When is a sailplane a sailplane? A simple question that I could not find an answer to:
When is a sailplane a sailplane (from a regulatory point of view), especially a powered one of the TMG (touring motor glider) variety?
EASA has some quite simple definitions, presumably modelled over the similar ones from ICAO, according to which:

‘Sailplane’ means a heavier-than-air aircraft that is supported in
flight by the dynamic reaction of the air against its fixed lifting
surfaces, the free flight of which does not depend on an engine.
‘Powered sailplane’ means an aircraft, equipped with one or more
engines having, with engine(s) inoperative, the characteristics of a
sailplane.
‘Touring motor glider’ means a specific class of powered sailplane
having an integrally mounted, non-retractable engine and a
non-retractable propeller. It shall be capable of taking off and
climbing under its own power according to its flight manual.

From this simple chain of definitions, it would seem that, for instance, a Cessna 152 could be classified as a TMG: It can take off and climb under its own power, and with the (non-retractable) engine inoperative it will do free flight sustained by its &quot;fixed lifting surfaces&quot;, otherwise known as wings.
Since a Cessna 152 is not normally seen as a glider, I presume that there are more rules that need to be respected before a plane can be called a glider (or sailplane in EASA's classification).
I know that CS-22 contains the specifics, but is there a simple set of rules to determine if an aircraft belongs to the class of (powered) sailplanes as opposed to normal aeroplanes?
There are indeed several steps between a pure glider and a powered aircraft. The criteria were historically quite simple:


Gliders with an auxiliary engine, which is just for getting them home in adverse weather, but not powerful enough for take-off. The propeller is in almost all cases retractable, so their aerodynamics is identical to their pure glider version, apart from a slightly higher minimum wing loading.
Motorgliders with an engine powerful enough for autonomous take-off, but with aerodynamics which gives them less than 1 m/s minimum sink with the engine switched off. This minimum sink decides if they are still powered gliders. This would be the TMG category.
Powered aircraft with an unpowered minimum sink speed in excess of 1 m/s. The Cessna 152 belongs into this category.


With the introduction of glassfiber motorgliders with their higher wing loading, the minimum sink rule could not be maintained, and manufacturers lobbied for a change in regulations. As it currently stands, CS-22 defines sailplanes to be within its scope if the maximum weight is below 750 kg. Powered sailplanes must not have more than two occupants, weigh 850 kg or less and have a span loading of 3 kg/m² (mass per span squared) or less.

CS 22.71 restricts minimum sink in unpowered flight to 1.0 m/s for single seaters and 1.2 m/s for two seaters.

Please note that suitable instrumentation should also be added for a motor glider to be counted as a sailplane, such as a variometer.