The flare, the check and the level manoeuvre

A successful touchdown occurs at the end of a series of manoeuvres designed to transfer the helicopter from a condition of moderate horizontal and vertical velocity to a condition of little or no velocity in either direction [2.6]. The following design features, along with an assessment of the acceptable level of damage, will affect the flare angle the pilot can employ and the aircraft attitude, sink rate and run-on speed allowed at ground contact: [1]

The idealized manoeuvre begins with a cyclic flare at constant collective pitch during which the increased rotor thrust (affected by any change in rotor speed) and aft disk tilt are used to decrease both the horizontal and vertical velocity components. The maximum pitch angle used by the pilot in the flare is often a matter of personal preference being affected by issues such as field of view. A maximum flare angle of 45° is used in the analysis given below as it gives a minimum theoretical result by ensuring that the sink rate is reduced by the greatest possible amount. At the end of this flare the aircraft should be close to the ground with its vertical velocity equal to zero (or at least below the design sink rate of the undercarriage) and with its horizontal velocity corresponding to autorotation at the angle of attack to which the rotor has been pitched. At this stage the pitch attitude will be typically much greater than that necessary to avoid a tail strike and considerable stored energy will remain in the rotor. The pilot will therefore rotate the helicopter nose-down towards a level attitude and use collective pitch to cushion the landing. Although in the idealized manoeuvre the pilot can use longitudinal cyclic and collective pitch in combination to level the aircraft whilst maintaining hovering thrust, at the expense of rotor speed, a different strategy is often adopted. The pilot will perform cyclic flare as previously described and will maintain it as long as possible. Eventually the flare effect will diminish to the point that the RoD reaches a minimum and starts to increase again. At this instant a rapid collective pull, or ‘check’, is made using some of the rotor energy to further reduce the horizontal and vertical velocity components. Shortly afterwards the helicopter is rotated nose-down and more collective pitch is used to minimize the sink rate as the aircraft is run on to the ground. This technique is summarized using the maxim ‘flare – check-level ’.