Aerotow Regulations and Rope Specifications

I am in the process of writing up some regulations for aerotowing to
standardize operations and ensure safe procedures for us here in
Colombia. The civil aviation authorities don't know a thing about the
intricacies of aerotowing so we are on our own with how we want to go
about it. Hence the desire to implement something that resembles best
bractices across the gliding world.

I have been reviewing information/regulations from Germany, the US,
the UK, Australia and New Zealand as well as manufacturer information
and manuals. The result of this excercise so far has been that there
are some considerable differences across countries even with such
basic things as aerotow rope requirements.

As far as a general guide is concerned I ran into the newly published
book "Aerotowing Gliders" by John Marriot (http://www.amazon.com/
Aerotowing-Gliders-towing-gliders-emphasis/dp/1456775154/ref=sr_1_1?
s=books&ie=UTF8&qid=1325720610&sr=1-1) which after a first quick read
looks like a good basis for general operating procedures.

As far as aerotow investigations and rope requirements are concerned I
had, among other things, a good look at

http://www.negevgliding.com/safety/airtow.pdf
http://www.dg-flugzeugbau.de/Data/TN.../2004-54-e.pdf
http://www.dg-flugzeugbau.de/sollbruchstelle-d.html (http://
translate.google.com/translate?sl=de&tl=en&u=://www.dg-flugzeugbau.de
%2Fsollbruchstelle-d.html&act=url)
http://www.dg-flugzeugbau.de/flugzeugschlepp-d.html (http://
translate.google.com/translate?sl=de&tl=en&u=http://www.dg-
flugzeugbau.de/flugzeugschlepp-d.html&act=url)

In addition and since I have years of past aerotow experience in the
US I looked again at the regulations in the FARs:

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FAR 91.309

Minimum Strength = 80 percent of the glider maximum certificated
operating weight
Maximum Strength = twice the maximum certificated operating weight

Note: Maximum certificated operating weight can be found in the glider
POH and may be the Maximum certificated gross weight at takeoff.

If the towrope has a breaking strength more than twice the maximum
certificated operating weight of the glider being towed, a safety link
has to be installed at the point of attachment of the glider and the
tow plane with the following breaking strength requirements.

Safety Link (Weak Link) Requirements

Safety link (Weak Link) at the glider end:

Minimum Strength = 80 percent of the glider maximum certificated
operating weight
Maximum Strength = twice the maximum certificated operating weight

Safety link (Weak Link) at the tow plane end:

Strength Requirements = Greater, but not more than 25% greater
than that of the safety link on the glider end, and not more than
twice the maximum certificated operating weight of the glider

------------------------

My conclusions that have come out of this so far a

Aerotow, especially of single seater light wing loading gliders,
should only be done with a nose hook (and not a CG hook) to avoid the
danger of the rapid onset of kiting that will not break your standard
glider weak link but stall the tow plane (see UK tests described in
http://www.negevgliding.com/safety/airtow.pdf).

Weak links at both ends of the rope to protect the glider but also the
tow plane in case the rope gets caught e.g. in a tree or power line
after glider release when close to the ground (I ran into several
accident reports where this happened, especially after a tow emergency
at low altitude). Interestingly enough the UK requires a weak link at
the tow plane end to protect the tow plane with an additional glider
end weak link being optional while the Germans only explicitely
require a glider end weak link according to the glider manual.

I have a personal distaste for the US arangement of using ropes as
weak links since it is not easy to tell which rope has which diameter/
is made of which material to then indirectly deduct the rope weak link/
aerotow rope strength (if no weak links are used). In addition, rope
wear can quickly become an issue limiting weak link/rope strength.
Coming originally from a winching background for me there is nothing
simpler than teaching and checking the weak link color required for a
specific glider, no second guessing, no hidden rope wear reducing the
weak link/rope strength.

Now to the recommended weak link strengths: The US idea of having a
slightly stronger weak link on the tow plane end seems to be a
sensible idea to have the weak link fail first on the glider end to
protect the glider from getting entangled with the rope which could
happen if it breaks on the tow plane end. However, the specific US
weak link strength requirements do not make much sense and e.g. in the
case of the Diamond DA-40 contravene what the aircraft manual permits
(http://www.diamond-air.at/fileadmin/...sales_support/
DA40-180/Airplane_Flight_Manual/Supplements/60101e-o1-r1.pdf):

DA-40 Manual states: Weak link of max. 899 lbf (400 daN) on tow plane
end required, on glider side optional according to glider manual, 750
kg/1653 lb max glider weight on tow, tow rope min breaking load 2248
lbf (1000 daN)

If I use the requirements of FAR 91.309 to determine weak link
strengths I could use between 80% and 200% of max gross weight of the
glider for the glider end but it would be limited to just below the
strength of the tow plane end weak link. So if the DA-40 manual limits
me to 899 lbf (400 daN) on the tow plane end my glider weak link has
to be less than that, best case 898 lbf (399 daN). The best I can do
now for max gross weight is a minimum of 80% of the glider's max gross
weight for the glider end weak link which would get me at the most
1122.5 lbs/509 kg for the glider. In short the FARs would not allow me
to tow any modern 2 seater with a DA-40 even though the DA-40 manual
explicitely allows for up to 1653 lb/750 kg gliders.

Now considering the UK tests mentioned earlier it seems to me like a
really bad idea to use a tow rope with no weak links approaching twice
the max gross weight as allowed by the FARs.

Research done in Germany suggests (see above links to documents on the
DG website) that weak links of no more than 661 lbf/300 daN are
necessary on the glider end even though most glider handbooks specify
the same weak link strength for aerotow as for winch launching (which
involves much higher forces).

So the quick summary of my take so far on all this with regards to
aerotow ropes is:

- Minimum aerotow rope breaking load of 2248 lbf (1000 daN)
- General purpose length of aerotow rope for standard daily ops of 50m/
165 feet
- Use of 2 weak links, one on the glider end, one on the tow plane end
- Weak links with the Tost reserve setup, 899 lbf (400 daN) on the tow
plane end, 661 lbf/300 daN on the glider end

Looking forward to comments, suggestions, experiences etc. Thanks in
advance,

Markus Graeber
Aeroclub de Colombia
Gliding Colombia/Vuelo a Vela Colombia