A flameout is the complete, unscheduled cessation of combustion inside a turbine engine's combustor section. Unlike a reciprocating engine that stumbles, misfires, or backfires before it quits, a turbofan or turbojet simply unwinds—smoothly, quietly, and sometimes deceptively. N1 and N2 coil down, EGT drifts toward ambient, and thrust disappears. That quiet character makes recognition a genuine skill, not a reflex. For ATP candidates and working airline crews, mastering the causes, cockpit signatures, restart envelopes, and procedural logic of flameouts is both an ACS requirement and a life-safety imperative.
How Combustion Works—and How It Fails
Turbine engine combustion requires three sustained inputs: compressed air from the compressor stages, atomized fuel metered by the fuel control unit, and a continuous, stable flame anchored inside the combustor liner. Modern annular or can-annular combustors maintain that flame through carefully shaped recirculation zones that keep a pocket of hot, rich mixture near the fuel nozzles at all times. When any of those three elements is degraded below a critical threshold, the flame extinguishes—and unlike a pilot light on a furnace, it does not relight itself.
The FAA's Pilot's Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25) discusses turbine engine operation and the general hazards that can disrupt combustion, though it does not organize flameout causes into a formal taxonomy. For study purposes, it is useful to group the primary causes into fuel-related, airflow-related, and contamination-related categories. Understanding each group clarifies both the symptom set and the appropriate response.
Primary Causes of Jet Engine Flameout
Fuel Flow Interruption
The most operationally common cause is a disruption to fuel supply. This encompasses fuel exhaustion from actual fuel depletion, mismanagement of fuel selectors or crossfeed systems, a failed fuel boost pump that drops delivery pressure below the fuel control unit's operating range, or ice crystals in the fuel lines blocking flow. At high altitudes, jet fuel can cool enough for dissolved water to precipitate as ice, a hazard addressed by fuel system anti-icing additives and fuel heaters. A sudden, total loss of fuel flow produces a rapid, nearly simultaneous drop in N1, N2, EGT, and fuel flow indication—all converging toward zero together.
Compressor Stall
A compressor stall occurs when the angle of attack on one or more compressor blade rows exceeds the critical angle, causing airflow to separate and reverse locally. The resulting disruption can be violent enough to destroy the stable recirculation zone in the combustor, quenching the flame. Compressor stalls often announce themselves with a loud bang or series of bangs, visible fire from the inlet or exhaust, and a rapid, uncommanded EGT spike—followed by flameout if the stall is sustained. This distinguishes a stall-induced flameout from a fuel-starvation flameout, where EGT drops immediately without an initial spike.
High-Altitude Lean Blowout
At cruise altitudes above approximately 35,000 feet, air density in the standard atmosphere is roughly one-third of sea-level density. The fuel-air ratio required to sustain combustion occupies a narrower range, and the margins against both rich blowout and lean blowout shrink. Rapid deceleration at high altitude—such as aggressive throttle reduction during an emergency descent—can lean the mixture past the stability limit and extinguish combustion before the fuel control unit compensates. Some older engine designs without automatic relight systems are particularly vulnerable to this phenomenon during steep, high-altitude descents.
Foreign Substance Ingestion
Large quantities of liquid water, hail, or ice—as encountered in severe convective weather—can rapidly cool and dilute the combustion gases, pushing the mixture outside the flammability limits. Volcanic ash is uniquely dangerous: fine silicate particles melt in the high-temperature combustor and re-solidify on cooler turbine nozzle vanes, simultaneously degrading combustion efficiency and potentially glazing the hot section. The AIM and numerous FAA advisory circulars warn crews to avoid areas of known volcanic ash activity precisely because ash-induced flameouts can affect all engines simultaneously with no reliable restart capability until the engine is cleared of resolidified material.
Recognizing Flameout in the Cockpit
Early recognition matters because altitude and airspeed at the moment of flameout directly determine which restart options remain available. Pilots should monitor for these concurrent indications:
- N1 and N2 RPM decelerating below idle toward windmilling speed without a throttle input
- EGT decreasing toward ambient temperature rather than stabilizing at an operating value
- Fuel flow dropping toward zero despite available fuel and an idle or higher throttle position
- Net thrust loss manifesting as unexpected deceleration or a descent at constant pitch
- Loss of bleed-air-dependent systems—cabin pressurization, anti-ice, and hydraulic pressure sourced from that engine may degrade simultaneously
- Engine fault or ENG FAIL annunciators, which illuminate on many modern flight decks based on a combination of parameters such as N1, N2, EGT, and fuel flow rather than a single generic threshold—the specific logic is aircraft-type dependent
On a multi-engine transport, an asymmetric thrust condition accompanies single-engine flameout. The rudder input required to maintain coordinated flight is itself a secondary cue alerting the crew to a power loss before every instrument is cross-checked.
Airstart Envelopes: Speed, Altitude, and Energy Source
Every turbine-powered aircraft's Approved Flight Manual (AFM) or Aircraft Flight Manual publishes airstart envelopes—graphical or tabular data showing the combinations of pressure altitude and indicated airspeed within which a restart is considered feasible. These envelopes typically define two distinct restart methods.
Windmill Airstart
A windmill airstart relies on ram airflow through the inlet to keep the compressor rotating at a speed sufficient for ignition. The minimum N2 required for successful ignition varies significantly by engine type and is specified in the applicable AFM/QRH rather than as a general industry figure. Because ram airflow increases with airspeed and decreases as air density falls with altitude, the windmill envelope has a maximum altitude limit and a minimum airspeed requirement. When conditions are within the envelope, the crew positions the thrust lever to idle, verifies fuel availability, selects continuous ignition, and opens the fuel shutoff valve. A successful light-off is confirmed by a rising EGT, followed by N1 and N2 accelerating toward idle. No bleed air or pneumatic power is consumed, preserving those resources for other systems on a multi-engine jet.
Starter-Assisted Airstart
When altitude is high or airspeed is insufficient to sustain adequate windmilling RPM, a starter-assisted airstart is required. The starter—usually a pneumatic air turbine starter supplied by the APU or a cross-bleed from a running engine in flight—motors the failed engine's compressor up to a minimum ignition speed before fuel is introduced. This method is more reliable across a broader range of conditions but draws heavily on pneumatic or electrical resources. Crews must confirm that the bleed air source is adequate before committing to this procedure; an APU operating near its own performance limits at high altitude may not generate sufficient bleed pressure.
In both methods, the critical sequencing rule is identical: ignition on before fuel in. Introducing fuel to an unlit combustor allows raw fuel to accumulate in the hot section and tailpipe, creating conditions for a catastrophic hot start or engine fire when ignition is finally achieved. The QRH step sequence enforces this order, and test questions frequently probe whether candidates understand why.
Key Numbers and Rules
- Windmill airstart envelopes are aircraft-specific; always reference the AFM—do not apply one type's envelope to another.
- Minimum N2 for windmill ignition varies by engine type; confirm the specific value in the applicable AFM/QRH rather than relying on a generic percentage.
- Volcanic ash can cause simultaneous multi-engine flameout; restart may not be possible until below ash-contaminated air.
- A hung start occurs when the engine lights off but N2 stabilizes below idle RPM; EGT may continue to rise, requiring immediate shutdown.
- A hot start occurs when EGT exceeds the AFM limit during the start sequence; immediate fuel shutoff is required.
- Continuous ignition should be selected during known icing conditions, turbulence, or heavy precipitation to reduce the risk of inadvertent flameout.
Common Test Traps
- Equating compressor stall with flameout: A stall disrupts airflow and may induce flameout, but the stall itself—characterized by a bang and EGT spike—is a separate event. Recognizing the sequence matters.
- Assuming windmill start works at any altitude: The windmill envelope has a hard altitude ceiling. Above it, a starter-assisted method is the only viable option.
- Overlooking bleed air availability: Selecting starter-assisted airstart without a functioning APU or surviving engine bleed source will not produce enough motoring RPM for a successful start.
- Confusing EGT behavior: In a fuel-starvation flameout, EGT drops immediately. In a compressor-stall flameout, EGT may spike first, then drop. Test scenarios often embed this distinction.
- Forgetting cascading system failures: A flameout is never just an engine event. Hydraulics, pressurization, and electrical generation may all be affected simultaneously, demanding a prioritized response per the QRH.