NATOPS CH2.1 Powerplant and Related Systems

Question 1

What is the name of the engine we have, and what are its sections?

Answer 1

We use the T700-GE-401C engine, a front-drive turboshshaft engine which consists of 5 sections.

• Inlet
• Compressor (5 stage axial and single stage centrifugal)
• Combustor
• Turbine (2 dual stage turbines)
• Exhaust

Question 2

What does the Turbine section consist of?

Which turbines drive what?

Answer 2

You have the N_g or Gas Turbine Generator which is going to drive the compressor and the Accessory Gearbox

The N_p or Power Turbine drives the Power Turbine Shaft, which runs inside the Gas-Generator Shaft extends through the front of the engine, where it connects to the Input Module which will in turn drive the rotors

Question 3

Where is TGT sensed?

Answer 3

It is sensed in between the Gas-Generator and Power turbine

Question 4

How much airflow is used for combustion?

What is the rest used for?

Answer 4

Approximately 30% of engine airflow is used for combustion. The remaining air is used for:

• Compressor Intel Temperature (T2)
• Compressor Discharge Pressure (P3)
• Combustor and Turbine Cooling (The big one here)
• Engine Oil Seal Pressurization

Question 5

What are the major components of the Engine Control System?

Answer 5

The components are necessary for the proper and complete control of th engine to maintain a constant N_p/N_r.

• Hydromecahnical Unit
• Overspeed and Drain Valve
• Digital Electronic Control Unit
• Engine Driver Alternator
• Series of fuel flow control valves

​In general the HMU will provided Gas-generator control while the DECU will trim back the HMU to satisfy the requirements of the power turbine work load.

Question 6

What are the two purposes of the Engine Driven Fuel Boost Pump?

Answer 6

1. Provide reliable suction feed from the fuel tanks to the engine, minimizing vulnerability and fire hazard in the event of damage to fuel lines
2. Provides discharge pressure to satisfy the min pressure requirement of the HMU

Question 7

What does the fuel filter do?

What does it NOT do?

Answer 7

The engine fuel filter provides filtration of solid particulate matter, but does not filter water.

Question 8

After leaving the Fuel pump how does fuel move through the HMU?

What is fuel tapped off for?

Answer 8

Fuel leaves the pump and passes through the METERING VALVE and SHUTOFF valve and is then directed through an external line to the Oil-Fuel Heat Exchanger.

Some fuel is tapped off for:

• Positioning the metering valve
• Positioning the servo piston that actuates the Variable Geometry Vane Servo
• Amplifying signals (T2/P3/N_g) which influence fuel flow

Question 9

Whar are the three inputs to the HMU?

Answer 9

There are two mechanical inputs to the HMU from the cockpit

• PCL via the Power Available Spindle (PAS)
• Collective via the Load Demand Spindle (LDS)

It also receives an electrical input from the DECU via the torque motor servo which precisely trims the HMU.

Question 10

What does the HMU respond to the PCL for?

Answer 10

1. Fuel Shutoff
2. Setting engine start fuel flow with auto acceleration to idle
3. Setting Permissible N_g up to Max
4. Fuel Priming
5. DECU override capability

Question 11

How do the DECU and HMU talk to each other and what are they communicating?

Answer 11

The metering valve schedules engine fuel flow commensurate with the current power demand and is trimmed to the required level by the torque motor servo in response to DECU Signals

(The metering valve is like a wild bronco trying to schedule more fuel that it needs based on what the power demands are, the DECU is like the cowboy calming the bronco down and getting him to run at just the right speed)

The HMU then provides feedback to the DECU via the linear variable displacement transducer (LVDT) to null the torque motor servo input, stabilizing metering movement and preventing engine oscillation and hunting

(the bronco then communicates with the rider to stop pulling on him once he is in a comfortable groove, for a smooth race to the finish)

Question 12

The PAS sets max available N_g, more than is required by the helo

Why?

Answer 12

Engine is intentionally scheduled for a higher-than-required power for two reasons:

1. Fail-safe to high power. If there is a failure of the torque motor servo then the engine will return to a higher than required setting to prevent power limiting
2. Power available with One Engine Inop. If one engine fails, the other will be sufficiently able to increase power up to its limit

Question 13

What does the HMU Provide?

Answer 13

• Rapid engine transient response through collective compensation
• Auto fuel flow scheduling for engine start
• N_g Overspeed protection (Mechanically limiting N_g to 110±2%)
• N_g Governing
• Acceleration Limiting
• Flameout and Compressor Stall Protection

Question 14

What does the ODV Provide?

Answer 14

• Provides main fuel to the 12 fuel injectors during engine start and operation
• Purges the main fuel manifold overboard after engine shutdown to prevent cocking of the fuel injectors
• Traps fuel upstream, keeping the fuel/oil heat exchanger full, so that engine priming is not required at every start
• Returns fuel back to the HMU if N_p Overspeed is energized (120% N_p) or the hot start preventer is activated

Question 15

Describe Engine Limiting and give some examples

Answer 15

Limiting is a condition defined by reaching a governing condition as a part of the T700 design. The Engine Control system is preventing further increase in powet output.

For example TGT-Limiting or HMU-Fuel Flow Limiting. Both will be realized when you increase collective and instead of getting more power, you get a drop in N_r

Question 16

What does it mean when you are Parameter Limited?

What are some examples?

Answer 16

Being limited means you are reaching a maximum value perscribed by CH4 and have the ability to exceed that limit. It is the pilots responsibilty to keep the engine and transmission within the perscribed limits.

N_p Limited, N_g Limited, Torque Limited, Pressure Limited

Question 17

What are the two types of fuel flow limiting?

Answer 17

1. When at certain combinations of pressure and temperature, high power settings from the collective could result in insufficient fuel provided by the fuel pump. You are limited by the physical size of the lines. In this case You will get an N_r decrease without hitting any other limit.
2. Engine Pressure Ratio (EPR) limiting is when the HMU tries to protect the compressor section by limiting fuel flow. (This is the compressor stall protection that the HMU provides)

Question 18

What does the alternator do?

Answer 18

It powers the essential engine electrical functions (DECU and Igniters), and provides the N_g signals to the cockpit

Question 19

What are the inputs to the DECU and where do they come from?

Answer 19

From the Cockpit

• Engine Speed Trim Switch
• Contingency Power Switch
• Engine Overspeed Test A and B Buttons

From the Helo

• Torque from the other DECU (load sharing)
• N_p demand
• 400hz Power (from Alt or Gens)
• HMU feedback (LVDT)

Question 20

What are the 14 Functions of the DECU?

Answer 20

• 400 Hz Airframe Power Capability
• N_p Governing
• N_p Overspeed Protection
• N_p Overspeed Test
• N_g Decay Relight
• Contigency Power
• Hot Start Prevention
• Engine Load Sharing
• Fault Diagnostic System
• Transient Droop Improvement (TDI)
• Auto Ignition System
• Signals to the Cockpit
• TGT Limiting
• Engine Speed Trim
• DECU Lockout

Question 21

N_p Overspeed Protection

Answer 21

An electrical signal from the DECU to the ODV will secure fuel flow at 120% N_p causing an engine flame out

Question 22

N_p Overspeed Test

Answer 22

When both buttons are pressed the N_p Overspeed limit is reset down to 96%. If one switch is pressed and the engine flames out, then the other test is faulty

Question 23

When does N_g Decay Relight come on?

When is it disabled?

Answer 23

If an engine flames out for any reason and exceeds a specific N_g deceleration rate, the Auto-Ignition System will turn the igniters on for 5sec.

The N_g Decay relight feature is disabled below 62% N_g

Question 24

Auto Ignition System

Answer 24

When an N_p overspeed condition is reached and during the overspeed test, the overspeed valve in the ODV is opened to reroute fuel to the HMU.

When N_p drops below 120%, the ODV overspeed valve is closed and the igniters are turned on for 5sec. This is repeated until N_p/N_r is controlled. Yaw kicks may be experienced.

Question 25

TGT Limiting

Answer 25

The legacy DECU will stop fuel flow scheduling when the temperature approaches 851°C, the IRP limiter will stop scheduling fuel at 839°C±10°C.

We have the EDECU so the it will protect the engine starting at 878°C beginning fuel flow limitting at 866±10°C

If more power is requested at this point, N_r will droop before more power is allowed.

Question 26

Engine Load Sharing

Answer 26

Torque signals are compared between the two DECUs, if they differ a torque error signal is generated. The system operates by increasing power to the lower torque engine.

Question 27

Engine Speed Trim

Answer 27

The INCR/DECR switch will trim both engines simultaneously controlling N_p between 96%-101% N_p

Question 28

Contingency Power

Answer 28

By turning on the C-Power switch we are allowing the TGT to increase up to 903°C, allowing normal fuel flow scheduling all the way up to 891±10°C

Question 29

DECU Lockout

Answer 29

Engine power is no longer controlled by the DECU, it is set by the PAS and LDS only. In lockout the Torque Motor Servo is disabled, deactivating

• TGT limiting
• N_p Governing
• Engine Load sharing

N_p Overspeed Protection is retained. In order to regain automatic control of the engine bring the PCL to idle before returing it to fly.

Question 30

When does Hot Start Prevention kick on?

When will fuel flow be restored?

Answer 30

Detects a hot start when TGT exceeds 900°C with N_g below 60% and N_p below 50%, and will stop fuel flow by tripping the ODV.

Fuel flow will be restored when either TGT decreases below 300°C or 25sec has passed. Whichever occurs first.

Question 31

400hz Power

Answer 31

The DECU can be powered by the Engine Alternator or aircraft power via the ENG OVSP circuit breaker. It will automatically determine the best power.

Question 32

Transient Droop Improvement

Answer 32

Designed to initiate power turbine acceleration early by using anticipator signals from the TDI N_r sensor on the left accessory module and a collective position sensor in the mixing unit.

If anyone asks THIS is the most important function of the DECU when executing Autos as it will “anticipate” the large collective pull at the bottom and provide lots of fuel at the pull to minimize drooping.

Question 33

What do the EDECU/CEDECU provide that the old DECU did not?

Answer 33

• TGT Limiting: increases the regular TGT fuel flow limit up to the 866±10°C
• Auto C-Power: will automaticall set TGT limit to 891±10°C when the torque from one engine is below 50% torque. C-Power on advisories will not illuminate
• Dual Auto Contingency: newer CEDECU will also activate auto C-Power if excessive N_r/N_p Droop is sensed

Question 34

Describe the path an engine oil particle takes from start to finish

Answer 34

Start in the oil tank, suction from the engine driven oil pump will suck me up and push me out through the engine oil filter (with a bypass)

From the filter it travels over an oil temp and oil pressure sensor then through the AGB to the 6 main sump bearings (each with 2 sets of oil jets)

Scavenge oil will then flow through the pump inlet and over a chip detector

Finally it passes either through the fuel/oil cooler and the scroll vanes, or via an oil/cooler bypass valve back into the oil tank

Question 35

Describe the Filter Element and Bypass

Answer 35

Oil is routed through a disposable filter element, as the pressure differential increases the PDI will popout providing visual indication that the filter needs to be replaced and fluid will be routed by the bypass instead.

A thermal lockout below 38°C will prevent the PDI from popping during cold weather starts. Oil may still bypass the filter until it warms up and reduces in viscosity.

Question 36

What is used to drive the engine starter?

Where can it come from?

Answer 36

The engine uses a Pneumatic start system, using air to run the starter. The air can either come from the APU or it can come from the other engine via Crossbleed. If the APU is on, it will be the source for engine starts regardless of switch position.

Question 37

When does the starter drop out?

Answer 37

The starter will drop out between 52-65% N_g

Question 38

What are the numbers associated with a crossbleed start?

Answer 38

When executing a crossbleed start, the RECEIVING ENGINE SHALL be a minimum of 24% N_g prior to advancing the PCL to idle.

This will require a donor engine of 90-94% N_g which will require you to go light on the wheels.

Question 39

How are the Engine Anti-Ice Valves held shut?

What happens if they are open?

Answer 39

Anti-Ice Solenoids are electrically held shut, and controlled by the ENG ANTI-ICE and DEICE MASTER switches. When the solenoid is deenergized, bleed air is taken from the 5th stage compressor.

When open you will experience power loss up to 18% torque, up to 49% if you have a malfunctioning Inlet Anti-Ice valves.

Question 40

What are the three ways to anti-ice the engine?

Answer 40

1. Vent Bleed air into the engine swirl vanes and Inlet Guide Vanes via the anti-ice/start bleed valve
2. Vent air into the engine inlet by the engine inlet anti-ice valve
3. Continuously pump emgine oil through the scroll vane case

Question 41

When do the engine anti-ice valves open?

How do you know if they are malfunctioning?

Answer 41

The valve will open when the anti-ice switch is on, and during engine starts. It will remain on while N_g is below ~90% to prevent compressor instability

The valves are malfunctioning if:

• ENG ANTI-ICE advisory is on with N_g is greater than 90% (94% if OAT is greater than 15°C)
• ENG ANTI-ICE advisory does not illuminate with N_g Less than 88% (variable based on OAT)
• No ENG ANTI-ICE advisory if switch is placed to ON
• No rise in TGT with ENG ANTI-ICE switch placed to On (expect 30-100°)

Question 42

Describe the Inlet Anti-Ice Valve and when it operates

Answer 42

The inlet anti-ice valve is controlled by the ENG ANTI-ICE switch along with the DEICE MASTER. When on, a set of Freon-Bellows will open variably based on OAT.

• When temperature is above 13°C the Freon-Bellows will be closed all the way.
• When temperatures are in between 4°C and 13°C the Bellows will be open variably
• When temperatures are below 4°C the Bellows will be fully open

When the inlet temperature reaches 93°C the INLET ANTI-ICE advisory will appear, when below that temperature the advisory will extinguish

Question 43

Explain why you lose 18-49% power when the Anti-Ice valves are open

Answer 43

• When the valve is open (and if you have a failed Inlet anti-ice valve) air is diverted from from the 5 stage compressor leaving less air to cool the combustion and turbine sections.*
• Less air for cooling means your engines will burn hotter, and if your engines are burning hotter then you will reach your TGT limit faster.*
• Effectively with the engine anti-ice valves open you will reach your TGT limits at a lower toque value than if you they were closed*

Question 44

What happens to the air that is warmed by the hot oil in the scroll vanes?

Answer 44

The primary function of the oil circulation is to cool the oil and provide anti-icing to the engine frame, the air heated in this manner is vented overboard by the IPS and does not provide any anti-ice protection to air entering the compressor.

Question 45

Where do N_p and torque signals come from?

N_g?

Answer 45

There are two N_p sensors located on top of the exhaust frame of the engine. They are identical and interchangeable but serve different functions. The left sensor will report N_p while the right sensor will provide torque. (Think Right and toRque)

N_g is derived from the alternator which sends it right to the display

Question 46

Describe the TGT thermocouple harness and its bias

Answer 46

It consists of 7 thermocouples used for measuring TGT which is sent right to the DECU. The signal is biased -71°C by the DECU before being displayed on the instruments.

If the DECU is not powered (CB pulled, or a failure) your TGT will indicate 71°C higher than expected.