# Why is 3000 psi the most common hydraulic pressure used in commercial aircraft?

As per the article on this website (from 2002), most of the aircraft use 3000 psi as hydraulic pressure:

Since the 1970s, military aircraft have been using 5000 psi hydraulic systems, which use lighter-weight components to actuate the flight control surfaces. Passenger aircraft have been relegated to 3000-psi hydraulic systems (with the exception of the Concorde, which features a 4000 psi system).

What is the technical rationale behind using 3000 psi? Is there any historical reason or regulation to use 3000 psi only?

"Above 3000 psi, using aluminum as a pressure vessel becomes risky," says Galloway. "5000 psi systems almost exclusively rely on steel or titanium. To minimize stresses, the sizes of passages can be reduced. Computational fluid dynamics is also used to minimize pressure drop in critical areas."

3000 PSI was the best you could get with technology in the 1950s, when standardization occurred. Without economical titanium tubing and reservoirs, using steel would incur a weight tradeoff that doesn't make sense.

The A380 was a trigger to the development of 5000 PSI civilian systems because of both the high demands and the long run distances, both due to the size of the plane. I read somewhere that they took a look at the military 5000 PSI systems and realized it would never stand up to commercial use, so little technology was carried over.

Every new design large transport from the A380 onwards (787, A350) is 5000 PSI.

The civil aviation industry likes to use what is proven and trustworthy, and will continue implementing designs that a are mature and safe. The design safety is 1 : 10$^9$ flight hour, and every time a change is made it takes a truly long time before the reliability and fault tolerance is proven.

So if 3000 PSI systems do the job on civil airliners, they will continue to be used in new design aeroplanes, unless there is a compelling reason to implement a fundamental change, which will result in deep R&D into applicability, reliability etc. Is there any fault condition that can impair passenger safety.

There is really old technology on board of civil airliners, for instance Intel 80186 and Motorola 68020 processors are still being mounted in new aircraft. A ripe old processor design sold 30 years ago, the stone age of the digital era.

Military aircraft are different. Safety is not the overriding factor, but mission capability is. Yes the pilot must be brought home if the plane comes down, but above all it must do its job competitively. So in military aircraft there is a drive towards modernisation, while in civil aviation there is a drive to not change anything that is working reliably, unless a specific problem must be solved.

3000 PSI was what the first hydraulic systems used for operating pressure, the initial hiccups were fixed, and there it remained - until there came an aircraft that was so huge that the line losses simply became too big. It had to be done. Fortunately the system parts for 5,000 PSI were already available, from the military suppliers. That is where a lot of new designs mature.