Engineer Norm

In my career as a hydraulics engineer I had constant, and sometimes trying, involvement with hydraulic fluid. My first ten years were primarily with Naval aircraft which used a petroleum-based fluid (MIL-H-5606). It was similar to kerosene and its flammability was always a concern. It was sometimes responsible for fires, particularly when overheated hydraulic wheel brakes were involved.
About 1951,  chemical company produced a water-based hydraulic fluid called Hydrolube H2. It contained about 30% ethylene glycol, so was pretty much like anti-freeze in your car radiator. The Navy thought it was the answer to their fire concerns, and mandated it’s use on new airplanes delivered to the Navy.
A good hydraulic fluid is also a good lubricant, is anti-corrosive, anti-foaming, and resists degradation caused by pumping at high pressures. Our chemists were not convinced of the anti-corrosion or the lubricity. Our chief engineer Ed Heinemann resisted the Navy’s mandate for airplanes we already had in production. We were preparing to use Hydrolube on the new A2D Skyshark airplane.
One day the Admiral of the Pacific Fleet sent a message to Naval Headquarters, “Don’t send me any more airplanes with Hydrolube!” Planes from other manufacturers were experiencing corrosion within their hydraulic systems. That essentially ended the experiment with Hydrolube, and Douglas, not having delivered any planes with Hydrolube installed, came out smelling like a rose. The A2D Skyshark never became a production airplane.
Commercial aircraft went down a different road. They used the petroleum-based fluid into the 1950s. Then a Douglas employee who whose hobby was chemistry, came up with a fire-resistant fluid based on phosphate esters. It initially had problems with viscosity at low temperatures, but when formulated with the right additives, it met goals for lubricity, anti-corrosion, anti foaming, anti-oxidation, and above all, fire resistance.
The new fluid was licensed to Monsanto Corp. and dubbed “Skydrol”. The FAA was happy to endorse this safety improvement, and required all the large commercial airplanes to use it.
We proposed to the Navy that we supply A4D Skyhawk airplanes with Skydrol. I was involved in preparing a study concerning costs, weights, maintenance practices, etc, and the Navy seemed very interested in achieving it’s long desired goal of fire-resistance. However, there were two major stumbling blocks: (1) All the O-rings and other rubber seals in the hydraulic system would have to be replaced, and (b) Skydrol attacks most paint. In the end, the Navy decided not to take us up on our offer.
Skydrol was priced at $17 a gallon in the 1960s, and Douglas received a royalty when a Boeing or Lockheed airplane used it. The formulation was changed periodically, and when I became involved we were using Skydrol 500A (which reflected the viscosity in centistokes at low temperature.)
Airplane fight-control valves are high precision spool-type assemblies, with clearances of about .0002 to .0005 inch between slide and sleeve to minimize leakage. We didn’t understand that with the filtration of the day, the fluid was loaded with tiny  particles (metal and O-ring wear, airborne dust particles, etc.) which tended to clog up those clearances and reduce the leakage to near zero when the valve was in neutral. It only took 2 or 3 seconds.
I was up to my neck in efforts to introduce ultra-fine fluid filtration to hydraulic systems at Douglas, and the filter manufacturers were pushing it at our competitor Boeing and at British firms. In the early 1960s a new phenomenon appeared. Control valves were being eroded to the extent that they soon had unacceptable rates of leakage.
I spent a lot of time examining eroded valve parts under microscopes, and trying to ascertain the cause of erosion. It seemed probable that electrostatic discharges were responsible.
The British developed a commercial airliner called the Trident. Their test airplanes passed their hydraulic system tests readily, but production models sometimes showed excessive valve erosion, even before being delivered to a customer. Why the difference? Detective work revealed that on the test airplanes, the hydraulic fluid had intentionally been contaminated with the maximum allowable water content (about ½ of 1%) to evaluate how detrimental that was to the airplane. Unexpectedly, the presence of the water seemed to protect the valves from erosion.
The next formulation of fluid was Skydrol 500AW, where the W stood for water. It worked for us. Boeing proclaimed it was all a hoax to sell more fluid, even though they had a backyard full of airplanes they couldn’t deliver because of valve erosion, and these were the first airplanes they made with high efficiency fluid filters. Cause and effect were there for those who looked.
Lab testing at Douglas and at Monsanto showed that the erosion was an electrochemical reaction due to static charges built up by the high velocity fluid escaping at the valve metering edges. Our new filters removed the particles which formerly impeded the leakage flow. Monsanto chemists eventually established the degree of conductivity of the fluid needed to dissipate the static charges and avoid the erosion (0.4 micro-ohms). Later they found another less corrosive additive to replace the water. Problem solved.
Boeing always chafed at having to pay Douglas a royalty for using the fire-resistant fluid, and as the Douglas patents were about to expire, they induced two other chemical companies to develop competing fluids. The competitors started with a lower density phosphate ester base stock (about 7% lighter), resulting in a fluid which saved a substantial amount of weight, particularly on a plane as large as a Boeing 747. This forced Monsanto to develop a similar fluid to remain competitive; Skydrol LD, for Low Density. There was a slight reduction in the fire resistance, but it was judged acceptable by all parties involved. The DC-10 was in development, and we had an “iron bird” in our lab with the three full-scale hydraulic systems installed. We put a different brand of fluid in each system, and evaluated them for a year. The biggest problem was with mixtures of these fluids, and in particular, maintaining the required conductivity.
The low density fluids caused increased O-ring swelling, We thought for a time this might be a show-stopper, but our year of iron-bird testing eventually proved it to be acceptable.
One day when I came to work, the sales representative for one of the fluid manufacturers was waiting for me with his feet on my desk. He demanded to know what “friction locks” we had in our hydraulic systems. The answer was none. This came as a relief to him. It turned out that Boeing 747s with his brand of fluid installed were having landing gear doors come open in flight. These doors were held closed by friction locks, and his fluid had an additive which greatly reduced friction when under pressure. But the upshot of this episode was that his company withdrew from the aircraft hydraulic fluid business.
With inflation, these fluids now cost much more than the original $17 a gallon, but Douglas, which has been absorbed into Boeing, gets no royalty. We reduced the system leakage, and through better filtration, reduced the replacement of fluids due to degradation, so the airplanes require less “makeup” fluid than formerly. Monsanto spun off this now lower-profit product line to a new company called Solutia.