17.3

Question 1

What are the three reasons that a prop governor will go into overspeed?

Answer 1

Aircraft goes into a dive increasing forwards speed; AOA is decreased.

When speeder spring force on flyweights is reduced.

When pilot increases fuel flow by moving the throttle forward

Question 2

What type of gearing is required for a turbo prop aircraft?

Answer 2

Usually some form of compound epicyclic design with reduction ratios of between approximately 6:1 and 20:1

Question 3

What are the three main types of engine for a turboprop?

Answer 3

Direct Connected (Integrated).
Compounded.
Free Power Turbine

Question 4

How does a direct connected engine power the prop?

Answer 4

The turbine of this type drives the compressor through the reduction gearbox, the propeller. A single power lever controls both the engine fuel flow and the
propeller pitch

Question 5

Disadvantages of a direct connected engine?

Answer 5

Requires fine (low) pitch stop for starting.

Complex interconnection between throttle and propeller control.

May be unstable with sudden changes in load due to the attitude of the aircraft

Question 6

How does a compound engine work?

Answer 6

A twin spool layout where the HP turbine drives the HP compressor and the LP turbine drives the LP compressor and propeller reduction gearbox.
Power is controlled by a lever which selects blade angle, and various automatic systems maintain RPM by adjusting fuel flow

Question 7

What is the disadvantage of a compound engine?

Answer 7

Only for big engines

Question 8

What are the advantages of a compound engine?

Answer 8

Engine free from surge.

Good flexibility.

Good acceleration

Low power starting due to twin spools

Question 9

How does a free power turbine work?

Answer 9

The compressor and propeller are mounted on different shafts and driven by independent turbines. The power lever provides direct control of the gas
generator fuel flow while the conditioning lever controls the propeller RPM

Question 10

Advantages of a free power turbine?

Answer 10

Easier starting

Independent control of engine and propeller

Engine and propeller can both operate at optimum speed which gives lower fuel consumption

Question 11

What is ESHP?

Answer 11

Although most of the energy of the gasses are used to drive the compressor and propeller, there will still be some residual thrust available from the exhaust. The sum of the SHP and thrust is termed Equivalent Shaft
Horsepower. This indicates total power available from the engine

Question 12

What are the 4 types of control arrangements for a turboprop?

Answer 12

Integrated Control of both Propeller RPM and Fuel Flow

Direct Control of Fuel Flow

Direct Control of Blade Angle

Twin Lever Control

Question 13

Explain Integrated Control of both Propeller RPM and Fuel Flow

Answer 13

Suitable for direct coupled and compound engine types. Propeller RPM is selected by the power lever which simultaneously adjusts fuel flow thereby
ensuring the correct fuel flow for a selected propeller RPM. At maximum RPM, further increases in power are achieved by increasing the fuel flow. The Constant Speed Unit (CSU) increases the blade angle to
absorb the extra power to keep the propeller RPM constant

Question 14

Explain Direct Control of Fuel Flow

Answer 14

Suitable for use on a free power turbine. The gas generator is controlled in the same manner as a turbojet. The propeller is maintained at a constant
selected RPM by the CSU

Question 15

Explain Direct Control of Blade Angle

Answer 15

Can be used on any type of turboprop engine. The power lever selects a blade angle and various automatic systems adjust the fuel flow to keep the
propeller RPM constant

The direct control of blade angle in a free power turbine system is also used on some helicopters. Operation of the collective pitch lever adjusts
fuel flow/engine RPM to suit the selected requirements

Question 16

Explain twin lever control

Answer 16

Installed on aircraft with single acting propellers. The engine is controlled by the power lever and the propeller RPM is dependently selected using
the propeller conditioning lever

Question 17

Explain ground fine pitch stop

Answer 17

A fixed abutment stop on the operation piston which allows the propeller to take up a zero degree position. This is known as the Discing position and
is used for starting and initial acceleration. The blades move to this position when the power lever in the cockpit is moved to ground idle

Question 18

Explain FFPS

Answer 18

This is a retractable stop used to limit the fine (low) pitch available during flight. It is always in the engaged position during take off and cruise power
settings. The FFPS is locked in position automatically as the power lever is advanced to take off. This stop is operated by oil pressure but controlled electrically via a
cockpit located FFPS lever with two positions, ENGAGED and WITHDRAWN.

Requirements
• To energise both engine lock withdrawal solenoids.
• To provide cockpit indication that the circuit is energised.
• To de-energise the electro-hydraulic stop circuit when the FFPS circuit is
energised

Question 19

Explain the electro-hydraulic stop

Answer 19

A pitch coarsening mechanism to prevent the propeller drifting below flight fine pitch should a failure of the FFPS occur. The system is activated by a micro
switch which will energise the system if the propeller blade angle should decrease below the FFPS by 2 degrees. The system is ARMED when the FFPS lever is ENGAGED and DISARMED when the FFPS lever is WITHDRAWN. Pitch coarsening is controlled by a Piston Valve Lift Solenoid

Question 20

Explain the hydraulic pitch lock?

Answer 20

A spring operated valve which is designed to prevent the propeller from moving to a finer (lower) pitch in the event of an operating oil supply failure. The valve is positioned in the coarse (high) pitch oil line and in the event of an oil pressure failure, the spring will overcome and close the valve thereby
trapping the oil in the coarse (high) pitch oil line

Question 21

What does manual feathering do to to the PCU?

Answer 21

Selection of the feathering lever overrides the propeller control unit (PCU) centrifugal flyweights by lifting the pilot valve to direct oil, supplied by an
electric feathering pump, to the coarse (high) pitch line and driving the propeller to the feather position

Question 22

When is up-trim automatically carried out?

Answer 22

• Torque of the local engine falls below 25%
• NP (as indicated by the torque sensor) falls below 80%
• PLA is in the rating detent
• MTOP is not set

Question 23

When is autofeather engaged?

Answer 23

Autofeather is triggered from the armed state when the torque of the local engine, as indicated by both torque signals, falls below 25% for at least three

seconds. Following a 3-second delay, an overriding drive coarse signal is input on both active and standby Control Lanes; the servo valve then select coarse
pitch. When one propeller is autofeathered, the autofeather function of the other propeller is automatically disarmed. The auxiliary feathering pump is also activated for approximately 30 seconds. This makes sure adequate oil pressure is available for propeller feathering