*Up to 15 passengers and 12,500 lbs ** Less than 6,000 lbs

JAR Class 1 (Group A) helicopters require no forced landing provisions if an engine fails. Class 2 machines have a limited exposure (that is, occupants and third parties must remain uninjured), while Class 3 types have to make a forced landing. Single-engined helicopters therefore come under Class 3. In addition, Class 2 ops must be done under conditions that allow a safe forced landing, in terms of weather, light and terrain – those done from elevated pads in non-hostile conditions must be done by day only, otherwise you must abide by Class 1. Class 3 ops must be done in sight of the surface, by day, with at least a 600-foot ceiling. The minimum visibility is 800m.
The screen height for JARs is 35 feet, for takeoff and landing. There are no distance requirements.
Group A helicopters must (with one engine out) clear all obstacles under the departure track within a defined area by a specified margin. In fact, they should be able to climb (after CDP) at 100 fpm to MSA with the gear down (most unfavourable C of G), then continue at 150 fpm to MEA with One Engine Inoperative (OEI). Naturally, if the remaining working engine is not powerful enough to lift the weight, the flight will not continue, so, as with fixed wing, there is a point during the takeoff procedure at which, if an emergency happens, you elect to carry on or reject, called, unsurprisingly, the Critical Decision Point, or CDP, which is the only point at which you have two choices.
Which group you belong to depends on Certification, Max All-Up Weight and the number of passengers carried, although the JAR classifications are based on the latter (see below). However, it may be more acceptable commercially to operate in a lesser group if it enables you to take more payload, and make more money – all you might need is longer takeoff runs or less obstacles.
In other words (just to reinforce the point), the conditions under which you operate determine how heavy your aircraft can be and, as a result, your payload. Over a whole trip, the weight could be dictated by:
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Maximum weight

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WAT limits (see below)

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Space available

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Obstacles

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The route

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Hovering OGE

Individual machine performance will vary due to such variables as the age of the airframe and engines, the standard of maintenance, or crew skill and experience, without the engines being adjusted for several seconds after the initial failure. What you can do on one day under a given set of circumstances may well be impossible another time. Performance is therefore a set of average values—particular machines may be better or worse.
The original testing, of course, is done with new aircraft and experienced pilots, which are known as unfactored. Fudge factors are applied to unfactored figures to produce net performance (and gross performance when they're not), so there is a margin if you have a tired engine, or a new pilot. Occasionally, performance data (as amended) in a flight manual will already be factored, but you will have to check the small print on the chart, in case they surprise you (JAR does not make a distinction between the two, except for a 1% margin for IFR). Also, figures and graphs are based on Standard conditions which allow for fixed reductions in pressure and temperature with height. As we all know, the real world isn't like that, so these assumptions may not always be true and due allowance must therefore be made for them (if your aircraft is performing sluggishly, you may find it's not the machine, but the conditions it has to work under that are at fault).