This maneuver is used to transition from forward flight to a landing on
the surface when there may not be sufficient power available to sustain a
hover. This might occur if the helicopter is underpowered, is at high
gross weight, or high density altitude. The helicopter might be underpowered
if it has sustained a partial engine failure in flight. This maneuver is
also useful in some helicopters for some emergency situations such as
certain tail rotor failures or stuck pedals.
This maneuver can be performed with either wheeled or skid equipped
landing gear. In the case of skid equipped helicopters, the skids are
equipped with skid "shoes" which are made of a very hard material. The
shoe can be replaced when it is worn through, preventing any wear of the
landing gear itself, which would typically be made of soft aluminium.
This description assumes skid gear, but is generally the same for wheels.
This maneuver can be performed using any approach angle, however a
shallow approach of 5 degrees or less is desirable because it minimizes
the amount of power required to arrest the descent rate prior to touchdown.
The shallow approach is normal in every way except that the pilot plans
his decelleration such that he arrives at his touchdown spot just above
translational lift airspeed. This insures that adequate power will be
available. The helicopter is transitioned onto the landing gear, and then
forward ground speed is bled off during the slide. In some helicopters
you can lower some collective to put additional weight on the landing
gear. The increased friction acts as a braking force to slow the helicopter
down more rapidly. Other helicopters require that the collective not be
lowered during the slide.
During the slide, the cylic controls the ground track, the pedals control
the heading, and the collective controls the braking force. The cyclic
should typically be frozen in longitudinal pitch during the slide.
Depending on power available, the pilot may want to maintain some speed
and surface taxi clear of the runway, otherwise the helicopter could
become stuck on the runway with insufficient power to move it.
The amount of slide and the touchdown speed are dependent on the wind.
A strong headwind will mean that there will be little if any slide. No
headwind, or even a tailwind can dramatically lengthen the slide. Touchdown
should be at the lowest possible airspeed above translational lift to
minimize the slide ground speed and length.
It may be possible to decelerate the helicopter below translational lift
with the skids just above the ground, in order to minimize the actual
slide. If this is done from too high an altitude, the helicopter may settle
hard onto the surface, and even flip over forward if the skids dig in.
First of all, many of the mistakes pointed out in Running Takeoffs apply,
allowing rotor RPM to get low,
not keeping the skids alighned,
not maintaining runway centerline.
Additional errors are:
Slowing below translational lift before touchdown
By slowing below translational lift before touchdown, the pilot risks a
hard landing if power available is insufficient to hover below
translational lift airspeed. Low time pilots are especially prone to this
because they are unused to touching down with forward speed and therefore
have a tendance to hold the aircraft off the surface while bringing their
ground speed to a halt.
Touchdown in a non-level skid attitude
The skids must be level during touchdown. Touching down tail low can
cause tailboom strikes, while touching down nose low can cause the
helicopter to tumble over forward.
Many helicopters are not in a level attitude during the final deceleration,
and the pilot often has to make a cyclic pitch adjustment before touchdown
to level the skids. The adjustment required will depend on the helicopter
make/model as well as the particular helicopter and it's center of gravity.
Moving the cyclic aft after touchdown
Pilots have a natural tendance to move the cyclic aft to help slow the
helicopter down after touchdown. However, while this is a natural thing
to do in the air, on the surface the tailboom can not move out of the
way and an exaggerated aft cyclic input could cause the main rotor to
tip back and strike the tailboom.
Moving the cyclic forward after touchdown
This is less common, but is sometimes done. It has the effect of making
the helicopter slide faster and further, therefore it is counter productive.
Rotor RPM - Robinson R22
The Robinson R22 has a particular problem with low rotor RPM during the
slide. At the low power settings encountered during the touchdown and
slide, the R22 correlator typically rolls off too much throttle, bringing
the RPM down low enough to activate the low RPM warning systems. This can
cause the pilot to overreact and roll on a lot of throttle, possibly
yawing the helicopter during the slide. Failure to increase the RPM
however, decreases tail rotor authority and increases main rotor flapping.
Therefore R22 pilots should be monitoring RPM during approach, and
hopefully adjusting throttle before touchdown occurs.
paul at copters.com
(replace " at " with "@" to email me - this avoids SPAMMERS I hope)