3.3 Engine Starting, Operating Limits & Malfunctions
Key Takeaways
- The standard turbine start sequence involves pneumatic starter rotation of the HP spool (N2), igniter activation, fuel introduction at minimum certified N2 (typically 18%–25% N2), positive light-off (EGT rise within 10–20 seconds), and starter cutout at self-sustaining speed (~50%–55% N2).
- A Hot Start is characterized by rapid, uncontrolled EGT acceleration exceeding certified start limits; immediate flightcrew memory action requires moving the Fuel Control switch to OFF while continuing starter motoring to purge fuel and cool the hot section.
- A Hung Start occurs when compressor rotation (N2) stagnates below idle RPM without accelerating to self-sustaining speed, caused by insufficient starter air duct pressure, premature starter disengagement, or fuel scheduling errors.
- Compressor stalls represent localized aerodynamic flow separation on compressor airfoils; a fully developed Compressor Surge involves instantaneous axial pressure collapse, explosive reverse airflow through the inlet, violent yaw/vibration, and rapid EGT excursion.
- Inflight relight procedures divide into Windmilling Starts (relying on ram air at high airspeeds >250–300 KIAS within a defined altitude envelope) and Crossbleed Starter Assist Starts (required at lower airspeeds where pneumatic starter torque is needed to achieve light-off N2).
Last updated: August 2026
Engine Starting, Operating Limits & Malfunctions
Turbine engine starting and high-altitude operation demand strict adherence to aerodynamic, mechanical, and thermal limitations. Flightcrews must instantly distinguish between normal transient engine parameters and critical malfunctions such as hot starts, hung starts, wet starts, tailpipe fires, and compressor surges. Prompt identification and decisive execution of non-normal procedures protect multi-million-dollar powerplants from catastrophic damage and ensure flight safety.
1. The Normal Turbine Ground Start Sequence
Starting a large turbofan requires external mechanical energy to rotate the high-pressure core compressor ($N_2$) to a speed where it generates sufficient airflow and compression pressure to support stable, continuous combustion.
+-----------------------------------------------------------------------------+
| TURBINE GROUND START PHASES |
| |
| [1. STARTER ENGAGEMENT] |
| - Start switch ON / Start valve OPEN (Pneumatic air drives ATS) |
| - Duct pressure drops ~5-10 psi, N2 accelerates, Oil Pressure rises |
| | |
| v |
| [2. IGNITION & FUEL INTRODUCTION] |
| - At certified N2 threshold (typically 18% - 25% N2) & positive N1 rotation|
| - Fuel Control switch placed to RUN / ON |
| - Dual igniters fire (continuous high-energy plasma spark) |
| | |
| v |
| [3. LIGHT-OFF & THERMAL ACCELERATION] |
| - EGT rise observed within 10 to 20 seconds (Light-off verification) |
| - Core accelerates rapidly through combustion expansion |
| | |
| v |
| [4. STARTER CUTOUT & IDLE STABILIZATION] |
| - At self-sustaining speed (~50% - 55% N2), starter cuts out / valve closes|
| - Engine stabilizes at Ground Idle (N1 ~20-25%, N2 ~60-65%, EGT normal) |
+-----------------------------------------------------------------------------+
Critical Start Parameters & Starter Duty Cycles:
- Minimum Duct Pressure: Pneumatic starters require a minimum pneumatic duct pressure (typically 30 to 40 psi on the ground) supplied by the APU, ground air cart, or crossbleed from an operating engine. Attempting a start with sub-standard pressure causes sluggish acceleration and hung/hot starts.
- Starter Duty Cycle Limits: Air Turbine Starters (ATS) generate enormous mechanical friction and heat. Typical transport category starter duty limitations mandate: 2 consecutive start attempts of up to 2 minutes duration each, followed by a mandatory 20-second cooling interval between attempts, and a 15-minute cooling period after the third attempt.
2. Start Malfunctions: Recognition & Abort Criteria
During any start sequence, the pilot monitoring (PM) and pilot flying (PF) must vigilantly track $N_1$, $N_2$, EGT / ITT, Fuel Flow, and Oil Pressure against time. Any departure from normal progression mandates immediate start abort.
+-----------------------------------------------------------------------------+
| START MALFUNCTIONS COMPARISON MATRIX |
| |
| MALFUNCTION | PRIMARY SYMPTOM | COMMON ROOT CAUSES |
| -------------+----------------------------+-------------------------------|
| HOT START | EGT rises rapidly and | Low starter duct pressure, |
| | threatens to exceed start | tailwind into tailpipe, |
| | limit (EGT redline) | fuel nozzle over-metering |
| -------------+----------------------------+-------------------------------|
| HUNG START | N2 rotation stagnates/stops| Low starter torque, premature |
| | below idle (e.g., at 35%); | starter cutout, excessive |
| | EGT slowly climbs | compressor friction |
| -------------+----------------------------+-------------------------------|
| WET START | Fuel flow indicated for | Dual igniter failure, fuel |
| | 15-20 sec with ZERO rise | nozzle blockage, water/air |
| | in EGT (No light-off) | contamination in fuel |
| -------------+----------------------------+-------------------------------|
| TAILPIPE | Fire/flames shooting from | Excess unburned pooled fuel |
| FIRE | exhaust duct after abort; | ignited in tailpipe casing; |
| | EGT elevated, zero FF | oil seal leak into turbine |
+-----------------------------------------------------------------------------+
In-Depth Abort Procedures & Memory Actions:
1. Hot Start Procedure:
- Recognition: EGT accelerates rapidly toward the redline starting limit before $N_2$ reaches self-sustaining speed.
- Immediate Memory Action: FUEL CONTROL switch $\rightarrow$ OFF / CUTOFF.
- Dry Motoring: LEAVE THE STARTER ENGAGED (or immediately re-engage within starter engagement speed limits). Continue dry motoring the engine for at least 30 to 60 seconds to blow cool ambient air through the compressor and combustion chamber, cooling the turbine blades and purging atomized unburned fuel.
2. Hung Start Procedure:
- Recognition: $N_2$ accelerates normally up to 30%–40%, but then stops accelerating well below idle speed while EGT steadily creeps upward due to inadequate compressor cooling airflow.
- Immediate Action: Move Fuel Control switch to OFF, continue starter motoring to purge unburned fuel, and verify pneumatic duct pressure.
3. Wet Start (No Light-off):
- Recognition: Fuel flow is displayed on the EICAS/ECAM, but no EGT rise occurs within 15 to 20 seconds after fuel introduction.
- Immediate Action: Move Fuel Control switch to OFF. Dry motor the engine for the full certified purge time (typically 30–60 seconds) before attempting another start. Attempting a second start without dry motoring will ignite pooled raw fuel in the combustor, producing an explosive hot start or uncontained combustor rupture.
4. Tailpipe Fire Management:
- Recognition: Tower or ground crew reports flames exiting the engine tailpipe, or EGT remains high after an aborted start with zero fuel flow.
- Critical Action: DO NOT DISCHARGE THE ENGINE FIRE BOTTLE! Engine fire extinguishing agent (Halon) discharges into the external engine nacelle cowl cavities, not into the internal core exhaust gas path. Discharging the bottle is useless for internal tailpipe fires and wastes the fire suppression agent.
- Correct Procedure: Move Fuel Control switch to OFF, ensure the starter is engaged, and continuously dry-motor the engine until the high-velocity air blast blows the fire out.
3. Compressor Aerodynamic Breakdown: Stall vs. Surge
Compressor blades are rotating airfoils subject to the same aerodynamic principles as aircraft wings. When local angle of attack (AOA) exceeds the critical stalling angle, boundary layer separation occurs.
+-----------------------------------------------------------------------------+
| COMPRESSOR BLADE ANGLE OF ATTACK DYNAMICS |
| |
| Rotational Velocity Vector (U) |
| <-------------------------------------- |
| | |
| | |
| | Relative Airflow Velocity (W) |
| Axial Velocity | / |
| Vector (V_a) | / Angle of Attack (alpha) = arctan(V_a / U) |
| | |/ |
| v +===================> [ COMPRESSOR BLADE ] |
| |
| * STALL TRIGGER: A DECREASE in Axial Velocity (V_a) or an ABNORMAL |
| INCREASE in Rotational Velocity (U) INCREASES Angle of Attack (alpha). |
| When alpha > alpha_crit --> BOUNDARY LAYER SEPARATES --> STALL OCCURS. |
+-----------------------------------------------------------------------------+
Stall vs. Surge Comparison:
| Characteristic | Compressor Stall (Transient / Rotating) | Compressor Surge (Full Axisymmetric Breakdown) |
|---|---|---|
| Aerodynamic Scope | Localized flow separation on one or more blade stages forming rotating stall cells | Complete, instantaneous collapse of axial flow across the entire compressor |
| Airflow Direction | Flow continues forward through unaffected sectors of the annulus | Instantaneous reverse flow: high-pressure combustor air blasts violently forward out the inlet cowl |
| Acoustic & Sensory Cues | Low-frequency rumble, mild vibration, minor EGT fluctuations | Loud explosive bangs/booms, violent airframe shuddering, severe yawing moments |
| Cockpit Indications | Slight rise in EGT, minor $N_1$/$N_2$ needle oscillations | Rapid EGT spike exceeding limits, severe drop in $N_1$ and $N_2$, loss of thrust |
| Recovery Action | Smoothly retard thrust lever to idle, turn on engine anti-ice | Immediately retard thrust lever to IDLE, verify bleed air valves open, monitor EGT |
[!CAUTION] Compressor Surge Recovery Protocol: If an engine surges during high-thrust climb or cruise: (1) Smoothly retard the affected thrust lever to IDLE to reduce fuel flow and relieve the combustor back-pressure; (2) Verify Engine Anti-Ice is ON (opening anti-ice bleed valves provides additional compressor stage bleed relief); (3) Confirm EGT drops below maximum limits; (4) If parameters stabilize at idle, slowly and cautiously advance thrust. If the surge recurs or EGT exceeds limits, shut down the engine and execute the Inflight Engine Shutdown Checklist.
4. Inflight Flameout Causal Chains & Auto-Relight Systems
An engine flameout is the unintentional extinction of the combustion flame inside the burner. Modern engines incorporate sophisticated electronic protections to prevent and recover from flameouts.
Primary Flameout Causes:
- Severe Weather & Water Ingestion: Heavy convective precipitation or hail can exceed the burner's allowable water-to-air ratio, quenching the primary flame.
- Volcanic Ash Ingestion: Sub-micron silica particles melt in the 1,500°C combustor, coating fuel nozzles and vitrifying into glass on HPT guide vanes, choking compressor airflow and inducing severe surges and flameouts.
- Severe Turbulence & Compressor Surges: Violent cross-flow shears at high angles of attack disrupt inlet airflow, stalling the compressor and depriving the combustor of air.
- Fuel Starvation / Aeration: Negative-g flight or uncoordinated turns with low fuel levels can unport boost pump pickups, introducing air pockets into the high-pressure fuel line.
Electronic Auto-Relight & Continuous Ignition:
- Auto-Relight Function: The FADEC continuously tracks core deceleration rates. If the EEC detects an uncommanded drop in $N_2$ below idle speed or a rapid drop in combustor pressure ($P_{s3}$) while the Fuel Control switch is in RUN, the EEC automatically activates both high-energy igniters instantly to re-light the fuel-air mixture before core RPM decays.
- Continuous Ignition (CONT): Airline SOPs mandate selecting continuous ignition manually during: (1) takeoff and landing on contaminated runways; (2) flight through moderate-to-severe turbulence; (3) flight through heavy precipitation; and (4) operating in severe icing conditions.
5. Inflight Relight Envelope & Procedures
When attempting an engine restart after an inflight flameout or shutdown, the flightcrew must consult the certified Inflight Restart Envelope in the Quick Reference Handbook (QRH).
+-----------------------------------------------------------------------------+
| INFLIGHT RESTART ENVELOPE |
| |
| ALTITUDE (ft) |
| 30,000 +----------------------------------------------+ |
| | | |
| 25,000 | +--------------------------+ | |
| | | CROSSBLEED ASSIST ONLY | | |
| 20,000 | | (Pneumatics Required) | | |
| | | 180 - 250 KIAS | | |
| 15,000 | +-------------+------------+ | |
| | | | |
| 10,000 | | WINDMILLING START | |
| | | ENVELOPE | |
| 5,000 | | (No Starter Req) | |
| | | 250 - 350 KIAS | |
| 0 +-----------------------+----------------------+ |
| 0 100 200 300 400 |
| AIRSPEED (KIAS) |
+-----------------------------------------------------------------------------+
Restart Methodologies:
| Restart Mode | Operational Envelope & Speed | Mechanical Process | Procedure & Crew Coordination |
|---|---|---|---|
| Windmilling Start | High Airspeed (250 to 350 KIAS), Medium/Low Altitude (<FL250) | High dynamic ram pressure drives the front fan and core compressor without external pneumatic starter assistance ($N_2 > 15%$–$20%$). | Fuel Control ON, dual igniters fire, monitor EGT light-off within 20–30 seconds. No starter button pushed. |
| Crossbleed Starter Assist | Lower Airspeed (180 to 250 KIAS), High/Low Altitude (<FL300) | Ram air is insufficient to achieve light-off $N_2$. Operating engine is advanced to high thrust to supply $\ge 30$–$40$ psi pneumatic duct pressure to the Air Turbine Starter. | Advance good engine to set crossbleed duct pressure, open start valve, verify $N_2$ rotation, introduce fuel at certified $N_2$ threshold. |
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Test Your Knowledge
During a ground start of a transport category turbofan engine, the pilot monitoring notes that EGT is climbing at an abnormally rapid rate and will exceed the redline starting limit before N2 reaches idle speed. What is the correct immediate memory action?
A
B
C
D
Test Your Knowledge
What is the primary physical and aerodynamic distinction between an isolated compressor stall and a fully developed compressor surge?
A
B
C
D
Test Your Knowledge
Ground personnel notify the flightdeck that sustained flames are shooting out of the engine tailpipe following an aborted engine start. Which action should the flightcrew execute?
A
B
C
D