Apples vs Oranges

[Head Turner]

In the Feedback section of Rotor and Wing October issue, Shawn Coyle's letter refers to piston vs turbine power.

Looking at two Flight Manuals one for the MD600 and the other the Agusta A119 the maximum weight to enable a 3-3.5 skid height AGL hover at Take Off Power is achieved using differing data.

The MD600 uses Hp and OAT to give Hd which is then used against OAT to give the maximum weight to achieve the Hovering Ceiling.
The Agusta A119 uses Hp and OAT alone.

Why the differences for the same situation?

The last paragraph of the letter also requires further explanation I quote.

The power required to hover (at the same weight and hover height above the ground) for both types is a function of density altitude. But the other half of the equation, the power available for the turbine machine, is not a function of density altitude

[Shawn Coyle]

Since you asked for further explanation, I'll give it.
There is a much longer explanation in my book, but in a nutshell-
A non -turbo-charged piston engine is basically a volume engine (the amount of air that can be sucked into a piston is fixed). The power that you can extract from this engine is based on the density of the air.
A turbine engine is not a volume engine, but in simplistic terms, a heat engine - the amount of air that can go in depends on the speed of the compressor, but the amount of energy that can be extracted depends on the temperature difference between the air and the temperature limit on the turbine (again, in very simplistic terms).
Look at the engine power check chart in any turbine helicopter. At a constant value of TOT, use the following conditions of pressure altitude and temperature to determine what the maximum power output of the engine would be:
9,000', -40C
5,000', +5C
2000' +40C
Unless the helicopter is obscenely overpowered and you are transmission limited in all cases, the power available from the engine will be wildly different, with the lowest power at the highest temperature.
All three of these conditions are the same density altitude. The power required to hover (ignoring and compressibility issues) will be the same.
In some helicopters, they will be able to hover at maximum weight at all three of those conditions, and in others, the hot temperature will preclude hovering.
Hope that clears it up.