Crash one.
Leuchars operated Lightnings until around 1969.
F4 Phantoms followed for the next several decades with Tornados only becoming operational from that Airfield in April 2001.
Since you mention the lake (loch) you must have been flying from the Scottish Gliding Union at Portmoak - I am left wondering who the instructor was as I must admit to never having heard of this incident. Perhaps you could PM me with the name as I do have an interest as a past CFI and safety officer at Portmoak.
It is possible that the aircraft were indeed Phantoms as the Americans also operated this type within the UK and were, quite often, involved in the low flying and other military operations with the Scottish region.
The usual route to/from Leuchars was to the North of Bishop Hill between that and the Ochils. Americans, unfamiliar with the area, might well have aimed for the gap between Bishop and Benarty Hills although that track would also have taken them low over Glenrothes airfield to the East of Portmoak.
Leuchars were well aware of Portmoak and there was a letter of agreement between them and the Scottish Gliding Union re their circuit requirements in Easterly winds as they came close to our own circuit in those conditions.
A somewhat out of context post but I am intrigued by your experience.
As an aside low slow or steep turns in gliders close to the ground, especially long span ones, were never a good idea. This was not so much because of the danger of the lower wing stalling from being in the lower slow moving mass of air on turning into wind but more from the differential lift between the tips at differing airspeeds. The lower wing in slower wind speed due to wind gradient effect plus moving at lower speed than the outer wing equals a considerable amount of extra lift on the upper wing. The early long span gliders were a bit lacking in aileron effectiveness and thus might not have had powerful enough ailerons to actually level the wings for landing.