Fatal crash Arizona

On 9/12/2016 8:34 AM, Ernst wrote:
Go to the accident docket and read the MATERIALS LABORATORY FACTUAL
REPORT. My conclusion is that the pawl spring had been missing for some
time.

Ernst


I found the 7-page Factual Report, which includes the statement, "A detailed
examination report for the glider release mechanism is contained in the
Materials Laboratory factual report located in the public docket."

I've been unable to find the "Materials Laboratory factual report." Pointer
help will be Seriously Appreciated!

Meanwhile, I'm still finding it hard to believe the accident aircraft was
successfully operated for ~26 hours without the release spring, though I can
believe the (light-in-tension) spring *might* leave very little in the way of
witness marks on the I.D. of the pawl's through hole.

Thanks very much.

Bob W.

On Monday, September 12, 2016 at 8:09:10 PM UTC-7, Bob Whelan wrote:

...I've been unable to find the "Materials Laboratory factual report." Pointer
help will be Seriously Appreciated!

The Docket Management System (DMS) has many good detail photos of broken aircraft. It is (or should be) every detail designer's go-to resource:

http://dms.ntsb.gov/pubdms/search/hi...docketID=58737

Thanks, Bob K.

On 9/12/2016 9:21 PM, Bob Kuykendall wrote:
On Monday, September 12, 2016 at 8:09:10 PM UTC-7, Bob Whelan wrote:

...I've been unable to find the "Materials Laboratory factual report."
Pointer help will be Seriously Appreciated!

The Docket Management System (DMS) has many good detail photos of broken
aircraft. It is (or should be) every detail designer's go-to resource:

http://dms.ntsb.gov/pubdms/search/hi...docketID=58737

Thanks, Bob K.


Many thanks!

Just to (sorta) complete the train of thought in my (bemused) posts preceding
this one. The photos of the release mechanism from the accident aircraft seem
to (pretty much) match my (oldish) memories of how it functions, the "pretty
much" exception being I remembered the hook retraction spring as a simple
tension spring (and not the dual-sided, probably custom-bent) coil type. It
was THAT spring force to which I referred when writing I couldn't understand
how the hook could have sensibly functioned in its absence. That force serves
dual purposes: 1) maintaining the hook cover against the back side of the
opening slot while in flight (while also allowing a back release in the event
of loss of rope tension combined with a Big Bow), and 2) (by
through-transmittal of the hook-opening-cover force) retracting the entire
hook mechanism after the pawl is released from the flat-plate/cable-hook
detent by the pilot pulling the release knob/cable.

As for the report's claimed missing pawl spring...I must be getting dense in
my old age, since I'm still puzzled by the intended function and line of force
of that implicated piece of (missing?) hardware. Using Figure 8 by way of
illustrating my puzzlement, it seems to me such a spring could either serve to
decrease or increase the pawl's contact force against the hook plate.
Decreasing the contact force would appear to be counter-productive, while
increasing it (arguably) might have served to make the incomplete contact
condition shown in Figure 9 even more likely.

In any event, my current working hypothesis is the hook likely back released
(as intended, for better or for worse) from a bow in the rope (gusty sink
being reported in that vicinity by the previously-towed pilot) at an
unfortunate/ugly towing-location, followed by loss of control. Having had two
such back releases during gnarly tows (one nearly too low to warrant an
attempted return, above head-high sagebrush, but fortunately not occurring
until later that same tow), I can relate. Whether or not the incomplete
contact condition between pawl and cable hook detent (shown in Figure 9) was a
contributor, I have no idea.

Back to the hook design - what am I missing? Thanks!

Bob W.

On Tuesday, September 13, 2016 at 10:34:20 AM UTC-4, BobW wrote:
As for the report's claimed missing pawl spring...I must be getting dense in
my old age, since I'm still puzzled by the intended function and line of force
of that implicated piece of (missing?) hardware.
Back to the hook design - what am I missing? Thanks!

Bob W.

If I understand correctly, the missing spring pushes the pawl
in the direction opposite of pulling the release knob.
Otherwise, the pawl is not secured in the "latched" position,
except by a bit of friction with the hook plate (from the
spring that is present and any rope tension).

Do I understand correctly??

On 9/13/2016 9:26 AM, Dave Nadler wrote:
On Tuesday, September 13, 2016 at 10:34:20 AM UTC-4, BobW wrote:
As for the report's claimed missing pawl spring...I must be getting dense
in my old age, since I'm still puzzled by the intended function and line
of force of that implicated piece of (missing?) hardware. Back to the
hook design - what am I missing? Thanks!

Bob W.

If I understand correctly, the missing spring pushes the pawl in the
direction opposite of pulling the release knob. Otherwise, the pawl is not
secured in the "latched" position, except by a bit of friction with the
hook plate (from the spring that is present and any rope tension).

Do I understand correctly??


Quite possibly. I suppose such a spring fairly might be considered the
"suspenders" to the hook-retract-spring's "belt." It's not obvious from the
photos (Figure 1 shows it best), but installed-geometry, plus gravity, in the
pawl's as-installed position/angle work "against" the pawl remaining
detent-seated...i.e. the pawl pivoting by itself (no other physical contacts)
would tend to flop its "business end" *away* from the detent due to the longer
cable-attach arm's length compared to the detent-engagement arm's length
(unequal length teeter-totter).

Nonetheless, whether the absence of a compression spring between the pawl and
receptacle/pawl-spring-housing was a crucial element in this accident is
debatable; it would take very little force on the rope to rotate the cable
hook from the barely-engaged position (Figures 9) to the fully engaged
position (Figure 8). Once there, further testing definitely required to
determine whether the design would be more or less prone to back-releasing in
the absence of the pawl spring, in the presence of a rope bow...

That said - and since a number of these hooks have been installed into the
noses of German-built ships originally entering the USA with only a CG hook -
owners of ships with these hooks SHOULD (and easily can) VERIFY the
presence/absence of such a compression spring by checking to see if the pawl
is positively forced against the rotating piece of the cable hook throughout
its rotation range. Positive engagement = spring-present. (Note that the
spring itself is hidden in the hook's assembled state...and might easily
escape unnoticed in the event of the hook being disassembled for any reason.)

Bob W.

At 17:28 13 September 2016, BobW wrote:
On 9/13/2016 9:26 AM, Dave Nadler wrote:
On Tuesday, September 13, 2016 at 10:34:20 AM UTC-4, BobW
wrote:
As for the report's claimed missing pawl spring...I must be
getting
dense
in my old age, since I'm still puzzled by the intended function
and line
of force of that implicated piece of (missing?) hardware. Back
to the
hook design - what am I missing? Thanks!

Bob W.

If I understand correctly, the missing spring pushes the pawl in
the
direction opposite of pulling the release knob. Otherwise, the
pawl is
not
secured in the "latched" position, except by a bit of friction with
the
hook plate (from the spring that is present and any rope
tension).

Do I understand correctly??


Quite possibly. I suppose such a spring fairly might be considered
the
"suspenders" to the hook-retract-spring's "belt." It's not obvious
from the

photos (Figure 1 shows it best), but installed-geometry, plus
gravity, in
the
pawl's as-installed position/angle work "against" the pawl
remaining
detent-seated...i.e. the pawl pivoting by itself (no other physical
contacts)
would tend to flop its "business end" *away* from the detent due
to the
longer
cable-attach arm's length compared to the detent-engagement
arm's length
(unequal length teeter-totter).

Nonetheless, whether the absence of a compression spring
between the pawl
and
receptacle/pawl-spring-housing was a crucial element in this
accident is
debatable; it would take very little force on the rope to rotate the
cable
hook from the barely-engaged position (Figures 9) to the fully
engaged
position (Figure 8). Once there, further testing definitely required
to
determine whether the design would be more or less prone to
back-releasing
in
the absence of the pawl spring, in the presence of a rope bow...

That said - and since a number of these hooks have been installed
into the
noses of German-built ships originally entering the USA with only
a CG hook
-
owners of ships with these hooks SHOULD (and easily can) VERIFY
the
presence/absence of such a compression spring by checking to
see if the
pawl
is positively forced against the rotating piece of the cable hook
throughout
its rotation range. Positive engagement = spring-present. (Note
that the
spring itself is hidden in the hook's assembled state...and might
easily
escape unnoticed in the event of the hook being disassembled for
any
reason.)

Bob W.

I am now confused by the "installed in German" part. Is the release
you are talking about a TOST release?

On 9/13/2016 4:24 PM, Don Johnstone wrote:
At 17:28 13 September 2016, BobW wrote:
Snip...
That said - and since a number of these hooks have been installed
into the
noses of German-built ships originally entering the USA with only
a CG hook
- owners of ships with these hooks SHOULD (and easily can) VERIFY
the
presence/absence of such a compression spring by checking to
see if the
pawl is positively forced against the rotating piece of the cable hook
throughout its rotation range. Positive engagement = spring-present.

Bob W.

I am now confused by the "installed in German" part. Is the release you are
talking about a TOST release?


Sorry for any confusion. A number of "Applebay releases" have been
subsequently installed in (on the fuselage bottom surface, near the front of
the nose of) non-USA-built gliders imported into the USA with only a single,
CG-mounted, release back by the wheel. This second cable attachment point
provided "a nose-hooked aero-towing option." Many - not all - such modified
ships were of German origin.

FWIW, I've been privately informed by a fellow Zuni owner (of S/N 28) that his
ship's release uses a(n easily visible) *tension* spring (not compression, as
on S/N 2) to positively seat the pawl against the rotating/indented cable hook
part...which is what my fallible memory kinda-sorta remembered from my own
(not recently looked at) Zuni (S/N 3).

In either case, any owner of a ship with an "Applebay nose release" can/should
easily confirm the presence of such a spring by verifying the business end of
the pawl is "somehow or other" positively forced against the rotating cable
hook as it operates throughout its range of motion. The truly curious can
disconnect it before operating their releases to get a better feel for what I
sought to describe in an earlier post. Please do reconnect it...or YMMV!

Bob W.