Understanding the ATR 72 Hydraulic System

Reading time: 10 mins

This week we’re looking at Hydraulics. The ATR is unique in some regard as it doesn’t rely on hydraulics to power the flight controls, so let’s have a look at what systems are hydraulically powered. The hydraulic systems are located in the aft section of the left landing gear fairing.

SUPPLY

The ATR has a total of 2 hydraulic systems; one blue and one green.  The liquids aren’t actually blue and green, this is just what the two sides are called.

The hydraulic fluid supplying both systems is stored in the Common Hydraulic Tank, located in the Left-Hand landing gear fairing. The tank is unpressurised, and there is a glass window in the tank, so you can check the level during your walk-around. 

Key facts:

• The tank is divided straight down the middle into two sides, blue and green. 
• Baffles stop fluid splashing and reduce foaming.
• Each compartment has a low level alert triggered when fluid quantity drops below 2.5L.

POWER – HYDRAULIC PUMPS

Each HYD system is pressurised by an electric pump powered by ACW (AC WILD). – Which usually is supplied when the engines are running. Read more about ACW.

However, the BLUE system also has an additional electric pump powered by DC power or directly from the battery . This is known as the Auxiliary Hydraulic pump (AUX HYD PUMP).

The pumps supply 3000 PSI of HYD pressure. 

In case of one hydraulic pump failure, the remaining operating pump can supply both Green & Blue systems via the crossfeed valve.

AUX HYD PUMP

The AUX HYD Pump’s job is to automatically pressurise the blue system if the ACW HYD pump fails. There is a small push button on the pedestal near the Captains hip. When the HYD AUX PUMP is manually activated on the ground the DC AUX pump runs for 30 seconds. It can be used even when the battery master switch is OFF – the danger is extensive use discharges the main battery. We use this on the ground to pressurise the blue system and allow use of the PROP BRAKE lock or unlock.

KEY FACT – In AUTO the AUX DC HYD Pump runs when ALL these conditions are met:

• Pressure of ACW blue pump is below 1500 PSI.
• Propeller brake is released.
• Gear handle is DOWN.
• At least one engine is running.

This means that if you have lost both main HYD pumps in flight, the AUX HYD pump will start to run automatically when you select the landing gear down. 

The AUX HYD Pump allows you to use:

• Flaps,
• Spoilers,
• Prop brake,
• Nose wheel steering,
• The emergency brake. 

This is because the AUX HYD pump powers the blue HYD system. 

POWER – ACCUMULATOR

Essentially accumulators are the storage of hydraulic pressure, rather than generation of pressure. Each system (Blue & Green) has a 0.2 Litre accumulator filled with nitrogen in the HYD bay.

The accumulator’s job is to reduce pump vibrations and pressure surges. They also compensate for the pump response lag-time when the demand is high. 

As part of the emergency braking system, there is an additional accumulator that stores enough pressure to allow for 6 applications of the emergency brake – allows you to land without HYD pressure, and still use the emergency brake to stop.

CONTROLS

– The picture above shows the Hydraulic Control Panel on the ATR 72-600.

“LO PR” lights illuminate when fluid pressure of the related pump drops below 1500 PSI. Pilots are made aware with a Master Caution, a Single Chime and LO PR message on the EWD.

“LO LVL” lights illuminate when HYD fluid quantity drops below 2.5 Litres. Each system Green/Blue has its own sensor. 

X FEED push button – Controls the cross feed valve separating both green & blue HYD systems. When selected ON the cross feed valve is open.

In case of LO LVL alert, the crossfeed valve:

• Remains closed, if it was in the open position.
• Automatically closes, if it was in the opened position.

– The above shows the Hydraulic system page on ATR 72-600.

• On ground with engines OFF, so ACW is not powering the GREEN/BLUE HYD pumps. DC power is available from DC BUS 2 to power the AUX HYD pump.
• Emergency brake accumulator is showing 2900 PSI.

Well, that’s it for this week’s technical blog covering the ATR 72/42 Hydraulic systems, read more blogs below. Until next time 🙂