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Thursday 24 April 2014 05:13 GMT

Aviation Theory

ILS Navigation part 2: The Glideslope

Previous Chapter ILS Navigation part 1: The Localizer -- Table of Contents -- ILS Navigation part 3: Flying the ILS Next Chapter

(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 localizer.

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.

instruments showing 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 VOR.

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 DH.

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".


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 or velocity.

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 and 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.


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.


Read on in part 3: Flying the ILS

Previous Chapter ILS Navigation part 1: The Localizer -- Table of Contents -- ILS Navigation part 3: Flying the ILS Next Chapter

Aviation Theory -- Flightsim Aviation Zone

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