HSI Glide path indicator not working

[ighazali]

I have a 1975 Mooney M20E with a gtn635Xi and everything works great such as lateral navigation and GPS performance (LPV/LNAV/LP+V) however on my HSI which is the Bendix King the glidepath indicators come alive but not in the middle like they should but rather a dot and half below the middle point (where they should be) and then when you pass the FAF to descent the glidepath indicators just shoot up and disappear. Again lateral navigation works well and the Bendix king HSI does precess quite a bit and I have to re slave it almost every time i take the plane out of the hangar, but it works really well once set. Any ideas what could be causing this issue for the glidepath? As well as for the constant re-slaving of the unit? 

[N201MKTurbo]

You should find an avionics guy with a GS generator who can slew the needle up and down. I suspect the meter movement is sticking. This can happen when the adhesive on the tape around the windings fails and starts to unwrap causing the movement to hang.

Your best bet is to replace it with a G5 or AV-30 or other modern display. Probably cheaper than having it repaired.

[Slick Nick]

That's a KI 525. Very robust HSI. 

Before you spend money on an avionics guy, go up and do an approach (in VMC obviously) on an ILS. Leave the GPS totally out of the picture. See if the behavior is the same, or if it's isolated to the signal coming from the GPS. 

The slaving system is part of the KCS 55A install, there are several components at work. It's entirely separate from the navigational inputs on the HSI. Is your slaving control set to slave, or is it in "free" mode?

[ighazali]

13 hours ago, Slick Nick said:

That's a KI 525. Very robust HSI. 

Before you spend money on an avionics guy, go up and do an approach (in VMC obviously) on an ILS. Leave the GPS totally out of the picture. See if the behavior is the same, or if it's isolated to the signal coming from the GPS. 

The slaving system is part of the KCS 55A install, there are several components at work. It's entirely separate from the navigational inputs on the HSI. Is your slaving control set to slave, or is it in "free" mode?

Slaving is set to slave once I adjust it. Also sadly the GTN 635Xi only has GPS approaches as it has no NAV. The HSI is tied to that!

[Slick Nick]

You shouldn’t need to align it yourself. Leave the switch in slave. On the KCS 55A power up it does a quick slave for the first 2 minutes, and then will fine tune from there. There are adjustments on the slaving accessory.
 

Which one do you have? A photo would help. The KA 51B was most popular.

Do you not have a nav radio or is it set to display on another instrument? 

[ighazali]

3 minutes ago, Slick Nick said:

You shouldn’t need to align it yourself. Leave the switch in slave. On the KCS 55A power up it does a quick slave for the first 2 minutes, and then will fine tune from there. There are adjustments on the slaving accessory.
 

Which one do you have? A photo would help. The KA 51B was most popular.

Do you not have a nav radio or is it set to display on another instrument? 

Nav Radio is set to display on a separate instrument which works well. The only issue is my glideslope needles on that come alive well after the FAF, on an ILS approach, which is a bit odd and I have a flag pretty much all the way down until 3.5-4 mile final where it will finally give me a reading. Localizer works great.  Here is the other side of the panel. There is a silver switch as well next to the slave unit I have no idea what it does. 

[Slick Nick]

I think that’s a KA 51A slaving accessory if memory serves. I can’t recall if there are adjustments on that unit or if you need to clock the magnetometer. 

One thing at a time. 

Either the HSI is getting incorrect information, or it’s getting good information and it has an issue displaying it properly. 

[ighazali]

8 minutes ago, Slick Nick said:

I think that’s a KA 51A slaving accessory if memory serves. I can’t recall if there are adjustments on that unit or if you need to clock the magnetometer. 

One thing at a time. 

Either the HSI is getting incorrect information, or it’s getting good information and it has an issue displaying it properly. 

I just looked in the books and that is the one! I am taking the airplane to my A&P IA to see what he can do about the glide path indicators he thinks that the system was not correctly calibrated properly to the GTN 635xi, or there may be a lose wire. He will take a look and I will also tell him that it precesses a lot as well and see what his opinion is. I flew the airplane today and again pulled it out of the hangar and then was off by 90ish degrees so had to reslave it but then it works quite well. On the way home it was only off by around 20ish degrees so reslaved it again and was fine in flight. Will probably replace it with a G5 or GI275 sometime next year. In the meantime I hope he can work some magic on the glidepath indicators. 

[Slick Nick]

Explain the sequence of events when you say its off when you pull it out of the hangar. Are you powering it up inside, and then taxiing out right away? Or are you outside when applying power? It's best to have the aircraft stationary during the initial alignment. 

There are adjustments available on the KA 51/51A to calibrate the 4 cardinal headings. Your A&P will find them in the manual. 

That all being said, if the gyro itself is worn, all the adjustment in the world won't help it, and usually you'd only need to adjust it if it was constantly off by the same amount. The fact that you're able to slew it manually and then have it stay when you switch back to slave tells me something else is up. The 525 is just the indicator, it could also be the KG 102 gyro in the tail or the KMT 112 magnetometer (unlikely.) 

If a G5 isn't in the cards right now, consider a used KI 525. They rarely fail, and used ones have come down in price since they are abundant with everyone ripping them out in favor of G5's etc. Then your downtime is limited to an hour, instead of a week for a full install. 

[ighazali]

I have some good news! I think the gyro is working correct. Any way to check it? My glideslope indicator starting magically working and have been working for the last few flights, not sure what the issue was or what was going on! Also the events are I pull the plane out of the hangar and then start up. Wait a minute or two for Gps to get signal and then start aligning it so the nav flag disappear. The other day it was off by 5 degrees so I moved it over manually and then it started going back to the initial heading it was at prior to me adjusting it. I will see if my A&P can recalibrate the slaving unit to the cardinal settings! I went to a junkyard and was able to get another Ki 525A for a pretty good price going to have the shop pop it in and see if it works, if not my current one has started to work a lot better. I do want to do G5's or the Gi275 along with the Gi275 engine monitor! 

[Slick Nick]

Have your mechanic clean and re-rack the KN-72 Nav converter. It should be located in the tail. Sometimes a bad connection there can cause issues. Definetly re-calibrate the KA-51. 

What do you mean you wait and then start aligning it? The gyro in slave mode should self-align once power is applied. It does a quick align in the first 120 seconds, and then fine tunes from there. You should not have to align it yourself just leave the switch in the "slave" position. It's going to be much more accurate than your heading from the whiskey compass. By the way, when was the last time your compass was swung? 

[ighazali]

On 11/10/2025 at 6:51 AM, Slick Nick said:

Have your mechanic clean and re-rack the KN-72 Nav converter. It should be located in the tail. Sometimes a bad connection there can cause issues. Definetly re-calibrate the KA-51. 

What do you mean you wait and then start aligning it? The gyro in slave mode should self-align once power is applied. It does a quick align in the first 120 seconds, and then fine tunes from there. You should not have to align it yourself just leave the switch in the "slave" position. It's going to be much more accurate than your heading from the whiskey compass. By the way, when was the last time your compass was swung? 

I usually start the airplane and then wait until it aligns itself and then fine tune it. However, after I make adjustments it goes back to the heading the unit had set. It is about 5 degrees off which is within the allowable limit so i have left it alone. I have no idea what you are referring to in regards to compass being swung  Also when you say recalibrate the KA-51 are you referring to having my mechanic go into the GPS unit and reconfiguring it there?

[Slick Nick]

11 hours ago, ighazali said:

I usually start the airplane and then wait until it aligns itself and then fine tune it. However, after I make adjustments it goes back to the heading the unit had set. It is about 5 degrees off which is within the allowable limit so i have left it alone. I have no idea what you are referring to in regards to compass being swung  Also when you say recalibrate the KA-51 are you referring to having my mechanic go into the GPS unit and reconfiguring it there?

Why do you seem to think that your GPS unit has something to do with it? It's not related whatsoever. When was the last time you had a compass swing? You're probably setting the gyro manually to a compass that's not been calibrated properly. Here in Canada it's more pronounced at our latitudes, so compass swings are required by law every 24 months. If you do a compass check and find out that it's off, there's your problem. If not, the next step is to have the KA-51 adjusted. There are adjustment pots on them that need to be calibrated, especially if it's been a long time since it was last done. How much has the magnetic variation changed at your location in the last 10 years? 

[ighazali]

On 11/18/2025 at 7:05 AM, Slick Nick said:

Why do you seem to think that your GPS unit has something to do with it? It's not related whatsoever. When was the last time you had a compass swing? You're probably setting the gyro manually to a compass that's not been calibrated properly. Here in Canada it's more pronounced at our latitudes, so compass swings are required by law every 24 months. If you do a compass check and find out that it's off, there's your problem. If not, the next step is to have the KA-51 adjusted. There are adjustment pots on them that need to be calibrated, especially if it's been a long time since it was last done. How much has the magnetic variation changed at your location in the last 10 years? 

Well the HSI is connected to the GPS for course guidance and there is no Localizer it is connected to. (GTN 635Xi) The only way to configure lateral and glidepath guidance is through the GPS. I am not sure with what compass swings are but we do have a compass deviation card which allows tells us what the magnetic deviation is. That was done in 2019. The HSI heading is only off by 5-7 degrees so it is still within the allowable limits. 

[Slick Nick]

1 hour ago, ighazali said:

Well the HSI is connected to the GPS for course guidance and there is no Localizer it is connected to. (GTN 635Xi) The only way to configure lateral and glidepath guidance is through the GPS. I am not sure with what compass swings are but we do have a compass deviation card which allows tells us what the magnetic deviation is. That was done in 2019. The HSI heading is only off by 5-7 degrees so it is still within the allowable limits. 

I think you may be confused on how the HSI works. It is two separate systems, integrated into one instrument. There is a heading component, and a navigation component. Other than being displayed on the same instrument, the two have nothing to do with one another. You could remove your GPS from the airplane entirely, and the directional gyro component would still work normally. 

A compass swing is a deviation check. 5-7 degrees is within limits, but I wouldn’t fly IFR if I was off by that much on the HSI. You can correct for this by having your KA-51 adjusted, which is what I suggested originally. Even the super old units with no internal adjustment, the Bendix King procedure was to actually clock the magnetometer in the wing accordingly to compensate. 

[Slick Nick]

In regards to your glideslope issue, have your avionics guy use a signal generator to determine if it’s the instrument or the antenna. Your post above mentioned that on an ILS your other instrument didn’t show glideslope until close to the station. This could also be an issue with your antenna diplexer. 

[LANCECASPER]

On 11/10/2025 at 4:01 AM, ighazali said:

I have some good news! I think the gyro is working correct. Any way to check it? My glideslope indicator starting magically working and have been working for the last few flights, not sure what the issue was or what was going on! Also the events are I pull the plane out of the hangar and then start up. Wait a minute or two for Gps to get signal and then start aligning it so the nav flag disappear. The other day it was off by 5 degrees so I moved it over manually and then it started going back to the initial heading it was at prior to me adjusting it. I will see if my A&P can recalibrate the slaving unit to the cardinal settings! I went to a junkyard and was able to get another Ki 525A for a pretty good price going to have the shop pop it in and see if it works, if not my current one has started to work a lot better. I do want to do G5's or the Gi275 along with the Gi275 engine monitor! 

Finding a used KI525, which may or may not have the exact part number might seem like a good solution, but how are you going to return the airplane to service without someone who is qualified to sign off?

Take the money you are paying on questionable used parts and pay a shop that does this every day to check everything and explain how all of it works together. If you want to fly precision approaches safely you need precision equipment calibrated properly to do so.