Weird KI256/KFC150 Behaviour

[zuutroy]

Can anyone shed any light on this? I am getting into IFR flying in my own aircraft after training on C172s and was flying an ILS last week with the KFC 150 to see how it performs. All good in heading mode but when it captured in APP mode it started behaving weird and the KI 256 started indicating a bank to the left in straight and level flight (test was done in VMC). At that point I hand flew the remainder.

Today I went out to diagnose. In the video below I’m tuning in a VOR and picking a nearby radial to see if the KFC will pick it up and track it. Same behaviour again where when I hit NAV on the KFC (bottom right) it doesn’t really do it and then the KI 256 is indicating a false left bank! I turned off the A/P and within a couple of mins the 256 is behaving correctly, continues to work fine in HDG mode back to base and perfect during the landing circuit. I certainly won’t be engaging anything other than HDG mode until I can figure this one out. The only other relevant information is that I have a new vac pump following overhaul and the 256 takes a couple of mins to erect on startup. Not sure if it always did this as I’m only really concentrating on it now.

The fact that a) it is reproducible and b) it slowly rights itself after disengaging NAV/APP suggests some signal is screwing with the KI 256 and maybe is some capacitive effect that decays away after removing the signal?

The other weird thing is why it just decides to return to level flight when this is happening. It seems like it’s telling the pilot one thing, but telling the KFC150 something else. It’s possible that it is indeed tracking the radial (although there seems to be about 1-2 degrees of deviation), and is obeying a KFC wings level command, but just telling me something different.

 

 

[Bob Weber]

That gyro is bad..

[zuutroy]

Thanks Bob. Could you tell me what leads you to that conclusion? Obviously when one sees an AI behaving strangely it's the prime suspect but the fact that it seems to be tied to the KFC150 confused me. I've just paid for an engine overhaul so I need to keep this repair bill to a minimum 

Edited July 21, 2021 by zuutroy

[Jake@BevanAviation]

Looks like the rotor speed is slow in the KI256.  On engine start you should see the AI card shake out, as the rotor speed comes up the gyro should become more stable.  If it is slow to erect (no shake out)  you can bet the bearings are stiff and might be slowing down the rotor speed.  FD bars work independent of the gyro assembly, the FD bars moving around should not influence the AI orientation.

[carusoam]

Zuu,

You got input from two guys with the most AP experience on the planet… 

For a third opinion… a private pilot opinion is sure to be helpful…

 

1) your AI is slow to start-up…

2) your AI is in disagreement with your TC…

3) your AI is an integral part of the KFC autopilot decision tree…

 

Interested in low cost ideas for OH or replacement?

It might be helpful to borrow an identical instrument and demonstrate what has been discussed here…

 

PP thoughts only, not an instrument or AP guru…

Thanks Bob and Jake!

Go MS!

Best regards,

-a-

[Airways]

@zuutroy I used this company for similar problems: https://aircraftinstrumentsltd.co.uk

It was a repair, not a complete overhaul.  But it took a looooot of time before I got it back.  If you are on a tight budget, you might consider them.

 

[zuutroy]

Thanks for the tip. SIBA in France have some in stock (€1950+VAT with Form 1)  and apparently they can also repair the 256. I've asked if they'll take mine as an overhaul exchange so I'll see what they come back with. Will also contact your guys.

[Bob Weber]

You might want to give a good check of the vacuum system and verify correct operation.

Jake's description of how it should look on start up is important to verify, it gives us an idea of the health of the system.

A stuck erector vane in the gyro can be indicated this way, as well as low vac level or restricted lines diminishing the volume.

When a gyro reacts like I watched yours in the video, the friction of the gimbal bearings is greater than the gyroscopic force of the spinning mass.

This can be caused by defective gimbal bearings, the bearings on the spinning mass, or low vacuum flow over the rotor.

[81X]

On 7/21/2021 at 1:46 PM, zuutroy said:

Thanks Bob. Could you tell me what leads you to that conclusion? Obviously when one sees an AI behaving strangely it's the prime suspect but the fact that it seems to be tied to the KFC150 confused me. I've just paid for an engine overhaul so I need to keep this repair bill to a minimum 

The attitude indication is for most intents and purposes, independent from the autopilot and is a function of the gyro, with failure modes typical of a vacuum-based gyro.
 

 If it’s an external vacuum issue, it’s probably worth the repair. 
 

These are typically hella expensive to overhaul so if it’s an internal gyro issue, it might be worth going solid state such as a GI-275.  

[jlunseth]

I just got my gyro replaced because of those symptoms. Plane was persistently right of course on AP. The 256 would indicate a left bank but the backup 275 would indicate level flight. This did not happen during approaches at first, but then I started seeing it during approaches also, so got it right in to the shop. The avionics shop recommended cleaning all the contacts on the AP motherboard at the same time. Gyro was bad.

On the suggestion to replace with a 275, bear in mind the thread that has been going on where one of our members experienced complete failure of a dual 275 system. I was going to replace my 256 with the dual 275, but for now have opted to replace the 256 and add a backup 275 for redundancy. Personally, I would not put in the dual 275 installation unless you have (1) two alternators, so a backup if one goes bad, and (2) another, third backup electric AI that uses a different technology than the 275, is not part of the 275 system, and has its own internal backup. Weight savings from removing the vacuum system is a myth, the electronics are not light. The 275 - yeah its cool, but the system is apparently not as well thought out as it should be.

[zuutroy]

So a different, serviceable 256 also wouldn't erect properly when I replaced it and ran the engine on the ground. Just showed some crazy attitude and then wobbled a bit back towards normal when the gyro was spinning down. Does this point to the vacuum not  being sufficient?

[carusoam]

3 minutes ago, zuutroy said:

So a different, serviceable 256 also wouldn't erect properly when I replaced it and ran the engine on the ground. Just showed some crazy attitude and then wobbled a bit back towards normal when the gyro was spinning down. Does this point to the vacuum not  being sufficient?

Sounds that way, what else could it be?

Our vac instruments use…

• vac pump
• air filter 
• lines
• sometimes a vac gauge… sometimes just an idiot light…
• sometimes an alternate vac system…

Things you can do…

• Test the instrument to know it is working before mounting it in the plane…
• install a vac gauge, at least temporarily to know what level of vac is getting to the gauge…
• inspecting vac lines… visually or with a smoke (?) test would probably be next…

If using in IMC…. Too important to not know the reason it isn’t working correctly…

PP thoughts only, not an instrument guru…

Best regards,

-a-

[zuutroy]

Thanks for the tips. For curiosity I connected the original to an external vacuum pump and it shakes out fine and indicates perfectly thereafter. Time to look for some vacuum gremlins I guess.