Summar so far - TST T/Plane Failure

[Guest TOSGcentral]

The in-flight failure of Robin’s TST tailplane will probably have caused some anxiety to readers of these forums.

 

 

Without pre-empting any inquiry results I want to give you a round-up of the situation as it stands from TOSG’s perspective and establish a responsible sense of proportion.

 

 

HISTORICAL. To my knowledge there have only been three fatalities in Thrusters. A factory test pilot was apparently killed in one on developmental test flying. I have no details on this. The second was the double fatal at Lethbridge in 2000 where the instructor and student both died.

 

 

There have been only two in-flight Thruster structural failures of any major structure. The first was the Lethbridge fatal when a lift strut attachment bracket to the leading edge wing spar separated and the wing consequently failed. TOSG classes this as a ‘one-off’ incident and believes the Coronial Inquest failed to establish full data that was in fact available.

 

 

The second has been this TST failure and a summary of what has been done follows below. Despite one half of the tailplane hanging vertically downwards some semblance of control could be maintained and while the aircraft arrived on the ground heavily, enough to distort the seat carry rails and do some other minor damage, neither occupant suffered injury.

 

 

To put this in context: Well over a 1000 Thrusters have been built and many of them have been engaged in the hardest form of airframe flying – repeated ab-initio training. They have not been falling apart! The Thruster is immensely strong and is virtually a roll over cage with wings!

 

 

Certainly people have been injured in them but the causes of this are well known. They group into two prime sectors – (a) engine failures near the ground on take-off where a heavily laden aircraft has been flown too slowly to maximise climb rate, went to (or close to) the stall and had insufficient height to regain control energy before bellying in. (b) loss of control on landing and this primarily revolves around inadequate basic flying training and understanding of tail-dragger dynamics coupled with some weird myth that tail draggers have to be three point landed all the time!

 

 

THE YARRAM TST FAILURE. A great deal of situation establishment work has been done by TOSG (who has a number of similar tail units to examine) plus the able support of Hihosland (David Hill) of these forums whose access to the damaged unit has been invaluable.

 

 

Four different tail units were measured, stripped and crack tested by TOSG with zero indication of any damage.

 

 

What we can definitely now say is that the failed tail unit appears to be factory standard. The internal sleeve/plug under the weld is of the correct geometry. The various components are of the correct material, wall thickness, o.d. and i.d. The weld itself visually appears to be of good quality.

 

 

The male insert tube of the tailplane halves interconnection appears to have seen better days and is a bit ‘ragged’ despite being in a well protected environment when the aircraft is fully rigged. I would presume that this is where any subsequent investigation would focus to seek indications of progressive failure and hence indicate what external elements could have permitted a cyclic fatigue bending load on the joint that failed.

 

 

There are two prime causes which individually, or combined, could have caused bending loads on the joint.

 

 

These are the ‘nylon’ main bushes that hold the front and rear tailplane spars and the tension of the upper and lower tailplane bracing (or flying) wires.

 

 

THE RAAus A.D. #280308-1. Chris Kiehn, the RAAus Tech Manager, has produced this A.D. and it is a totally barricading procedure to keep anyone from harm – without being onerous.

 

 

The A.D. requires that, prior to next flight, you paint strip the suspect area and dye penetrant crack test it. From reports on these forums a crack test kit is expensive. I am amazed as mine cost me only about $50 but that was a few years ago and they do last a long time if used correctly.

 

 

This will show you if you have a resident fatigue crack there that will be growing. It has (or should have) nothing to do with the way you personally maintain and rig your aircraft – it may have a lot to do with how previous owners have treated it. No matter what the cost of a crack test kit it is far cheaper than a funeral – or the lower price you will get trying to flog off an untested aircraft! You may not be as lucky or skilled as Robin was in getting it down!

 

 

Next are the bushes that the two halves of the tail unit slide through on the boom and these are the main stability of the union between the two halves. They are normally very tight to get in and the tailplane will sit there (once in) unsupported. If there is play in these bushes then you are getting bending loads at the failure area.

 

 

Next is the bracing (flying) wires to the tail unit. No matter how tight the main bushes may be you will get vertical cantilever forces on the tailplanes from either elevator and/or landing shocks (not to mention taxiing over rough paddocks). These equate to bending loads at the failure point that will not break it but will contribute to progressively fatiguing it until it has no option but to break!

 

 

The Thruster factory never came out with cable tensions so TOSG did! Using known safe tensions a very simple measuring procedure was designed that is inexpensive and easy. Do not be dismayed by the electronic gadget that Lois is using on the A.D. photograph – I used some sound stuff to get real accurate readings but you can do it safely with a simple spring balance scale.

 

 

The RAAus A.D. covers all this. Just go through it and you will be totally safe from a repeat failure and have peace of mind in flying your aircraft. I did some test flying on a T300 this morning. I did not climb into the cockpit in fear and trembling that a tail or wing was going to come off – I did not actually even think about it! My aircraft is thoroughly checked out (It was used for the data to protect yourselves) so I just got on with the flying and enjoyed myself.

 

 

Aye

 

 

Tony

 

 

[TechMan]

Hi Tony,

 

The BMAA reports that the Thruster UK facility currently has their approval suspended untill the CAA are happy with the change in personnel and facilities after it was sold by Gordon Pill.

 

Maybe that is why they never replied to my questions regarding cable tensions ( for which I have a read receipt), or maybe they just don't have a figure and it was always just done with a calibrated finger and the twang method.

 

Chris

 

 

[Guest TOSGcentral]

Thanks for the update Chris.

 

The UK situation is a bit of a concern - especially to the Brits. The new factory was scheduled for a CAA inspection in March but the CAA postponed for some reason.

 

Have to wait and see

 

Tony

 

 

[Guest TOSGcentral]

Final Watts Bridge Thruster Inspection.

 

Today I completed the inspection on the last of the Watts Bridge Thrusters (A TST as it happens) I.A.W. RAAus AD 280308-1. No defects were found on the aircraft and it was signed off for normal flying.

 

This is the fifth aircraft inspected at Watts under the AD and none have shown defects.

 

While this is good news it does not mean that other defective aircraft may be out in the field so please comply with the AD and check your own machine out.

 

Aye

 

Tony