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Customer notes _ u/c beams

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The purpose of these notes is to give owners of these new carry beams a good grasp of what they are about and why. The notes should be read carefully and then you should examine your own machine carefully and compare what you have with what the notes have to say. You should do this prior to commencing work on fitting the new beam.






The carry beams are designed for any of the ANO/CAO 95.25 two seat Thruster models. The beams replace the original design alloy box section beam AND the optional chrome moly heavy duty insert. The new beams combine both these functions into a one piece unit plus are designed for a 450kg MTOW.






The new beams are manufactured under a CAR35 Engineering Order # 29-E0-008. The beams are manufactured by Aviation Welding Australia of Archerfield (who are a well respected and licensed LAME aviation welding and repair business). Each beam has an individual serial number (issued and recorded by TOSG {Thruster Operator Support Group}) and has a Release Note. Serial Numbers and Release Notes must be affixed to the aircraft’s airframe log book to preserve CAO 95.25 Compliance. The work must also be either done and/or signed off by an RAAus L2 Maintenance Authority holder if the machine is (or maybe in the future) operated by an RAAus Flying Training Facility. For private use only the work involved may be presently done by an RAAus L1 Maintenance Authority holder.






These new beams are rated for 450 kg MTOW but they DO NOT give you leave to increase the flying weight of your current aircraft beyond it’s present certified design limits! They may be fitted to existing Thrusters as replacement units for defective beams but in fact form one part of a larger kit that is being designed to take the overall aircraft to a new 450kg MTOW limit. This kit will be complex and will, among other things, involve entirely sleeving the wing spars and increasing bracing cable number and size to deal with flight loads. The new beam just deals with landing loads at this weight so therefore will deal with lesser weights as a replacement unit.






The first two seater Thrusters had undercarriage carry beams of 2†x 1.5†rectangular section T6061 T6 aluminium with a 3 mm wall thickness. This material is no longer easily (or at all) commercially available.



While adequate for private operations the beams proved inadequate for the demands of flying training and not infrequently bent. The Thruster factory therefore introduced an option of a ‘heavy duty chrome alloy insert’ to the alloy outer beam to stiffen them. This was quite intricate and was expensive.



At the time of writing the Australian Thruster factory appears to be defunct so the inserts are no longer available. The outer section is also no longer available.



The solution to this situation was to produce replacement beams privately under a CAR35 Engineering Order to enable any individual aircraft to preserve CAO95.25 compliance. This has now been done as a single piece unit.



Unfortunately obtaining the correct grade of chrome molly at the right dimensions was also found to be impossible (or not economically viable to have them made for a limited number of units). The new beam has therefore been reduced in cross sectional area to 1.5†x 1.5†section and a reinforcing strap welded to the lower surface. The beam has ten internal chrome alloy spacer inserts welded into it for additional strength and anti-crushing from bolt tightening.



The beams are internally treated with Fisholene for anti rust and externally treated with primer and then a heavy duty gloss enamel metal protective paint. Surface protection integrity must be maintained throughout the life of the beam as chrome alloy readily rusts if exposed to the atmosphere.






Successive types and models of Thruster two seaters saw minor changes to fittings the beam also may carry. These differences have not been recorded in any master documents for referral that would come down to owners. There is potentially the additional challenge that the aircraft may have been ‘casually modified’ by previous owners. The status of individual aircraft have to be determined in advance of commencing work.



It is therefore ESSENTIAL that individual aircraft are examined to establish their actual configuration. This may be most usefully done by an image record via digital camera and good workshop practice of laying out removed parts in their correct orientation pending reassembly.






Gemini – X, A & B models.













The following lists provide further data on fitting.






The basic geometry of the types given above has not changed. The new undercarriage beams are pre-drilled and internally spaced/reinforced to these primary structure dimensions. They will therefore go straight onto the types listed above.



The main fitting locations are as follows:



(a) 4 x main attachment points u/c beam to cockpit floor rails.


(b) 4 x main attachment points u/c beam to main u/c spring legs.


© The support clusters to take the various strut attachment plates on the ends of each beam.


(d) The central mounting point for the elevator circuit reverse pulley housing.






The following must be checked carefully on what you actually have. Hole orientations have to be carefully copied across from the existing beam to the new beam such that all the original fittings are reinstated and secured. Some you may have, some you may not. Any of them may be sufficiently different that we cannot pre-drill the new beam for them when the beams are made!



(a) Port side. Main throttle lever housing


(b) Port side. Throttle travel range control front and back stops/guides


© Port side. Throttle forward and rear throttle lever range stops


(d) Port side of beam centre. Mounting for rudder cable fairlead pulley


(e) Starboard side. Dual control throttle lever mount housing.


(f) Starboard side. Possibly a rear stop for throttle lever travel range but this will probably be mounted on a floor tube rail rather than the u/c beam.


(g) Starboard side of beam centre. Mounting for rudder cable fairlead pulley.






The correct orientation of your new beam is as follows:



(a) The bottom of the beam is the side which has the bottom strap welded to it.


(b) At this point the beam does not have a front or back until you begin drilling into it for fittings. When you do this then clearly mark the front or back of the beam (ie with chalk).






There are a few consequences of the changed cross sectional size of the u/c carry beam reducing in height from 2†to somewhat less.



(a) .The main datum for the beam is the TOP. When copying across locations of fitting holes measure DOWN from the top of the beam


(b) There may be exceptions to the above depending on your own fittings. In which case a ‘best fit’ may have to be used to obtain a clean drill hole and avoid the underneath wall of the beam.


© Because of the reduced height of the new beam four spacers are provided in the kit which will enable you to re-use (if serviceable) your existing main attachment bolts between the beam and the four floor rails.






1 x new beam ready for fitting.


2 x long spacers for underneath the two central floor rails.


2 x shorter spacers for underneath the two external floor rails.


A copy of these notes.






The weight of the original outer casing of the beam was 1.9 kg



The weight of the original inner heavy duty insert was 2.3 kg



The combined weight of the two was therefore 4.3kg



The weight of the replacement beam is 5.0 kg.



This is a major airframe repair/modification. The aircraft must therefore have a new weight and balance survey performed and entered into the log book. If you are unsure how to do this then you can have a free copy by email via requesting this from TOSG. If you do not have email then you can have a hard copy print out mailed to you for $10.






  • Read ALL of these notes until you are certain that you have a full grasp of the task.



  • Take a good look at your existing beam layout such that you know where various fitting will be required to go on the new beam.



  • Position the aircraft, with the wings off, in a suitable open work area.



  • Hoist the aircraft with the mainwheels clear of the ground. Disconnect any wheel brake control assemblies that may be present.



  • Go to the rear of the aircraft, disconnect and slacken the turnbuckle tensioner in the elevator drive circuit.



  • Unbolt the elevator circuit reverse pulley from the beam and lift it clear.



  • Drill out the rivets holding all of the remaining fittings attached to be beam.



  • Disconnect the two throttle drive cables from the Port throttle lever.



  • Disconnect the beam from the cockpit front A frame and the two engine support struts. Remove the two plates each side of the beam ends (noting their orientation).



  • Remove the Port & Starboard mainwheels and undercarriage spring legs.



  • Unbolt the beam from the four cockpit floor rails.



  • Carefully re-check that all fittings have been removed from the beam and slide the beam out of the side of the pod.



  • Check all removed fittings for damage and wear plus bolts for serviceability.



  • Put the old and new beams together and transfer to the new beam the locations of drill holes for fittings that have not been prior drilled.



  • Dry fit the fittings and when satisfied drill out for the fittings. DO NOT begin reattaching any fittings until the new beam is in the aircraft.



  • Install and re-build the new beam in the reverse order to that given above. You may wish to lower the fuselage onto a mattress for additional stability while doing this. During reassembly use a barrier cream (eg Duralac) to barricade against dissimilar metal corrosion. Use stainless steel rivets.



  • Rebalance the carburettors and check all adjustors are locked



  • Have a final check around for tightness and ‘In Safety’ (At least one and a half threads protruding through nuts). Also that no tools or debris have been left in the cockpit.



  • Retension the elevator circuit and wire lock the turnbuckle.



  • Rig the aircraft, drain the fuel tank and then perform a centre of gravity position calculation on the empty aircraft.



  • Record the work in the aircraft log book, along with the serial number of the beam (engraved on the top of the beam) and the Release Note. Also record the new weight and balance data in the log book.



If you experience any difficulties then telephone TOSG on 07 5423 1963 or by email to [email protected].



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