LOC Signal Question

ARFlyer

ARFlyer

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ARFlyer
I have noticed that the LOC needle swings while doing apps in a constant speed prop aircraft. The effect is most apparent in the full RPM setting. I have to pull the prop back about a 1/4" to settle the needle down while on a LOC or a ILS. Does anybody know why this happens? Is it because of the location on the aircraft or some odd interference from the prop?
 
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Well, first off, I'd check to ensure that there is not a problem with vibration- induced connector issues.
 
There are two signals coming from the ILS ground antenna. One is AM modulated at 90Hz and the other at 150Hz. When lineup with the runway both have the same signal level at your receiver. However if the receiver antenna signal is shadowed by a three blade propeller rotating at 2400RPM it introduces a 120Hz component that can cause momentary deviations. To avoid this problem on single engines the LOC/VOR antenna is placed far back on the vertical fin. On twins and jets the LOC antenna is located on the nose just behind the radar dish. Check the location of the LOC/VOR antenna.

José
 
I don't think I've ever seen a LOC antenna located in the nose of a jet, they've always been on the horiz stab. The GP antenna has been in the nose under the radar dish, but never the LOC. Which jets were you referring?
 
Captain said:
I don't think I've ever seen a LOC antenna located in the nose of a jet, they've always been on the horiz stab. The GP antenna has been in the nose under the radar dish, but never the LOC. Which jets were you referring?
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All the Boeing and large corporate jets have the LOC antenna on the nose.

On these aircraft the ILS (LOC/GS) is separate ARINC 700 series receiver from the VOR thus separate antennas

See http://www.ab9il.net/aviation/737-radio.html for antenna locations.

José
 
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Piloto said:
There are two signals coming from the ILS ground antenna. One is AM modulated at 90Hz and the other at 150Hz. When lineup with the runway both have the same signal level at your receiver. However if the receiver antenna signal is shadowed by a three blade propeller rotating at 2400RPM it introduces a 120Hz component that can cause momentary deviations. To avoid this problem on single engines the LOC/VOR antenna is placed far back on the vertical fin. On twins and jets the LOC antenna is located on the nose just behind the radar dish. Check the location of the LOC/VOR antenna.

José
Click to expand...

The antenna is on the front and top of the vertical fin. Our props are two bladed on the Arrows. So I would guess that would change the modulation?

It's more of a swinging of half deflection then back the other direction to half. It is very smooth like a metronome.
 
ARFlyer said:
The antenna is on the front and top of the vertical fin. Our props are two bladed on the Arrows. So I would guess that would change the modulation?

It's more of a swinging of half deflection then back the other direction to half. It is very smooth like a metronome.
Click to expand...

If you have dual LOCs on the next approach have both tuned to the same ILS and see if both do the same. If only one does it you may have a defective LOC receiver. Must likely an AGC problem on the LOC receiver. This type of problem is difficult to troubleshoot by the average avionics shop unless you teld them the symptom. If both LOC receiver do it check the antenna cable and grounding.

José
 
You might want to check them in VMC too...
 
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