A recent Facebook post on the “A.I.R. Atos” user group
brought up the suggestion that a twisted control cable incident may have caused
a crash:
From Face-Book, post crash, the "cable twist" :
I was skeptical that the twist would disable the system, but
my curiosity got the best of me. I
decided to give this a closer look (in particular, the suggestion that this “wrap”
caused both spoilers to deploy at the same time).
With my wing setup for its spring pre-flight I tried to
simulate the wrap shown above (on one side, then both sides). Admittedly, it
was not easy to get this much twist to happen. The only way I was able to create
a fixed twist in the cable was to jam some short sticks in both wire sets to
get them to hold the “twist”. Without
the artificial effort, they tended to completely un-twist with any amount of
control input.
Eventually I managed to
simulate the twist and yes, it did cause resistance and yes it did cause both spoilers to lift off the wing (or, more accurately, did not allow them to return
to the normal closed position).
What was obvious during the control check, the control bar
movement was not smooth and there was substantially more resistance than a
normal control check. This unusual resistance would have inspired some further
investigation.
So lets start with the obvious (and I'm pretty sure the pilot involved would agree....), a proper PRE-FLIGHT
INSPECTION was not done and would have caught this twist (if indeed the twist
was present prior to launch). In this scenario, the site is windy and exposed,
as I understand it, pilots make a habit of leaving the side wires disconnect
(to avoid wear of the nose control cable).
I am not totally convinced the twist was present pre-launch.
Finding the twist post crash may not be
proof that the twist was there at take-off. (tho… the pilot’s description does
suggest something was wrong with the controls during takeoff).
There exists the possibility that the crash had stretched the
spoiler cables. Any impact strong enough to damage a control bar weak link creates
the opportunity to cause control cable stretch (more precisely, tighten the twist
and or “set” the twist of the cable). Of
course these control cables have very little stretch, but because of the
length of this cable, they can develop and store a substantial amount of twist
(force) and in this situation may have caused the cables to twist “post crash”.
Here is some basic information on cables and the type of
stretch related to its construction:
https://www.cmacable.com/wp-content/uploads/2017/05/1704constructional-stretch-1.pdf
A fresh set of “new” control cables will not tend to twist
the connections of the control system. A few high load incidents can cause cable
stretch and create enough twisting effect in the cable that it will cause the
support block to roll over and twist the cables (and yes, even with the spoiler
end of this cable being a soft/flexible mount of the spoiler cord on the
outboard section of the control system).
During setup, there should be no need to twist the shackle
fittings to straighten or compensate for any twist found in the spoiler cable
and its support block. Even with a very weak bungee support cord. These cables
should line up without any need to force or twist any part of the
connection. If you find that the block
twists more than 90 degrees when the system is connected, you need to correct
the problem.
A new cable might be in order, or at the very least, a careful
inspection. If you are sticking to the
original cable set, then it will be necessary to disconnect one end of the
control cable, let the cable “relax” and roll / twist to its normal alignment, then
re-connect. The spoiler cord end is one option, but I find the better option is to correct the issue at the nose/support block end. This makes it allot easier to figure out
how much twist is involved and access is allot easier then mucking about with
the cords, knots and settings of the outer attachment.
Outboard control cable end (buried inside the sail):
Inboard end connection (cable removed):
The other reason you might want to work with the support
block end is you will need to cut off the shrink wrap anyway to refresh your
support bungee. If your replacing he bungee for the first time, be sure to use
the material used by A.I.R., some bungee material may look the “same” but may
be too strong for this application.
Caution…There is a bunch of work that might put you off,
like needing to use thread locker products, the lock tab, even the possibility
that the bolt strips out of the soft aluminum connecting block... such is life,
if your uncomfortable working on your glider, get help.
Cable connection, assembled and with a proper natural
alignment:
Another point to consider, do not try to solve a twisting
issue by simply refreshing or installing a stronger support bungee (to get the
bungee to pull the twist out of the system).
The only purpose for this bungee is to support the weight of the connection
block and maintain a light tension on the spoiler cable.
As to the suggestion that a twisted control system caused
the crash in the Face Book post? Yes, it is possible. A combination of way too much control system play (greater than 1” of travel in the control bar movement before actuation), a weak bungee cord , a twist loaded into a stretched wire and a lack of pre-flight inspection could be just the right combination of things to get this very simple control system to fail.
If there is any tendency to twist when you are making the
control system connection, you need to correct the problem.
Of course, this incident could have been caused by other known issues such as the factory advisory
regarding nose pins:
Or if you have an older wing with Dyneema control nose line:
Or as mentioned on a earlier blog post, your control wire gets pulled back up through
the sail access and you inadvertently cross your assembly sweep cord: (http://towforce.blogspot.com/2018/04/air-atos-q-series-set-up-error.html
)
Without question, the ATOS control system is amazingly
simple and safe if you do a proper pre-flight inspection.
If you are not going to complete the
setup of the control system (because of circumstances related to where you fly), the nose cone must be left off and lower surface unzipped (An ATOS is this configuration must be considered as, not ready for flight and a complete control system check is required).
Any time you connect the
control wires to the control bar, you must visually inspect the internal wing
connections and routing, you must complete your control system check and of course, you must close the zipper and install the nose cone for flight.
DO A PROPER PRE FLIGHT.
Believe it or not, I have had pilots tell me that during
their pre-flight something felt odd, but they flew anyway! Yikes!… if something
does not “feel right” track it down! Remember, the spoilers on an ATOS are kind
of important (just a bit of an understatement?)… don’t leave home without them 😉
Cheers and Fly Safe!
