Hydraulic Systems

Question 1

Draw the hydraulic system

Answer 1

See image

Question 2

How many and what are the different hydraulic systems?

Answer 2

4
First stage
Second stage
Wheel brakes
Rotor brakes

Question 3

What is unique about the first stage system?

Answer 3

It contains a low level switch and a tail rotor shut off valve. Combined they will maintain fluid for the main rotor servos in the case of a leak in both systems in the tail section.

Question 4

What is unique about the second stage system?

Answer 4

It contains a priority valve which connects the utility system, landing gear and pedal trim/yaw damper, prior to the shut off valve.

Question 5

How many pressure switches are in each stage?

Answer 5

5 in each

One after the shut off valve and one on each of the four servos to sense a jam.

Question 6

What is contained in each of the identical hydraulic modules?

Answer 6

Pressurized reservoir
Pressure switch
Solenoid operated shutoff valve
Pressure transmitter
Pressure and return filters w/ LRBs
Reservoir relief valve
Fill and pump case check valves

Question 7

What does the priority valve do and which hydraulic system is it in?

Answer 7

The priority valve restricts flow to the utility system depending on system demand giving priority to the flight control servos.

It is in the second stage.

Question 8

What does the low level switch do?

Answer 8

Senses fluid loss in the first stage and isolates the tail rotor servo and lines aft of the tail rotor shut off valve. This is done to preserve fluid for the main rotor servos in the event of dual hydraulic system leaks in the tail section.

Question 9

Describe the hydraulic pumps.

Answer 9

Two constant pressure, variable delivery, main gearbox driven pumps.

3000psi constant pressure

Full pressure available about 10% Nr

Pump case lines return fluid to reservoir for lubrication and cooling.

Question 10

How many main rotor servos are there?

Answer 10

3

Forward, aft and lateral

Question 11

How many pressure switches are on each servo assembly?

Answer 11

2, one for each stage

Question 12

What is a pilot valve and what do they do?

Answer 12

The pilot valve converts mechanical control inputs into hydraulic pressure to act on the servos.

They automatically bypass if jammed and release hydraulic pressure so the remaining stage can move the servo.

Question 13

What do the hydraulic pressure switches do?

Answer 13

Switches on servos illuminate the associated Servo light when pressure drops below 1600 psi, the light extinguishes when pressure returns to 2100 psi.

They also disable the shutoff valve of the opposite system when pressure is below 1600 psi.

Question 14

When is the priority valve fully open and closed?

Answer 14

Fully closed at 2250 psi and below.

Fully open when pressure returns to 2700 psi.

Question 15

What is the target airspeed in the servo problem emergency checklist?

Answer 15

125 or less

Question 16

What is the target airspeed for the fixed pitch emergency procedure?

Answer 16

80 kts, which also complies with the unsafe gear EP.

Question 17

What bus are the hydraulic shutoff switches on? What happens if power to that bus is lost?

Answer 17

DC Essential Bus

Servo systems are fail safe on, so without the DC essential bus the pilot cannot turn them off.

Question 18

What pressures correspond to the hydraulic pressure display?

Answer 18

Normal (green) is 2800-3200 psi

Max pressure (red) is 3300 psi

Minimum pressure (red) is less than 2700

Question 19

Which hydraulic filter can bypass?

Answer 19

The return, at 100 +/- 10 psid and the LRB will pop.

The pressure side does not bypass to prevent servo contamination. The LRB will pop at 70 +/- 10 psid

Question 20

How is a servo jam indication cleared?

Answer 20

Since they are magnetically latched, the circuit breaker must be reset.

They are also connected to the landing gear so upon landing the light will extinguish if the jam has cleared.

Question 21

What type of landing gear does the S-76 have?

Answer 21

Retractable
Tricycle type
Non-steerable nose wheel

Question 22

Describe the landing gear retract system.

Answer 22

Electronically controlled and hydraulically actuated.

Emergency pneumatic extension

Question 23

What bus is the landing gear system on?

Answer 23

DC Essential bus

Question 24

When is the landing gear up warning system activated?

Answer 24

Below approximately 60 kts or when the radar altitude is lower than the highest radio altimeter decision height and the gear is up.

Question 25

Which side of the helicopter is the weight on wheels sensor located?

Answer 25

Left, PNF side

Question 26

What does the landing gear damper do?

Answer 26

Improves the touchdown stability and reduces the possibility of ground resonance by centering the landing gear laterally.

Question 27

What systems are on the weight on wheels sensor?

Answer 27

Landing gear retract function
Servo jam warning system
Engine out audible warning system

Question 28

Where is the landing gear control valve located?

Answer 28

Right main gear well

Question 29

How much pressure should be in the emergency gear blow down bottle?

Answer 29

3000 psi, recharge if below 2700 psi.

Question 30

What allows the air to lower the gear in place of the hydraulic fluid?

Answer 30

Shuttle valves located on each of the gear actuators.

Question 31

When will the nose wheel not self center?

Answer 31

If swiveled more than 180 degrees, if greater than 90 degrees a slight forward roll should be used to straighten it before lift off.