Knife Edge Countersinks

[liaisoneng]

On the subject of knife edge countersinks,everyone agrees they're poor practice but the actual criteria vary. I know a Boeing standard is 66% sheet thickness for max allowable countersink depth (with Briles being 75%). I was wondering what other criteria are out there (other companies, government, etc). Also, does anyone have real world examples of the dangers of knife edging? I can't find anything in the Aloha incident that definitely confirms it as an issue. Any info would be greatly appreciated.
Russ

 

[chuck1]

Russ,
I know that when I worked at McDonnell Douglas, the design allowables for aluminum flush head rivets installed in machine-countersunk sheet with greater than 70% depth of countersink were reduced. The reason was that head tilting will occur under relatively low loads, which adversely affects fatigue. Off-hand, I'm not sure how much the allowables were reduced.
Chuck