In a web application under Tomcat with AsyncAppender in use, StatusPrinter reports "Setting discardingThreshold to 51". The AsyncAppender is purely default; no discardingThreshold value has been set. The documentation at http://logback.qos.ch/manual/appenders.html indicates that events will be dropped when the blocking queue has 20% capacity remaining. Although it never explicitly says, I take this to mean that the default value should be 20, not 51.