How an L2 saved my life .....

[Guest rocketdriver]

Well, I think he did! ......

 

On Friday we had a condition report done on one of the aircraft we recently imported. Our intention was, and still is, to fly the thing in a month or so, and both Ray (my partner in crime) and myself felt the time was right to start the formal preregistration activities.

 

We had already extensively surveyed the aeroplane (we thought), had found and fixed several things and I guess we felt that she was pretty right for flight .. just a few little thing left to do ......

 

So there we were, the inspector poring all over the aircraft, me thinking “don’t find anything ...its more work and anyway, everything is ok .... but then, if there is something wrong, please tell me so I don’t crash on first take off ....” and Mick, politely and professionally ignoring the owners’ emotion, carefully surveying the aircraft. It really is a two edged sword ... you don’t want anything to be wrong, but if there is, you want it pointed out to you ....... but please, don’t find anything wrong! .....

 

Anyway, amongst other things , (not many but enough!), Mick said we should look carefully at the elevator trim tab, in particular the connection between the trim tab and the trim tab control cable .... The steel clevis looked quite rusty. So today I took it all apart ....

 

The rusty bit was just rusty ....cleaned it up and painted it with kill rust .....

 

BUT. The control horn, made of aluminium, was a bit corroded ..didn’t look very serious, but sandpaper wasn’t making much impression.... so I got a small screwdriver out to scrape away the worst of the “surface” corrosion, and, to my HORROR ..... see the picture, photo1. Photo 2 is what it is supposed to look like ... The pin attaching the cable tom the control horn would have just pulled straight through..... This was just a gentle scraping with a screwdriver on something that looked at first (inexperienced) glance to be perfectly serviceable ..... Never seen exfoliation before. I was able with very little effort to separate

 

layers of aluminium from something that looked for all the world like a normal piece of aluminium sheet .... like peeling off layers of plywood .... Heard of it before, but never seen it .....

 

Hate to think what might have happened if I had gone flying with it like that.

 

THANK YOU MICK

 

Needless to say, a new control horn is in process of being assembled to the trim tab ....

 

 

 

 

[Guest Maj Millard]

Good story Rocketdriver, Chances are the corrosion was initiated by the steel pin, also referred to as disimiliar metal corrosion. And chances are your L2 has a lot of experience, and has seen this type of problem many times before.

 

The thing with an experienced person is they know where to look, so it's really money well spent.Good find Mick !....................................Maj...

 

 

[Guest Andys@coffs]

My trade wasnt framie so not too sure but the thing that would concern me is, if there was corrosion in that location to that extent is it indicative that a more thorough inspection for corrosion is required elsewhere?

 

I understand the dissimilar metals point that major made, but wonder if the conditions were right for corrosion here whether that would hold true elsewhere as well...

 

Andy

 

 

[Gribble]

One question I have is, had it given way mid flight what would the resulting situation be? Elevator flutter due to an oscillating trim tab? Would the tab have been sucked into the low pressure side of the elevator causing a sudden up/down movement? Would it have been something easially controlable via the stick?

 

Definately a good thing you didnt have to find out the hard way of course.

 

 

[Guest rocketdriver]

Hi Andy ... big thing for me is how inconsequential the slightly rusty clevis looked and how the real problem was completely hidden behind the fitting .... And , yes, there are a couple of other corrosion type issues, but the a/c is 30 years old, has not done a lot in the last 10 years and has been sitting in a hot / cold Canadian hanger for most of that time I suspect ...

 

 

[fly_tornado]

pull test every wire if you haven't done it yet. bound to be a few loose connectors in your plane waiting to catch you out.

 

 

[Guest rocketdriver]

Hi Gribble

 

Had it given away mid flight, I don't suppose the pilot would be here to talk about it, and nor would there be many bits in one spot on the ground ....

 

 

[Deskpilot]

Hi GribbleHad it given away mid flight, I don't suppose the pilot would be here to talk about it, and nor would there be many bits in one spot on the ground ....

When I was in the RAF, we put up a 19 a/c formation flight for some reason or other. Anyway, as they streamed in to land, one blew a tyre and was stranded on the runway, all the others diverted to local RAF fields. When ready, they flew back, at low level (like 1000ft). One aircraft, a T4 trainer (these were Lightnings) suddenly started a slow roll that could not be countered. roll after roll, at low level. It was a case of get out or get dead, but, eject at the wrong time and your dead anyway because they were so low. The 2 pilots waited until they were up-side down and then ejected. By the time the canopy and seats fired, they were right side up and both lived to tell the tale.

 

Reason for the accident. During ground testing of a newly serviced/fitted aileron actuator, it was necessary to put a connection bolt in backwards (don't ask why) which meant the nut couldn't be fitted. The technician then forgot to reverse the bolt and secure it correctly. It simply fell out during flight. I wonder what would have happened if it fell out during the formation flight. Probably the biggest bomb burst ever. Needless to say, the techie concerned, was severely reprimanded.

 

 

[Spin]

Sounds like money well spent RD and proof that a new set of eyes on a situation are often able to see what someone intimately involved cannot.

 

As to the results of a break in flight - hmm wouldn't want to be in the aircraft when that happened! My closest experience of this was in a Zenair 601 where someone apparently hit the trim tab, which protruded from the main control surface in this installation and in so doing bent the thin push-pull rod actuating the tab. It seemed to have been semi straightened and the bent portion pushed into the end of the cable sleeve. The fault was missed on preflight, trim repositioned pre-takeoff, which would then have exposed the bent portion and the thin rod duly let go the first time trim was altered in flight. This set up a terrific buzz through the stick and to a lesser extent the airframe, the frequency of which altered part way through the remainder of the circuit. On landing we found that the trim tab had partially torn away under the force of the rapid oscillation, hence the change in the buzz through the controls. I'd like to think the tab would merely have torn away completely had the aircraft been further from the field, but I'd not like to bet my life on it - flutter in an airborne context has a negative effect on my bowels, something I discovered in a paraglider many years ago.

 

(And yes I am aware of the difference between fluttering fabric and the nasty type)

 

 

[Guest Maj Millard]

Good point [email protected] thought the same thing after my prev post. If it's in one spot there certainly could be more.................Maj...

 

 

[dazza 38]

Galvanic Corrosion ( also known as Dissimilar Corrosion). Needs 3 things for it to happen. A) Electrochemically dissimilar metals must be present eg- Steel pin touching aluminium. B)These metals must be electrical contact Ie- touching each other. C) The metals must be exposed to a electrolight Ie- Moisture If you break one of the above, you can avoid the problem.

 

Example- A barrier between the two parts- eg- paint, grease etc. preventing both metals touching each other.As one metal is more Noble than the other one.

 

Possibly in the above case- coating the pin in paint, and keeping a eye on it. Cheers

 

Ps- This is for general information.

 

 

[Thruster87]

Exfoliation is a special type of intergranular corrosion that occurs on the elongated grain boundaries. The corrosion product that forms has a greater volume than the volume of the parent metal. The increased volume forces the layers apart, and causes the metal to exfoliate or delaminate. However, in certain conditions, the grain boundaries can undergo marked localized attack while the rest of the material remains unaffected. The alloy disintegrates and loses its mechanical properties.

 

This type of corrosion is due either to the presence of impurities in the boundaries, or to local enrichment or depletion of one or more alloying elements. For example, small quantities of iron in aluminium or titanium (metals in which iron has a low solubility), segregate to the grain boundaries where they can induce intergranular corrosion. Certain precipitate phases (e.g. Mg5Al8, Mg2Si, MgZn2, MnAl6, etc.) are also known to cause or enhance intergranular attack of high strength aluminium alloys, particularly in chloride-rich media.

 

 

[eightyknots]

Congrats to Mick for picking this up in his aeroplane inspection!