Helicopter take off from oil rigs
 

Hi, apologies in advance if this is a silly question but it came up during our coffee break and as you know, coffee breaks are the theorist’s platform to share everything they don't know with confidence.Question was (in very non aviation terminology) Chopper taking off from a helideck offshore, we have watched and sat on thousands of them
1) Increase in power, chopper lifts and hovers at let’s say 5/6 meters above the deck. Totally understand this is to see that everything is good in the gauge and functionality world whilst over a very small dance floor.
2) It shimmy's about and aligns its self for its heading and fly away.
3) And this is the point raised, almost every time, it always seems to drop (slowly) down from the hover height to the point of the rear wheels almost touching the deck again, then it’s up and away..... why does the chopper drop down before lift ? what is the transition at that point ?
4) To clarify, its merely a question, not a complaint, not a criticism only a question........in perspective, yesterdays was why is there a window in your washing machine door but not in your tumble dryer door !! is life shaping stuff lol

Thanks in advance, this could close a chapter

I would suggest this may may be specific to the type of helicopter, or company procedures. It sounds as though it may be some sort of power/performance capability check. Ask one of the pilots, they would be only too happy to clarify for you.

It's not something we did on the type I flew (S-76C). Our procedure was hover at five feet, check N1 margin is >=3%, climb vertically to 25 feet with a rate of climb >200feet/min, rotate to maximum 15° nose down, accelerate to 60 knots, climb to 250 feet before commencing any turn, undercarriage up on reaching 60 knots, disarm floats approaching 75 knots. Job's done.

PS: Our tumble dryer door has a window.

I think Megan has it and UKCP has misread your description. Some types specify a delta power application above that required to hover at a certain height - more like 10 ft, not 5-6m but I accept height perception as pax is difficult. having established the power required - normally as Torque figure but on some modern types a non-dimensional value, on a First limit Indicator or Power Indicator, the aircraft is lowered back to a wheels light state - which will require less power - and then the delta application applied. E.g at 10 ft you require 80% torque and the procedure is to apply 10% torque above that value, but you lower down to wheels light and only need 75% torque. The actual value you pull to will be 90% torque, not 85%. Make sense?

Many thanks for taking the time out to answer. It makes perfect sense and I relayed accordingly. Very much appreciate it, stay safe, thanks