What should the relationship between RAAus and CASA be?

[Dafydd Llewellyn]

Clearly I am no expert in this area but there are a couple of things one could deduce. Firstly, an RA-Aus registered J230 with a design MTOW of 700kg is unlikely to have ever been flown close to its MTOW. Combine that with your view that it is well constructed in the first place and it would seem to me that you have an aircraft likely to do well compared to most others regarding fatigue life. But even a J230 will get to the geriatric stage one day.If you apply the same logic at the other end of the Jab scale, a J120 with an MTOW of 500kg that, quite possibly, is frequently flown above its design MTOW could you have a significant fatigue issue much earlier?

 

And then there is the exposure to UV - kept in a hangar or outdoors?

 

We have Jabiru celebrating its 25 years in the air on the cover of the October SportPilot and we certainly don't hear much (anything?) about Jab structural issues in their older aircraft. That has to be encouraging. Now, if only the engines . . .

Yes, I've been watching a restoration of an early Jabiru, and whilst the original wings had been damaged beyond economical repair in an overturn (following a forced landing, caused by through-bolt failure) the bushes in the wing lift-strut lugs were still unmoving, despite the crash loads, indicating that the wing had not reached its fatigue life. The basic composite structure (once one gets below the many layers of unsuitable paint that had been indiscriminantly applied to the thing over the years, which should be done to the extent necessary to effect repairs, and no more) is in unexceptionable condition. The original lift-struts had been replaced by a later design, and they have to be replaced again, so there's no fatigue issue there. The only real deterioration was indentation of the bearing plates on the back of the firewall, that carry the engine mount bolts. It's of course, one of the early solid-foam-filled wing models. So if it's any indicator, I'd not expect to see fatigue issues with the composite structure of those early Jabs for quite a while, yet. Most of the damage was ham-fisted maintenance, actually.

 

The main danger would be, if some idiot had chemically-stripped the aircraft prior to re-painting it; one should also keep solvents such as Acetone and MEK away from it. If we see an airframe failure in a Jabiru, it will in all likelihood be from that cause.

 

Alan Kerr (who amongst other things, designed a Boron-patch repair for the F-16) did the basic structural substantiation of the early Jabirus (up to about the 230/430) and he did some impromptu fatigue tests with induced damage; the indications were that the Jab airframe was quite resistant to fatigue. That seems to be being borne out in practice. If I owned one, however, I'd dye-check the lift-strut end fittings from time to time. The Jab. has some very intelligent detail design, though it does not look anything special to the casual observer. I can't say the same for quite a lot of other aircraft.

 

Yes, they should be kept indoors rather than being permanently tied-down in the open.

 

Re the engines, I think you will find some help coming along before much longer. Can't say more about it just yet.

 

 

[facthunter]

Strutted hiwing has a design advantage over thin section low cantilever type wings especially with steel truss carry through sections buried in the fuselage out of sight. For simplicity of maintenance the Jabiru structure appears excellent for the non expert owner. A few good clues from people like Dafydd would help, because you probably take notice of him. I can't remember a plane easier to look after than a jabiru. Nev

 

 

[turboplanner]

If it's any help there are tens of thousands of FRP boats which have gone to their maker over the years, and hundreds of FRP refrigerated vans which have probably been installed on their tenth cab/chassis, the only difference being the quality of the resin, and the original laminating skill.

 

 

[Dafydd Llewellyn]

If it's any help there are tens of thousands of FRP boats which have gone to their maker over the years, and hundreds of FRP refrigerated vans which have probably been installed on their tenth cab/chassis, the only difference being the quality of the resin, and the original laminating skill.

Not quite - the Jabiru (like any composite aircraft) is laid up using various grades of uniaxial and biaxial or multiaxial cloth, not chopped strand mat - and the load-bearing skin of the wing is typically less than a millimetre thick. Also, it's epoxy, not polyester or vinyl ester. So the stress levels are usually quite a bit higher than for a boat or a refrigerated container. The boats, on the other hand, are affected by increased-intensity UV, and also osmosis; and the refrigerated container, by thermal stresses, to a greater degree than a recreational airframe. So the situations don't really compare well enough to be useful. However, there's a "rule of thumb" that E-glass/epoxy has a good fatigue life provided the strain level (i.e. how much the laminate is stretched) does not exceed 0.003 inches per inch (i.e. 0.3%) at the limit load. If this limit to the strain level is maintained, it makes a composite airframe rather heavy compared to a metal airframe - so this is not very likely to be the case for recreational aircraft that have extremely "tight" MTOW limits; and it also should give pause to people who tend to fly their aircraft over their certificate MTOW (and is no doubt a large part of the reason why CASA has clamped down on the overweight registration of aircraft by RAA. The morals of importers who pursue such weight increases can be judged accordingly).

 

This is not the full story, of course; it's also highly desirable to keep the shear stress between individual layers in the laminate as low as possible (the Jabiru form of wing spar construction achieves this par excellence) and to avoid points of local compressive or flexure stress that tend to force the layers in the laminate apart. To the extent that the designer:

 

(a) Understood these points, and

 

(b) Managed to achieve them in his detail design, and

 

© His efforts have not been nullified by either poor detail design or exposure to solvents or environmental aspects such as UV,

 

a composite airframe can have better fatigue properties than a metal airframe.

 

However, composites have no ability at all to locally yield at points of stress concentration and thus relieve themselves; and they are extremely sensitive to stresses in more than one direction, and this means the quality of the detail design is much more critical in a composite aircraft than it is in a metal one, and much more difficult to assess by inspection.

 

The overall effect is that it is very dangerous to make sweeping generalisations about composite airframes - as somebody once put it, "when they are good, they are very, very good; and when they are bad, they are horrid." So if the purchaser "only buys the paint", he may be in for a rude surprise. Caveat Emptor.

 

 

[Dafydd Llewellyn]

Clearly I am no expert in this area but there are a couple of things one could deduce. Firstly, an RA-Aus registered J230 with a design MTOW of 700kg is unlikely to have ever been flown close to its MTOW. Combine that with your view that it is well constructed in the first place and it would seem to me that you have an aircraft likely to do well compared to most others regarding fatigue life. But even a J230 will get to the geriatric stage one day.If you apply the same logic at the other end of the Jab scale, a J120 with an MTOW of 500kg that, quite possibly, is frequently flown above its design MTOW could you have a significant fatigue issue much earlier?

 

And then there is the exposure to UV - kept in a hangar or outdoors?

 

We have Jabiru celebrating its 25 years in the air on the cover of the October SportPilot and we certainly don't hear much (anything?) about Jab structural issues in their older aircraft. That has to be encouraging. Now, if only the engines . . .

Re the engines, keep an eye on http://camitaeroengines.myshopify.com/pages/new-camit-aero-engines