Have you ever flown an ILS approach and had the localizer or glideslope jump back-and-forth erratically? Here's why it happens, and what you can do about it.
Airline Crew Report: Missed Approach In Newark
The following NASA ASRS report was written by an airline crew descending on final approach into the Newark International Airport (KEWR). At what point would you have initiated a missed approach in the scenario described below?
While being vectored for the 22L approach at EWR airport we were cleared for the approach. While initially getting a good signal from the ILS, it became unreliable and unreadable. We were told by New York approach to contact Newark Tower. When we contacted the Tower we were told that we were right of course and just a few minutes we were told that the Tower had received a low altitude warning and were told to execute a missed approach. We executed the missed approach and contacted New York Departure then were vectored for another approach.
Unreliable Signals: Traffic, Terrain, And Your Antennas
Aircraft and vehicles operating near ILS antennas can disrupt the signal's integrity, causing unreliable approach guidance.
To help prevent the problem, some airports have ILS critical areas. When the ceilings are 800 feet or less and the visibility is under 2 miles, you can expect ATC at tower controlled airports to direct you to hold short of the ILS critical area. But keep in mind, many airports don't have ILS critical areas.
At airports without ILS critical areas, aircraft can (and will) taxi up to the hold-short line while another aircraft is on an approach. While it's rare for that to be a problem, you need to be prepared for an interrupted signal from your localizer, glide slope, or both.
Terrain can also be a factor in signal quality, because terrain, power lines, buildings, water, and even dense vegetation can impact the quality of an ILS signal. (read more about ILS operations guidelines from the FAA here.)
ILS Course Scalloping
So what does it look like when your localizer signal is being impacted? Take a look at the video clip below. This was an autopilot-coupled approach we flew into Salem, OR. As we intercepted the localizer back course, the localizer signal started swinging erratically back-and-forth (often called course scalloping).
Since the autopilot was coupled on the approach, the aircraft was chasing the needle left and right, until we disconnected the autopilot and executed a missed approach.
Note: The video below is sped up 400%.
Unreliable Signals: Aircraft Receivers
Your aircraft could be the cause of an unreliable ILS signal too. If you experience problems during approach and other aircraft don't experience the same problems, you might need to write-up the LOC/GS receiver for maintenance.
Localizer Service Volume
While you might receive localizer signals outside of the service volume, the localizer is only guaranteed to be accurate up to 10 degrees on either side of the runway to 18NM. At an angle of 35 degrees on either side of runway centerline, the useful volume is limited to 10NM. Keep in mind though, like in the video above, just because you're in the service volume doesn't necessarily mean you'll have a good signal.
The glide slope is normally usable up to 10NM. However, at some locations, the glide slope has been certified for an extended service volume which exceeds 10 NM. The glideslope works the same as a localizer, but just turned on its side. The equipment still transmits 90 Hz and 150 Hz lobes, which are interpreted by the ILS receiver. The beam is 1.4 degrees thick, with .7 degrees of glidepath projected on either side of the beam. A typical glideslope will take the airplane down towards the runway at a 3-degree angle.
Outside of these service volumes, you'll need to pay close attention to your navigation instruments to make sure you're following the approach path accurately. This becomes a serious reality when flying 25+ mile final approaches into the busiest airports in the country.
The Solution To Degraded Course Guidance?
So what should you do if your ILS signal is acting up? If you're in IMC, going missed and setting up the approach again is a good idea.
Often times ILS signals are impacted by vehicles or aircraft on the ground and near the runway. When you set up the approach a second time, chances are the interfering vehicle is gone.
If you have trouble with the approach a second time, try a different approach, preferably a GPS approach that isn't subject to the same ground-based signal interference.
Has This Happened To You?
Have you ever experienced unreliable approach guidance from an ILS course? Tell us in the comments below.
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