evidently going pear-shaped is different to having pear-shaped tones. the word detective ties the former phrase to the Royal Air Force, where learning to fly apparently includes doing acrobatic loops. Difficult for even an experienced pilot, these loops as performed by a novice are more than likely to appear lopsided and "pear-shaped." One can imagine observers on the ground saying, "Good. Good. Oh rats, he's gone all pear-shaped."

it's somewhat of a contranymic phrase, what?