Imagine your glider collapsing asymmetrically just over the ground. Would you react correctly? Not just by using massive brake which might result in a full stall, but by letting the glider fly again, and catching the dive and spin with appropriate brake and weight-shift actions? If you are not sure of your answer, well, you are not alone; many, even experienced pilots often make mistakes in this situation. The main problem revealed in the accident statistics (see DHV 2010) is that pilots don't manage to stop the glider from turning or spinning because they don't react quickly enough, or don't react at all. Therefore it's very important to know what your glider does, when you don't do anything.
The DHV test pilots performed several (generally at least six) asymmetric collapses according to the LTF requirements both at trim-speed (brakes off) and full-speed (speed bar pushed to its limit) for every glider. Particular attention was taken to ensure that only collapses were judged which fulfilled the LTF requirements exactly. Here the canopy must be collapsed to within a clearly defined set of markings, and the trailing edge must also be completely affected by the collapse. With the data loggers accurate measurements of the canopy and pilots movements were recorded, which is more than the requirements defined in the LTF norm.

Figure: the red lines indicate where the canopy is to be marked. On the right are minimum and maximum markings for the large 75% test collapse, going through to the trailing edge. On the left, the 50% collapse line.

Figure: schematic of an LTF-norm collapse. The collapse should fold according to the markings on the canopy, and must deform from the leading edge through to the trailing edge. Collapses which do not fold along the marked lines do not usually include the trailing edge of the canopy. Such collapses are  associated with less dynamic reactions and would falsify judgment if used in certification tests.

Asymmetric collapse table information

Height loss in meters: the entire height loss from the beginning of the collapse until stable normal flight was regained is given.
Pitch angle in degrees: the maximum angle the canopy dived to after collapsing is given.
Pitch change rate in degrees per second: processing the pitch angels and time signals provides us with this result. Higher values of pitch change rate indicate dynamic gliders, lower values show more dampened canopies.
G-Force in G: measured at the pilot. In stable straight flight 1G (gravitational pull of the earth) acts on the pilot. Dives and spins induced by asymmetric collapses accelerate the pilot and increase the acting forces. At 2G the pilot feels twice the gravitational pull of the earth and is “pressed” with twice the weight into the harness.
Course change angle: the total angle through which the glider turns from the beginning of the collapse until stable normal flight is regained is given.
V/sink max. in m/s: the maximum recorded sink velocity in m/s is given.

We were excited to see if the results from the data loggers would back up the opinions of the test pilots and video analysis. The first surprises were the relatively large pitch angles recorded when the glider dive forward after the initial collapse. According to the LTF norm, class A and B gliders should not dive forward more than 45°. Only a few of the tested gliders managed to roughly remain within this limit. Many of the LTF-B gliders had much greater recorded pitch values, some in excess of 60°. An explanation for these discrepancies are that during testing the pitch angles can only be roughly judged by the test pilots and from video material. Here there are no fixed reference points, hence the results are likely to have large margins of error.
In the tables above, the recorded angles give a good indication of whether a glider reacts moderately or dynamically to a collapse:
Large negative pitch angle – fast pitch change rate – high G-force – high V/sink => large height loss
Small negative pitch angle - slow pitch change rates – low G-force – low V/sink => moderate height loss
From the tests with the gliders in the table we note:
The friendliest LTF-A gliders Paratech P12, Gradient Bright 4 and Skywalk Mescal 3 all need about 30m to regain normal flight after a norm asymmetric collapse. The Nova Prion was a little more dynamic. Niviuk Koyot and Ozone Mojo 3 required almost twice as much height to recover and recorded significantly higher sink velocities. Such a bandwidth under class A gliders is unwanted. In the safest glider class possible, pilots ought to be able to expect the maximum in technically possible passive safety from their gliders. Reactions to asymmetric collapses range from “manageable” to “unexpectedly dynamic”. On the positive side, none of the tested gliders showed any particularly undesirable tendencies (e.g. to cravat after collapsing or similar).

Under the LTF-B gliders the differences were, as expected, greater. Low-end gliders Paratech P28 and Swing Arcus 6 were almost identical to the LTF A models. Others were somewhat more dynamic, but all within acceptable limits.
Under the high-end LTF-B gliders things are different: the bandwidth here, of possible reactions to a collapse is much greater. All gliders demonstrated reactions applicable to the test norm for some   collapses, but could also react far more dynamically on occasion. The Skywalk Chili 2 collapses particularly softly and unremarkably. On recovery the outer part of the collapsed wing inflates first, which occasionally lead to a cravat. The Nova Mentor generally reacts very well to collapses of all sizes, but again can react particularly dynamically on occasion. Large collapses, on the outer limit of the norm measurement field can lead to cravats, and require active recovery from the pilot to prevent entering a spiral dive. Cravats, generally the “worst case scenario” for hobby pilots occurred on Ozone's Rush 3 when collapsed massively. This glider passed the LTF tests with folding lines attached to the front of the canopy, and reacts more dynamically to collapses induced via the A-risers. We note here critically, that folding lines are not necessary to collapse the glider according to the LTF-norm. The Niviuk Hook 2 was particularly dynamic, with fast rotations, high sink velocities and large height losses of approximately 70m after collapsing.
Some of the gliders found in the high-end B sector clearly have the potential to expect too much from a hobby pilot. It is evident that the line between “appropriate behavior according to the norm” and “not certifiable” is very thin in some instances.

From accident investigations we know there are two main problems when dealing with frontal collapses: if a glider recovers very slowly, often combined with collapsed ears then should the pilot apply brake at the wrong moment, this can lead to a full stall. If pilots attempt to open the collapsed ears by applying symmetric brake before the glider has regained sufficient airspeed, then this may result in a full stall with all its consequences and height loss. For such gliders it is particularly important to adhere to the approved school of thought: “hands up” until the glider has regained normal airspeed.
Another problem which is becoming more evident with the popularity of higher aspect ratio gliders, is the tendency of a canopy to deform in the middle “horseshoe” after a frontal collapse. Re-inflation often occurs asymmetrically, and the collapsed part of the canopy may then hang in the lines or cravat. It is often not possible to stop the following rotation without resorting to a stall. These characteristics are nothing new, but are often not revealed though the LTF norm tests, especially when the tests are conducted at the minimum folding limit. Advanced pilot skills are required to deal with such situations: a short jab on the brakes is required at the right moment to prevent the glider entering a horseshoe. Woe is in store for the pilot who stays on the brakes for too long – the canopy is prevented from regaining airspeed and can directly enter a full stall.
In our tests the Niviuk Hook 2 demonstrated a tendency to horseshoe, combined with fast rotation speeds and a danger of twisting.
“Normal-speed symmetric recovery” from a massive frontal collapse as demonstrated by e.g. Nova Ion Light and Prion, Skywalk Mescal 3 and Gradient 4, are the least challenging for the hobby-pilot. A significantly delayed recovery, or the tendency to horseshoe require much better pilot skills to manage safely, and are always associated with greater loss of height.

Serious injuries and fatalities from spiral dives are one of the biggest safety problems in paragliding. Glider characteristics are one area which needs to be addressed, but for many pilots the far greater danger comes from the unexpected forces acting on ones body. Most spiral dive accidents result from pilots loosing consciousness or blacking out due to the high G-forces experienced. A big step in paragliding safety could be made if untrained pilots simply avoided this manoeuvre.

Spiral dives are subject to constant and repeated discussion at the LTF and EN working group meetings. No other manoeuvre can be so easily manipulated by the actions of the test pilot; different entry speeds or slight weight shifting from the neutral position can make for big differences in the test outcome.
To date, paragliders are tested to sink velocities of max. 14m/s under the LTF norm. In future, the recovery behavior of the glider after a spiral rotation of 720° will also be judged. Most gliders have significantly higher sink velocities at this point.
We followed these plans in our tests, so the results are not directly comparable with the current LTF tests.
Two turns at maximum speed in the spiral and then “hands up” to see what happens – requirements on gliders (and test pilots) are tough. Results show that the normally “friendly” gliders can react particularly differently under these conditions – some gliders reacted with further acceleration and only a very gradual recovery to normal flight on their own. It is known that compact low aspect ratio glider designs are often more associated with recovery problems than higher aspect ratio gliders.
The recorded datasets from our loggers provided interesting information after processing. For example, the rate at which rotation speeds and sink velocities increase gives a clear indication of the dynamics present in the manoeuvre. High sink velocities attained in short periods of time are typical for gliders which quickly begin to dive. Gliders which enter the spiral more moderately can be identified with the lower build up of sink velocities. The rightly feared “dive point” in a spiral, where the canopy suddenly begins to markedly accelerate can also be clearly identified in the data. Datasets show an abrupt increase in sink and rotation velocities between the first and second turn.
G-forces are usually greater than 3.5G in a massive spiral dive. According to medical studies (see under  dhv.de), forces such as these are border-line for untrained pilots and can lead to blackouts or loss of consciousness. Experiences made at the G-force training station also validate this. Some gliders reached even higher G-forces, top was the Swing Mistral 6, with 5.5G at 27m/s sink velocity. As a side note: the length of a gliders lines also play a role in the G-force felt by the pilot: short line gliders do not develop as much G-force.

Things usually work out well, as long as the ground is far away enough, as we found in our tests. The Sing Mistral 6 was the only glider which entered a stable spiral dive and required active pilot reactions to recover from it. All the other tested gliders recovered (some only after severe height loss) on their own. Further evidence, that spiral dives should only be conducted with sufficient height reserves.

Many experienced pilots do not think highly of B-line stalls as a descent method. However, in the LTF-A and B classes, clean glider behavior in a B-stall is very important. For pilots who are not confident in flying spirals over prolonged periods, the B-stall is an important alternative in an emergency. “Good behavior” in respect to B-stalls is defined as follows: the B-risers should be clearly marked to help prevent danger of confusion, the force required to hold the stall should be moderate, sink velocities should be significantly higher than those in a big-ears descent, the canopy should not deform or horseshoe while in the B-stall and recovery should occur with no deep or parachutal stall phase.

In our tests, B-stall of over 10 seconds were performed. This is important to identify if a canopy has a deformation or horseshoe tendency – LTF tests are often kept far too short.
Data loggers recorded how far back the glider pitched on entering the B-stall, and how far forward it dived on recover. In addition the sink velocity and height required for recovery was recorded.

Big-ears is probably the most important descent method for hobby pilots. Her we don't suffer from high G-forces as in a spiral dive, or have to control a glider in a B-stall . While descending with big-ears the glider still flies forwards – and this is often just as, or more important than the descent itself. Via weight-shifting, directional control is possible at all times. This manoeuvre must be simple and safe for all beginners and low-airtime pilots: simple entry, easy to maintain over prolonged periods, the canopy should remain stable and not thrash around, the sink rate should be effective and exiting should occur without any further problems. In addition, no deep stall tendencies should be apparent when flying this manoeuvre.
None of the tested gliders had real problems with this manoeuvre. Differences were noted in the maximum sink velocities. Most gliders sank at around 2.5m/s when flown at trim speed. Both Nova gliders, the Prion and Ion Light sank at about 3.5 m/s; these gliders only have two main A lines, and so the reduction in area on big-ears is greater than by other gliders with 3 main A-lines. Once the gliders were accelerated to full speed with the speedbar, the faster gliders also had the highest sink velocities.
Top was the Nova Mentor with 5,5m/s, the rest in the B class reached sink values between 4 – 4.5 m/s. A class gliders were between 3-3.5 m/s – top here was the Nova Prion at 4 m/s.

Flying with big-ears naturally reduces the gliders airspeed. The additional drag causes a reduction of between 5-8 m/s in airspeed, depending on how much area has been folded. Pushing the speedbar to its limit brings the LTF-A gliders back to their normal trim speed, LTF-B gliders fly between 5-10 km/h faster than trim.

None of the gliders showed a tendency to enter a deep or parachutal stall. At full speed, the Niviuk Hook 2 tended to thrash a little, all other gliders remained stable.
All gliders have separate A-risers for entering big-ears, but even in the A class, not all gliders had clear labeling or colouring on these risers. Nova and Skywalk have taken the worries of flying schools to heart, and help to reduce the danger of confusion by clearly marking the big-ears risers.