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Instrument landing systems

Radio signals to guide you home

Instrument landing systems enable pilots to make safe landings in both visual and instrument meteorological conditions.
Illustration by Steve Karp.
Illustration by Steve Karp.

You’ll find them at airports with ILS instrument approaches. There are at least two components to the system. They operate independently but in tandem to provide pilots with lateral and vertical precision guidance to a runway. Proceeding to an airport to perform what’s known as an ILS approach, the pilot tunes in the correct radio frequency on an airplane’s navigational radio to pick up that guidance in the form of radio signals. An ILS may also include high-intensity lighting arrays to help the pilot spy the runway when in instrument conditions.

The first component, the glideslope, resembles a cellphone tower and is stationed close to the approach end of the runway near any visual approach slope indicator lights. The glideslope transmits a UHF signal to the aircraft modulated by signals on two frequencies: 90 Hz and 150 Hz.

The frequencies overlap at the proper glideslope, so when an airplane is descending and its glideslope receiver is picking up those frequencies, they will be translated into vertical guidance displayed on a VOR receiver whose horizontal needle rises or falls to show you where you are in relationship to the glideslope.

Lateral guidance is provided via a localizer, located at the opposite end of the runway and resembling a cluster of TV broadcast antennas on their sides. Similar to the glideslope, a the localizer sends a VHF signal and modulating 90-Hz and 150-Hz signals at the approaching airplane. Where the signals intersect is the course that will guide the airplane down the centerline of the runway. The airplane’s receiver translates those signals to show the pilot where the airplane is in relationship to the course. Drift too far to the right, and the needle on the localizer will swing to the left—telling the pilot to adjust course to the left.

VFR pilots can use an ILS too—not in instrument conditions, because that wouldn’t be legal unless you declared an emergency—but under visual conditions or at night. If you’re flying to an unfamiliar airport, you can dial in the ILS frequency and pick up the localizer to help you navigate to the runway. At night, keeping on or above the glideslope will prevent you from running into obstacles such as trees, wires, or buildings before you touch down.

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Jill W. Tallman

Jill W. Tallman

AOPA Technical Editor
AOPA Technical Editor Jill W. Tallman is an instrument-rated private pilot who is part-owner of a Cessna 182Q.

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