Why Does the Localizer or Glideslope on ILS Approaches Jump Around?
Unpredictable Instrument Landing System Signal Leads to Aborted Landing Attempt
Have you ever found yourself on an ILS approach, only to have the localizer or glideslope jump back-and-forth like a yo-yo? This annoyance has most likely crossed the paths of many aviators at some point in their flying careers. Here's the scoop on why it happens and what you can do about it.
Aircraft Crew: Missed Approach in Newark Exemplifies the Issue
Let's take a peek at an NASA ASRS report written by an airline crew descending on final approach into the Newark International Airport (KEWR). As they were vectored for the 22L approach, they initially received a strong ILS signal, which soon proved unreliable. While trying to navigate the unsteady signals, they were told by New York approach to contact Newark Tower. Upon contacting the Tower, they were informed that they were off-course and to execute a missed approach. The missed approach was carried out, and they were vectored for another approach.
Traffic, Terrain, and Your Antennas
Aircraft and vehicles operating near ILS antennas can disrupt the signal's integrity. To combat this problem, some airports have designated ILS critical areas. At tower-controlled airports with low ceilings and poor visibility, ATC will direct you to hold short of the ILS critical area. However, not all airports have this luxury.
At airports without ILS critical areas, taxying vehicles can disrupt the ILS signal, making approach guidance unclear.
Terrain can also impact signal quality, as power lines, buildings, water, and even dense vegetation can influence 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 disturbed? Watch 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%.
Your Aircraft and ILS Receivers
Your aircraft may also be the culprit of an unstable ILS signal. If you encounter problems during approach, and other aircraft in the same location do not, you may need to write up the LOC/GS receiver for maintenance.
Localizer Service Volume
Although it's possible to receive a localizer signal outside of the service volume, the localizer is only guaranteed to be accurate up to 10 degrees on either side of the runway for 18NM. Beyond 35 degrees on either side of runway centerline, the useful volume is limited to 10NM. Keep in mind that, just because you're within the service volume, it does not automatically mean you'll have a stable signal.
Now, when it comes to the glideslope, it's usable up to 10NM. However, at some locations, the glideslope has been certified for an extended service volume that goes beyond 10NM. To put it in layman's terms, the glideslope operates similarly to the localizer, but flipped on its side. The equipment still transmits at 90Hz and 150Hz lobes, which are picked up by the ILS receiver. The beam is 1.4 degrees thick, with .7 degrees of glidepath projected on either side of the beam. Normally, the glideslope guides the aircraft towards the runway at a 3-degree angle.
Got your head spinning yet? Just remember: outside of these service volumes, you'll need to pay close attention to your navigation instruments to ensure you're on course.
The Solution to Degraded Course Guidance?
If you find your ILS signal acting up, what should you do? If you're in IMC, go missed, and set up the approach again. Chances are, the interfering vehicle will be gone by then.
Should the approach be problematic a second time around, try a different approach, such as a GPS approach that isn't susceptible to the same ground-based signal interference.
Have you experienced unstable guidance from an ILS course? Share your stories in the comments below.
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- During an ILS approach, the localizer or glideslope may jump around, causing frustration for many pilots.
- An incident involving an airline crew at the Newark International Airport showcased this issue, as they encountered an unreliable ILS signal upon final approach.
- Vehicles operating near ILS antennas can disrupt the signal's integrity at some airports, making approach guidance unclear.
- Terrain, power lines, buildings, water, and dense vegetation can also influence the quality of an ILS signal.
- ILS Course scalloping occurs when the localizer signal starts swinging erratically back-and-forth.
- If your aircraft is encountering problems during approach and others in the same location are not, it may be necessary to write up the LOC/GS receiver for maintenance.
- The localizer is only guaranteed to be accurate up to 10 degrees on either side of the runway for 18NM, and beyond 35 degrees, its useful volume is limited to 10NM.
- If you find your ILS signal acting up, particularly in IMC conditions, go missed and set up the approach again, or try a different approach like a GPS approach to avoid ground-based signal interference.
