For many years MIL-PRF-23699 approved engine lubricants were employed in helicopter gearboxes, ensuring a common lubricant for both engines and transmission systems. More recently, high-load-carrying oils have been evaluated and found to offer advantages over conventional-load-carrying MIL-PRF-23699 oils. These higher-load-carrying oils, complying with specification DOD-PRF-85734, were originally intended only for engine applications. The two MIL spec oils are thus of identical viscometric properties. Operators considered that lubrication performance of gearboxes could be enhanced by a higher-viscosity, high-load-carrying lubricant possessing improved corrosion-protective qualities.
Eastman HALO 157 is a clear, amber-colored fluid with a faintly aromatic odor reminiscent of turbine engine oils. It is based on selected polyol esters, the inherent characteristics of which are enhanced by additives. It is also fully compatible and miscible with other oils approved to specifications MIL-PRF-23699 and MIL-PRF-7808.
Multiple tests have been completed in bearing and gearbox rigs and flight evaluations, culminating in the NATO standard (AIR STANDARD ACS 4035) for a 9-cSt advanced helicopter gearbox lubricant.