However, if say flying in NAV mode and you either need to turn 30R to avoid or ATC asks for some such, it is reasonable to set the bug 30R and press HDG. The autopilot should do a Rate 1 turn to the right.
On King ones the STC specifies a specific roll angle (there is no TC input) so e.g. on mine it is something like 22 degrees (~150kt cruise). It should not do 30.
@wigglyamp will know all this since it is/was his speciality (and hopefully he will be along soon) but there is normally a calibration page where this can be set up. With some autopilots (e.g. my KFC225) some parameters cannot be accessed by anybody and these come on a “certification diskette” whose content gets loaded into an EEPROM module. This module enables the autopilot computer to be swapped and the new one doesn’t need recalibration. I don’t know if STEC use the same system. Unfortunately I don’t have the 55X IM. I have emailed you flyingfish something which may illuminate it. I found an STEC30 IM on google and emailed you the URL; best to not publish it in case STEC demand its removal.
I have had similar experiences in Cirrus with the STEC 55X
One thing I would do in your Extra 400 is to simulate a trim runaway and figure out the control forces. Just trim it to the max position in a safe altitude horizontal flight and see how the control forces change to get an idea how critical such a situation would be.
Flyingfish wrote:
If the heading bug is set significantly off the present course and the autopilot’s HDG mode is engaged, it banks the aircraft into a dangerously aggressive turn (over 30 degrees)
The autopilot must never bank more than what is required for a rate 1 turn (2 minutes). Most likely something is wrong with your turn indicator which is the input sensor of the S-TEC55. The wings should align with the markings on the instrument. A qualified shop should be able to adjust this behavior. I have spent a gazillion of dollars on S-TEC autopilots and can pretty much only say negative things about them but the turn rate was never an issue.
I agree, there is something not quite right with that behaviour. The S-Tec AP’s should never do further bank angle than needed for a rate one turn. If they do, then either some gain value or the turn indicator is not doing what it should.
Thank you all. Will digest / test/ report back. Peter: emails arrived thx.
FWIW I just did a LFEY-LSGG today.
https://www.flightradar24.com/data/aircraft/n121ag/#d3e5571 .
There was a nice tailwind on take-off and in the first part of the cruise, but even on pure TAS, the plane performed remarkably well.
(I did several engine setting changes for breaking-in which explains the changes in airspeed.)
At one point I was calibrating the JPI EDM 930 so I set exactly 75% power. TAS settled on 203 KTAS at FL180 which is very near book value.
Fuel flow must be kept a tad higher than book because of a more restrictive TIT limitation which I have decided to follow: 1650F.
Book limit is 1750 F but several people told me that it was bad for the turbo to inflict such high temps on it.
I tried a LOP scenario, but had difficulty understanding what the EDM 930 said so I aborted it.
Also did an autopilot coupled ILS approach. The STEC overshot twice but eventually intercepted correctly and managed the glideslope very well.
I just love this plane. It is challenging down low but very rewarding as soon as the gear is retracted.
Just wanted to consolidate my understanding of the situation and share it for others’ benefit.
The STEC 55X must be used with extreme care when “downgrading” from an aircraft that has a GFC700 or similar modern gear.
The main issue is its inability to accurately intercept courses which seems to be related to its concept.
Furthermore, a number of problems can be caused by peripheral devices:
>The turn and bank indicator is a known cause of fishtailing and general instability. A function check is inexpensive though.
> Another issue is friction due to dirt accumulation in the servos. This causes them to need more voltage than normal to initiate movement. Normally they should move at less than 2 Volts. My roll servo needed 4 volts. This caused nasty behavior such as throwing the plane in a 30 degree bank or more once the servo finally reacted to the increasing input from the controller.
These servos CAN be opened up while installed (at least on my AC) by a good specialist and cleaned / lubricated. This really helped in my case.
> I was shocked by the poor quality of connectors in the exposed area where the servo sits. Mental note that this could become a problem.
I still have an item on my to do list: the trim runaway survival test that achimha suggested!
All in all here is how I operate it now:
> Use mostly the flight director in departures, arrivals, approaches. The Extra 400 is so nice to hand fly that I really don’t mind the additional workload.
> Engage the AP in cruise in nav/gpss mode which is much less sensitive to the T&B caused slalom.
> Engage the autopilot to fly straight and level, climb/descend to a given altitude in good weather.
> For IMC with icing potential, I have found the following alternative to the dreaded VS-mode climb: engage only HDG or GPSS mode and set a certain indicated airspeed by trimming the aircraft. This way the plane will never pitch up. It simply gives me whatever VS it can under the circumstances. Only problem is the lack of intercept…
This is working OK for me. I will send the T&B for check/overhaul at the next annual and will also check and clean all servos annually.
The plane flies beautifully (no fishtailing) when hand flying in both climb and cruise, it is fast (200 easily) and climb has become excellent after fixing the cooling issues. Separate thread though.
Thanks to all who contributed here. I feel a lot more confident now…
I have to say that my initial enthusiasm for the system has somehow faded following the failure of the roll servo first and the computer now. Servo was on warranty but the computer is not. Less than 200 flight hours into operation this is not what I expected…
