What is the name of the engine we have, and what are its sections?
We use the T700-GE-401C engine, a front-drive turboshshaft engine which consists of 5 sections.
- Compressor (5 stage axial and single stage centrifugal)
- Turbine (2 dual stage turbines)
What does the Turbine section consist of?
Which turbines drive what?
You have the Ng or Gas Turbine Generator which is going to drive the compressor and the Accessory Gearbox
The Np 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
Where is TGT sensed?
It is sensed in between the Gas-Generator and Power turbine
How much airflow is used for combustion?
What is the rest used for?
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
What are the major components of the Engine Control System?
The components are necessary for the proper and complete control of th engine to maintain a constant Np/Nr.
- 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.
What are the two purposes of the Engine Driven Fuel Boost Pump?
- Provide reliable suction feed from the fuel tanks to the engine, minimizing vulnerability and fire hazard in the event of damage to fuel lines
- Provides discharge pressure to satisfy the min pressure requirement of the HMU
What does the fuel filter do?
What does it NOT do?
The engine fuel filter provides filtration of solid particulate matter, but does not filter water.
After leaving the Fuel pump how does fuel move through the HMU?
What is fuel tapped off for?
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/Ng) which influence fuel flow
Whar are the three inputs to the HMU?
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.
What does the HMU respond to the PCL for?
- Fuel Shutoff
- Setting engine start fuel flow with auto acceleration to idle
- Setting Permissible Ng up to Max
- Fuel Priming
- DECU override capability
How do the DECU and HMU talk to each other and what are they communicating?
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)
The PAS sets max available Ng, more than is required by the helo
Engine is intentionally scheduled for a higher-than-required power for two reasons:
- 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
- Power available with One Engine Inop. If one engine fails, the other will be sufficiently able to increase power up to its limit
What does the HMU Provide?
- Rapid engine transient response through collective compensation
- Auto fuel flow scheduling for engine start
- Ng Overspeed protection (Mechanically limiting Ng to 110±2%)
- Ng Governing
- Acceleration Limiting
- Flameout and Compressor Stall Protection
What does the ODV Provide?
- 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 Np Overspeed is energized (120% Np) or the hot start preventer is activated
Describe Engine Limiting and give some examples
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 Nr
What does it mean when you are Parameter Limited?
What are some examples?
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.
Np Limited, Ng Limited, Torque Limited, Pressure Limited
What are the two types of fuel flow limiting?
- 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 Nr decrease without hitting any other limit.
- 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)
What does the alternator do?
It powers the essential engine electrical functions (DECU and Igniters), and provides the Ng signals to the cockpit
What are the inputs to the DECU and where do they come from?
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)
- Np demand
- 400hz Power (from Alt or Gens)
- HMU feedback (LVDT)
What are the 14 Functions of the DECU?
- 400 Hz Airframe Power Capability
- Np Governing
- Np Overspeed Protection
- Np Overspeed Test
- Ng 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
Np Overspeed Protection
An electrical signal from the DECU to the ODV will secure fuel flow at 120% Np causing an engine flame out
Np Overspeed Test
When both buttons are pressed the Np Overspeed limit is reset down to 96%. If one switch is pressed and the engine flames out, then the other test is faulty
When does Ng Decay Relight come on?
When is it disabled?
If an engine flames out for any reason and exceeds a specific Ng deceleration rate, the Auto-Ignition System will turn the igniters on for 5sec.
The Ng Decay relight feature is disabled below 62% Ng
Auto Ignition System
When an Np overspeed condition is reached and during the overspeed test, the overspeed valve in the ODV is opened to reroute fuel to the HMU.
When Np drops below 120%, the ODV overspeed valve is closed and the igniters are turned on for 5sec. This is repeated until Np/Nr is controlled. Yaw kicks may be experienced.
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, Nr will droop before more power is allowed.
Engine Load Sharing
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.
Engine Speed Trim
The INCR/DECR switch will trim both engines simultaneously controlling Np between 96%-101% Np
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
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
- Np Governing
- Engine Load sharing
Np Overspeed Protection is retained. In order to regain automatic control of the engine bring the PCL to idle before returing it to fly.
When does Hot Start Prevention kick on?
When will fuel flow be restored?
Detects a hot start when TGT exceeds 900°C with Ng below 60% and Np 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.