ILS Approach: Instrument Landing Systems Explained

When you make the leap and decide to earn your instrument certification, one of the first new systems that your CFI will be introducing you to is the ILS, or instrument landing system. The ILS has been around and being used to help IFR pilots land for more than fifty years.

What is an Instrument Landing System?

An instrument landing system is one of the primary landing navigation aids used by pilots flying under instrument flight rules (IFR). The FAA describes the ILS saying it provides aircraft with “precision vertical and horizontal navigation guidance information during approach and landing.” They also note that, “associated Marker Beacons and/or Low Power Distance Measuring Equipment (LPDME) identify distance to the runway.”

How does an Instrument Landing System work?

The two key physical components of the Instrument Landing System are the localizer and the glideslope. They are aided by marker beacons and the approach lighting system (ALS). Both the localizer and glideslope communicate with aircraft via radio beams and together they provide the navigational guidance that pilots need to safely land.

If the localizer is inoperative, the airfield is not authorized for ILS landings. An inoperative glideslope transmitter means that pilots can still fly a non-precision localizer instrument approach.

There are three categories of Instrument Landing Systems, each with their own minimums. Most Instrument Landing Systems are Category I with a decision height of no less than 200 feet and visibility minimums of one-half mile or 2,400 feet of runway visual range (RVR).

Some airlines and corporate flight departments are FAA authorized to use Category II instrument landing systems. A Category II ILS has additional equipment, facilities, and pilot training which are required since the minimums are lower. Pilots landing using a Category II ILS have a decision height minimum of not less than 100 feet and 1,800 to 1,200 feet RVR is required.

The Category III ILS with its three subclasses is more rarely used and comes with very low minimums. A Category IIIa approach can have as low as a 50-foot decision height and down to a 700-foot RVR, while a Category IIIb allows an RVR down to just 300 feet. The FAA created Category IIIc approaches but has yet to authorize them or certify any aircraft to make the approach that would allow a completely blind landing with 0 feet of runway visual distance required.

VHF Localizer

The localizer is a piece of equipment that is usually located 1,000 feet from the opposite end of the runway so it does not present a collision hazard to approaching aircraft. The localizer transmits a VHF (very high frequency) signal from an antenna that is aligned with the centerline of the runway. The localizer transmits in the range between 108.10 and 111.95 MHz at a power of about 100 watts. Pilots can pick up the localizer signal from a distance of approximately 18 nautical miles away and an altitude of 4,500 feet above the antenna.

The purpose of the localizer is to provide pilots with horizontal guidance to help them maintain alignment with the runway centerline throughout the final approach and landing. To do this, in addition to the primary radio signal, the localizer uses horizontal polarization and a different modulation frequency for each side of centerline. A pilot who is lined up with the center of the runway will receive both a 90 Hz and 150 Hz signal simultaneously. If a pilot is too far to the left, they will receive only the 90 Hz signal. Too far to the right is indicated by 150 Hz. The acceptable course width that the localizer cues pilots to stay within is very narrow, usually between 3- and 6-degrees.

Glideslope Transmitter

The vertical component of the Instrument Landing System’s navigational guidance is provided by the UHF (ultra-high frequency) glideslope transmitter located about 750 to 1,250 feet down from the approach end of the runway. The glideslope transmitter helps pilots stay on the appropriate glide path that will set them up to fly a straight, sloped descent line from the final approach fix (FAF) to the touchdown zone. The frequency range for the Glideslope transmitter is 329.3 to 335.0 MHz at a power of about 5 watts.

The glideslope transmitter uses vertical polarization and modulation signals to advise pilots when they are off course vertically. If a pilot is on the glideslope, they will receive both a 90 Hz and a 150 Hz signal simultaneously which will cancel each other out. If a pilot is flying above the glideslope, they will receive only the 90 Hz signal. Flying below the glideslope is indicated by receiving only the 150 Hz signal. The standard glide slope angle is 3-degrees, although it may be higher based on local terrain near the approach end of the runway. A 3-degree glideslope equates to a descent rate of roughly 500 feet per minute.

Marker Beacons

Depending on whether the ILS is used for Category I or Category II approaches, there will be one or two VHF marker beacons installed along the localizer course. Outer and middle beacons are used for Category I systems and an additional inner beacon in use as well for Category II.

The outer marker (OM) is positioned 4 to 7 miles from the airport and indicates the point at which an aircraft flying the localizer will intercept the glide path. The middle marker (MM) is situated 3,500 feet from the landing threshold, on the localizer centerline where the glideslope centerline is 200 feet above the elevation of the touchdown zone. The inner marker (IM) is placed between the middle marker and landing threshold to mark the decision height point of a Category II approach glide path.

Approach Lighting Systems (ALS)

While ILS approaches are initiated using solely radio guidance, as the pilot approaches the runway, they must make visual contact and transition to a visual landing. The ALS provides pilots the directional, glide path, and distance lighting they need to make a smooth and safe transition from instrument to visual approach.

To tie it all together, here is a simulation of flying an ILS approach and how the ILS system works in flight.