spayne59

thanks, I'll ask about that

Hi fellow Mooniacs Looking for advice from anyone with a KAP150 autopilot in their J model who has replaced their AI with a Garmin GI275. As a project with the local Garmin dealer, we decided to remove the original vacuum powered KG258 AI and KI107 DG, and replace them with a pair of GI275 instruments, one as the ADI and one as the HSI, and remove all of the vacuum system while we were at it. Very happy with the functionality and appearance of these new instruments. They are fully reversionary and battery backed for 60 mins; and also the ADI emulates the analogue pitch and roll signals of the KG258 that drive the autopilot. We have a KAP150 autopilot which has performed flawlessly in the past. It has pitch, roll and trim servos controlled by the KC191 flight computer. It is not the A/P version that drives a KI256 indicator with Flight Director. From a roll/heading perspective, the system works perfectly. Set the bug on a heading and the autopilot rolls the aircraft and then levels off to fly that heading beautifully. The GI275 also includes GNSS and if you select it, the aircraft follows the GPS flight plan like a go kart on a race track. Our GPS source is a GTN650, btw. But…the problem is pitch control. Whenever the autopilot is engaged (even just in wing leveller/pitch control mode) prior to engaging either HDG or ALT modes, we get “pitch porpoising” happening. Clearly this is being driven by the autopilot via the pitch servo (I have confirmed it is not the trim servo doing this). We have removed and cleaned/serviced the pitch servo in case it was the cause – but it was not. The GI275 has analogue pitch and roll signal emulation to the KC191, so it behaves like the KG258 that we removed. The output is 50 mVac per degree of pitch to the KC191. Given that the GI275 analogue output pitch signal level has been confirmed as very like that from the previous KG258, the only difference I can see is the response time of the signal – and that potentially this is upsetting the control loop tuning to the extent that oscillation starts. The new GI is presumably much more responsive at detecting and reporting pitch changes than the KG258 it replaces. It is asking for pitch adjustments faster than the A/P can respond? When engaging the A/P, the oscillation starts and is of mild amplitude but at a frequency I estimate between 0.5 and 1 Hz. It is not even detectable in turbulence, but in clear air the pitch stability between A/P Off and A/P On is very noticeable. It is mildly nauseating to passengers. It is particularly nauseating to the owners considering the amount we just invested, and that this has resulted in a problem we never had before! We are now at the point where the local Garmin agent are pretty much out of ideas, and Garmin are saying “you need to ask King” and King are saying “the A/P is fine, you need to ask Garmin”. Or, of course, we could just spend another $30k and install a Garmin digital autopilot ☹ Recognising that there are a lot of KAP150 autopilots in M20Js, and hearing of good sales of GI275s to the GA market, I really hope to hear of others that have seen this problem, and pray that someone has a (preferably easy and cheap) solution to it!

Latest update: Mooney factory advised they are talking to a spring manufacturer about making a batch of (reverse engineered) no back springs for the Plessey actuators. Still only talk at this point, but a positive sign.

it varies. Dan at LASAR has one that I've reserved for $8,500. All because a spring that cost $19.20 in 1992 has failed . Ah, the pleasures of ageing aircraft.

Yes, I tried LASAR first, no stock. Buying a reconditioned actuator the only option with them

Donkaye - do you still have those springs?

Our no back spring is worn and we need another, AOG. Actuator is the Plessey type and is not the same spring as in other J models. I can’t find one anywhere! Can anyone help?