New ACI episode.

[Garfly]

This one, on the crash of a Twin Otter - Air Moorea Flt 1121 - is closer to home for our type of operations.

 

https://www.youtube.com/watch?v=sr_FtoVXyac

 

 

[facthunter]

Illustrates the need for two methods of pitch control. Many can fly on pitch trim in the event of primary pitch control failure. A necessary feature of a good design, even a U/L. I am a bit surprised the Otter didn't allow this. Someone endorsed may tell us. Nev

 

 

[Garfly]

Nev, according to the full BEA report, yes, the trim wheel might well have saved the day but there wasn't time for him to work that out and try it.

 

Actually, I was wondering what the effect might be in a typical ultralight of an elevator cable/linkage failing in flight.

 

Let's say for both cases; where there is and is not an independent trim pitch control.

 

Is it likely that the elevator would just align with the slipstream or, rather, tend to snap up … or down? Or go into flutter mode? Or, heaven forfend, all the above, in turns!?

 

And what, then, are the range of possible pitch outcomes. (Okay, we now know one.)

 

And is it likely (let's assume 4,000' rather than 400' ) that we might re-establish a degree of pitch control by way of flaps and throttle?

 

Good if we could be better prepared for such a fail than this poor guy was.

 

By the way, the full BEA report on the Air Moorea crash is available here.

 

It's a great read but it's an almost 30Mb download so here's a screen shot of the bit related to your question.

 

http://www.bea.aero/docspa/2007/f-qi070809.en/pdf/f-qi070809.en.pdf

 

 

 

[Old Koreelah]

Thanks for the link, Garfly. A sobering story. Cables are an easy way to link controls, but they need maintenance. There have been recent corrosion issues with swaged SS cables, with replacement required. Not an easy job. Another big worry is tension. Too loose and you invite flutter; too tight and you damage pulleys and anchor points.

 

 

[Old Koreelah]

I hear that the Qantas A-380 engine failure is the subject of a doco after 60 Minutes next Sunday.

 

 

[Neil_S]

Very interesting documentary. Once again a combination of issues - the wear caused by friction AND the strain caused by jet blast.

 

Thought-provoking stuff.

 

Thanks for posting, Garfly.

 

Cheers

 

Neil

 

 

[Garfly]

One of the interesting recommendations of the French investigation was that training in the handling of a control system failure ought to be introduced into both the CPL and the PPL syllabus.

 

Probably all the more recommendable for us in our very lights.

 

Old Koreelah wrote:

 

"Cables are an easy way to link controls, but they need maintenance. There have been recent corrosion issues with swaged SS cables, with replacement required."

 

Yeah, subtle metallurgical faults in small easily overlooked parts can so easily turn nasty on us.

 

Just last year, for example, BestOff Aircraft Australia urgently warned the SkyRanger community that a bad batch of Stainless might have found its way into some imported kit shackles and elevator joiners and that they all should be closely checked before further flight.

 

http://fr.scribd.com/doc/150567880/Service-Bulletin-2013-02-GB

 

The Service Bulletin includes this little video suggesting a handy little test by magnet:

 

 

 

Paul Dewhurst, of Flylight, the UK SkyRanger importer posted this about it:

 

"As I understand it, it isn't directly a corrosion issue. - although a secondary give away is that the material will corrode gently. The problem according to Aeros is that the problem material changes its crystalline structure when bended or welded, which weakens it, and it weakens further over time when subjected to a steady load.

 

As for the merits of 4130 or similar steel, its not a strength issue here - as designed it is well over strong.

 

And for steel corrosion is a perennial problem, and hence needs careful treatment, and coatings, which can then hide any cracks and makes inspection more difficult, compared to stainless steel. Also if the coating isn't done well rust forms underneath, and any scratches that break the surface also let the rust form. So much higher maintenance and less inspectability than stainless. Lots of problems with older aircraft with steel fuselages and wing struts rotting from the inside out in our small wet island home

 

Connector parts like shackles that bear and move in contact to the cable thimbles are better off in stainless than steel for the rust issue too.

 

So for home maintenance type little planes the wysiwyg properties of stainless are useful.

 

The problem here is not the properties of stainless steel per se, its that a batch of an unsuitable grade was erroneously supplied.

 

Regards

 

Paul Dewhurst

 

Flylight Airsports ltd "

 

A video on the subject - how to go about checking and testing the suspect parts in situ - was posted by a french SkyRanger guy:

 

https://www.youtube.com/watch?v=_jPYY1qk6sI

 

We don't need to understand the French language to know what he's on about. Thanks to the Norman Conquest, we can easily get the drift, especially of his conclusion: "You must be extremely vigilant. You can just imagine the consequences!"

 

And more vividly now, after Terror in Paradise.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[facthunter]

Garfly, Thanks. The control should not snap to any limit, and if mass balanced should not flutter in any aircraft designed properly. Loss of pitch control is the most serious of all. Perhaps the use of beta range might have helped in the Otter but would have been tricky. Staying in trim at all times might be a good idea. I have had elevator failure in a largish plane and due to the trim system it was a total non event. I would not own a plane that doesn't have a duplicated control capability in the pitch mode, though I have flown many that don't I check things well before flight. Some trim systems are only a spring and if the main system malfunctions , the trim does too. Nev

 

 

[Garfly]

Nev, there's one thing that caught my eye in that accident report that you might shed some more light on :

 

"2.3.1. Management of the malfunction

 

 

 

The malfunction occurred at the top of the initial climb, passing through 350 feet, when the pilot retracted the flaps then adjusted the engine parameters. While he was adjusting the parameters with his right hand, he was holding the wheel with his left hand while pulling back to counter the pitch-down moment and stabilise the airplane on its flight path. It was only at the end of this process, which lasted nine seconds, that he would adjust the trim. It should be noted that the pilot was applying the procedure recommended by the manufacturer; the operator had reversed the sequence."

 

What it seems to be saying is that this pilot's technique - the one recommended by the maker - was to forcefully hold the stick back against pitch change - due flaps up - whilst busy adjusting power for the climb. I think they're saying that this typically took fully nine seconds. (The report's precision seems to have lost something in translation.) Now, presumably, the other method - the one in the ops manual? - was to take some load off the elevator with trim before attending further to the thrust. Is that what they mean? As it happens, I suppose, had the pilot done it that way on the day, the flight may well have ended normally 8 minutes later. (Albeit, just postponing the inevitable.)

 

Anyway, are these alternative methods a point of debate in the industry? What's the thinking behind them?

 

I was also wondering if the preferred method in the DHC-6 might relate to the positioning of the controls. I believe the trim wheel is right down by the right side of the captain's seat while the thrust levers are way up in the ceiling.

 

But, in any case, Nev, I'm sure there'd be a lot of us here more than keen to hear the yarn about your own elevator failure in an airliner. You say it was a non-event … but how did you manage the approach and landing? They must have had the equipment standing by for you, at least, no?

 

P.S: While we're on about the old Twotter … herewith, shots of one in and out of St. Barth (where, famously, the final approach slope and the slope on final approach are pretty much the same thing ;-)

 

 

 

[Garfly]

Actually I've discovered that there's a good essay by Phil Stonebanks on control system failure on this very site.

 

It's in Tutorials (Pilot Tools tab) in the Decreasing your Exposure to Risk section:

 

http://www.recreationalflying.com/tutorials/emergencies/stonebanks.html

 

And relevant others here:

 

http://www.casa.gov.au/wcmswr/_assets/main/airworth/awb/27/011.pdf

 

http://www.pilotoutlook.com/airplane_flying/loss_of_elevator_control