They sent me a letter regarding a recent disturbing incident of a UK based Twister.
The letter reads:
I am writing to let you know that there has been a serious in-flight control failure involving a Silence Twister and to explain, as far as I'm able, what happened and what you need to do during your pre-flight inspection to ensure that a similar event doesn’t happen again.
The investigation is at a very early stage and discussions haven’t taken place between the aircraft’s kit manufacturer or the UK agent so we’re not sure whether it would be possible for this event to be repeated on other machines; nonetheless, we feel that it is important to act early by letting you know about the incident... ‘just in case’.
The aircraft had been de-rigged to conduct minor maintenance and was subsequently re-rigged by the maintainer for the owner; both the LAA Inspector and the owner conducted a thorough pre-flight inspection, including normal post-rig control checks.
During the post-maintenance test flight, conducted by the owner, an experienced pilot but fairly new to the type, a loud bang was heard and the aircraft simultaneously yawed and rolled to the right.
The pilot was able, just, to keep the aircraft in a relatively level attitude using full opposite rudder and nearly full opposite aileron.
The pilot noted that the aircraft’s rate of decent was ‘quite high’ and application of power didn’t appear to change this very much.
The pilot was wearing a parachute and considered evacuating the aircraft but felt that he was too low for this to be done with any real confidence; he therefore selected a field and conducted a forced landing.
During the subsequent landing the aircraft’s undercarriage gave way and the underside of the aircraft, engine cowling and propellor suffered damage. The pilot was uninjured.
The pilot, having evacuated the aircraft, noted that the starboard tailplane and elevator assembly had rotated on its tubular main spar and was at 90 degrees to the normal airflow. (see pic below).
The aircraft was recovered back to its operating base by road and, on inspection, it was found that the wire securing pin (4800115) had left the side of the plastic guide tube before the end of the tube and hadn’t, therefore, been in a correct position to direct the pin into the securing spigot (480053) thus the tailplane was not mechanically secured to the fuselage before flight. (see pics below of the plastic guide tube and spigot).
It is highly recommended that before you next fly your aircraft that you ensure that the tailplanes are securely locked in place; this can be done by firmly trying to remove the tailplane, it would be appropriate, to be absolutely sure, to engage the services of an assistant to hold the fuselage in place during this process.
One owner has commented that he always checks the pin is actually passing through its plastic guide correctly before finally fitting the tailplane... sounds like good advice.
If, when you check your tailplane’s security, you do find anything amiss please let me know as soon as you are able.
Thank you for your attention to this matter.
Yours Faithfully
Malcolm McBride
Airworthiness Engineer.
LAA
malcolm.mcbride@laa.co.uk
After reading this I sent Malcolm a reply with some pics from this blog of the construction of the securing wire (pin).
See my Week 96 post - to refresh your memory.
I explained the use of a metal bush and how it makes the system flawless - as copied from Pete Wells.
He has since replied back with another letter and says:
"I think this failure was caused because there was a lack of familiarity with the rigging system although the build quality of this particular item wasn't very good".
I have to agree with Malcolm - if you look at the photos below you can see that the plastic guide tube is not completely encapsulated in flock (as per the instructions in the manual). This allowed the plastic guide to move around a bit and also wear out it seems as the wire was able to leave the tube before the end.
Of course the overall message is - don't let someone else rig your plane!
Fly safe everyone.
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| Tailplane rotated 90 degrees |
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