Menu Sign In Contact FAQ
Welcome to our forums

KI-525 glide slope failure mode

Just discovered this article that describes how the KI-525 HSI has a failure mode for the glide slope indicator in a low voltage situation. Having suffered alternator failure recently (in the London TMA, but fortunately in VMC) I shall be sure to watch the HSI during the next self-test sequence to see if I am affected.

It’s also the first time I’ve heard of a “bipolar tantalum capacitor” – that beats “fluxgate magnetometer” for an impressive-sounding piece of technology

EGBJ / Gloucestershire

That’s a good one

C110 is only 2.2uF so could easily be something decent like a 10% ceramic or a film cap of some sort.

Regards the sensitivity to the supply voltage, it’s true, but only if the supply drops below a certain value. The unregulated supply goes to the collector of an emitter follower and that insulates the output from the effects of supply voltage variation – so long as the supply remains above some value, which was probably assumed to be +12V or some such. I have the KI525 MM but on a quick read can’t find the relevant figures.

A sticky glideslope pointer is a common fault on the KI525. I had it too. It often shows up on a ground test (no engine running) but such units usually work OK airborne because the vibration overcomes the stiction.

In higher grade aircraft instruments one didn’t have the moving coil movement (a weak mechanism, with a spring return) used in these “GA” units; the better stuff often used synchro movements where the pointer is positively forced into a given position, using what is basically a 2-phase AC motor which revolves only through a small angle. I have a UMA backup oil pressure gauge which is such a synchro movement. Other approaches use miniature DC servo motors; I have a Castleberry AI (KI256 lookalike but electric) in the RHS and that uses closed-loop DC servos to position the flight director (which on a KI256 is yet another potentially sticky moving coil mechanism).

The KI525 uses a servo loop too, measuring the GS pointer position using two LDRs (light dependent resistors) illuminated by an LED, but it still sticks sometimes.

The dodgy GS pointer was one of the things which finally made me install the Sandel SN3500 EHSI which is a superb piece of kit. I now have two of them.

Shoreham EGKA, United Kingdom

This is a long known problem and there has been a King SB published years back to change this capacitor.

I am told that the problem was discovered by one of the avionics guys at a well known north east London Avionic shop who alerted King to the problem and King embodied his idea in the SB issued a few months later…………… Needless to say King have never admitted that the fix for this problem was not their idea.

The capacitor problem is not the one described in the article (though it’s mentioned). The problem is the failure of the GS indicator to operate in a low voltage situation, and the observation that the prescribed test only stipulates testing at full voltage.

[Edit] To clarify – the problem does not affect all units.

Last Edited by Rich at 21 Jan 06:40
EGBJ / Gloucestershire

Rich wrote:

The problem is the failure of the GS indicator to operate in a low voltage situation

Do note that is a REAL low voltage. At 12 Volt the system should behave normally and should pass the selftest. I have seen this issue, too quite rare, and definitly doesn’t show up at every unit, when you do a ramp check without external power. Nothing wrong with performing the selftest if you have a GNS or IFD, far too many people just acknowledge without checking, soms are even unaware of this selftest actually also testing indicator interface.

In my opinion it shows the reason for a good low voltage detection, the UK requirement shows the UK CAA see and understands this problem, while low voltage detection on all older Cessna’s is useless (it senses if the regulator receives voltage from the battery), You can have this light off, even with your alternator removed. On older Pipers the systems is slightly better, but also quite useless. The Socata and Robin seems to have a better system, measuring bus voltage, which is they way to go.


At this point it might be useful to say that I have a King HSI system for sale ( with EASA form 1 for all the parts ).

The HSI has an RMI bootstrap, two needle RMI and the inverter to drive it.

The kit was removed for a G500 upgrade.

6 Posts
Sign in to add your message

Back to Top