The Art of the Helicopter John Watkinson - Karatunov.net

320 The Art of the Helicopter
the runway and transmits a pair of beams that are equally off the axis of the correct
approach path. These are sensed with equal strength when a correct approach is being
made. The glide slope transmitter is sited at the approach end of the runway. This also
radiates two beams, this time equally off axis with respect to the ideal glide slope. These
two beams are sensed with equal strength on the correct glide slope. There are also two
highly directional antennae radiating beams directly upwards into the approach path.
These are the outer and inner markers.
In order to make an instrument approach the helicopter will initially be on autopilot
flying with altitude hold. Once in the vicinity of the approach path, the ILS receiver
is tuned to the correct frequency and once identified the autopilot will be coupled to
the locator using the LOC position of the selector. The locator alone is not enough
to ensure a correct approach and the desired heading of the autopilot needs to be set
with the heading of the runway. In this mode the autopilot is influenced by both the
localizer signal and the heading error.
As Figure 7.53 shows, the heading could be correct but the helicopter could be
approaching parallel to the runway. In this case the heading error would be zero but
the localizer error would be issuing a steer left command. The machine will steer left
until the localizer error balances the heading error. As the approach path is acquired,
the localizer error becomes smaller and so the balancing heading error also gets smaller.
The result is that as the machine flies onto the localizer beam centreline it will adopt
the correct heading with both errors zero.
The machine will then fly along the beam centre with altitude hold still on. At some
point the receiver will intercept the glide slope beams. The point at which this occurs
will depend on the altitude. When the glide slope beams are detected, the autopilot
coupling must be set to GS (glide slope) mode. The horizontal coupling is unchanged,
but the altitude hold is disabled and replaced with coupling from the glide slope receiver.
The glide slope information will act on the autopilot by reducing the desired altitude
if the machine is above the beam and increasing it if it is below. The result is that
the helicopter descends down the glide slope. As the flight path is already aligned
with the localizer beam, the helicopter must be on a direct course for the glide slope
transmitter.
The markers indicate the progress of the approach. The outer marker is about four
NM from the threshold and transmits continuous Morse dashes. The inner marker is
about 3500 feet from threshold and transmits alternating dots and dashes.
Using the ILS transmitters the helicopter can make an automatic approach. A
machine without an autopilot could make a manual instrument approach with the
pilot using left/right and up/down indicators to fly down the glide slope. In the case of
an autopilot failure the pilot could elect to continue the approach manually in this way.
At some point the so-called decision altitude is reached. In the case of a manual
instrument approach or an automatic approach with a single channel autopilot, if the
landing site is not visible by the time the decision altitude is reached, the landing must
be aborted. Only a suitably equipped machine may carry on to make a blind landing.
Such a machine must carry a triply redundant voting autopilot, a RADAR altimeter
and Doppler RADAR to measure groundspeed. A pressure altimeter is simply not
accurate enough. At a RADAR height of a few hundred feet, the constant glide slope
and airspeed are both modified to produce a flare. The disc attitude is pitched back to
slow the machine down and collective is lowered to prevent zooming.
As RADAR height falls, forward speed control is switched from dynamic pressure to
Doppler RADAR so the groundspeed can be brought to zero a little before RADAR
altitude reaches zero. This gives time for the hull to level before touchdown which will
be programmed to be within the landing gear limit.