Help with Glide Slope Problem

[Jimbus]

I am looking for help from someone with greater avionics knowledge than me ...not difficult....or my local avionics expert.. I hope somewhat harder.

I have a C182 with a KN53 as Nav1 and KN155 as Nav 2 and a KN53 compass/HSI on Nav 1 and a standard ILS/VOR on Nav 2 both fed from the same arial.

Glide slope tests ok on the ground/checked against each other. After flying for an hour or so I intercept a localiser for an ILS approach (coventry). GS comes in at the right place and then sits in the middle of the display. Both flags are away. I am thinking I am flying a perfect approach until I look at the standby which has a full scale fly up confirmed by an altitude/DME check. This happened on my IR test and had to fly the ILS on the standby which on my plane is obscured by the control column. This did nothing for my already heightened anxiety state.

So back on the ground ... into the shop, HSI and KN53 both tested fine on the ground. Pulled out both instruments and bench tested for several hours... no failures. Replaced in the plane and flew 2 ils approaches at my local field with no problems ... a flight of about 30 mins. Then fly a longish flight... 2hours ... back home to fly an ILS approach... glide slope stuck in the middle again, standby working fine. My avionics people are puzzled and want to strip the whole panel down.

Question is...has anyone out there experienced this fault or something like it?
... or have any idea why I should have both flags away...i.e. receiving both signals yet the glide slope not only wont work but sits in probably the most dangerous place it could be..i.e. telling you you are flying perfectly down the glide slope.

[mad_jock]

HAve you tried giving the instrment a thump when its showing bang on and the other standby out?

[A and C]

Two things can happen to this indicator, first there is a modification kit that King issued to stop the problem of sticking GS bars.

Second there is also a recomendation that a capasitor in the unit is replaced to stop the GS indication from becoming voltage sensitive.

Dont go digging around in the panel untill the HSI has had a looking at, I would recomend the avionic workshop at Stapleford for this but dont go to the company's head office in Lingfield because they know very little about GA avionics.

Oh ! dont try Mad jocks idea the polorised glass in the front of the unit costs £200......................... you can guess how one of my customers found that out !

[Jimbus]

Thanks for the replies. The GS isnt actually 'stuck'... it just sits in the middle and moves down gradually from the top of the instrument... and then just stays there moving slightly. Oh ... and I did try tapping it.

I ommited to mention in the first mail that the shop did replace the 'capacitor' in the HSI on the first test just incase this was the problem. Clearly not. My suspicion is that it is something to do with the KN53... but I am not a good enough electronics expert to know what sort of signal would give this behaviour in the HSI.