More of a 'I Learnt About Flying From That' was the Wessex pilot who parked his aircraft on a sloping parade square and hung his helmet by its strap from the rotor brake handle in the cockpit roof. Went off for a brew. The brake lever disengaged itself from its clip, helmet dropped and flicked the parking brake lever to off. With no chocks, the Wessex trundled down the square and trashed its tail rotor on the wall at the bottom.