When starting a turbine engine, pilots and crew must monitor several engine parameters simultaneously and be prepared to abort the start if conditions indicate a problem. Three of the most important abnormal start conditions are the hot start, the hung start (also called a false start), and the wet start. Each represents a distinct failure mode with its own signature on the engine instruments, its own cause, and its own corrective action. Failure to recognize and respond promptly can result in severe turbine or combustion section damage, a costly engine overhaul, or — in the worst case — an uncontained engine failure.
These conditions are covered in aircraft-type training materials and AFM/FCOM abnormal-start procedures, but the foundational definitions and recognition criteria for turbine starting systems are found in the FAA-H-8083-32B (Aviation Maintenance Technician Handbook — Powerplant), while ATP-level pilot guidance on engine starting procedures is more properly found in the Airplane Flying Handbook (FAA-H-8083-3) and the applicable aircraft-specific AFM/FCOM. Understanding the thermodynamic and mechanical reasons behind each abnormal start is what separates a pilot who memorizes a checklist from one who can adapt when an unusual situation does not fit neatly into a procedure.
Turbine Engine Starting Basics
A normal turbine engine start follows a predictable sequence. The starter (pneumatic, electric, or hydraulic) accelerates the engine core to a minimum rotational speed — typically expressed as a percentage of N1 or N2 — at which point the ignition system fires and fuel is introduced into the combustor. As combustion begins, exhaust gas temperature (EGT), also called turbine inlet temperature (TIT) or inter-turbine temperature (ITT) depending on the sensor location, begins to rise. Simultaneously, N1 and N2 (or Ng — gas-generator speed) accelerate toward idle. The starter disconnects automatically or is cut off by the crew at a design speed, after which the engine must be self-sustaining. Monitoring these three parameters — EGT/ITT, N1/N2, and fuel flow — during the start sequence is the pilot's primary task.
Hot Start: Too Much Heat
A hot start occurs when EGT (or ITT/TIT) exceeds the manufacturer's maximum allowable limit during the start sequence. The temperature exceedance can be brief or prolonged, but any exceedance is significant because turbine blades, nozzle guide vanes, and combustion liner components operate at the very edge of their metallurgical limits even during normal operation. Exceeding those limits — even momentarily — can cause oxidation, creep, warping, or cracking of hot-section components.
Hot starts are most commonly caused by one or more of the following: excessive fuel flow during light-off (too much fuel introduced too quickly), insufficient airflow through the compressor (low starter speed, compressor stall, or bleed-valve issues), or a late light-off in which unburned fuel pools in the combustor before ignition, producing a sudden temperature spike when it finally ignites. Hot starts can also occur in high ambient temperature environments, at high-altitude airports where the air is less dense, or when residual heat is present in a hot-section from a recent engine shutdown (a condition sometimes called a heat soak). This is one reason many turbine engine starting procedures require a mandatory cooling period between shutdown and restart.
Recognition is straightforward if the crew is monitoring instruments: EGT rises rapidly and approaches or exceeds the red-line limit before the engine reaches idle speed. The corrective action is immediate — abort the start by closing the fuel shutoff (condition lever to cutoff), continue motoring the engine with the starter to purge residual fuel and cool the hot section, and do not attempt a restart until the engine has cooled and the cause is identified. The logbook must reflect any EGT exceedance so maintenance can evaluate hot-section serviceability.
Hung Start: Insufficient Acceleration
A hung start (false start) occurs when the engine lights off and EGT rises normally, but N2 (or Ng) stabilizes at a sub-idle speed and fails to accelerate to normal ground idle. The engine is producing combustion but is not generating enough thrust or power to overcome internal friction and accessory loads and accelerate itself to idle. EGT may simultaneously climb toward limiting values because the turbine is working hard with insufficient airflow to keep temperatures in check at that low rotational speed.
Common causes of hung starts include insufficient starter energy (low pneumatic pressure from the APU or ground cart, weak battery, or premature starter cutoff), a fuel control unit malfunction that meters too little fuel, an air bleed that has not closed causing excessive parasitic load, or an underlying mechanical binding issue. High-altitude airports exacerbate hung-start risk because starter performance and combustion efficiency both degrade with reduced air density.
Recognition: N2/Ng stabilizes well below idle and does not continue to rise within the time window specified in the procedure. The specific stabilized-idle speed and hung-start threshold vary widely by engine model and are defined only in the applicable AFM or engine manual — there is no generic FAA percentage that applies across engine types. EGT may be normal or climbing. Corrective action is again to abort: fuel off, continue motoring to cool and purge. The crew must not simply add more fuel in an attempt to coax the engine to idle — doing so will almost certainly produce a hot start on top of the hung start.
Wet Start: Fuel Without Ignition
A wet start occurs when fuel enters the combustion chamber but ignition does not occur, resulting in raw (unburned) fuel accumulating in the engine and exhaust system. The telltale sign is that EGT does not rise as expected after fuel introduction — there is simply no combustion. The engine may emit visible fuel from the exhaust, and the smell of raw fuel is often detectable. The danger of a wet start is twofold: the raw fuel can damage seals and non-fire-rated components in the exhaust path, and — far more critically — if ignition occurs late or the crew attempts a second start without purging, the accumulated fuel can ignite explosively, producing a severe hot start or an engine fire.
Wet starts are caused by igniter failure (fouled, failed, or improperly connected igniters), fuel contamination (water, ice crystals, or contaminants preventing atomization), a malfunctioning fuel nozzle that does not properly atomize fuel, or starting in conditions where fuel has gelled or partially frozen. Low fuel pressure can also prevent adequate fuel delivery for ignition.
Corrective action: immediately close the fuel shutoff and motor the engine with the starter for the full prescribed purge time specified in the applicable AFM to expel accumulated fuel before any reattempt. Attempting a relight without purging risks a catastrophic ignition of pooled fuel. Maintenance must inspect igniters and fuel nozzles before the next start attempt.
Why It Matters: Safety and Economics
All three abnormal starts — hot, hung, and wet — share a common thread: they demand immediate recognition and decisive action. Turbine engines are extraordinarily expensive assets. A single hot-section inspection triggered by an EGT exceedance can cost tens of thousands of dollars; an undetected hot start that causes internal damage can cost hundreds of thousands. Beyond economics, an engine weakened by an undetected abnormal start may fail in flight. At the ATP level, understanding these conditions is not academic — it is a direct airmanship and safety-of-flight competency.
Pilots must also understand environmental and procedural factors that elevate risk: hot and high airports, hot-soaked engines, marginal ground-support equipment, and cold-weather fuel issues all increase the likelihood of abnormal starts. Briefing the start sequence, verifying ground-cart parameters, and confirming cooling time requirements are practical mitigations.
Key Numbers and Rules
- EGT limits: Specific redline values are aircraft/engine-type specific. Know your aircraft's AFM limits. Any exceedance requires a maintenance logbook entry.
- Hung start threshold: There is no standard FAA-defined N2 percentage for a hung start. Failure to reach stabilized idle within the AFM-specified time window constitutes a hung start, but the threshold is entirely aircraft/engine specific and defined only in the applicable AFM.
- Purge time: After a wet start or any fuel-off abort, motor the engine for the full AFM-specified purge time before reattempting a start. Purge duration is aircraft/engine specific and must be taken from the applicable AFM or maintenance manual.
- Cooling time: Many engines require a defined cooling period between shutdown and restart to reduce hot-start risk from heat soak. Required cooling times are engine/aircraft specific and defined only in the applicable AFM or maintenance manual.
- Ignition check: During the start sequence, confirm EGT begins to rise within the AFM-specified window after fuel introduction. This timing is aircraft specific and stated only in the individual AFM. No rise = wet start; excessive rise = hot start developing.
Common Test Traps
- Confusing hung and hot start: A hung start is about RPM failing to accelerate; a hot start is about temperature exceeding limits. They can occur together (a hung start producing secondary EGT exceedance), but they are defined separately.
- Thinking a brief EGT exceedance is acceptable: Any exceedance of the manufacturer's EGT start limit requires a maintenance entry and evaluation — duration is not the deciding factor for whether to log it.
- Attempting to save a wet start with a second ignition attempt without purging: This is dangerous. The correct action is always to purge before reattempting, no matter how inconvenient.
- Forgetting the cooling-period requirement: Exam questions may describe a rapid restart scenario and ask if it is acceptable. If the AFM requires a cooling period, it is not negotiable.
- Assuming all turbine starts are monitored the same way: EGT sensor location varies (TIT, ITT, EGT, TOT). All measure similar phenomena but at different points in the gas path. Know which parameter your aircraft uses and where its limits are defined.