Pilots have to be able to handle a wide range of emergencies in the air. For powered aircraft, the obvious example to me is recovering from an engine failure, but obviously this can't happen on an unpowered glider. Many gliders have no electrical systems, so an electrical failure can't happen. A stall, I believe, can, and I assume the recovery would be similar to in a powered aircraft. What other emergencies should a glider pilot be able to handle? I'm especially interested in any emergencies that powered aircraft don't have to deal with.
The number one emergency in a glider is a tow rope break at low altitude, less than 200 feet above the ground with no appreciable headwind. Below 200 feet, the proper technique is to land straight ahead, which usually means landing off airport.
Between 200ft and 500ft, it is possible to make a U-turn back to the runway, and land in the opposite direction in which you took off. The main hazard with this kind of landing is landing with a tailwind, and once the wheel touches the ground, the flight controls operate in the reverse direction.
Above 500 feet, it is possible to fly an abbreviated pattern and land on the runway you took off from.
Add 50 feet of altitude for each 10 knots of headwind.
Some other tow emergencies are inability to release to tow rope, in which case the tow pilot needs to release the rope on his end; engine failure of the tow-plane; and a double tow rope release failure, which means the glider needs physically break the tow rope (it has a pre-determined breaking strength of 80-200% of the weight of the glider) or to land behind the tow-plane.
You can also run out of altitude and have to land in a field, which is called an "off airport landing." Someone has to come and get you with a trailer, and you disassemble the glider and drive it away. One hazard is the off airport landing in a field, and the farmer comes out with his shotgun.
Other hazards are flying into the clouds or into severe turbulence, which can happen when the glider pilot is flying "wave," a kind of lift generated by strong winds in proximity to mountains.
When thermaling, you are flying very tight steep turns, often with other sailplanes who are also trying to gain altitude. Everyone is supposed to fly the thermal in the same direction, and there is the hazard of colliding with other gliders.
In addition to the emergencies mentioned by rbp, there's also spin recovery. Presumably power pilots need to know this as well, but it is more relevant to glider pilots because thermalling can take you close to the incipient spin territory. In Australia, at least, real spin recovery is practiced until the pilot is proficient at recovery.
Another very important accident that can occur is what I know as the Kavalierstart. It occurs on winch launches with some airplanes when the pilot is not pushing the joystick all the way forward. The airplane gaining speed really fast will pitch up and as it is going up with a very steep angle, the lift will actually pull in the opposite direction of where the winch rope is coming from. Due to the excessive load the rope will break, and as the hook is before the CG, the missing force of the rope will cause the glider to continue pitching up bringing it into a upside down attitude (as it is already flying up pretty much vertically). Without the rope pulling the glider it will almost immediately lose its airspeed and drop down. There is no way over recovering this and it usually ends up in killing everyone on board.
Answering an old question but there are issues with the winch launch that have not been covered; one is a wing drop on the ground run. If the wing touches the ground while being winched then the glider can cartwheel if the pilot is not quick enough or able to pull the cable release. The winch can keep pulling the aircraft until the winch operator spots a problem and stops the launch or the pilot pulls the release. This happened at my club about 3 years ago (2012) with an experienced pilot in a Nimbus 3. He died of his injuries.
There is also the possible issue with cable fouling, although unlikely it is possible in a shallow fast climb to dip the nose on release and impact the cable if the cable parachute has opened. That will ruin your day.
Pulling too far back on the stick on the ground run can cause an overly-quick rotation into a steep climb causing the tail to hit the ground. While removal of the tail will reduce drag considerably it will cause one of two control issues.
In winch launches you are very much at the mercy of your ground crew and winch driver. All it takes is for a misheard instruction or someone to mistake the glider model and you could be launched on the wrong winch setting. An underpowered or overpowered launch can cause a lot of problems for a pilot but then that is why we drill for problems. I have to admit to signalling a launch for the wrong type of glider, thankfully it was spotted by the pilot who radioed a correction but I have seen gliders launched on the incorrect winch setting on a few occasions.
Pilots of Petrol Pigeons don't have a cable to worry about.
Most gliders do have battery and/or solar-powered avionics and comms, so there can be electrical failures. In fact, one must take battery life into account for long flights.
Gliders often fly much higher than other general aviation aircraft, so failure of the oxygen system with attendant hypoxia is a risk.
Let me add some more possible failures:
- Incorrectly assembled aeroplane ( actual reason for some tragic deaths ). Avoided by following check lists and instructions before even contemplating to start.
- Items dropped in the cockpit may lock up controls ( I did drop a water bottle down among the cables, luckily not locking up the controls )
- In air collision, especially when flying in clouds ( One reason flying in clouds generally not allowed in competitions )
- Heat stroke: the cockpit has plexi glass all around and in sunny weather it will be very warm. Never forget you most important personal safety item, the hat.
- Out of oxygen: we from the "low lands" can not stand any long time up high. Once above 10.000 feet (3.000 meters) we should have Oxygen along. Note that in wave flying a glider might reach 10.000 meters.
- Out of energy, already mentioned in other answers. Pilots train for this and are supposed to commit to landing once reaching minimum height above ground. Add a bit to the minimum when over rocky terrain or over dense woods.