I have choquett avocados. They ripen at the crown turning brown and mushy.
Before the rest of the avocado. My wife tells me I'm bringing them down too early
In October. Presently you just have to cut away the mushy part, the rest of the avocado
Is OK.

The link below explains...
---------------------------
Tony Tantillo - Produce Tips - Fruits (http://tonytantillo.com/producetips/fruits/avocados.html)
Hard, unripe avocados should be stored at room temperature and usually take three to six days to ripen. Put avocados in a paper bag with an apple, banana, tomato, or pear, all of which give off ethylene gas, which speeds up the ripening process. Never put hard avocados in the refrigerator because they will never ripen. Ripe avocados will keep in the refrigerator for up to one week.
---------------------------------

The problem is not the ripening. But the way they ripen. Always
At the crown. They do not ripen evenly.