I think you need to be clearer whether you are discussing 'drift down' or 'drop down' they are totally different.
Drift down is the ability to fly at an altitude with the knowledge that, if you lose an engine, you can drift down to maintain a safe altitude - whilst clearing obstacles during the descent by the prescribed distance. This can be calculated but the glide angle has to be used (i.e. wind has to be known); unless it is done offshore when it is a simple calculation.
Drop down is associated with Category A procedures - it represents the one-engine-inoperative descent from TDP to a point (usually called min-dip) from which obstacle clearance can be assured and continued take-off is possible. Drop down graphs are provided by manufacturer which have these type of procedures; where elevated helideck/heliport procedures are provided the deck-edge miss is guaranteed by the procedure and drop down below the take-off site is permitted.
Most twin engine helicopters can hover on one engine - only the best can do it at close to MAUM. For civilian helicopters, the AB139 gets the prize and most of the clever small twins (EC135, MD902, A109 - power and grand, Bell 429 etc) are getting closer (at sea level). Most States require OEI HOGE performance to be calculated and used when CAT hoisting is being performed.
Last edited by JimL; 2nd September 2005 at 14:49.