Per Barry J in Horabin v British Overseas Airways Corporation  2 All ER 1016 at 1019:
Wilful misconduct is misconduct to which the will is a party, and it is wholly different in kind from mere negligence or carelessness, however gross that negligence or carelessness may be. The will must be a party to the misconduct, and not merely a party to the conduct of which complaint is made. As an example, if the pilot of an aircraft knowingly does something which subsequently a jury find amounted to misconduct, that fact alone does not show that he was guilty of wilful misconduct. To establish wilful misconduct on the part of this imaginary pilot, it must be shown, not only that he knowingly (and in that sense wilfully) did the wrongful act, but also that, when he did it, he was aware that it was a wrongful act, ie, that he was aware that he was committing misconduct.