Techie Stuff

Twins
Flying twin-engined helicopters requires a different philosophy in many ways, certainly getting used to not dumping the collective every time an emergency happens, and their complexity, although there is no real change in flying characteristics as there would be if an engine fails in an aeroplane. You also have takeoff and landing profiles, in case something happens, and performance charts, with generally more shallow approaches to comply with them.
The regulations require you to ensure that your aircraft has adequate performance for any proposed flight. The "performance" of an aircraft describes its ability to maintain certain rates of climb against distance, so you can avoid hard objects (obstacles), particularly when you can't see them. As a result, the charts will emphasise rates and angles of climb very strongly (climb requirements are established with one engine working hard for a specified time).
There are reasons for multiple engines, of course. One is that you get more power and can lift more, but another is for safety – failure of an engine should not affect the continued safe operation of the flight, or the other one, which is why there are isolation arrangements in the engine compartment. It follows, therefore, that the less the weight of the machine, the better it can fly with less power. In fact, with reference to the profiles above, you may find different max all-up weights for helipads and clear areas (there is no definition of a "helipad" for performance purposes – rather, it's any area that isn't a clear area, or one that allows operation inside your chosen performance group).
The take-off and landing phases of any flight are the most critical, demanding the highest skills from crews and placing the most strain on the machine. Because of this, strict regulations govern the information used for calculating take-off or landing performance. Of course, in the old days (say during the war, or when the trains ran on time), having enough engines to lift the load was all that mattered and no priority was given to reserves of power and the like. Now it's different, and you must be able to keep your machine a specified distance away from obstacles and be able to either fly away or land without damage to people or property (and the machine) if an engine fails.

Performance requirements will be worked out before a C of A is issued, over a wide range of conditions. They are subsequently incorporated in the Flight Manual, which forms part of the C of A.
Helicopters are certified in one of several groups. For example, JAR classifications are 1, 2 and 3, which are broadly equivalent to the UK Groups A, A(Restricted) and B (see the table below). It is important to realise that these are different from Airworthiness groups, which dictate how well the airframe stands up to a forced landing.
Passengers JAR Class UK AN(G)R
Over 19 1 A
9-19 2 A (Rest)*
Less than 9 3 B**