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Envelope protection - safe or unsafe (STEC3100 autopilot)

I’m in the middle of upgrading my autopilot from S Tec 55X (rate based) to the latest S Tec 3100 (fully digital). S Tec has an offer at the moment to make this upgrade and a the turn coordinator is getting lazy and needing replaced I thought I’d make the upgrade especially as I have all the other elements like G500 and two GNS430W to drive the 3100.
The 3100 has many great features including a level button and envelope protection. Envelope protection is becoming an issue because of the non standard nature of my modified 182.
The problem starts with the STC for the 3100. Like many STCs for modifications they state the sometime like “Performance is better than the original” so the original POH is often not changed due to the huge costs involved.
Consequently the envelope protection has to be pre programmed to the original POH numbers of a standard 182Q (in my case) rather than the numbers for a King Katmai. So, the envelope protection stall speed will be set at 50 knots not the actual stall speed of 31 knots. This is fine at first glance but once we started thinking about it, it’s not fine.
Firstly landing: the approach speed for me to safely land in my field is between 55 and 45 knots. The EP is not going to allow me to do this set at 50 knots. Interestingly, the only way to disable the EP is to turn off the autopilot master switch, just disconnecting the AP but leaving it on does not disable the EP.
Secondly tight turns: one of the great advantages of slow flight is the turning circle. If I fly up a blind valley by mistake a 45knot rate one turn gives me a 180 in 200-300ft. The EP won’t allow that.
Thirdly take off: the Katmai rotates at 40 knots. What will the EP do then, drop the nose?

Todd Peterson who builds the Katmai’s in Kansas is aware of this. They have fitted recently two aircraft with the 3100 and two with the Garmin G600 AP. They have persuaded Garmin to allow the EP to turn off with the disconnect button and give a warning that the EP is off. We are not sure yet if S Tec can do the same. Seems to me I need one of two options, disable the EP completely or allow the disengage button the disable the EP. Nobody likes having to turn the AP off completely to achieve EP disablement, there are several scenarios where this would be more dangerous.

We are yet to have the discussion with S Tec so I’ve no idea yet how this will play out. I never saw this one coming when I decided to upgrade. I may be getting my 55X back yet.

EGNS/Garey Airstrip, Isle of Man

Hmmm this is a tricky one. It’s a pity it came to light during the 3100 installation.

I am really surprised that the autopilot STC got granted for such a different aircraft. But then STEC have a long history of doing that, which is why they have so many STCs. Avidyne had a lot less luck getting their DFC90 STCd for the whole range of C182 variants.

Administrator
Shoreham EGKA, United Kingdom

I am really surprised that the autopilot STC got granted for such a different aircraft

Well, I guess it’s not like that. This aircraft is still a 182Q. And the STEC STC is for, inter alia, the 182Q (whatever STC has been previously applied to it).
AFAIK, STC’s don’t usually include a list/matrix of mutually-exclusive STC combinations. That would be impossible to keep track of.

Frankfurt (EDFZ, EDFE), Germany

Yes, unfortunately the STC is for a 182Q so the envelope protection figures have to reflect that. If the EP can be disabled with the disconnect button leaving the AP on that would be entirely acceptable. I’ll know more next week.

EGNS/Garey Airstrip, Isle of Man

boscomantico wrote:

I am really surprised that the autopilot STC got granted for such a different aircraft

Well, I guess it’s not like that. This aircraft is still a 182Q. And the STEC STC is for, inter alia, the 182Q (whatever STC has been previously applied to it).

Exactly. Compatibility/interactions between multiple STCs is the installer’s responsibility. Not the FAA.

ELLX

That is really just playing with words though.

An STC should contain sufficient instructions to be able to perform the installation correctly, assuming a reasonable level of installer competence. EASA tends to be a lot more prescriptive in this than the FAA.

If an STC inevitably results in a nonfunctioning-to-spec installation (as in this case) then the STC holder should not have applied for the STC. One cannot say the FAA should not have granted it – because the FAA does not personally inspect a specimen installation. Neither does EASA. So STC holder incompetence or even malpractice can be used to get an STC granted. I once bought some K&N air filters which had a TB20 on the STC but they fitted only the TB10 (IIRC).

What would be interesting is how the 3100 gets the airspeed data. Does it have a pitot tube fitting (and an internal pressure sensor) or does it get it via RS232 or ARINC429 from an air data computer? Obviously somebody knows; what is not known is whether they will post it

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

That is really just playing with words though.

No, it isn’t. The working assumption is that the autopilot STC is for a plain C182Q, not specifically for a King Katmai. This plane has another (single or series of) STC(s) installed (the “King Katmai” STC(s)). Applying a STC meant for a plain C182Q to a King Katmai is the installer’s judgement. Compare:

  • Customer brings a C182Q with a JPI electronic engine monitor installed under STC number NNNNN. Is that STC compatible with the new AP STC? Yes, engine monitor has nothing to do with AP.
  • Customer brings a C182Q with engine/aerodynamic changes (vortex generators?) that change the stall speed, and more. Is that STC compatible with the new AP STC? Well, I’ll need to change the envelope protection settings. Can I do that, technically and legally? If not, can I disable envelope protection altogether?

Peter wrote:

If an STC inevitably results in a nonfunctioning-to-spec installation (as in this case) then the STC holder should not have applied for the STC.

The working assumption is that he didn’t. He may apply for an STC for “Cessna C182Q modified as per STC number XXXXX, AKA King Katmai”, and then his installation manual would contain the envelope protection settings for a King Katmai.

ELLX

then his installation manual would contain the envelope protection settings for a King Katmai.

Indeed it should, but it does not

Administrator
Shoreham EGKA, United Kingdom

If an STC inevitably results in a nonfunctioning-to-spec installation (as in this case) then the STC holder should not have applied for the STC.

Completely impracticable as this would increase the cost for any STC to absurd amounts.

How should someone who applies for an STC check if the new device does not interfere with any potential combination of other installed STC in such a mainstream type as the C182? If we would really demand this, STEC had to individually ensure compatibility with literally thousands of potential combinations of installed STCs.

Current approach to STCs is exactly the right one: The manufacturer of the equipment has to make sure it works in the standard configuration of the Airplane (i.e. all configurations that have been delivered under the original TC) and the installer is responsible to manage interactions if they wish to install more than one STC in parallel.

Germany

I think this is semantics.

Obviously an STC cannot cover every possible accessory fitted to the plane.

But a Katmai C182 is quite different from a normal C182. An autopilot STC does have to take such a huge performance change into account.

Administrator
Shoreham EGKA, United Kingdom
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