https://en.wikipedia.org/wiki/Aircraft_hijacking cites two figures for the U.S. rate of hijacking attempts near its peak. The higher one is "Between 1968 and 1977, there were approximately 41 hijackings per year." meaning once per 9 days, an order of magnitude below yours. The other is once per 13.3 days over 1968-72. If both Wikipedia numbers are reliable then the peak must be somewhat higher than 1/9 in 73-77.