Can this happen in an HSI??

[AmigOne]

A friend from South America, also a Mooney driver, has the following issue in his King KCS 55a HSI. He says that whether tuned to a VOR or an ILS, the NAV red flag in the HSI comes down intermittently yet the signals to the CDI and GS (on an ILS practice) continue to operate normally when compared to his #2 VOR indicator from the #2 navcomm. Is this possible?? I said it should not happen but if it can happen, is it an issue with the HSI internal electronics or, in his case, the Garmin 430 to which it is connected?.

[takair]

As I recall, the 525 can be wired with a "Super Flag" that can cause this to happen. Unfortunately it has been years since I have had to troubleshoot one. Most avionics shops should be able to troubleshoot it. The maintenance menu of the 430 gives some ability to manipulate the flag to help with troubleshooting.

Did he mention what the flag does with the GPS driving the HSI?

[takair]

Just looked it up. The flag on a KI 525 requires 14 vdc on pin D (assuming 14 v plane), unlike most CDIs. There is also some circuitry inside the unit. As I recall the op amp within would occasionally fail like this. However, if that was the case, it would likely occur on GPS as well. If it is only on VOR/ILS then it may be a sensitivity issue, where you don't quite lose the needle, but the flag is in view. With a centered needle, you may not notice an issue with the CDI. If you fly an ILS with some offset (let's say half scale), then you should notice that the needle starts to center as the flag comes in view. So, ask your friend the behavior of the flag on GPS, if normal then he can try the above tip, or better yet have the shop test it on the ground. Hope this helps.

[AmigOne]

Thanks for the advice, in fact I was resorting to the forum because this issue has stomped the local technicians. They bench tested the HSI and declared OK, the same with the 430 but knowledge in the area of interaction between the older vintage HSI and newer GPS units is still evolving. I will ask my friend for more information.

[Piloto]

Just disconnect the related connectors and spray contact cleaner on the contacts and reconnect. On some installations were the HSI can be switched to either NAV 1 or NAV2 the HSI is connected to a transfer relay. In many instances the culprit is the relay contacts or relay connector. Clean the relay contacts and if possible open the relay and clean the contacts. These transfer relays pose so many intermitent and erratic problems that I would not recomend its use. Instead wire the HSI and autopilot directly to NAV 1 and the CDI to NAV2.

 

José    

[dennis pinkerton]

On 7/5/2013 at 1:56 PM, Piloto said:

Just disconnect the related connectors and spray contact cleaner on the contacts and reconnect. On some installations were the HSI can be switched to either NAV 1 or NAV2 the HSI is connected to a transfer relay. In many instances the culprit is the relay contacts or relay connector. Clean the relay contacts and if possible open the relay and clean the contacts. These transfer relays pose so many intermitent and erratic problems that I would not recomend its use. Instead wire the HSI and autopilot directly to NAV 1 and the CDI to NAV2.

 

José    

Jose, thank you for this valuable information. Is the transfer relay external to the units or built in? Thanks for your help.