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Backbeam approach

Old 6th Aug 2011, 10:35
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Backbeam approach

Just asked in the pub, what is & how do you fly an ILS backbeam approach. My answer, fuzzied by a lot of ale, left, I think, the questioner more confused than ever. I believe we convinced everyone that one would have to be inverted & follow only L/R indications on the Loc indicator ! Can anyone offer a good explanation & I will try & enlighten, again. Thanks. PS, I know, it was a long time ago.
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Old 6th Aug 2011, 12:08
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an ILS back beam approach uses the ils beam for the opposite runway. You cannot use the glideslope feature as the glidesope is calibrated for the opposite runway. You can use the localiser signal, but if using flight director guidance it will turn you the wrong way to establish on the centreline. To fly one, the flight director is switched off, and rather than steer towards the displaced centreline beam bar to correct to the inbound course, you steer away from it to drag the beam bar back into the centre. Simplified and basic. Modern systems can compensate and show correct displacement signals at the flick of a switch.
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Old 6th Aug 2011, 17:03
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I'll use USA language terminology since that's what I know. Hopefully you can translate to English!

Back course approach procedures have caused confusion since their inception so don't feel badly about it. The most important thing to understand is that some method must by applied to account for the apparent reverse sensing of the course deviation indicator needle. How a pilot does this is of course dependent upon the type of instrument installed in the airplane.


If flying a localizer back course approach in an airplane equipped with a good old fashioned OBS, you have to fly away from the needle to center it since changing the bearing selector has no effect on needle displacement when selected to a localizer frequency. Might as well set the final approach course in the OBS for orientation since reversing it to the front course setting won't make any difference whatsoever in the needle behavior. The needle sensing will always be the same regardless of what course is set. Unless a back course mode switch is provided to reverse the needle sensing.

With HSIs, the front course is set in the course selector. By setting the front course, (opposite to the actual final approach course) you are effectively flipping needle upside down because the CDI rotates within the heading display thus reversing the direction of apparent needle movement. If you could unfasten the old OBS from the panel and flip it upside down it would have the same effect.

So on an HSI:

Set the published front course in the course selector to fly inbound on a back course procedure. Fly towards the needle to intercept.

Set the published front course in the course selector to fly inbound on a normal LOC/ILS procedure. Fly towards the needle to intercept.

Notice a common theme here?

Yup, when using an HSI to fly an approach, set the published FRONT COURSE if tuned to a LOC frequency!

Way too simple isn't it?

Note: HSIs EHSIs and PFDs will generally display a BC annunciation when the NAV receiver is tuned to a LOC frequency and the indicated heading is more than 90 degrees different than the course set in the course selector.

Last edited by westhawk; 6th Aug 2011 at 17:14.
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Old 7th Aug 2011, 09:29
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Fantastic replies. Thanks very much. I will re-try the explanation this evening without embibing first!
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