NOTE: If driver’s hair extends appreciably from beneath helmet, it is MANDATORY that the driver wears a “balaclava” or head-sock to prevent the hair from extending outside helmet. …….
To me, ‘appreciably’ is rather vague, but ‘to prevent the hair from extending outside helmet’ is concise.
If I was doing pre-tech and suspected someone’s hair of being too long, I’d have ’em put on their helmet and if hair was below the bottom edge of the helmet, tell them to get a sock. If none available, I’d give them my unused spare.