Atsb AO-2013-126 Cirrus electrical event

[VH-XXX]

I know this isn't a full blown investigated incident, mainly because the aircraft didn't crash, however I am trying to understand the actions of the pilot on this occasion.

On approach to Kingaroy, at about 500 above ground level (AGL), the pilot extended the flaps and shortly after, disconnected the autopilot (AP). Upon disconnecting the AP, the pilot reported that the aircraft pitched-up violently due to trim runaway. The AP pitch trim was trimming the aircraft for a nose-up position, even though the AP was disconnected. This required the pilot to use a large amount of forward physical force to maintain stable flight. He attempted to resolve the problem by pressing and holding the autopilot disconnect switch (AP DISC), however, this had no effect. The pilot conducted a go-around.


So let me get this straight...

- The pilot flies from Archerfield to Kingaroy and engaged auto pilot during that flight
- He approaches Kingaroy, descends from altitude X, either with or without a circuit using the autopilot electronic controls
- In an Avidyne equipped aircraft this would involve rotating the altitude bug, heading bug and pressing VS and Alt on the autopilot system
- Pilot descends to 500 ft at what would be less than 120 knots for stage 1 flap extension
- At THIS point when the aircraft is on final, although the article says "approach" the flaps are extended
- After the flaps are extended, autopilot is turned off and the aircraft pitches up due to an alleged electrical problem with a runaway rear trim

This sounds a little suspect.

- Flying the aircraft onto a 500 ft final on autopilot when they could be better served looking out the windows
- A normal approach would generally entail applying flaps when turning onto base from 1,000ft and not 500ft
- Relying on autopilot to keep the aircraft stable with flaps is inherently not good practice ,as the autopilot will not keep the aircraft as stable as hand flying, particularly in turbulence

Thoughts?

One of the dangers of (an assumed) straight in approach perhaps?

[Captain Nomad]

I wonder how long between selecting flaps and disengaging autopilot?

It matters not the aircraft type, if you change the configuration and follow it rather quickly with an AP disconnect before the auto-trim has finished its job you can be guaranteed a surprise... A pitch up after flap extension (especially if it was on the upper end of the flap speed envelope) would have to be expected, surely? It does sound like it could have been purely normal physics combined with poor airmanship/timing to cause an event like this. I have flown aircraft where the AP can spit the dummy and take longer to disconnect when interrupted while auto-trimming. It's a bad time to interrupt an AP!

If it was a true trim runaway one would think the only thing that would isolate the situation would be to remove power from the pitch trim mechanism through a 'trim interrupt' type switch or by opening a CB. There doesn't seem to be any info about such action which would seem to suggest that after control was ultimately regained from the AP the trim worked normally?

[VH-XXX]

It would be nice if there was a little more info in the report so the readers could use the information to get a clearer picture for their own safe flying.

For starters:

- pilot experience
- VFR or IFR (which has a significant impact on the procedures used)
- timeframes between flaps and ap disconnect

Otherswise there's no point making this a proper report, it may as well have just appeared in the weekly summaries.


It is interesting to note that the trim was found all the way back after the flight. Also interesting the number of stability systems in the aircraft, perhaps they were somehow all fighting each other plus you'd have to multiple breakers potentially.

[Arm out the window]

Flying the aircraft onto a 500 ft final on autopilot when they could be better served looking out the windows

The two aren't mutually exclusive...

[bodybag]

"I can't help but think that the pilot has not used good judgement or airmanship on this flight"

The report is very light on relevant factual information which is reason to exercise good judgement in starting PPRuNe threads slandering the pilot with comments such as this.

The pilot in this case has made a report to the ATSB in a situation where VERY few private pilots would. Most would simply fly home with the circuit breaker pulled and get the problem fixed. This pilot is proactive about safety in a commendable way!

I am concerned that a PPRuNe discussion about the actions of this pilot could actually have detrimental effects on safety reporting culture. This person certainly did not expect to have a nation of armchair critics evaluating his / her performance (based on extremely limited information) when they submitted this report and I dare say that had they known they were going to be slandered by you, they may not have made the submission.

[Captain Nomad]

I would not call the comments so far 'slander.' Speculation based on limited information, yes.

If you put something out in the public domain it will become the subject of speculation - especially when it gets discussed on a 'rumour network.'

Having said that, you are quite entitled to your valid opinion regarding the situation.

If the trim was indeed full nose up at the end of the flight the pilot would no doubt have had his hands very full flying a go-around and subsequent circuit in that condition. Does the Cirrus have a backup manual trim?

I hope for his peace of mind (and everybody else) they come up with a suitable explanation. Sometimes software companies and manufacturers get involved in these cases. I wonder if Avidyne, Cirrus or anyone else is getting involved?

[VH-XXX]

It doesn't sound like they are taking this any futher, to the manufacturer or otherwise. For a "serious incident" I thought the ATSB would follow it up further and or give more information. Perhaps they weren't given all information by the operator. I still maintain that appyling flaps on a 500 ft final with the auto-pilot engaged isn't normal, but I'm not an expert.

I note that there was a mechanical issue noted:

The elevator trim was subsequently adjusted to correct an observed slight nose-up situation when the AP was disconnected.

That must mean that when the auto-pilot is disconnected it must revert back to a central location, otherwise this wouldn't have been an issue? That doesn's seem to make sense. My time in the Cirrus tells me that when the AP is disconnected it simply stops where-ever it is at the time, but doesn't neutralise, or at least I've never noticed that behaviour.

So going slow'ish with the flaps down, would require forward trim subject to W&B loadings, so disconnecting the AP should only cause this IF the trim neutralises when you disconnect.

Any Cirrus pilots care to comment?




slander (plural slanders (http://en.wiktionary.org/wiki/slanders#English))

a false (http://en.wiktionary.org/wiki/false) or unsupported, malicious (http://en.wiktionary.org/wiki/malicious) statement (http://en.wiktionary.org/wiki/statement) (spoken or published), especially one which is injurious (http://en.wiktionary.org/wiki/injurious) to a person's reputation (http://en.wiktionary.org/wiki/reputation); the making of such a statement
The comments are neither malicious or unsupported.

[bodybag]

"I can't help but think that the pilot has not used good judgement or airmanship on this flight"

If you can determine this from the report that you have referenced then you are being very creative.. otherwise it is 'unsupported'
'ESPECIALLY one (comment) which is injurious to persons reputation'

You may not know this person but that does not change the fact that they have a reputation.

[duncan_g]

Does anyone know if this aircraft was equipped with the Garmin or Stec autopilot? I would suspect the Garmin... but nevertheless, I think the below is applicable to each type.

The flight manual (http://www.safety.airshareselite.com/images/13772-003_SR22T_POH_thru_Orig.pdf) says:
Pitch trim is provided by adjusting the neutral position of the compression spring cartridge in the elevator control system by means of an electric motor. It is possible to easily override full trim or autopilot inputs by using normal control inputs

An electric motor changes the neutral position of the spring cartridge attached to the elevator control horn.

Elevator (pitch) trim operates on 28 VDC supplied through the 2-amp PITCH TRIM circuit breaker on ESS BUS 2.


The autopilot interfaces the pitch trim system via a pitch trim adapter (also powered via the PITCH TRIM CB), allowing the AP to drive pitch trim. A failure of this module could cause trim runaway as described, which is not solved by hitting the AP disconnect. By selecting approach flap, the autopilot could have been attempting to maintain VS and subsequently feeding in pitch servo, meanwhile commanding forward pitch trim. However if the forward pitch trim function had failed, this would result in a pitch up when the AP was disconnected.

I also see that the autopilot minimum use height on approach is 400' AGL, so it would seem sensible that this pilot was practicing its use in VMC, prior to finding himself in that situation while in the soup....

[duncan_g]

would be interested if anyone thinks I got the wrong end of the stick with my previous post :}

[27/09]

Hmmmm, I don't think it was an electrical event at all.

The autopilot was disconnected before it had finished re trimming the aircraft after the flaps were lowered and the aircraft was still trimmed so as to require forward pressure on the control column to maintain the correct nose attitude.

[VH-XXX]

Yeah, but..... After the flight the trim was all the way back so pressing forward during the go-around means it shouldn't have been back there...? That would suggest that the trim actually stopped working completely and the pilot was unable to push it forward until the electrical system was shut down and restarted. Either way, a moderately interesting event.

[27/09]

After the flight the trim was all the way back so pressing forward during the go-around means it shouldn't have been back there..

Where did this information come from?

Also pressing forward on what? The control column or the electric trim switch? May take is he was pressing forward on the control column not the trim switch.

In the narrative you posted in your first post

He attempted to resolve the problem by pressing and holding the autopilot disconnect switch (AP DISC), however, this had no effect.


If the aircraft was requiring nose down trim when the autopilot was disconnected, i.e. needing forward control column pressure, pressing the AP DISC isn't going to make any difference. You need to manually trim the aircraft and probably need to do that with the manual trim wheel (rather than the electric trim), depending on how the electric trim is deactivated.

Did he in fact ever re trim the aircraft properly but instead fly the aircraft back around the circuit using a strong forward pressure, hence the trim still aft after landing as you suggest.

[duncan_g]

need to do that with the manual trim wheel (rather than the electric trim)

no manual trim on this aircraft - only electric trim :eek:

[27/09]

no manual trim on this aircraft - only electric trim

Therefore one must assume it is possible to fly the aircraft with the trim at one end of the range without being able to re trim in the event of an electric trim fail?

Can the trim be disconnected in the case of a trim run away?

I still contend the aircraft was out of trim at autopilot disconnect requiring control column input from the pilot to maintain attitude.

Did this pilot assume trim runway and not bother bother trying to re trim?

[duncan_g]

I see what you are getting at 27/09.

I wonder if it is possible to accidentally grab the trim switch whilst thinking you are hitting (and holding) AP DISC..

I've only flown an older SR20 and remember finding the trim switch awkward...


This photo is from a slightly different model, but I think the yoke switch layout would be the same. The AP DISC switch is above the trim hat.

http://www.pcaviator.com.au/store/images/D/Carenado-SR22-GTSx-HD-FSXP3D-PCAviator2.jpg

[VH-XXX]

I wonder if it is possible to accidentally grab the trim switch whilst thinking you are hitting (and holding) AP DISC..

Based on my Cirrus time, yes this is absolutely a possibility, as they are the same button, BUT... you clearly know that you have disconnected the autopilot because a very loud series of beeps comes through your headset, probably 8? from memory, but if you press and hold the AP disconnect, you only get 1 beep. It would be possible to have several goes at pressing the button whilst pushing back the trim, however you'd simply push it forward afterwards.

It would be really good to know how experienced the pilot was in this aircraft type so we could better understand what might have happened.

http://cdn.avweb.com/media/newspics/325/CIRRUS_AP_DISCONNECT.JPG