Quick Windmill Relight was a regulatory reaction to come incidents where a pilot inadvertently shutdown one (or both) engines when they intended to do something else, and quickly returns the fuel switch to 'RUN'. This first became an issue after introduction of the 767 - the supervisory EEC switches were on the aislestand near the fuel switches. There were two cases where - during Takeoff - one of the EECs failed and the pilot reached down to turn them off but instead pushing the pushbutton EEC switches, the pilot moved both fuel switches to CUTOFF
. Quickly realizing their error, the fuel switches were quickly returned to 'RUN' - in the first case (JT9D engines), one engine recovered, the second went into a start stall - they did an immediate return to land. The general consensus was that the pilot was an idiot (push button switches vs. fuel condition levers) and it wouldn't happen again. Then it did
. This time CF6-80A engines on takeoff out of LAX - this time both engines recovered (but not before they got within a few hundred feet of the Pacific Ocean) and the flight continued on to Chicago (
) - when the passengers disembarked, some were still wearing their lifejackets... Anyway, after the second incident it was obvious we had a serious human factors problem, and the EEC switches were moved from the aislestand to the pilot overhead and to the best of my knowledge it hasn't happened since on any aircraft type (except for a certain Egypt Air crash, where unless you're an Egyptian investigator, it's believed it was an intentional act).
However the feds jumped on this and insisted we needed to demonstrate "Quick Windmill Relight" capabilities on all new engine types. QWR is a very challenging requirement - especially if the engine was at power prior to the inadvertent shutdown. If the engine was at power prior to shutdown, the compressor is hot and heats the incoming air during the relight attempt - which has the effect of 'overfueling' the engine (compared to a normal windmill start) leading to a start-stall. FADEC has the huge advantage of being able to recognized it's a QWR and adjust the starting fuel flow, but it's still a very challenging design requirement (it's also very, very hard on the engine since shutting it down at power usually results in a major compressor rub due to the rapid, differential cooling of the rotor and case).