3 degree split
Hi, for my 2 cents worth.
For about 5 years, around the turn of the century, on our Jurrasic 737-200s the company SOP included the procedure of Anti-Ice ON when OAT at +10 C or below and dewpoint within 3 degrees. I was told that this was a problem on the 727, mainly the centre engine, and with some of our tech guys having a wack of three holer experience, the same limitations were applied as a precaution. Around 2002 IIRC, this was dropped with no noticeable problems after the change, though I think the JT8D made and ate a lot more ice under normal ground operations than most people realise.