Glider towing limitations question

[Pilot DAR]

I am doing an approval of the installation of a tow hook on some 182's. Transport Canada is asking me to state the maximum breaking strength of the tow rope (or weak link), and the "limitation" I will place on the 182 for maximum glider weight to be towed. The very comprehensive operations manual this operator already uses for 182 tow operations, does not specify either of these values.

I have suggested to TC that such a limitation is not normally is not a part of the airplane approval, but they are equating it to a helicopter having a limitation on the cargo hook, which does make sense to me.

What is the collective experience with tow planes having maximum glider weight limitations, and stated breaking strengths for the tow rope? Is the pilot expected to assure that these two characteristics are compliant before flying?

How is the breaking strength of a tow rope or weak link "certified" in the aviation sense, as they are probably commercial products?

[Prop swinger]

Seeing as you're in North America I would just go with FAR 91.309.

Glider weight will vary between about 300kg to 850kg, so glider operations might have to use separate ropes for different gliders.

The BGA's Aerotowing Notes (https://www.gliding.co.uk/bgainfo/clubmanagement/documents/aerotownotes.pdf) is a little more relaxed, specifying a weak link of about 5000N at the tug end & a little less at the glider end (p23). It does say that the limit should be specified in the manual or a supplement.

Using the rope as a weak link strikes me as a bad idea. Sooner or later it will get worn & weakened; if it's not replaced it will break & leave the glider up a creek. I've only ever been towed using a strong rope with commercial weak links (including a spare) at both ends.

Have a look at TOST (http://www.tost.de/Eindex.html).

[thing]

I din't know tow ropes were certified. Certainly the ones I've been towed by have been just that, ropes.

[Jim59]

UK
All the aircraft I have towed with have either had the limitations, in terms of maximum weight of glider and maximum breaking strength of the rope, or a weak link in the rope, specified in the flight manual (Robin DR400) or as a UK CAA flight manual supplement (DH82A, Rallye 180T, Super Cub, Piper Pawnee and Citabria). Some towing approvals also put a limitation on the take-off weight of the tug.

The weight limitations are there to ensure that a minimum climb rate is achieved when towing and the rope/weak-link strength to ensure that the tail is not pulled off the tug if the rope snags in e.g. a hedge. The weak link in the rope MUST go at the tug end - not the glider end to protect the tug. Normally the glider weak link requirement is much greater than that of the tug - so only one weak link is required.

[POBJOY]

The 'ball park' weak link on Cessna's for Banner Towing is 300 kg, and that assumes the use of the 'tie down' fitting as the Hook mount.
The use of a Nylon rope is preferable as it allows a 'stretch' factor that reduces snatch loads.
The size of the nylon rather depends on what wear factor you require as even 6mm will be strong enough but may wear too quickly if used on tarmac.
As i recall the factoring for the Cessna was based on the following.
Ultimate load tail hook fitting 3600 lbs
Ultimate load Aircraft release 1200 lbs
Safe factor for use 600 lbs

Why are you using 182's with all those 180's around !!! (much better cowlings for cooling)

[Prop swinger]

The weak link in the rope MUST go at the tug end - not the glider end to protect the tug.Other way around - the weak link should be weaker at the glider end. Obviously the maximum strength should protect the tug but there should be a presumption that the rope will stay with the tug, not the glider.

[Pilot DAR]

Thanks for your replies, it helps. I get the sense that there is a lot of wisdom and experience which is "out there" but not necessarily being captured in flight manual supplements.

My client already uses 182's, and is adding some more. I don't know the reason for the type choice, though I can imagine the ease of training inexperienced pilots to fly them is probably a factor. It's not my job to reccomend type, just approve what I'm presented with. In Canada the value of a 180 is greater than a 182, as the 180 will go [back] onto floats much more easily than the 182, and certainly is much more in demand as a skiplane.

Is there much risk of snagging the tow rope in a hedge to the point of it breaking the weak link, or otherwise threatening safety? I was presented with the perceived risk of the glider being dragged into the trees, and snagging down the tow plane. I countered that I believe that the glider generally flies higher than the towplane, so the risk is very low of the glider snagging something that the tow plane did not.

AC43.13-2A requires a placard for the maximum breaking strength of the rope, I'm going to try to approve it that way. But all wisdom on this is very appreciated.........

[thing]

Glider flies lower than the tug.

[BackPacker]

PD, wouldn't you perhaps be better off to contact, for instance, the BGA about this? They must have some specific documents about safe towing operations which might give you a running start.

I have the equivalent documents from the Dutch glider association, but they're all in Dutch, unfortunately.

Additionally, at least here in Europe, the main manufacturer of winches, cables, attachments, fittings and so forth is Tost. They may already have some of the expertise/documents/certification preparation work you're looking for.

Tost Flugzeuggertebau (http://www.tost.de/)

Anyway, I think you will need to distinguish two "maximum" values here. One is the value for the continuous load, and is very proportional to the glider weight and glide ratio. That's the weight that the airplane must pull into the air. So the 180/182 needs to have an engine that's strong enough to achieve a certain minimum climb gradient while towing a glider with that weight. According to my documentation here, the minimum climb gradient is 5%, and minimum V/S is 1.5 m/s. (No doubts in my mind the 180/182 can do that - I was twice towed by a DA20 Katana with a 100 HP Rotax 912, while sitting in a two-seater K21, so a 180/182 should have no trouble at all towing even the heaviest glider.)

The second value is the maximum rated strength of the "weak link". This will play a big part in turbulence when the transient loads on the cable may exceed the cable or attachment strength. In which case the weak link should go first to prevent damage elsewhere. Similar to an electric fuse. Again, I have no doubt that the 180/182 is able to work with the strongest weak link that's in use for glider towing. (According to my documentation the strongest weak link for glider towing is the red one, rated at 7.5 kN. There are two stronger ones, brown and black, but these are only used for winch launching. In addition, my documentation specifies that the tow cable has to be at least 1.5 times stronger than the weak link. To compensate for wear, a factor 2 is used in practice.)

And while looking at "things that can go wrong" and that should be addressed in certification, as far as I'm concerned, here's another point.

When the glider gets out of position the tug pilot might not have sufficient elevator/rudder authority anymore to maintain the proper attitude. So there has to be a mechanism for the pilot to release the cable NOW. (Or, if the cable is on a winch, to cut the cable.) This mechanism has to work reliably even with tension on the cable up to the maximum permitted weak link (plus a safety factor), and from all conceivable angles. I know Tost did a lot of work making sure this works properly, so I would not be too concerned if it's a regular Tost hook. But somebody else might come up with a different attachment, and I would take a good hard look at that.

Glider flies lower than the tug.

True, but the glider normally flies *above* the slipstream (at least here in Europe - Wikipedia claims Australia does it differently). That's why it's called a high tow, despite being lower than the tug. (By the way, I was told to keep the tugs wheels on the horizon. That would automatically lead to the proper position.)

And of course things can and do go wrong. Keeping proper position behind the tug is hard, the first few times. So the glider may well end up above the tug momentarily.

Additionally, the glider Vs is typically a lot lower than the tug Vs so in the later stages of the ground roll you will typically find the glider is already in the air, flying along in ground effect, while the tug is still firmly on the ground. As a glider pilot you need to be very aware of the danger this poses to the tug, and stay as low as possible until the tug is well and truly airborne, after which you follow it up.

Edited: I was browsing the Tost website and they've got a whole bunch of EASA certified installations for various aircraft. Including the C172, but not the 180/182. But it must be possible to work from that, and come up with appropriate paperwork for the 180/182.

[astir 8]

I can't help too much with the technical bits but by way of explanations to previous posts|:-

The only aerotow I've done in Australia was in the "low tow" position (under the slipstream. The reasoning is apparently that in the very strong thermals encountered in Australia (and I'll vouch for that!) there is more risk of the glider "upsetting" the tug from the high tow position. ("upsetting" = dragging the tug tail upwards and sending it into a steep dive) .

The other comment is that you need a weak link at the tug end of the rope mainly to avoid nasty damage if the trailing rope hooks on something (perimeter fence etc) as the tug is landing. Tugs are not built as strongly as carrier aircraft!

You also need a weaker weak link at the glider end of the rope because the glider pilot doesn't want a faceful of rope if there is a break resulting from turbulence or ham fisted flying. Those ropes can contain a lot of stretch and energy so the rope should have its weakest point at the glider end.

That's another good reason for using graduated weak links and not a "weak rope"

[thing]

You can fly under the slipstream as well. It's splitting hairs to talk about flying over the slipstream but under the tug. The point is it's highly dangerous to fly above, you can easily stall the tug, or indeed on the ground run tip it nose down.

[BackPacker]

The point is it's highly dangerous to fly above, you can easily stall the tug.

I would not call "sending the tug into an uncontrollable nosedive" a stall. If anything, you are *reducing* the tugs AoA.;)

But yes, there is a sort of "cone" behind the tug in which you need to remain as glider pilot. Outside of that "cone" the tug runs out of rudder or aileron authority and things go very bad very quickly.

The other comment is that you need a weak link at the tug end of the rope mainly to avoid nasty damage if the trailing rope hooks on something (perimeter fence etc) as the tug is landing. Tugs are not built as strongly as carrier aircraft!

You also need a weaker weak link at the glider end of the rope because the glider pilot doesn't want a faceful of rope if there is a break resulting from turbulence or ham fisted flying. Those ropes can contain a lot of stretch and energy so the rope should have its weakest point at the glider end.

I agree in principle that you need two weak links. A weak weak link at the glider end, and a stronger weak link at the tug end. I'm just wondering how this works in case of a retractable rope. I was looking at the pictures of this system at the Tost site and can't find anything like that.

[thing]

I see your point but it depends how experienced the tuggie is. A new guy may well be liable to keep pulling back on the stick to keep the combo flying if the glider gets high, I've seen it happen where the tug is barely scraping through the air before the glider pilot gets him/herself sorted. Those sort of moments where you hold your breath....:)

[Jim59]

The weak link in the rope MUST go at the tug end - not the glider end to protect the tug.
Other way around - the weak link should be weaker at the glider end. Obviously the maximum strength should protect the tug but there should be a presumption that the rope will stay with the tug, not the glider.

Sorry you are wrong; with a non-retractable rope the tug will virtually always specify a weaker strength than the glider. In fact many gliders specify only the weak link to be used for winch launching. The weak link must be at the tug end.

If retractable winches are used the weak link would be at the glider end, however it is assumed in this case that when landing the rope will have been retracted (with a pilot indicator to confirm this), so it can be assumed that it will not catch on a ground obstacle.

Typically tugs used in the UK specify a Yellow weak link for towing (400 daN +/- 10% - within 2% of 400 kg). I have also cone accross White (500 daN). ( 1 decaNewton or daN = 1.02 kg force.)

Typical glider weak link values
winch aerotow
2 seat K21 1000 daN 600 daN
2 seat Grob 103 745 daN 745 daN
2 seat Puchacz 750 kg 500 kg
1 seat Discus 1 680 daN 680 daN
1 seat ASW24 660 daN 660 daN
1 seat Pegasus 590 daN 590 daN

If a tow rope breaks at the tug end the chances are that it will end up dangling well below the glider (assuming the glider is in the towing position above the slip stream). In that case it is usually recommended that the glider fly back to its home airfield and release the rope in a suitable place there rather than on an unsuspecting member of the public.

[Jim59]

You also need a weaker weak link at the glider end of the rope because the glider pilot doesn't want a faceful of rope if there is a break resulting from turbulence or ham fisted flying. Those ropes can contain a lot of stretch and energy so the rope should have its weakest point at the glider end.

That used to be true with nylon ropes, pretty well all operations that I've seen in recent years use less stretchy rope such as pre-stretched polyester that avoids that problem.

For what it's worth most of the rope breaks I've been aware of have been on the ground either if the tug pilots takes up the slack in the rope with excess vigour and it fails when it jerks tight, or on grass when the ground is soft, the glider's wheel has sunk into the ground, and the tug is pulling it out of its hole. In the air I've only known of rope breaks when the glider has got very badly out of position, a large bow has developed in the rope and it has failed when the rope has gone tight again. Mainly when training glider pilots to aerotow - but occasionally if a more experienced pilot is distracted by doing non-essential tasks on aerowtow such as raising wheels or adjusting instruments.

[Prop swinger]

Hi Jim59,

Quote from the aerotow guidance notes linked to above:There is an argument for having weak links at both ends, chosen so that the glider end weak link would fail before that of the tug end.The reason being that tugs are used to landing with a dangling rope, glider pilots are not.

Pilot DAR,I was presented with the perceived risk of the glider being dragged into the trees, and snagging down the tow plane.A more likely danger is the tug pilot cutting it a little fine on approach & wrapping the rope round a tree/hedge/fence.

[Pilot DAR]

Thanks for the replies so far, I'm finding it very informative!

A more likely danger is the tug pilot cutting it a little fine on approach & wrapping the rope round a tree/hedge/fence.

This gets to the heart of one of the original questions. Is it really a hazard? Does this happen? I imagine a rope whipping along at 80 MPH, would it have the frredom of motion to actually wrap around or otherwise snag on something to the point where it created a hazard? I more imagine it slipping past angrily but not having time to snag.

Schwiezer hooks are approved on many Cessnas as a part of their tyoe design (they appear in the parts catalog), but I cannot find anything authortative of approved which describes limitations or other oprtating requirements for Cessnas so equipped. The Cessna drawing for the Schweizer hook on the 150 does say maximum glider weight 1200 pounds. There is no statement if this is a structural or performance based limitation. I have spoken with the Cessna Tech Rep, but information is sparse...

Anyone seen data on this?

[vihai]

Pilot DAR, in my experience at our gliding club it happened badly that a cable end wrapped around a tree while doing the low pass to drop the cable.

There has been a loud bang but since the towplane was fast the cable broke without the towplane losing too much speed.

No, we didn't have the weak link at the tug side.

A pilot here also tells that he had the cable wrapped around the gear leg of another tug but I hardly can imagine how it could have happened.

[Jim59]

Pilot DAR
Hi Jim59,

Quote from the aerotow guidance notes linked to above: Quote:
There is an argument for having weak links at both ends, chosen so that the glider end weak link would fail before that of the tug end.
The reason being that tugs are used to landing with a dangling rope, glider pilots are not.

I'm aware of that view, but the practical problem is that if the tug is using a Yellow weak link (400 daN) - because that is the max. permited to protect most tugs - then the glider will have to use the weakest weak link Tost make - a Green that breaks at only 300 daN. This will increase the likelyhood of ropes breaking in the air unnecessarily and creating real hazards, whereas the actual numbers of occurrences of weak link failures in the air during normal flight are very few at present. There is a risk of a fix for a non-existent problem creating additional danger. The ropes I make-up only have one weak link - at the tug end. The cordage I use is about six times stronger than the tug's weak link.

[FullWings]

This gets to the heart of one of the original questions. Is it really a hazard? Does this happen? I imagine a rope whipping along at 80 MPH, would it have the frredom of motion to actually wrap around or otherwise snag on something to the point where it created a hazard? I more imagine it slipping past angrily but not having time to snag.
Yes, it really is a hazard.

Last year I was standing about 50m from a fence when a tug approached too low over it. There was a sort of "BzzzWRRRpttttttTTT!" noise, bits of wood went flying and a fence post got pulled out of the ground, trailing barbed wire.

Interestingly, the weak link at the tug end didn't go. The energy to uproot the fence came from the flying speed of the tug, so a sustained strong pull could have had interesting consequences...

We have a weak link at the glider end and a slightly stronger one at the tug end, so that the rope stays with the tug after after a break rather than dangling off the bottom of the glider (or dropped on people, cars, houses, etc.)

When I've flown from mountain sites, or ones with little/no immediate landing options, they generally don't use weak links, for obvious reasons.

[mary meagher]

John Mariott has written a fairly comprehensive book on tugging, I recommend it (as he asked my advice while writing it!}

In the UK the weaker link is at the glider, the slightly stronger one at the tug.

I found myself pushing a rope once that hadn't been correctly fastened to the tug, while flying with a student; this happened about 220 feet on a nil wind day climbout, so we were fairly far from the airfield already, so I took control (when in doubt there is no doubt), turned and had a look and we managed to return and land in a corner of the airfield; dropped the rope before crossing the hedge, and it took 5 hours to find the blame thing - kept imagining it all wound up in the combine harvester....

The student grumbled, he said HE could have done the return! I said, yes, but I wasn't sure I could do it!

We think it quaint that the Australians insist on flying the curious low tow, giving another set of problems early in the launch. Here in the UK, the glider lifts off gently with flying speed, waits for the tug to begin the climb, and then with the TUG POSITIONED CORRECTLY ON THE CANOPY OF THE GLIDER, carries on following NOT TOO HIGH, but above the slipstream. Low tow and boxing the wake are exercises in skill and recovering from oscillation, etc.

Reason we teach tug position on the canopy, rather than the picture of the tug wings on the horizon, Backpacker, is that we sometimes fly in mountains, or possibly with clouds etc obscuring the picture. Flying up into a nice big ridge at Talgarth, in Wales, how on earth could you check your relative position by tug wings on the horizon?

But the big no no is DON'T PULL UP HIS TAIL! That's how to kill a tug pilot. Brian Spreckley and Chris Rollings practiced upsetting tugs at 6,000 feet or so, and it takes the tug at LEAST 2,000 feet to recover. Trouble is, especially with a belly hook on the glider, or too short a rope or too stupid a glider pilot, is that it is likely to happen lower than that. The tug plane is in effect suspended from the glider until the rope breaks, WITH NO FORWARD SPEED AT ALL! It is not in a dive, it is STALLED!

So first of all, training new pilots to fly the glider on airtow, make sure they can hold position correctly, recover from every kind of oscillation, and when in doubt, let go. Tuggies, your left hand should be very very near the rope release handle. And if in doubt, dump him!

New tug pilot at our field forgot he had a rope on the back, and pulled up 3 fenceposts with barb wire attached. Didn't damage the aircraft. The farmer - who always turned up when somebody did something stupid, presented him with a post hole digger and he had to spend the rest of the afternoon mending the fence......