Right, let's pick this up with the indication side of the hydraulic system — Page 73, Lesson 102

Right, let's pick this up with the indication side of the hydraulic system. We've covered the pumps and the valves that move the fluid, but now we're looking at how the crew actually knows what's happening inside the system. On smaller aircraft, the indications are fairly basic. You'll have a light or an indicator showing that the electric pump motor is running, and you'll have the undercarriage and flap warning lights. But on larger aircraft, the picture is much more complete. The crew gets a full readout of the system's contents — that's the quantity of fluid — plus the pressure and the temperature. And there are various ways of dealing with abnormal conditions. Now, let me walk you through the specific instruments. First, the Quantity Indicators. The reservoir itself has a clear window fitted into it. That's your basic, physical check — you can look at the fluid level directly during servicing. But for the flight deck, the reservoir is also fitted with a float-type contents unit. This is a float that sits in the fluid, and as the level changes, it electrically signals the fluid quantity to an instrument on the hydraulics panel in the crew compartment. So you have two ways of checking: the physical window for ground servicing, and the electrical signal for the pilots in flight. Next, the Pressure Relays. A pressure relay is a component that transmits fluid pressure to a direct reading pressure gauge, or to a pressure transmitter. Now, the pressure transmitter is the key part here — it electrically indicates the pressure on an instrument on the hydraulics panel. So the relay is the component that takes the physical fluid pressure and converts it into something that can be read, either directly or electrically. Then we have the Pressure Gauges themselves. Electrically operated pressure gauges are fitted on the hydraulics panel to register both main and emergency system pressure. So you have two separate readings there — the main system and the emergency system. But for servicing, direct reading gauges are often fitted to the accumulators and reservoirs. That makes sense — when you're on the ground working on the system, you want a simple, physical gauge you can read without needing electrical power. Now, on a large aircraft like an Airbus, this all comes together on an electronic display. I want you to look at the ECAM hydraulic system page here — that's the Electronic Centralised Aircraft Monitor. This is the electronic display showing the hydraulic system configuration and indications. You can see three separate systems displayed along the page — on an Airbus, these are typically the green, yellow, and blue systems. For each one, you can see the relevant valve positions, the fluid quantity, the pump status, and the pressures. And notice the accumulator for the 'green' system — it's showing a low air pressure caption. That's a warning to the crew that the accumulator's air charge has dropped, which affects the system's ability to maintain pressure. So the whole picture — quantity, pressure, temperature, valve positions, pump status — is all presented electronically on this one page, giving the crew a complete overview of the hydraulic system's health at a glance.