should carefully nraintain the helicopter position relative to the ground with the flight cmtrols. Maneuvers should be p&formed snoothly and the cyclic should be trirr~~M (force trim/gradient) so that no pitch or roll movement rates kild upr especially roll rate. If the bank angle starts to increase to an angle of a~raximately So to 8' and full corrective cyclic does not reduce the angle, the collective shcruld -&! reduced to diminish the unstable rolling edition. 7 . SLXIP'E TAKBOFFANDLANDIFJGS. when performing slope takeoff and landing maneuvers, the published procedures should be followed and care shmld be used to keep roll rates small. The pilot should slowly raise the downslope skid/wheel to bring the helicopter level and then lift off. If landing, the pilot should land on me skid/wheel and slowly 1-r the downslope skid/wheel using cmbined movesnents of cyclic and collective. If the helicopter rolls to the upslope side (approximtely So to 8"), the pilot should decrease clollective to correct the bank angle and return to level attitude and then start the landing procedure again. (See Figure 2.1 2 Par 4 ### Page 3 l/27/86 FIGURE 2. UPSLOPE ROLLINGMUTION AC go-87 Excessive application of cyclic into the slope, in coordination with collective pith application. During landings or takeoffs, this condition results in the downslope skid rising sufficiently to exceed lateral cyclic control limits and an upslope rolling motion occurs. TAL 8 USE OF COLLECTIVE. Collective is more effective in -trolling the rolling m&an than lateral cyclic because it reduces the main rotor thrust (lift), A smooth, moderate collective reduction (at a rate less than approxixmtely full up to full down in 2 seconds) is adequate to stop the rolling motion.-' Care should l be taken, hiver, not to dump collective at too high a rate thus causing fuselage - rotor blade contact. Additimlly, if the helicopter is on a slope and the roll starts to the upslope side, reducing collective too fast may create a high roll rate in the opposite directicn. When the uphill slope skid/wheel hits the ground, thedynamics of themotion can cause the helicopter to bounce off the upslope skid/wheel and the inertia a cause the helicopter to roll about the downslope ground -tact point and over on its side. The collective should not be pulled suddenly to get airborne, as a large and abrupt rolling mt in the opposite direction will result. This movenent may be unc~trollable. If the helicopter develops a roll rate with one skid/wheel on the ground, the helicopter can roll over on its side. (See Figure 3.1 FIGURE 3. DUNNSLOPER~INGMUI'IoN FULL OPPOSITE ’ f0 PREVENT Ofi’ I ING NOTION ExCeSSive application of collective pitch in coordination with cyclic application into the slope. When the downslope skid is on the slope, excessive application of collective may result in the upslope skid rising sufficiently to exceed lateral cyclic limits and induce a downslope rolling motion. Par 7 ### Page 4 AC 90-87 l/27/86 9 HExJoWTERSWFECFED. &id type helicopter, While this advisory circular prtirily addresses the dynamic rollover can occur in .either the skid or whe& equipped helicopter. 411 types of rotor systems, rigid, sgni-rigid, or fully * articulated are affected to some extent. Tail rotor thrust and wind drag cn the fuselage ccntribute to roll manent. In helicopters that are equipped with main - rotor systems that turn clo&wise (when viewed fran above), tail rotor thrust . would be in the opposite direction and right pedal instead of left pedal control J would increase that thrust. 10 PILUTTEQNIQUE. when landing or taking off, with thrust (lift) approxima&y equal to the weight (light m the skids or wheels), the pilo\: shmld keep the helicopter cyclic trimned (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 folluws: a. Less lateral cyclic control will be available during crosswind operations when the wind is caning fran the upslope direction. b * Tailwind conditions should be avoided when conducting slope operations. c. when the left skid/heel is upslope, less lateral cyclic mitrolwill - be available due to the translating tendency of the tail rotor. d If passengers or cargo are loaded or unloaded, the lateral cyclic requir&tent will change. If the helicopter utilizes interconnecting fuel lines that would allow fuel to autmatically transfer fran me side of the helicopter to the other, the gravitational flow of fuel to the downslope 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 fran a slope, if the upslope skid/wheel starts to leave ;he ground before the downslqe skid/wheel, the pilot should smoothly and gently 1-r the collective and check to see if the dmnslope skid/'eel is caught on sanething. Under these conditions vertical ascent is the only acceptable method of lift-off. **11 LEVEL,FLATFI2BDSURFKES.** as'well. Dynamic rollover can occur on level surfaces There are documented reports that indicate a skid/w;heel has been caught on a fixed object of the ramp, or stuck with ice or in soft asphalt, and resulted in rollover. Failing to rmve a tiedom or skid securing device has caused dynamic rollover. 4 Par 9 ### Page 5 1/27/M AC 90-87 12 . FLOATIJYGPIATF'ORMS. Reports have been submitted indicating the probable cause of accidents involved flight operatims of helicopters on a floating platform. If the platform is pitching/rolling while attempting to land or takeoff, the result could be dynamic rollover. Acting Director of Flight Standards Par 12 --- From OpenAirDocs.org, an independent open copy of FAA data. Only the Federal Register edition on govinfo.gov is the official legal text of an AD.