Pre-flight ice check
Do you remember a SAS MD-80 crashing at ARN a few years ago? The cause was clear ice on the upper wing detaching after take-off and damaging both engines.
To assist the crew in spotting clear ice (especially at night) the MD-80 now carries a small set of steps for the crew to stand on to see over the wing. If the red string is not visible and moving they need de-icing.