AGK Engines - Control & Operation Flashcards
Normal Start Sequence
Start selected
Ignition commences
Air start valve opens (N2 rotation)
Fuel delivery (minimum)
Light up occurs (EGT Rise)
~Increase fuel flow to increase N2]
N1 begins to rotate
Self-sustaining speed
Starter circuit disconnects (air start valve closes)
Fuel flow increases to accelerate engine
Stable ground idle
-Once at stable ground idle, they’ll be a notice difference in speeds of the spools
~N2 spins faster than N1
Types of starter motor?
Two types of starter motor, both connected to the accessory gearbox
Air (pneumatic starter)
Electric (Starter/Generator)
Air (pneumatic starter) - starter motor
Driven by compressed air and controlled by an air start valve
~Air supplied from external source, cross bleep or APU
~Incorporates an overrun clutch to prevent damage from failing to disconnect
HP spool rotates first to initiate starting
Electric (Starter/Generator) - starter motor
Used on More Electric Aircraft (MEA) such as B787
~During the start sequence AC electric spins the motor, turning the accessory gearbox
~Once at ground idle, the system flips to generator mode
HP spool rotates first to initiate starting
Start malfunctions, engine may suffer the following…
Dry Start
Hung Start
Hot Start
Wet Start
Dry Start characteristics
Low and stagnating RPM
No EGT rise
No fuel flow
~Maybe due to malfunction of the HPSOV
Hung Start characteristics
Low and stagnating RPM
High EGT
Low fuel flow
Maybe the result of:
~Compressor stall
~Low bleed pressure
~Engine failing to reach self-sustaining speed
~Early disconnection of the starter motor
Hot Start
Normal increase In RPM
Rapid and excessive EGT rise
Normal fuel flow
Possible following a previous failed start attempt
Wet Start
Low and stagnating RPM
No EGT rise
Some indicated fuel flow
A dry run (blow through) should be carried out before attempting a restart
Multi-spool engine start
Ground idle
~ N1 - 20-25%
~ N2 - 50-60%
Flight idle is a higher idle setting allowing the engine to respond to any speed increase quicker
Airborne Relight
Same requirements as ground start
Starter motor not required as the engine will be windmilling
A manual start may be used to better control operation of the start valve and igniters
Altitude ans Airspeed considerations must be taken into account
~Usually below 25,000ft and 250kts
~Known as the relight envelope
Thrust indications
Max t/o: maximum allowed; usually time limited
Max continuous: Maximum thrust than can be used continuously
Go Around: thrust value close to , but less than, maximum thrust
Max climb: used for routine, all engine climbs
EPR and thrust indications
Engine Pressure ratio (EPR)
Accurate method to indicate pressure thrust
Compares the pressure rise in the exhaust with static pressure in the intake
If a sensor becomes unavailable (iced up) the engine will revert to N1 Thrust setting
N1 and Thrust indications
N1 Thrust Setting
Thrust set by percentage of Fan RPM
FADEC will rotate N2 to a desired value in order to provide an N1 rotation and given Thrust setting
Turbo-prop Torque Indications
Percentage of maximum - Torque
usually measured using soil pressure or a electronic Wheatstone bridge
In t/o or high power conditions, percentage may exceed 100% for a limited time
De-rate and Flex thrust
To maximise engine life and reduce costs, reduce thrust is available via FMC
~Engines will provide thrust in a worst case scenario
~Under all other conditions, especially cold and sea level environments the engine will perform better than required
~Under these conditions the crew can legally plan a reduced power t/o
Two methods available:
-De-rating
-Flexible (Flex) thrust.
De-rating
Reduces the thrust available from maximum value
Has the effect of imposing a lower thrust limit on the engine
~The Vmcg and Vmca speeds will change and are available through the performance manual
DISADVANTAGE- full thrust not available should it be required
Flex Thrust
Another alternative to reducing engine thrust on t/o
Common method - use assumed temp
As max thrust decreases, air temp increases (eng can be fooled into producing less thrust)
Using assumed temp for which this would be produced (at full thrust) is interned into FMC as OAT for the day (e.g 67°C)
~ENetring OAT of 67°C will reduce then thrust by a given amount
BENEFIT- if full thrust is required, thrust levers can be set to TO/GA to disregard the flex configuration
As full thrust is available, Vmcg and Vmcd must be calculated based on full thrust used (worse case scenario)
Thrust in flight - high/low by-pass
High
~Reduced thrust is more pronounced as there’s a lower RAM effect in the by-pass duct
~Fuel consumption ∝ temp = more fuel efficient in cruise
~thrust can be reduced (less drag)
~fuel flow reduction
Engine seizure
Is when one or both spools stop rotating
~e.g. due to lack of lubrication, deformation of engine component or failure of compressor/turbine blade
Detected by vibrations, rapid rundown of spool(s),
Increase in EGT or engine internal temperature
Adverse yaw from asymmetric effects
As vibration increases, as a precaution - shut down engine
