Props

Question 1

Headline paragraph for the props

Answer 1

Each engine has fitted a four-blade, constant speed, full-feathering, reversing propellor. Each has a primary, overspeed and fuel-topping governor, autofeather and propeller synchrophaser.

Question 2

What is propeller blade angle?

Answer 2

The angle between the propeller chord line and its plane of rotation, which can vary from low pitch (max reverse) to high pitch (feather).

Question 3

What are the three forces acting on the propellers?

Answer 3

Spring Tension from a feathering spring mounted in the propeller dome.
Counterweights mounted to each propeller blade.
Both act to increase blade angle (coarsen/feather).
Oil Pressure from the engine, boosted by primary prop governor and routed to the propeller dome, fines the prop.

Question 4

Relationship between Oil Pressure, RPM, Torque and Blade Angle

Answer 4

As oil pressure increases, RPM increases, torque decreases and blade angle decreases.

Question 5

What are the blade angles from max reverse to feathered?

Answer 5

Max Reverse -14, Ground Fine -3, Ground Low Pitch Stops +2, Flight Low Pitch Stops +12, Cruise +30-45, Feathered +79.3

Question 6

What is the normal propeller RPM operating range?

Answer 6

1450-1700 rpm

Question 7

What controls blade angle in both the alpha and beta ranges?

Answer 7

Alpha (flight) range, pilot valve controls blade angle.

Beta (ground) range, the beta valve controls blade angle, linked directly to the power lever.

Question 8

What N1 %s does Autofeather arm and the Landing Gear Warning Horn sound?

Answer 8

Autofeather >90% N1

Landing Gear Horn <84-86% N1

Question 9

What is the ‘deadband’?

Answer 9

Between ground fine (-3) and the ground low pitch stop (+2).

Question 10

Describe the primary governor’s operation

Answer 10

Controls prop RPM by varying the amount of high-pressure engine oil going in/out of the hub. Governor pump increases engine oil pressure to a max of 375psi so it can operate the propeller. Mechanically compares RPM set by pilot, with actual RPM, adjusts blade angle to achieve and maintain desired RPM. Between 1450-1700rpm.

Question 11

Describe the low pitch stop’s purpose and operation

Answer 11

Prevents the primary governor from reducing prop pitch to undesirable angles, by closing the beta valve, thus preventing any further oil pressure through the prop, creating a hydraulic low pitch stop preventing blade angle being further reduced.

Question 12

What are the two low pitch stops, and how does the system differentiate?

Answer 12

Flight LPS is at +12 and prevents excessive drag in flight. Ground is at +2 and prevents prop resonance during ground operations (prop <400rpm or >1050rpm). The low pitch stop solenoids energise when on the ground to reset the LPS to +2.

Question 13

How do the low pitch stop solenoids energise?

Answer 13

Right main squat switch (WoW) indicating the aircraft is on the ground.

Lifting the power lever acts as a backup.

Question 14

How do the low pitch stop solenoids de-energise?

Answer 14

Right main squat switch (WoW) indicates the aircraft is airborne.

Microswitch on the power levers ~68-70% N1, above which the LPS returns to the flight stop.

Question 15

Describe the overspeed governor’s purpose, operation and set limits

Answer 15

Prevents excessive prop RPM, through reduction of oil pressure, increasing blade angle and reducing prop RPM. Set to limit at approx 1768rpm (1770rpm visual to us). Testing range set to 1500-1610rpm when test switch held to GOV.

Question 16

Describe the fuel topping governor’s purpose and operation

Answer 16

When blade angle can’t be changed, or in reverse thrust, it limits prop RPM by limiting engine power. Built into the primary governor and limits prop RPM to 106% of primary governor setting airborne and 95% when on the ground. Once at these speeds, it automatically reduces engine power to limit prop RPM.

Question 17

Describe how autofeather is armed and tested

Answer 17

Once switches armed and power levers above 90% N1. Green AFX caption above 17% Tq. De-activates when power lever brought back below 90% N1, but ground test bypasses that and arms approx. 22% Tq.

Question 18

How should autofeather operate?

Answer 18

Detects engine failure by a reduction in torque not associated with power lever movement. 17% Tq opposite extinguishes, 10% Tq starts to feather.

Question 19

RVS NOT READY

Answer 19

Prop levers aren’t fully forward with the landing gear extended, therefore the fuel topping governor will limit the amount of reverse thrust available (not optimum).

Question 20

L/R PROP PITCH

Answer 20

Prop blade angle is less than the flight low pitch stop.

Question 21

Explain expected RPM associated with governor failures, and subsequent flight limits

Answer 21

Above 1700rpm indicates failure of the primary governor. Up to 1768rpm (1770 indicated) caught by overspeed governor, engine torque limited to 96%. Overspeeds above 1768rpm not approved, as failure of both governors.

Note: fuel topping governor should catch at 1802rpm (1800 indicated)

Question 22

PROP GND SOLENOID

Answer 22

One or both low pitch stop solenoids are energised in flight, meaning the prop could drop to the ground low pitch position when power levers are retarded. Pull PROP TEST GOV CB to de-energise the solenoids, but longer landing distance as props will remain at flight low pitch stop.