(Continued from part
1: The Localizer)
The glideslope typically gives a 3° approach to the runway and
intersects the runway approximately 1000 ft from the threshold.
The 3° slope means a descent of about 300 ft per nautical mile is
required to stay on the glideslope. This gives a reasonable rate
of descent for most aircraft, e.g. 600 fpm at 120 kts and 450 fpm
at 90 kts groundspeed. The approximate height above the ground for
the glideslope at any given point can be approximated by multiplying
the distance from the runway in nautical miles by 300. i.e. at 3
nm, the glideslope should be 900 ft.
Another handy calculation to be familiar with is how to figure
a descent rate that will keep you at the correct rate for any given
groundspeed. The rule of thumb here is 5 times your groundspeed.
The glideslope antenna is usually situated 750-1250 ft down
the runway from the threshold and offset by about 300 ft to the
side of the runway. The distance from the threshold is to assure
adequate wheel clearance over the threshold. The aiming point when
flying the ILS is not the "numbers" but the designated touchdown
zone, where the glideslope intersects the runway.
Like the localizer, the glideslope signal is composed of
two overlapping lobes. The area of overlap is about 1.4° or plus
or minus 0.7° from the centerline. The glideslope is calibrated
to 10 nm, but, you may receive it further out. Descent should not
be started until the localizer is intercepted.
The cockpit indicator for the glideslope is the horizontal needle
on the VOR indicator and usually some type of vertical ribbon gauge
on an HSI. Like the VOR, the red "Off" flag should not be visible
when a usable signal is received. The glideslope frequency is paired
with the localizer frequency and is automatically set when you tune
The glideslope is a command instrument, i.e. fly toward the needle.
The following illustration shows the too high, on glideslope, and
too low on both the Cessna glideslope indicator and the HSI from
the Lear 45 in FS98.
The only time you would see reverse sensing on the glideslope is
during inverted flight -- not a recommended procedure and leads
to very short rollouts on landing. <g>
A full scale fly-up indication means that the aircraft is 0.7°
or more below the glideslope. Like the VOR, the glideslope deflection
is shown by dots. The recommended procedure is to not go below a
half scale deflection in order to assure ground clearance. A full
scale deflection of the glideslope indicator is not acceptable on
an ILS approach.
The full scale deflection of the glideslope is 1.4°; the localizer
is 5°; and the VOR is 20°. This means that the glideslope is 3 times
more sensitive than the localizer and 12 times more sensitive than
The glideslope is only approved for navigational use down to the
DH (Decision Height) for any particular ILS. Any reference to the
glideslope after that point must be supplemented by visual reference.
Category 1 ILS is approved for use down to a DH of 200 ft HAT (Height
Above Touchdown). Cat 2 ILS has a DH of 100 ft and Cat 3 has a 0
If the glideslope fails, the approach can usually still be flown
as a localizer only approach, but, the minimums will be higher.
The allowable descent profile will be indicated on the approach
chart and will use such things as the markers or DME to indicate
the stepdowns in altitude. If only the full ILS is allowed, the
chart will be marked "LOC ONLY N/A".
FLYING THE GLIDESLOPE
The key to flying the glideslope is a constant rate of descent. Applying
our rule of thumb from earlier is one way to decide on the proper
rate of descent for a given groundspeed. Be aware that your groundspeed
may change during the approach due to changes in the wind direction
The glideslope should be held with small pitch changes. The process
is similar to bracketing a track, but in the vertical plane rather
than the horizontal. For example, if you are a little low, raise
the pitch slightly until you are back on the glideslope and then
reduce the pitch slightly. Remember that you are only dealing with
1.4° full deflection, so not much change is necessary.
Airspeed is maintained with power. The changes in pitch may require
slight changes in airspeed, which should remain constant. Variations
of ±5 kts are acceptable. A typical power adjustment is made in
increments of 100 rpm or 1 inch of manifold pressure for a piston
engine. Strong or gusty winds may require greater changes.
Energy management is a key to flying a good ILS approach. If you
are a little low and fast, increasing pitch will slow you down as
well as increasing your altitude. Conversely, if you are a little
high and a little slow, decreasing pitch and flying the aircraft
down will decrease altitude and increase speed. Remember, hold the
glideslope with pitch and hold airspeed with power.
Like the localizer, the glideslope indicator will become more sensitive
as you near the runway and your corrections should become smaller
The marker beacons, also called fan markers because of the signal
shape, are highly focused vertical signals which can only be received
when directly overhead. It is not possible to "track" a marker beacon
like you can other navaids.
The typical ILS has two marker beacons positioned along the localizer. The
outer marker (OM) is positioned between 4 and 7 nm from the
runway threshold and the middle marker (MM) is at 3500 ft
from the threshold.
The airborne equipment is the marker beacon receiver which has
both aural and visual indicators. A series of low frequency (400
Hz) dashes, transmitted at 2 per second, and a flashing blue light
indicate passage of the OM. The MM is indicated by a series of medium
pitched (1300 Hz) alternating dots and dashes and a flashing amber
light. There is sometimes also an inner marker (IM) between
the MM and the threshold with has high pitched (3000 Hz) dots at
6 per second and a flashing white light.
Marker beacons (fan markers) are also sometimes used as enroute
fixes along airways and have a dot dash dot pattern at 3000 Hz.
The same white light that marks the IM is used for the fan markers.
Some localizer back courses also use the white light and a pattern
of dot dot, dot dot, dot dot at 3000 Hz
The marker beacons are very narrow and hence are only heard for
a few seconds as the aircraft passes overhead.
OTHER GLIDESLOPE CHECKS
Not all ILS approaches have an OM or MM. In this case, you can check
your altitude at a known point, such as the FAF (Final Approach
Fix) or with DME checkpoints. DME stepdowns are often provided
through the use of an required altitude at a specific DME fix.
Various visual components are a part of the ILS approach and are used
to help in the transition from instrument to visual flight. Approach
Light Systems (ALS) extend from the runway threshold out from
the approach end of the runway. ALS act as a lead in to the runway
and are a standardized series of red and white lights, consisting
of extended centerline lighting and crossbars at specific intervals.
They extend 2400-3400 ft for precision instrument approach runways
and 1400-1500 ft for non precision.
Another form of approach lighting is the sequenced flashing
lights (SFL), or runway alignment indicator lights (RAIL).
This appears as a ball of white lights traveling toward the runway
along the extended centerline at a rate of two flashes per second.
The SFL and RAIL are sometimes referred to as the "rabbit".
The threshold is marked with a row of green lights. Some runways
may have flashing strobes on either side of the threshold. These
are called runway end identifier lights (REIL).
The Visual Approach Slope Indicator (VASI) may be
seen in several forms. There are 2 bar, 3 bar, PAPI (Precision
Approach Path Indicator), PVASI (Pulsating Visual Approach
Slope Indicator), Tri-color VASI and T-VASI. The purpose of all
of these is to provide visual glideslope information during the
last phase of the ILS approach. The 2 bar VASI is seen in FS98 and
consists of two rows of lights, usually at 500 and 1000 ft from
the threshold. When you are low, both bars are red. When on glideslope,
they are red over white. When high, they are both white.
This completes Part 2 of the ILS Navigation Lessons.
on in part 3: Flying the ILS