Inadvertent pitch trimmer input causing structural failure

[Genghis the Engineer]

There's a lot of talk in the little aeroplane world at the moment specifically about this accident and report...

https://assets.publishing.service.gov.uk/media/5a5f150c40f0b65266e77b87/EV-97_Teameurostar_UK_Eurostar_G-GARB_02-18.pdf


Not unusually for little aeroplane accidents, the paucity of data means that investigators can at best just offer some indications of possible cause, and there's no certainty to this. However, the suggested "least unlikely" cause is that there was an inadvertent sudden input on a mechanical pitch trim lever located between the seats.

The argument if not overwhelming, is strong. AAIB show that a 2.7cm trim lever movement could create 2g (delta 1g), and there was 7.2cm of movement left. A very crude extrapolation says that therefore full sudden back-trimmer might have caused a delta-g of about 3.4g from 1g. Limit load is 4g, and you'd expect a 1.5 safety factor.

So, in an aeroplane that you'd expect to suffer catastrophic structural damage at 6g or thereabouts, this sudden pitch trimmer input might have caused around 4.4g. And of course, all of this is very much in the "gross extrapolation" area, so a structural failure in this way certainly doesn't look impossible (nor does the possibility that this is a complete red herring of course).


So - question. has anybody seen anything of this nature in certification / testing / other types. The potential for a problem does at least seem to be demonstrated by this AAIB report. Also the fact that the vast majority of aeroplanes with mechanical trim systems use wheels - where it's physically impossible (usually) to make sudden step trimmer inputs, suggest that somebody has thought along these lines before.

Has anybody any prior experience of the issue, or any useful thoughts they care to share?

G

[gordon field]

G. I am very wary of electric trim systems in light aircraft and always check them prior to flight for full and free movement over their full range and noting the movement of control surfaces and importantly that the cut out switch works. On more sophisticated aircraft they can usually be cut out by switching off by the avionics master switch and as a last resort by switching off the Master switch.

[Genghis the Engineer]

Indeed - I used to have a (share in a) PA28-161 with an electric trimmer; it was placarded not to be be switched on below 1000ft IIRC, for just such reasons. I can also recall a Tucano loss in flight that was partially attributed to an electric pitch trim runaway. What I'm less sure of is whether there's anything out there in the way of formal reporting on the topic? I don't think I've ever seen anything at all about the airworthiness implications of lever versus wheel for a mechanical trim system.

G

[LOMCEVAK]

Genghis,

The Tucano accident was not a trim runaway but an inadvertent full nose down trim input during an inverted spin. I have actually done this during the initial spinning trials and the aeroplane was easily recoverable. As you say, there were other factors.

L

[Genghis the Engineer]

Thanks for the correction L, I thought it had been a runaway, but yes, knew about the spin and some other factors.

G

[NutLoose]

There was the Cessna 182 accident that sadly killed those on board at Leicester, they believe pilot accidentally operated the autopilot button when advancing the throttle or adjusting the transponder, thus allowing the trim to run all one way. This was fixed with audible warnings and a requirement to hold the button for a given time period to turn it on and I had to replace the unit on our aircraft..

The report is here

https://assets.publishing.service.gov.uk/media/5422ec04e5274a13170000d5/dft_avsafety_pdf_501522.pdf

[anchorhold]

From memory the C182 accident at Leicester was electric trim runaway, I'm not sure if that happened independantly of the autopilot. But it had been previously reported by another pilot.But in that case it would not have been instantanious, but also at the time I was told that under certification that the pilots should of had the strength to still control the aircraft. Sadly at the time neither pilot would have been trained for trim runaway, having trained on C152s.

[Genghis the Engineer]

A somewhat different point - but how many pilots of anything are trained for a pitch runaway? I've flown several types with electric trims, and don't recall anybody ever teaching me about strategies for dealing with that, nor much by way of advice in the POH. Maybe it's just supposed to be instinctive?

G

[LOMCEVAK]

With respect to pitch trim runaway training, my Tucano currency requirements call for a landing with full nose up or nose down trim once a year. I have done this in the aeroplane but we normally do this in the sim. I am currently doing a PC21 type rating, and flying at medium level and selecting full trim in each axis in turn and varying the IAS to experience the resulting control forces is part of the syllabus.

In SEP types, perhaps this should be part of differences training.

Rgds

L

[Genghis the Engineer]

In SEP types, perhaps this should be part of differences training.
At least in EASAland and FAAland, there's no mandatory training on type within the SEP class, ditto (as the EV97 in my first post) within the microlight class. Differences training is only for "coarse" differences such as nosewheel .v. tailwheel, VP props, retractable gear, and so-on within SEP and not even that within microlight. I can't see there ever being any appetite to change that.

So where a difference can be made, primarily I think that needs to be in certification, and the composition of the POH. The latter can of course then give explicit advice about risks and how to manage them, and consciencious pilots may then make use of that advice.

My interest here, particularly, is in certification as I think that perhaps there are some questions that should be addressed in certification with regard to electric trimmers, that aren't being addressed as a matter of course.

G

[anchorhold]

Ghenghis.... until the the C182 trim runaway accident, I do not think many instructors were teaching trim runaway or issues with the autopilot. Those who read the C182 accident report probably do.

In terms of certification, I agree that it should not be possible to 'snag' the trim from a certication point, yes trim runaway should be in the POH and it should be made clear that through brute strength you can maintain attitude. I think in the case of the C182 accident had the two pilots (husband and wife) had trim runaway in their mindset they would have had a better chance, likewise they could have thrown the master switch. On that point I have come across some pilots on renewals think turn off the master switch will stop the engine! This problem with trim had previously occured, but the engineer could find any fault. I wonder if this flagged up anything with the C182 pilots in terms of a discussed gameplan if this happened again.

I am not sure the CAA or EASA should mandate that we train or examine in respect eletric trim problems, but the good examiners and instructors will go the extra mile as they did when I did my SEP and MEP training.

It is interesting that the RAF train for out of trim onditions on the Tucano, my concern on a civil type (particularly microlights) is the stress that it might put on the trim tab in the short or longer term. I wonder if the Tucano has a history of trim runaway.

[happybiker]

My interest here, particularly, is in certification as I think that perhaps there are some questions that should be addressed in certification with regard to electric trimmers, that aren't being addressed as a matter of course.

G

Electric trim certification is addressed in CS 23 including the runaway conditions and associated disconnect switches. Any trim interrupt/disconnect switch is considered a safety system control and the function and operation should be described in the POH or AFM. Electric trim systems are not referred to in Section S or CS VLA.

[Genghis the Engineer]

It is, and thanks for pointing that out, but briefly - here's the wording from CS.23.677 (pretty much the same wording is in FAR23)...

(d) It must be demonstrated that the aeroplane
is safely controllable and that the pilot can perform
all the manoeuvres and operations necessary to
effect a safe landing following any probable
powered trim system runaway that reasonably
might be expected in service, allowing for
appropriate time delay after pilot recognition of
the trim system runaway. The demonstration must
be conducted at the critical aeroplane weights and
centre of gravity positions.

Checking, an interesting point is that not only is this wording not in VLA or Section S (nor 22 or the old Section K), it's not in part 25 either.

Even here, there's nothing about immediate behaviour in the event of a runaway - of course a good certification team will look into this anyhow, but it's a bit of a hole, and in regards of sudden manual deflection of a mechanical system, as in the case of the EV97 fatal, there seems no guidance anywhere, for any class of aeroplane.

G

[happybiker]

Genghis

It could be debated that any trim control that can result in sudden manual deflection with catastrophic effects would not be in compliance with CS23.677 (a) Proper precautions must be taken to prevent inadvertent, improper, or abrupt trim tab operation.

The certification criteria to be met following electric trim runaway is expanded upon in CS 23 Book 2, AMC, Flight Test Guide. Chapter 3 No 139. Does this provide the information you are looking for?

Re BCAR Section K, it is stretching my memory somewhat but there were a number of amendments published as " Blue Papers" by the CAA which I do not believe were consolidated into the main body of Section K. I do recall that there was information published by CAA on this subject as checking compliance for trim disconnect was usually part of the process for issue C of A on imported aircraft.

[LOMCEVAK]

anchorhold,

The Tucano does not have an abnormal history of trim runaways and I cannot remember why this training requirement was introduced. It has a 'Trim Isolate' switch which cuts out the trims in all 3 axes simultaneously. I have had an intermittent pitch trim runaway in one whereby it ran away but I was able to then trim back. After a couple of minutes it did it again so I retrimmed to what I thought were the trim settings required for the approach and selected Isolate. The landing was uneventful.

Rgds

L.