Skydrol 500 series fluids have the longest service history among phosphate ester products. The first version, Skydrol 500, was released in 1952. Continuous improvements in the formulation led to Skydrol ™ 500B-4, the current version containing the same groundbreaking anti-erosion additive and acid scavenger found in Skydrol ™ LD-4 in 1977. Skydrol 500B-4 is the most worker friendly of aviation phosphate esters. This has given the product enormous popularity for use in workshops and indoor test benches.