The frequency-wild generators
drop off line when the propeller rpm falls below 70%. In
operational terms, this means that when the propellers’
rpm are reduced prior to feathering following the
aircraft’s arrival on stand, the Green and Blue system
AC pumps will cease operating. This will cause the
DC pump, powered from starter/generators on the
high-speed engine spools, to cut in, thus maintaining
pressure in the Blue system.
From:
http://www.aaiu.ie/sites/default/fil...2009-10-21.pdf. Not sure if it is the exact same model of ATR-72, but pretty close I guess.
Also:
Each hydraulic system is provided with a 0.2 litre
accumulator, which damps out pressure surges and
compensates for pump response time in the event of
high demand.
Between the accumulator(s) and the possibility that the DC pump was supplying pressure to the Blue system, which feeds the flap actuation system, the flaps extending from 15 to 30 degrees appears to be possible with both props feathered based on the description in the report linked to above.