Clandestino,
That is exactly the danger with the ATR because the anti- and de-icing equipment is doing a terrific job. Nevertheless in icing conditions more than light you will start to accumulate ice all over the plane including the underside of the belly the pack inlets antennae etc. I'd like to know how much icing the boots on the ATR could really take... and make you believe everything is fine.
I have not been able to find your reference on FAA severe icing elsewhere than on the NOAA page.
The ridge that formed behind the boots on the Roselawn ATR may be strong evidence that they were flying in heavy icing conditions.
The real danger lies exactly in this equipment being so efficient that the crew may think they are safe. But runback ice is always a problem. So I fully agree with your last statement that at that time they probably could not have known.
I'm happy that ATR had to change the design of the boots and other measures which make the plane even safer. What probably led to the fact that icing incidents and accidents have nearly been eliminated on the ATR may be the specific procedure changes calling for: Leave severe icing conditions immediately.
Nick NOTOC,
I fully agree and actually do the same. As long as you see ice accumulate get out of it as long as you still can. Unfortunately ice has the tendency to build up on the window frame, wipers, ice evidence probe to quite remarkable sizes but will also brake off from time to time suggesting less icing than actually present. I usually start a timing at first ice accretion and after 10 minutes I'll get nervous if the accumulation is still strong.