Glider Hours

On 4/26/2011 4:13 PM, Walt Connelly wrote:
I have seen a number of glider advertised and most tell you the total
time. Unlike a powered airplane which has a tach or a Hobbs meter,
gliders seldom have any built in way to determine the actually time on
the air frame. While people might try to inflate their personal flying
time, I would not be surprised if many of these gliders are low balling
their actual air frame time which I presume has a lifetime limit in one
way or another. Is there any reasonably foolproof/accurate way to
determine the true time on an airframe? I cant think of one.

Walt

A couple of thoughts...

- My understanding of 'German glass' (the glider world 'certification
pioneers') is that early (Glasflugel & Schleicher & probably others)
pre-carbon-ed airframes were LBA-overseen-tested to 18,000 hours, then
(originally) certified to 1/6 of that, or the 'magic' 3,000 hours you'll
sooner or later encounter in the glider world. As airframes/types reached
3,000 hours, additional certification depended upon them passing detailed
inspections with the results forwarded/blessed by the LBA in 3,000 hour
increments. I believe some airframes have now been certified up to 12,000
hours (Twin Grobs? LS-4's?). It'd be great if knowledgeable Europeans will see
fit to chime in here...

- 'Pure glass' gliders are necessarily 'overstrong' (i.e. designed to
stiffness, rather than strength criteria [the latter being typical of aluminum
and wood gliders and airplanes]), in order to demonstrate 'usefully high'
flutter-free useable airspeeds.

- I'm unaware of any evidence of fatigue-related aging issues in any
first-generation glass ships' composites. (The metal bits are a different
story, of course...)

Based on the above, my conclusion is potential owners of 1st-generation glass
gliders have little to fret about in ship-life terms, at least of the plastic
bits, regardless of whether one is purchasing from an apparent 'squirrel' or
from Diogenes' sought-after human.

Regards,
Bob W.

On Wed, 27 Apr 2011 11:51:52 -0600, BobW
wrote:


- My understanding of 'German glass' (the glider world 'certification
pioneers') is that early (Glasflugel & Schleicher & probably others)
pre-carbon-ed airframes were LBA-overseen-tested to 18,000 hours, then
(originally) certified to 1/6 of that, or the 'magic' 3,000 hours you'll
sooner or later encounter in the glider world.

When glass glider were invented, things were a little more simple than
what you described.
The wings of glass gliders were (and still are) tested to destruction
in bending tests, but no large-scale load cycle tests were carried
out.
The only real long-term test article is that Janus C (carbon) wing
that is stored outside Stuttgart university and tested every couple of
years.


As airframes/types reached
3,000 hours, additional certification depended upon them passing detailed
inspections with the results forwarded/blessed by the LBA in 3,000 hour
increments. I believe some airframes have now been certified up to 12,000
hours (Twin Grobs? LS-4's?). It'd be great if knowledgeable Europeans will see
fit to chime in here...

Basically no need to do that - you explained the procedures perfectly
correctly.

The only "glass" glider I'm aware of that has an airframe hour limit
is the Pegase (but not based upon technical issues).

As you describe correctly, most German gliders need a thorough
inspection every 3.000 hrs (newer Schempp Hirth gliders 6.000 hrs)
which sometimes includes the replacement of certain parts (usually the
only mandatory replacements are rudder cables and release cables).

I've heard of an ASK-21 that recently passed its 24.000 hrs check.


- 'Pure glass' gliders are necessarily 'overstrong' (i.e. designed to
stiffness, rather than strength criteria [the latter being typical of aluminum
and wood gliders and airplanes]), in order to demonstrate 'usefully high'
flutter-free useable airspeeds.

.... and in contrary to aluminium the materials of a composite glider
are not fatigue critical - an aluminium structure needs to be
overengineered to extend its fatigue life, but glass, carbon or kevlar
fibre don't need this.


Based on the above, my conclusion is potential owners of 1st-generation glass
gliders have little to fret about in ship-life terms, at least of the plastic
bits, regardless of whether one is purchasing from an apparent 'squirrel' or
from Diogenes' sought-after human.

100% agree.



Cheers
Andreas