Backcourse

eyton402 · Trainee Joined: Mar 30, 2007 Posts: 78

Could someone explain the function of the Backcourse option?
Thanks

99jolegg · Posted: Thu Mar 27, 2008 12:20 pm Post subject:

The backcourse function is a similar entity to the ILS because the backcourse approach is executed from the signal eminating from the back of the ILS on the opposite end of the runway.

For example, an airport has a single runway (two ends of the runway makes two possible runways for take off or landing). This runway is runway 09 and runway 27. Runway 09 is equipped with an ILS, meaning that aircraft landing on runway 09 benefit from the localiser (lateral beam to locate the runway horizontally) and the glideslope (the 'vertical' beam that helps you make a safe descent to the runway). These beams will eminate 20-40nm from the runway. So this has runway 09 sorted for instrument approaches. However, what do we do if the wind is not in our favour? You should always land into the wind where possible so if we have wind and velocity at 262/21 then runway 27 is the obvious runway to choose.

The ILS system on the opposite end of the runway, runway 9, produces a 'by-product' signal; in effect, a signal that can be used to make a non-precision approach to runway 27. In most cases, the backcourse only consists of the localiser, and not the glideslope which is why it is a non-precision approach. Because you are tracking the beam from the ILS installation at the other end of the runway, things become reversed, known as reverse sensing. This makes left = right and right = left. So, when you look at the EHSI (Electronic Horizontal Situation Indicator) for the raw backcourse approach information, where on a standard ILS approach, the bug indicator going to the left would mean you need to apply left aileron and rudder, on a backcourse approach, you need to apply the opposite: right aileron and rudder.

The backcourse approach button you see on the MCP (the part with all the autopilot buttons on) will serve as the APP button for the standard ILS approach. It'll hold the backcourse down to a certain height. This height will be higher than that of the decision height on an ILS approach because the backcourse approach is non-precision and will require you to locate the airport visually at a greater height.

I hope that all makes sense, let us know if not.
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eyton402 · Trainee Joined: Mar 30, 2007 Posts: 78

Thanks very much for such a detailed answer. I will study it carefully.

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