STEC55X advice - what can go wrong?

Flyingfish wrote:

Can this thing bite? How?

As always when you use VS mode for climb, you have to both monitor airspeed and select rate of climb carefully so that the AP doesn’t stall the aircraft with the trim in a nose-up position.

Would it be smart to add some kind of big red push button to disable elevator trim located within reach while pulling on the yoke?

Having the yoke AP disconnect button also disable the electric trim (while pressed) would be a good idea. Then if something happens, you don’t need to think about what button to push.

Flyingfish wrote:

I have only a few hours with the STEC55X. I found it simple, even basic and only offers a VS climb mode.
Mine has the yaw damper option

I have been flying with the STEC55X for the past 8 years and have about 1500 hours on it, I have had zero ‘MOMENTS’ with it and it has worked fine.

I also have the yaw damper but sad to say, I have never used it.

Like you initialIy I was concerned about failures/ runaway trim/ etc. but so far…so good, its always been spot on.

As you say there is no VS mode which means (in climb especially) you need to keep one eye on the ASI….but you are doing that anyway, if you watch that and am not dialling in stupid climb rates, you wont have a problem with the aircraft stalling.

There are several ways to disconnect the autopilot should you have any problems such as runaway trim and you should familiarise yourself with all of them and practice them as well while on the ground.

Unlike some newer autopilots the STEC55X struggles to intercept a sharp intercept angle onto a localiser especially if you are over 150kts so you may need to hand fly onto the localiser to stop yourself going through it and having to intercept from the other side.

There is a known problem with the altitude hold porpoising in cruise, at FL270 it would be +150ft / -150ft but they came up with a fix in which something was
reprogrammed and its worked perfectly since.

Hope this helps.

- Can this thing bite? How?

Be sure to familiarize yourself with the CWS button and its effects, in particular when released. It behaves quite different compared to a typical King AP, like King KFC150. The latter has a quite useful CWS implentation, I am not so sure whether I would dare to say the same with respect to the STEC 55X.

quatrelle wrote:

Unlike some newer autopilots the STEC55X struggles to intercept a sharp intercept angle onto a localiser especially if you are over 150kts so you may need to hand fly onto the localiser to stop yourself going through it and having to intercept from the other side.

Yes, this is quite a biggie in normal operations. If ATC vectors in quite tightly (which is good for low track miles) an overshoot of the LOC is almost guaranteed. Dialling in a HDG which gives a rather small intercept angle (say 20°) as a last step prior to intercept is a good cure.

quatrelle wrote:

There is a known problem with the altitude hold porpoising in cruise, at FL270 it would be +150ft / -150ft but they came up with a fix in which something was
reprogrammed and its worked perfectly since.

That one is really a nuisance. I see this a lot if CG is in the rear half of the envelope (i.e. Pax on board). They hate it. So, @quatrelle, I would be delighted to learn anything about a fix. Could you elaborate a bit, please.

Having the yoke AP disconnect button also disable the electric trim (while pressed) would be a good idea. Then if something happens, you don’t need to think about what button to push.

What exactly does the autopilot disconnect button do on the 55X?

On the King units it

• tells the autopilot computer to disconnect (this will fail if the software has crashed, and these boxes have no watchdog!)
• while the button is being held down it interrupts power to all the servos, so you can control the plane as required, while reaching for the Autopilot Master switch

There is a known problem with the altitude hold porpoising in cruise, at FL270 it would be +150ft / -150ft but they came up with a fix in which something was
reprogrammed and its worked perfectly since.

That is “typical STEC” issue because – unlike King – they got their STCs without testing in every airframe and all around the loading and speed envelopes…

Unlike some newer autopilots the STEC55X struggles to intercept a sharp intercept angle onto a localiser especially if you are over 150kts so you may need to hand fly onto the localiser to stop yourself going through it and having to intercept from the other side.

That is normal for all the autopilots where the box is getting basically the HSI deviation bar signal, with no predictive behaviour (e.g. roll steering via ARINC429).

AxelF wrote:

I would be delighted to learn anything about a fix.

Part of the autopilot was sent back to the manufacturers to be reprogrammed and it wasnt an expensive fix but it now works 100%, it doesnt fluctuate 10ft.

It may take into early next week but I will email a maintenace andget the information.
AxelF wrote:

. I see this a lot if CG is in the rear half of the envelope (i.e. Pax on board). They hate it

I agree it would even make a sailor seasick !

quatrelle wrote:

Part of the autopilot was sent back to the manufacturers to be reprogrammed and it wasnt an expensive fix but it now works 100%, it doesnt fluctuate 10ft.

It may take into early next week but I will email a maintenace andget the information.

Sounds promising. Looking forward to that info.

I suppose different airframes will behave differently but anyway:
Did the porpoising you observed commence already in the climb at higher levels? In my experience (PA-46 airframe) it starts in the climb passing FL170-200.

AxelF wrote:

In my experience (PA-46 airframe) it starts in the climb passing FL170-200.

Ditto P46T

No never noticed it in the climb just when levelled off, my friends P46T was worse +/- 400ft, it was like a rollercoaster, since mod - rock solid

Will give you the info as soon as they get back.

quatrelle wrote:

As you say there is no VS mode which means (in climb especially) you need to keep one eye on the ASI….but you are doing that anyway, if you watch that and am not dialling in stupid climb rates, you wont have a problem with the aircraft stalling.

Providing you have not suffered a partial engine failure. If the engine is not delivering normal power, for whatever reason, many autopilots in VS mode will happily take you into and through the stall, which, especially in cloud, can then develop into a spin as the autopilot also tries to respond to wing drop.

Also, it is worth looking in detail at the Dunkeswell report . That was a KFC150, not an STEC 155X, but I believe that the same error mode could have resulted from pushing against electric trim when just below cloud at low altitude.