In an interview, a sister of 90 years old told me that at the end of the war, an airplane feld down near the village of Nieuwmunster - Belgium. The Germans, who lived in the village said to each other : "Grossmutter ist gefallen, grossmutter ist gefallen ?" Can somebody give me more info about this "grandmum" ?

At a rough guess, when you recall the German people talk of "the Fatherland", it is reasonable to associate a maternal figure with their ethos, and with "grandmother"- the old and wise protector gone, they had nobody to fall back on in stressful times. Suddenly THEY were the elder generation...with no backstop or wicket-keeper,
and no 'home' to run to. Pork chops at a Jewish wedding, as it were.

Sorry, Neville, there was no such a 'grossmutter'. The expression 'grossmutter ist gefallen' means 'grandma has fallen'. There are two meanings: Grandma is dead or Grandma has fallen down. As this saying was done after an a/c crash I would suspect that the 'fallen down' meaning is the best.
Sometimes aircraft where given personal names. Maybe the crashed aircraft was an old, large, slow type with an high number of flying hours. Without any details nothing more can be said.

That is very likely...BTW, there was a plane called "Grosspapa" (Fokker D III 1612/16 at Kesta 4b), so I wouldn´t be surprised at an aerial Grossmutter ;)