Does auto rotation in helicopter always help?

nope.

there area of airspeeds , heights on terrain that's labeled "to avoid" such
http://upload.wikimedia.org/wikipedia/en...
has 2 fundamental sub-areas: 1. 1 in high airspeeds @ low height on terrain (reduced reaction time before plumet ground, ,
2. above safe height hover.. put simply, twinengined airframe between 20 , 200 ft.. if you're hovering in height, helicopter's properties largely prevent pilot initiating proper autorotation reasonable vertical speeds.
above hover height, hover deemed "safe" because in case of engine failure, have enough time , enough height on terrain trade increasing forward speed.

vertical autorotation less safe performed forward flying autorotation, because maintain same rotor rpm in vertical autorotation have larger sink rate , therefore poorer chances recover autorotation.

mi-171 sink @ 2500-3000 ft per minute @ best speed.

trained [on simulator] perform both engine dead true autorotation 200 meters hover; below that, it's problem simulator instructors because part, there's of vertical speed when want flare autorotate vortex ring , crash.

@ common ferry flight heigts (we're flying @ 500 ft above terrain) both engines die, you've got enough time pick field in 90degrees section ahead of you, drop collective , flare.

when there engine failure in helicopter, auto rotation help??

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