There is more to sending students solo than just completing the flight curriculum and the proper presolo tests and paperwork. At some point the instructor must judge whether students have acquired sufficient skill, knowledge, habits, and the confidence to proceed by themselves. Good judgements are based upon understanding the pilot, the aircraft, and the environment. It is worthwhile reviewing each of these in detail with respect to sending students solo.
The students should know by now how to set up the aircraft properly so it is easy for them to control. However, instructors should check that this is done properly as the students may be excited and miss something at this stage. Also before solo in a two-seater, the instructor’s area of the cockpit should be secured, so it will not interfere with the actions of the student. Additionally with tandem seating, the weight and balance will be different. At a minimum, the student should be briefed about any changes in controllability or other factors that may arise when they are solo. Extra weight may have to be added to certain aircraft to more closely match the performance seen with two on board.
The student should be also only be sent solo in familiar weather conditions. This usually means good visibilities and ceilings. However the key word is familiar weather conditions. If the majority of training flights have been in high winds, then it may be acceptable to solo the student under these conditions, as this would be the norm for them. There can be considerable variation in what is considered normal from one geographic location to the next. Again what is more important is judging the effect of any changes from what the student considers normal. The response to these changes might vary from a briefing, to some familiarization flights, or even a postponement of the solo. Remember, even experienced pilots have difficulty in adapting to changes in the prevailing conditions.
For most students the best conditions are moderate winds (5- 10 knots), no more than 10 degrees off the active runway, good visibility (>15 miles) with a distinct horizon, and a good ceiling (> 2000 feet). While calm conditions may be considered good, some caution must be given to soloing near sunrise or sunset when glare may be a problem. While usually a minor factor, runway condition, width, and length may be important under some circumstances. Choosing a low traffic time and extra effort at clearing aircraft from the runway is very helpful to students.
While most glider pilots want to get to the soaring part as soon as is possible, this may not be best for the first solo. Novice pilots are better served by a couple of quick flights with good circuits and landings rather than a single longer soaring flight.
The readiness of the students must match the above. The student should be reasonably well rested. Students should not be soloed if they have had more than 4 flights that day. The total time at the airfield can be a factor as well. The student may have only done a few flights but was active all day and has become fatigued.
Mental readiness is an entirely different matter. Aside from flying skills, pilots ready for solo generally show a greater attention to the details of their environment. They have some definite opinion about the effect that it will have on their flight and a general plan. While they may question whether they are ready to solo, they are generally only looking for confirmation from their instructors.