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What does back-course approach mean ? How much signal strength should the back lobe of ILS should have in order to trace it and commence landing. How common is this type of landing for a pilot. What all instruments are required for this type of landing?

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  • $\begingroup$ Hello Huntkil, welcome to Aviation.SE $\endgroup$ – DeltaLima Dec 1 '16 at 8:32
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A localizer back course is simply flying on the back side of the localizer to the runway. How a localizer works is it sends out two signal lobes: one at 90Hz and the other at 150Hz. The localizer needle in the airplane compares the relative strength it receives from both lobes and then shows a deflection.

See this figure from the Instrument Flying Handbook (2015) published by the FAA. Figure 9-33

Notice the top left of the picture shows the same lobes on the same side of the runway. Now imagine flying the front-course, the blue lobe is on the right. If you are flying the back-course, the blue lobe would be on the left.

The localizer needle doesn't know if you are on the front-course or the back-course and will show the same deflection for both situations. The problem comes in for the back-course is that the needle deflection is deflected the wrong way.


To answer your questions.

  1. What does back-course mean? It simply means you are flying on the opposite of the localizer signal. If there is a back-course published, the signal strength will be strong enough to fly the approach as required.
  2. How common is a back-course approach? There are currently 69 back-course approaches in the United States. There are approximately 1,893 ILS approaches. This is around 3.5% of back-course approaches as compared to ILS approaches.
  3. The same flight and navigation instruments required to fly a front-course ILS is required to fly a back-course ILS.

From a technique standpoint. Whenever I fly a back-course approach, irrespective of whether I am using an HSI or VOR receiver, I will always set in the front course. For pilots that use an HSI this will provide for "positive sensing" meaning the needle will deflect correctly. For pilots using a VOR, the needle will shade bottom numbers that correspond to heading that need to be flown to get back onto the back-course approach.

VOR Indicator

If this was a back-course approach, notice that the needle is deflected to the right. Conventional wisdom says you need to turn away from the needle to re-intercept. I say to select a heading under the needle to fly. That heading will be approximately 120°. Take a look at the heading indicator and determine that you most likely need to turn left to re-intercept the back-course. Try it and you will see it works. Conversely, I also teach the same concept for an ILS except the heading to fly is on top of the VOR indicator instead of on the bottom.

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