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PC12 N950KA accident - a curious KFC325 autopilot disconnection

This report on the 6x fatal PC12 accident in 2012 has come out recently.

Reading it, and they way they talk about not finding anything much of relevance in the autopilot, I can see echoes of the mysterious behaviour of my KFC225 and its many failures.

Specifically, phrases like

are just pure bull when it comes to Honeywell GA autopilot software. It is astonishing that the NTSB takes this at face value, but I suppose they have to… I do find it surprising given the huge volume of email I have on this topic with aircraft owners and others and the fact that a google on the obvious term pops up my pretty damning report as #3 and has done so for best part of a decade! I know Honeywell know all about this, from emails accidentally CCd to me after I reported one of the failures to them c. 2005 and from a meeting I had with them in 2013 at EDNY.

The autopilot disconnection might have been caused by something unrelated but the investigation should not have taken this “info” at face value.

A few years ago I spoke on the phone to a pilot whose KFC225 equipped Baron was inverted before he could regain control of it.

It is possible that the KFC325 is a completely different (and properly executed) design to the KFC225 but how many will believe that?

Ultimately, this accident happened apparently because the pilot could not take over the flight manually, for whatever reason (most likely IMHO because he was not current on type and got caught off guard, in IMC) but there are very few sky-gods around and autopilot failures do not help.

Administrator
Shoreham EGKA, United Kingdom

The other thing that struck me in the report was the relatively light g loading tolerances for the PC12.

“plus 3.3 to minus 1.32 g’s. ", and the highest recorded g during this tragic tale was +4.6g’s.

This seems unusual to me for an aircraft of this type. I thought normal catagory was +3.8 to -2.0?

Regarding the autopilot. I do feel a lot of reliability issues are airframe installation specific. I have been flying behind the same KFC225 autopilot that Peter has had such poor experience with for the past 6 years and only recently had a servo fail after some 1,600 hrs. I only recall one mysterious disconnect during that time. Other than that touch wood it has been very reliable and accurate, and generally regarded as the best autopilot for the PA46 airframe.

E

eal
Lovin' it
VTCY VTCC VTBD

I came across the PC12 stall video in a discussion elsewhere on this accident. It seems clear that the AP disconnected due to activation of the stick shaker, which will disconnect the AP in most types I am aware of.

The pilot then failed to fly the aircraft and when he tried to the aircraft was in a spiral dive, and he just pulled, which broke the aircraft (similar to the guy who stalled his Mustang near Leeds but survived despite having bent the wings to the extent the aircraft was a write-off). He had almost no proper instrument time and was straight out of the PC12 training program.

London area

Very sad story. Spins and spiral dives training should be mandatory for PPL.
I fly gliders and it’s part of the training for SPL.
However, other similar aircrafts could have better coped with it.
TBM are certified at +3.8g and were successfully tested at 6.1g during development. Socata seem to have paid special attention to structural strength of the wing spar.

LFLY, France

It seems clear that the AP disconnected due to activation of the stick shaker,

Why do you think the stick shaker came on during autopilot flight?

In a SEP that’s dead easy to achieve – just set up +1000fpm in VS Hold and leave it there for some minutes

But a PC12 will hold a reasonable rate of climb all the way to FL quite a lot. Also you have loads of time to detect the reduction in the IAS, because it bleeds off gradually. You have to be incredibly busy, or asleep, to not notice.

I do feel a lot of reliability issues are airframe installation specific

Yes – I agree. Some airframes are far better than others. The Caravan for example has had a ~100% failure rate; similar to the TB20GT/21GT.

My comments were really applicable to the autopilot firmware and to the comments in the report re the (IMHO bogus) self diagnostics, not to the servo burnouts which plagued the aforementioned airframes and which were ruled out in the accident report here.

Administrator
Shoreham EGKA, United Kingdom

From the report, the AP disconnected at an IAS of 109kts. Those with experience on type suggest normal climb speed around 150-160 with that being within the stall warning range. There’s no hard proof, but it seems a reasonable inference.

Climbing in VS mode will get you in a lot of aircraft, even those with supposed reversionary modes to prevent you stalling. It is also believed to be the cause of the Mustang incident I mentioned. Plenty of older cheap autopilots (including those fitted to many bizjets) display an unpleasant oscillation in speed hold mode, and thus VS is the preferred climb and descent mode for comfort. It requires focus flying two crew, fairly easy to see how a single pilot with potential distractions and little instrument experience made the initial error.

London area

The VS of a PC12 is about 65kt, so should the stick shaker come on at 109kt?

The stall warner should come on well before the stall, and that really wakes you up – again, unless you are really busy in which case you might filter it out.

Administrator
Shoreham EGKA, United Kingdom

Josh, it wasn’t a Mustang, it was a CJ2 in the incident that damaged the airframe and returned to Leeds. We operate a similar aircraft and allowed the AAIB to take some measurements on ours. For interest I tried a stall at climb power and FL430 (in the sim). It wasn’t difficult but a lot of height is lost in the recovery. Maybe the sim isn’t accurate that high

I don’t buy the idea of stick shaker or pusher activation at over 100kts in this sad accident, but have never flown the type myself so wouldn’t know for sure

Last Edited by Neil at 04 Dec 23:16
Darley Moor, Gamston (UK)

Ultimately, this accident happened apparently because the pilot could not take over the flight manually, for whatever reason (most likely IMHO because he was not current on type and got caught off guard, in IMC) but there are very few sky-gods around and autopilot failures do not help.

Peter, you do love this sky God term and i am not sure it adds anything to these discussions. I just don’t accept that hand flying an aircraft IMC after an autopilot failure involves super-pilot skills. It is normal flying if you are an instrument rated pilot. If you can’t do it you should not be flying in IMC.

EGTK Oxford

Agreed — I’ve done most of my flying in actual IMC without an autopilot and lots of it enroute.

ESKC (Uppsala/Sundbro), Sweden
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