I had to find this. There's an excellent article in the June 98 FlightFax on the US Army's safety website that deals with this issue. The article is on page 5.
http://safety.army.mil/pages/media/p...98archive.html
BTW, since the US Army's aviation effort is primarily helicopters, the monthly FlightFax magazines that their flight safety group puts out, are just outstanding reading. These are some of the best helicopter flight safety publications that I've ever seen anywhere. Here's a link to the main FlightFax index page.
http://safety.army.mil/pages/media/pubs/ff/
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Safe flying to you...