Pre Flight Briefing
The Characteristics of Autorotation
When established in autorotation, in a typical light helicopter a high rate of descent will be seen, typically 1600-1800 feet per minute.
The engine and rotor RPM needles wil be 'split'. A forward airspeed in the 60-80 kts range is normal. Higher airspeeds result in higher rates of descent and higher RRPM. Lower airspeeds result in higher rates of descent and lower RRPM.
Safety Checks (including lookout and verbal warning)
Before commencing the autorotation exercise it is important to make sure that the block of sky you are in is free of hazards (HASELL check), and that the aircraft is in a safe state to carry out the autorotation. Care should be taken to ensure that the collective lever friction is not applied. The exercise is always preceeded by a verbal warning.
The affect of AUM, IAS, Disc Loading, G Forces and Density Altitude on RRPM and rate of descent
AUM will increase RRPM.
IAS will increase RRPM and ROD.
An Increase in Disc Loading will increase RRPM.
An Increase in G-Force will increase RRPM.
Higher DA will reduce both lift and drag forces.
Rotor and Engine Limitations
The helicopter manifacturer specifies limits for engine parameters and Rotor RPM in the power on and power off conditions.
The rotor RPM limits are normally structural at the high end of the RRPM range and aerodynamic at the low speed end of the RRPM Range.
Control Of Airspeed and RRPM
The flight controls still work a normal sense in autorotation, with the exception of the collective lever which normally will increase engine power output as well as increasing blade pitch. In autorotation the collective lever is used for RRPM control, raising the lever will decrease the RRPM and lowering will increase the RRPM. Care must be exercised not to allow the RRPM to exceed either the minimum or maximum limits when practicing autorotation.
Recovery to Powered Flight
In a piston engine aircraft, autorotation is facilitated through the use of an overrunning clutch which allows the engine to be disengaged from the rotor system. When the engine is disengaged and autorotation is entered the needles will normally be split, engine low rotor high(er).
Throttle Override and Control of ERPM and RRPM during re-engagement
Piston engine helicopters have a means of disabling the corellator, to prevent ERPM changes whilst in autorotation or during an engine off landing. This throttle override or 'detent' as it is known is normally activated by closing the throttle through the idle position and against the spring force which can be felt at idle. The collective lever can now be raised and lowered with no noticable ERPM change, enabling it to be used to control just RRPM. It is not necessary to roll the throttle into the override position whilst carrying an autorotation provided the needles are split far enough to prevent accidental re-engagement. If the throttle override is selected it is important to reset the ERPM before re-engagement to just below the ERPM range to prevent an underspeed condition as the lever is raised.
Danger of Vortex Ring Condition During Recovery
During the recovery to the climb, there is a risk that if the helicopter is allowed to pitch nose up (which it will try and do naturally due to an increase in MR Thrust) that a descending, increasing power and slow airspeed condition will develop. These three factors are the required factors for
vortex ring state. It is important to maintain an attitude, to hold the normal climb speed for the aircraft involved during the recovery, this can be achieved by the application of forward cyclic to hold the attitude as the nose tries to pitch up.