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Guest TOSGcentral



FOREWORD: TOSG is NOT an Airworthiness Authority! But I will pass on potentially life threatening data as soon as I have them and have done some precautionary questioning.


The following was deduced from a talk with the pilot who managed to land a fatally crippled aircraft with only undercarriage damage and no injuries to himself or student.


SITUATION. One half of a TST tailplane failed in flight and rotated 90 degrees downwards. This gave severe stability and elevator control problems that had to be fixed (but were fixable by careful throttle use – ie thrust). There was insufficient elevator authority present for a normal landing upon the duty runway and a leg strut broke on the third bounce.


The occurrence happened at 500’ agl on downwind in the circuit and the aircraft was not being subjected to anything unusual in the way of stressing other than local turbulence that apparently was mild.


STRUCTURE. The TST has a welded chomolly space frame tailplane and elevator assembly. The design is common to the TST, TST L, T300 and T500.


The main tailplane frames, at the leading edge, have a male tube welded onto them that are inserted and make union within the boom. The union is confirmed by a transverse bolt that locks them together outside the boom.


The rear of the tailplane, and consequently the attachment with the elevator, is most robust and comprises a large diameter pivot pin that, via transverse bolt flanges, both locks the rear of the tailplanes together and also secures the elevator drives.


Up the front, the male section is potentially weaker as it is of smaller diameter than the tailplane main leading edge structure so is welded into position.


FAILURE. The failure occurred directly adjacent to the weld union of the male tube with the main side of the tailplane tube it was attached to. The images indicate a clean break which could be accumulated fatigue after heat treating from the welding. However, it is unlikely to have JUST broken and a fatigue crack, steadily growing, could have been present for some time.




RECOMMENDATIONS. I repeat that TOSG is not an Airworthiness Authority but I am going to say the following in all prudence:


All operators of TST, TST L. T300 and T500 Thrusters should, before next flight, dismount their tailplanes and critically crack test analyse the area between the male union and 2†down the shaft of the male tube for any cracking.


SUBSEQENT. All defects found should be reported promptly to RAAus Tech Office (and I would appreciate a personal notification).


The Thrusters are built like tanks and wildly over-engineered, so this one has me baffled. However let us go through it sensibly and see what emerges. I will be doing the three of the applicable types presently in my care tomorrow. But note that Gemini owners are not included in this advice as their structure is different.







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It is now weeks since this event and, if my information is correct, since notification was made to RAAus.


I have been seriously concerned that there has been no evidence of comment nor action from RAaus concerning this event.


One can only compliment Robin, the pilot/instructor, in managing to get his aircraft safely to the ground with nil injury to himself or his student. As a direct result of his efforts a wife has a husband and children still have a father. Had he mishandled that event there could well have been two more fatalities to add to our unfortunately long list of deceased pilots.


How many of us have actually sat down and thought through the consequences of a tail plane failure and what we are going to do about it.


Robin did.


Robin's degree of professional airmanship needs to be acknowledged.





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Guest TOSGcentral

I am in total agreement with David's comments - it was a skilfull and cool piece of flying on the part of the instructor Robin Sidebotham!


I am also profundly concerned and disgusted with the situation. Inclusive of the UK Thruster fleet over 400 aircraft are potentially affected by this apparent fatigue failure.


It is worth pointing out that this appears to be the first genuine Thruster airframe failure of any note and we need to find out why!


The double fatal at Lethbridge a few years back was a fatigue failure but the causes are understood and were a result of prior damage even though the Coroner did not uncover this.







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Guest brentc
However, it is unlikely to have JUST broken and a fatigue crack, steadily growing, could have been present for some time.

Surely diligent maintenance practices should have uncovered such a fatigue crack? If this kind of thing is going to happen and keep happening, we'll be heading down the LAME path in no time and push costs out of everyone's reach! 049_sad.gif.af5e5c0993af131d9c5bfe880fbbc2a0.gif



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Surely diligent maintenance practices should have uncovered such a fatigue crack? If this kind of thing is going to happen and keep happening, we'll be heading down the LAME path in no time and push costs out of everyone's reach! 049_sad.gif.af5e5c0993af131d9c5bfe880fbbc2a0.gif

The point in contention is should other Thruster owners have been advised officially and immediately of this occurance in order that their maintenance programme is based on best available information???





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Guest TOSGcentral

While I agree in principle Brent - that area does not actually concern me overly.


The failed item is quite a robust bit of gear and takes very little load relative to its size. It is also not scheduled for any of the scheduled maintenance inspections.


This would be an easy one to miss, particularly as the area is quite heavily painted and crack propogation could be happening under the paint. Probably one would simply not expect this sort of crack in that position - while you would certainly be watching for cracks on undercarriage leg bend points, wear on engine struts supports, or cracks in the stainless steel boom fittings that hold the cockpit front and back A frames and are under quite considerable forces from time to time.


This particular failure is most curious and it will be interesting to see what comes out of it.





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Guest TOSGcentral

OK – with the forums down I have still been busy. To save myself more time I am copying the bulk of an email sent just now to RAAus that rounds the situation up as it presently stands from TOSG’s perspective.



This is the action so far taken by TOSG.



  • An alert post was placed on the Thruster section of the Recreational Flying forums.
  • The UK Thruster Club forums were similarly notified.
  • They have informed BMAA and the UK Thruster factory.
  • Four tail units (2 x T500, 1 x T300 and the T100 Prototype) were gathered yesterday afternoon and crack tested by myself. I have a TST tail coming in today for similar inspection.
  • The situation has been considered and a provisional reason for what is an apparent fatigue failure is opined below.






The certified Thruster range of two seat trainers were modified from the 1986 all-alloy Gemini into the TST which incorporated considerable amounts of chromealloy for strengthening . This included the introduction of a welded chromealloy tail unit that has remained unchanged until the arrival of the UK T600 and Vision 600 models.



The models affected by this failure therefore encompass (Australia) any model of TST, T300, T500, T100 Prototype and the 19 series registered Flying Fox and Bilby. In UK the types affected are the 1987 TST, T300, and Super T300.



We are looking at over 400 aircraft that could potentially have this failure Chris!



Types not affected are the Glasshouse, Gemini X and any of the single seaters other than the T100. These are all alloy and have totally different tailplane fittings.






None of the aircraft so far tested have proven defective. My UK Thruster counterpart has a veritable fleet of the affected types under his airworthiness charge plus a stock of second hand units – he is testing the lot of them.






From the images that were promptly sent to me I deduce that this is a fatigue problem with a reasonably even and total metal separation along what would be the heat treated area line as would result from welding the male tailplane connection tube to the larger diameter main starboard tailplane panel leading edge.



It is unlikely that this fitting simply ‘snapped’ but would have been a progressive fatigue failure until it gave up!






On the pilot’s verbal report to myself the aircraft was in normal circuit at 500’ agl and was subjected to only mild turbulence. It was not being thrown around or otherwise subjected to unusual aerodynamic loads.






The area under examination is buried within the boom and the plastic bush. It is impossible to see unless you take the tail unit off the aircraft. It is highly unlikely that anyway much curiosity would be aroused as the fitting is itself so robust and apparently not subject to a great deal of bending loads as it is supported by stay wires top and bottom.






I have had less than 24 hours to think on this but would now opine the following:



Although the tailplane does not have much mass it is constantly subjected to vertical bending loads as a result of elevator use. If the aircraft is being flow with ‘slack’ tail unit bracing wires then it can move vertically.



This would provide considerable cantilever bending loads on the area that has failed – and continue doing so in a cyclic manner that engenders fatigue. Such movement would progressively wear the plastic mounting bush and begin escalating the problem.



I feel that the root cause of this occurrence (if you will excuse the pun) is not the aircraft’s structural strength but poor factory documentation. Until the T600 came along, which at least had a means of correctly instating elevator drive cable tensions, there was NO data on any cable tensions on the Thruster. So it has come down to what owners feel is reasonable to themselves!






The following is the full procedure that I have adopted:


At the moment you only have to be concerned with the male (starboard)


side of the union.




1. Dismount that side of the tailplane (you can leave the other one




2. Do this by first disconnecting the two tailplane halves (a clevis


pin and safety pin on the port side close to the boom)


3. Disconnect that side elevator from the transverse elevator drive


shaft and then reversing the velco gap sealers between elevator and


tailplane (ie pull the vecro union apart and force them to be smooth


side adjacent rather than sticky side which will limit sideways


movement that you are going to need).


4. Drive the inboard end of the elevtor off the drive shaft that runs


through the boom. Slide inwards to disconnect the out edge of the


elevator from the outboard fixed mounting pin. Put the elevator to


one side.


5. Disconnect the top and bottom tailplane bracing wires (both are


simple clevis pins and safety pins.


6. Pull the tailplane out of the boom such that it is free in your




7. Use either paint remover or suitable abrasives to clean the


smaller diameter tube about 1.5" inboard from the weld with the


tailplane leading edge until you are back to clean bare metal. This


will encompass the heat treated area from the original weld that the


fatigue crack appears to have run along.


8. Then apply the dye penetrant crack testing kit.






All owners of affected aircraft types should be immediately alerted to check the area of faliure prior to further flight.









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Guest Teenie2

Just a word of caution ,in step 7 above you mentioned removing the paint using paint stripper or abrasives.I have spent many years in A/C heavy maintenance and from my experience the best bet is stripper,as you use abrasives (mechanical or by hand)the edges of a fine crack have a tendency to merge or peen effectively making a very fine crack disappear.



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Guest TOSGcentral

Good one Teenie 2. Some of these inspections are likely to be a bit 'agricultural' so any tips such as these are most valuable.







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Thanks Tony for all you do and for the TOSG.


Tony knows about this incident because Tuesday I was doing some back ground checking on a Thruster I was looking to buy. I called Robin as he had knowledge of the plane I was looking at. Robin was very helpful and obviously knowledgeable and during the course of the conversation he told me about the incident with his Thruster.


I had been speaking to Tony from TOSG earlier that day as I'm trying my best to get the most information I possibly can about the aircraft type and even specifically individual plane history.


As I'm sure is the case for all of those who have purchased or wish to purchase an aircraft, you do all the homework you can to make the most informed decision you can.


As for Thrusters I am very grateful to Tony for providing such a thorough and available source of information regarding these aircraft.


The one thing that leaves me wondering and more than a little bit uncomfortable is the lack of incident information provided by the RA AUS, I'm new to all this so someone may need to manage my expectations.


Thanks again Tony for letting me pester you.


And I'm glad Robin lives to tell the Tail ;)







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Guest TOSGcentral

Further information for you.



Chris Kiehn has been back to me. The apparent silence from RAAus is due to them not having received Robin’s incident report that he sent in. I have asked Robin to send another one. Chris is back in the office today and will be chasing up the situation – so I expect we will be hearing something official later today rather than just my informal quacking.



Robin has given me more information on what actually happened.



There was no warning or apparent flight reason for the failure.



When the tailplanes separated the starboard one detached entirely from the boom at the leading edge and rotated downwards around the tailplane rear spar until it came up against the lower bracing wire. There it stayed and acted as a massive down elevator as well as an airbrake.



The other tailplane stayed more or less in position until the landing attempt was made when it too detached at the leading edge and rotated downwards as had done the first one.



The design of the 1987 onwards two seat Thrusters is such that the tailplane rear spar is held in position by the elevators being firmly bolted to the transverse elevator drive shaft that runs through the boom. So the tailplane rear spars cannot come out of the boom with elevators still connected and take an elevator with them.



The tail unit is braced top and bottom from the tailplane rear spar via wires and, while with the leading edge detached, the tailplane would be free to rotate around the rear spar but that spar would be constrained by the wires from folding upwards or downwards.



There was evidently just enough power of up elevator and up engine thrust to counter the ‘down elevator’ effect of the failed tailplane. A very great amount of power would anyway have been required to counter the massive amount of drag being produced.



It was a cool and great piece of flying on Robin’s part and certainly not something that I would wish to experience.



NOTE TO JIM: The photos did not come through mate. Can you try again my email is [email protected]



If anyone else has photos of the TST after it landed but prior to it being moved I would appreciate copies please.









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This is why it is so important that we send incident reports in to the RA-Aus and confirm that they have been received so something can be done about things. These forums are great to let people know about things but we do need to be sure of facts.


Well done Tony for getting in on this and I am sure Chris at the RA-Aus will now be able to do something when he gets the incident report



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Tony did say


""Chris Kiehn has been back to me. The apparent silence from RAAus is due to them not having received Robin’s incident report that he sent in. I have asked Robin to send another one.""




It seems my e-mail to a board member suffered the same 'gobbled by the gremilins' fate as Robin's notification.




Truly unfortunate.






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Guest TOSGcentral

The situation may actually be a bit worse than you paint it David, on additional information I received this morning.



Now let us try and get some oil on to calm potentially troubled waters! There are a number of factors to put in balance.



I personally do not want to see this turned into a political football – at least not at this stage. From our personal correspondence David you are well aware that I was extremely irate about what appeared to be very grave omission on the part of RAAus.



The prime concern is Flight Safety. I believe that has been achieved (or is well in the process of happening). TOSG reacted immediately it found out and within hours there was both a National and International alert made, photos of the failure, responsible and informative reporting of circumstances and a potential reason for the failure was given that is anyway a routine part of aircraft serviceability surveillance – or should be.



The fact this came from a member Support Group rather than waiting a year for an official report is neither here nor there – the information was got out promptly and could have saved lives and movement image.



The other important factor in my opinion is that Flight Safety is entirely reliant on reporting. It is essential that the members have confidence that RAAus do, and will, act promptly on all reports received and recycle information back out to the membership – otherwise the members will not send in reports and all we get are the majors and fatals that the media dangle in public view.



Equally we may say that RAAus cannot formally act without receipt of a formal incident or accident notification.



However we can learn from the administration aspects of what has happened. Clearly we need some positive means of establishing that a responsible report or notification has been received and reasonably can be expected to be actioned – rather than it slipping through cracks in the floor as this one appears to have done.



We also need some leeway that our RAAus staff have some freedom to chase up partial or casual reports if they are deemed to have significance to the Flight Safety area!



Equally (and I will be blunt because I have many times been on the receiving end of this one) we need assurance that our staff are dispassionate, driven primarily by Flight Safety, and are not swayed by personal opinions of individual members that they may find themselves dealing with!



Having said that I wish to comment on the RAAus Tech Manager – Chris Kiehn.



Chris has ever been a friend to TOSG and I have no reason to disbelieve that he personally did not receive, or be made aware of, this occurrence.



Chris is in a helluva position at the moment so cut him some slack and let him do his job – that I can assure you he is now really getting on with as far as this TST is concerned.



Flight Safety, Sportsfans, is what this is all about and we all – Board Members, Staff and general members – have to work together as a team, with mutual respect, to get the best results.









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Guest airsick

It is not a solution that will solve all the problems outlined here but perhaps read receipts on email correspondence should be used. These will fire back a read receipt from the recipient acknowledging when they read the email assuming the reader doesn't actively interevene to stop the receipt being sent.


Another not so well known confirmation that can be requested is the delivery receipt. This is a receipt that is automatically issued by the recipients mail server to acknowledge receipt of the email. It does not confirm that the email has been read but will let you know that it has arrived in the destination organisations email system.


Receipts assist in confirming whether the email has been received, read, etc. and can help in tracking down where things are falling apart. If the email has been received but not dealt with then clearly the fault lies within RAAus. I am not suggesting this is what has happened in this instance but using these simple tools we can eliminate this as a potential bottleneck.


I won't go into details on how to use these receipts as it varies from one email client to another. Do a quick google search on it and you will soon find out how easy they are to use.



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Rest assured that the situation is being looked into.


There are other factors that are involved in this failure that need to be looked into. Age of aircraft, landing cycles, environment and maintenance practices.


We all know that the back end of any aircraft utilising a tail gets vibration from propwash and certain areas need to be looked at during regular maintenance.


Is this part a definite fatigue area? Is it a one off? Was this aircraft involved in a previous 'knock' to the tail section? RA-Aus knows that this particular aircraft suffered moderate damage back in 2000. Was that a contribution to this failure?


As you can tell, there are many factors to consider.


As this was a serious airframe failure, the positives on this issue are that a) the pilots were unhurt and b) we get to learn something important which we can inform and advise the rest of the world on.


I will work with Tony on providing a solution for the Thruster community. Let's hope it is a one off, though let's rather be sure of that after inspecting the affected area.







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Guest TOSGcentral

This is the latest news on this failure situation:



Yesterday David Hill produced some excellent close up photos of the failed tailplane component. This updates what I had to say previously and demonstrates that the separation occurred around the weld itself – not a little down the shaft as the earlier photos suggested.



David has kindly allowed me to give the address of his web site and you can view the new pics on - http://www.davidsusan.com/pict.html



Chris Kiehn did not waste any time once he got his hands on this situation and by late afternoon yesterday had produced an AN (RAAus AN 280308-1) requiring that TST, T300, T500, Bilby & Flying Fox types have the suspect area cleaned off with paint stripper and crack tested prior to next flight.



That seems to have entirely contained the situation in just a few days and we should have no more failures if owners act prudently.



David’s photos do however prompt a further line of enquiry. They show an inner sleeve or plug at the weld failure. This appears to be doing little and is hardly long enough to give stability against bending or cyclic fatigue loading. I will contact the UK Thruster factory and see if I can get hold of an extract of a technical drawing to see what this assembly should look like before we go bananas and begin thinking about X-rays on the existing fleet.



I would like to acknowledge and thank the several people who so promptly responded with assistance, including a few that privately supplied background information that may have a bearing. It was a good effort on everybody’s part and shows the value of contributions the members may make when they have a chance to.



In conclusion I would like to re-affirm that TOSG is committed to Flight Safety in the Airworthiness and Operations of any and all Thruster types. The information and support the Group is able to provide is freely available whether you are a Group member or not.



However we are only as good as the information that we receive so please consider carefully your overall responsibilities – not just to yourself and family – but to our Movement itself and preserving the priceless freedoms it gives us! Major contributions start with prompt, accurate and responsible reporting of incidents and Tech issues. They may seem minor but collectively can demonstrate a trend towards something more serious that could take a life!









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CrMo welding methods and cracks


Hi All.


Hats off to Robin for averting tragedy, I've practised attempting to land with simulated control failure. But this just involved letting go of the joystick, and just using throttle and rudder, this is very difficult, however it's not possible to flare properly, so the best you can hope for is to get ~6feet off the ground at low airspeed and go for the controlled crash.


As for the actual cracked section I have four points to raise:


(a) Has someone, who knows what to look for, looked at the cracked section? Fatigue cracks are usually shiny (or with clam shaped ripples), and extend usually around 80% of the section, with the last 20% or so being a matt grey colour where the material ripped. On the other hand shrinkage cracks from the original welding will be matt and brown from rusting. Welding defects such as undercut will crack along the edge of the weld.


(b) The insert section looks to be too short? is it meant to be longer?, it would act as a doubler if longer. The short length as shown can only act as a stress raiser. Tony may some other examples to measure to see if the problem is a one-off manufacturing defect.The presence of this internal piece would act as a "chill" during welding, which is undesirable.


© Chrome-Moly has a tendency for what is called "hot shortness" Essentially this results in shrinkage cracks, usually within the weld or just on the weld edge, particularly a problem when the metal is not preheated and/or a thin section is welded to a thick section or you are welding two short stiff sections at two ends (i.e. a tube passes through a tube and is welded at both sides), or current is too high with weld traverse speed too fast. It should also be noted that oxy-acetylene welding is much less critical. With TIG it requires some considerable experience to minimise thermal gradients.


We had a particular issue at work trying to weld a CrMo bracket to a stiff piece of CrMo tube, it was welded at two places, the first weld was OK, but the second weld would invariably unzip, in the end we just brazed it. (Brazing of CrMo is the preferred method for bicycle frames anyhow)


(cc) Mil-std-2219 requires preheating for welding of 4135 and 4140 CrMo (these are 0.35% and 0.4% carbon, the CrMo normally used is 4130 at 0.3%) and for Argon (or He) gas. Postheating for stress relieving and normalisation is also recommended. It is possible by poor welding technique to raise the carbon content in the weld bead e.g. carburising flame, oily steel, using CO2 weld gas.


(d) Some kind of safety wire could be used to ensure the front of the tailplane wouldn't separate even if cracked. Heck even a cable around the boom would keep it together.


I've had two case of weld cracking in my T500 (TTIS ~800hrs now) , one was on the vertical fin where one of the thin diagonal braces broke off. The second case was in the CrMo axle tube, this had ~1/2" OD x 3/8" ID bushes for the spring securing bolts. The bushes were welded at the top and bottom of the axle RHS section, all four bushes were at least partially cracked. I had them re-welded by an aircraft welder for good measure, although the axle assembly is perfectly functional even with cracked bushes. There are probably T500's out there with cracked bushes now, you just can't see the cracks until you pull the CrMo insert out.


Cheers, BobT



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Stress Corrosion Cracking ???


Hi Guys,


I've had another look at the photos, the bottom one caught my eye, as it appears to be badly corroded.


It is possible that water has got inside the tubing, and been retained in the gap between the short insert, and the part of the tube that cracked. In that case you have ideal conditions for stress corrosion cracking, this normally requires some existing stress in the metal part, but post-weld stress that has not been stress relieved would be sufficient. In this scenario the crack would have initiated on the inside of the welded tube. The 4th photo also show a forked crack.


Stress corrosion cracking can usually be distinguished as


  • the edges of the stress corrosion crack are jagged (like a lightning bolt) and will sometimes fork,
  • weld cracks are straight, generally longitudinal but fuzzy edged
  • fatigue cracks are generally fairly straight and shiny, although fatigue can finish the job that other modes start.


It should be noted that in "real" aircraft, "tube oil" is run through the inside of the CrMo sections after manufacture (it's basically fish oil), to minimise internal corrosion.


Cheers, BobT



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Guest TOSGcentral

Thank you Bob for that weld information - very valuable.


I would certainly like the damaged piece professionally examined and opinon given on the type(s) of cracking which would point a finger at the cause.


The internal sleeve or plug was initially a mystery and mismanufacture was suspected as it is so short. However I measured four tail units here and came up with exactly the same dimensions as the failed unit.


We are looking at a number of other factors and I will have some data later today on which to make some more comparisons against serviceable units.


We are also looking into potential causes and working up some simple barricading procedures to ensure that this does not happen again.


I will post again when I have new information.





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G'day All,


I located an engineers supplier today with a dye penetrant crack inspection kit, but the cost!!! $100... mygod. Anyone know of anything available for cheaper price? Good work Tony and others, regards Don.



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G'day All,I located an engineers supplier today with a dye penetrant crack inspection kit, but the cost!!! $100... mygod. Anyone know of anything available for cheaper price? Good work Tony and others, regards Don.

I rang Dave Mullett from Lectromax, as mentioned in the AN (on the RA-Aus website), they have dye penetrant there in the aerosol form for $75.00.


He did mention that if bought by the box load, it works out cheaper. Perhaps people could let TOSG, or another individual who is willing to put their hand up, know who would want a set and try and organise it that way to try and reduce the cost?





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