Right, let's pick this up — Page 52, Lesson 79

Right, let's pick this up. We were looking at the seals squeezed between the two moving surfaces of a hydraulic actuator. Now, I want to talk about what those seals are made of and, crucially, the fluid they're working with, because the two are completely tied together. Seals are made in a variety of materials, and the choice depends entirely on the type of fluid they'll be used with. If you fit a seal of the incorrect material into a system, the sealing quality will be seriously degraded, and that can lead to failure of the component. So it's a hard rule: the seal material and the fluid must be compatible. Also, seals are easily damaged by grit. That's why a wiper ring is often installed on actuators—it prevents any grit that may be deposited on the piston rod from contaminating the seals as the rod moves in and out. Now, the choice of an aircraft's hydraulic fluid is influenced by the materials used for the glands, seals, rings, seats, and so on. There are two fluids in common use, and I want you to know both of them in detail. The first is DTD 585. This is a refined mineral-based oil—a petroleum product—and its colour is red. It's used with synthetic rubber seals, specifically Neoprene. Now, a note on that: DTD 585 is an obsolete specification. The British specification that replaces it is DEF STAN 91-48. There are other specifications for the same super-clean grades: H515 is the NATO designation, OM15 is the Joint Service designation, and MIL-H-5606F is the U.S. specification. The second fluid is SKYDROL. This is a phosphate ester-based oil. Its colour depends on the type: Type 500A and 500B are purple, and Type 700 is green. It's used with synthetic rubber seals, but a different rubber—Butyl. Skydrol is fire-resistant, and it's less prone to cavitation because of its higher boiling point. That's an important operational advantage. Now, handling. Hydraulic fluids should be handled with care because they're considered a skin and eye irritant. They also have a detrimental effect on paintwork, sealing compounds, rubber materials, perspex, and so on. And they should never be mixed. That's absolute. It is of major importance that only the specified hydraulic oil, or its approved alternative, is used in a hydraulic system. If the incorrect fluid is added, you get breakdown of the seals, which causes fluid leakage—both internally within components and externally from the actuators. The colouring of the fluids assists in their identification, and it also helps you find hydraulic leaks. But the specification can only be confirmed by two things: consulting the aircraft manual, and only using fluid from sealed containers or the appropriate replenishment rig. Colour alone is not enough. Finally, let's look at the ideal properties of a hydraulic fluid. It should be relatively incompressible—up to 27.6 MN/m², which is 276 bar—so that operation is instantaneous. It should have good lubricating properties for both metal and rubber. It should have good viscosity with a high boiling point, which helps prevent vapour locking and cavitation, and a low freezing point—for example, a temperature range of +80°C to -70°C. It should have a flash point above 100°C. It should be non-flammable. It should be chemically inert. It should be resistant to evaporation—low volatility. It should have freedom from sludging and foaming. It should have good storage properties. It should be non-corrosive. And it should be reasonably priced and readily available. So, to tie it together: the fluid and the seal material are a matched pair. Mineral oil with Neoprene, phosphate ester with Butyl. Get that wrong, and you're looking at seal breakdown and leaks. And remember, the fluid's boiling point and freezing point directly affect things like cavitation and vapour locking, which we'll come back to.