How often was a sub attacked by aircraft on patrol?

[AkbarGulag]

This is probably the greatest place to ask this, as many of you here have read books or actual reports/patrol-logs, for US subs in the "Pacific Theatre". How often/frequently did subs and their crews face air threats?. Clearly some would have been more or less fortunate than others, but there must have been some accepted norm while in theatre. How high was the risk for American boats?