Glider pilot reported a canopy malfunction resulting in a partial separation from the aircraft during a training maneuver. The pilot along with the check pilot passenger stabilized the aircraft and returned to the airport safely.

Narrative

The following is a description of an unintended canopy opening and partial separation that occurred during the flight portion of a Practical Test for an Initial Flight Instructor-Glider. I was the Applicant for the certificate; as well as Pilot in Command for the flight. The aircraft being flown was a Schweizer SGS 2-32 glider. The Designated Pilot Examiner (DPE) occupied the front cockpit while I operated the aircraft from the rear cockpit. I performed a thorough pre-flight inspection; per the manufacturer's guidelines; prior to ground towing the glider to the launch area. We then boarded the glider; and I performed the pre-flight checklist; which included closing and latching the canopy. I would like to note that due to the experiences I have heard and read about where other gliders have had canopies unlatch in flight; the last item on my personal checklist is to ensure that the canopy (and means of entry and egress for both cockpits) is secured; with all four pins fully in place. That includes the two hinge pins on the left side as well as the two latch pins on the right. The launch and tow to altitude was normal; with a release at 3;000 ft.AGL or 8;500 ft. MSL; followed by a thermalling climb to approximately 11;500 ft. MSL. At this point; I was asked by the DPE to demonstrate an incipient spin; followed by a fully developed; 1 rotation; upright spin. Prior to performing the maneuvers; the DPE again verified the canopy latch was secure; and I performed clearing maneuvers; followed by the incipient spin. We then entered the full spin and performed the normal recovery procedure; ending with a pullout from a diving condition. During the recovery from the dive; when G-force seemed to be the greatest; I noted a slight gap; approximately 0.5-1 inch; between the latch side of the canopy and the fuselage/cockpit wall. This condition lasted for a time I estimate to be less than 1 second before the canopy fully opened to the limit of its travel. The canopy over the rear cockpit (my location) shattered and departed the aircraft; leaving a hole approximately 12-18 inches lengthwise; and fully from one side of the canopy frame to the other. It is uncertain what caused the fracture; whether it be from the shock of the opening and hitting the limit strap; the air-pressure; or possible contact with the wing leading edge. The latter is doubtful though; as it could only happen if the limit strap were to stretch (and then rebound to its previous length); and there were no apparent contact marks on the wing. It should also be noted that prior to the flight; the canopy had a vertical crack 3-4 inches long that had been stop-drilled; and that this crack bordered a portion of the missing area of the canopy.At this point the aircraft went back into the slight dive; and while I do not remember performing the action; I believe this was an unintended control input on my part due to both the startle factor; and the fact that I had just used this input to recover from the stall/spin. The aircraft briefly exceeded maneuvering speed; but before nearing VNE the DPE assumed control and pitched nose up; reducing the speed. I used the opportunity to grab and close the canopy and frame; though it was too distorted to latch correctly; and had to be held in place. I resumed control; and while the DPE held the canopy shut; and I performed an approximately 180 degree turn to a heading towards the gliderport facility. Due to the unknown extent of the damage; and his prior military experience; the DPE and I switched duties for a moment again so that he could perform an Aircraft Controllability Check; and based on his judgement recommended a speed range well over stall and under or at recommended landing pattern speed. I then resumed control; informed the ground crew of our situation over the radio; and started a descent to pattern altitude. Due to the significant wind entering the rear cockpit; and the resulting noise and distraction; I asked the DPE to relay any radio calls (emanating from the speaker in the front cockpit). He informed me that the ground crew had made sure all runways were available and which runway the winds were favoring. I also asked that he give me airspeed and altitude callouts; so as not to divide my attention further during the final phases of landing. I was able to make an abbreviated pattern and normal landing at the gliderport without further incident. I believe the possible contributing factors to the incident were: -The age; wear; and tolerances in the canopy latching system -The extra load placed on the latch during the spin recovery -The pre-existing crack in the canopy -The startle factor of the incident on myself; in allowing extra airspeed to develop.I would also like to add that the following factors contributed to the flight being completed safely: -The assistance from the Designated Pilot Examiner in noticing the excessive airspeed. -The Examiners experience in performing an Aircraft Controllability Check. -The assistance of the gliderport ground crew in ensuring all runways were clear and available. -The willingness of the Examiner to act as a valuable resource. -The training I have received to be ready to utilize all available resources; focus on the important aspects of maintaining safe flight in an emergency; and the decision to terminate the test and land as soon as possible.