Engine smoke

[fishermanivan]

I like the added smoke on startup, but it should only happen once fuel is introduced, not on the starter motor.  

[richboy2307]

Hi,

Thanks, we’re open to reviewing this further. Do you have any references to support it? Our current understanding is that the smoke occurs before internal combustion, caused by residues and oils burning off after the engine has been sitting idle. So basically during ignition prior to fuel being introduced.

[Acktu]

I’m no expert, but visible exhaust smoke generally requires combustion. During motoring, the engine is just spinning from the APU air, so you wouldn’t expect actual smoke. Any oil or residue burning off would happen once there’s enough heat from ignition, not before fuel is introduced.

 

[Swagger897]

On 4/17/2026 at 11:42 AM, richboy2307 said:

Hi,

Thanks, we’re open to reviewing this further. Do you have any references to support it? Our current understanding is that the smoke occurs before internal combustion, caused by residues and oils burning off after the engine has been sitting idle. So basically during ignition prior to fuel being introduced.

Licensed A&P here. The only circumstance that would cause visible smoke/vapor to be seen before fuel flow is the engine was previously wet motored on a cool engine (fuel lever on, spar valves open, fuel dispersed through injectors, NO ignition). Residual fuel will pool in the exhaust and combustion chamber and on next start attempt, regardless of wet, dry motoring, will you see fuel vapor out the back.

I have been meaning to create a bug report about this where the sims TGT is rapidly decreases back to ambient immediately after shutdown. There doesn't seem to be any residual heat in the engine where IRL combustion and turbine case sections, as well as the core of the engine beneath the cowlings, remain hot to the touch for several hours past shutdown. It's not uncommon for an aircraft to arrive to the gate at 9PM, have a boroscope inspection slated to be done and not be able to perform it until 1-2AM where combustion case temperatures have dropped below 50C. We have specialty tools where we can take jetbridge air and blast it into the exhaust to accelerate cooling, but only if we're located at the ramp still.

I say this because if there were to be any residual fuel left in the cases, that fuel will evaporate very quickly, before the next engine start attempt no-doubt. The above video is a great representation of older style fuel system before the days of HMU's that more efficiently meter the fuel to the combustion cases. Modern day engines will produce near zero visible smoke on a standard day. You can view my video of an HMU change for a trent 7000 here where the preservation oils that burn off are about as close that you come to visible vapor. Even comparing an engine nearly 5 decades apart, the principle of fuel turning to visible vapor has not changed.