The -34C operating restriction for the Rolls-Royce Model 250-C30 in the S-76 aircraft is due to the fuel specs. Jet A and JP-8 both have a maximum viscosity rating of 8 centistokes where JP-5 has a maximum viscosity of 8.5 centistokes. By referencing fuel spec ASTM-D-1655, you find that at -32C you reach the 8.0 centistoke level for Jet A and JP-8.
This is where the note in the Model 250-C30 Operations and Maintenance Manual comes from, stating "The engine will operate satisfactorily on JP-5, JP-8, Jet A and Jet A-1 at fuel and outside air temperatures down to -32 degrees C (-25 F)." For JP-5, the maximum viscosity is 8.5 centistokes, which translates to -34 C.
Operation of the –C30 / S-76 at lower temperatures is not limited by the engine – most operators simply follow standard cold weather fuel mixture instructions, such as adding Av-Gas to the jet fuel. JP-4 and Jet B do not have maximum viscosity requirements, and are thus also used by some operators operating in extreme cold weather environments.
Steve76 - Email me if you have any additional Qs.
T/shaft