Sounds like some sort of surging problem?
But in the cruise? A bit unlikely.
707, just knackered motors I reckon.
Never had that sort of thing in the DC 8.

Too many too young to remember the good old CJ610 that powered the original Lear Jets, Jet Commanders, Jet Stars, Sabre Liners just to name a few.

When ever you heard one of those motors cranking up and the igniters clicking you only stood behind the exhaust once, and only once. 3 foot lick of flame would come out of the back when light off occured. Pretty exciting to say the least, especially for the chap standing behind the engine.

YouTube - Learjet CJ610 Turbojet Engine In Test Cell

About 20 seconds into the start sequence.

Ah, a great deal more info:ok:

Well it probably wasn't reverser operation :}

The color yellow is significant (most reports are bright white sparks)

White would go with titanium blades (being FODed or rubbing their tips against the case). Yellow goes with steel blades. If its arround their tips its rubbing (screwed up bearing) if its random then its chewing up something loose like a rivet or a stone. Common on the ground but not at cruise.

On occasion there is are small licks of flame around the turbine wheel, but your correct it is mainly a red hot glow.

Nice video.:ok:

Have you seen this one: YouTube - TNA Metro Startup
In both videos after combustor ignition the turbine wheel begins to pulse for some reason. After a few moments the pulsing stops and the turbine wheel glow becomes stable. A few moments later the turbine glow disappears despite the engine still running.
I think part of the reason for this could be associated with temperature spikes which are quite normal during engine start-up, also a the combustor fueling profile (rich start mixture perhaps) could also contribute.

Obi

It's explained in that video ->

The Garretts pulse in more fuel to accelerate the engine faster during start. with doing that they'd either hang or take about two minutes to start.

Cheers.:ok:

I won't pretend to know anything about fuel injection, is there a particular reason why the fuel is pulsed into the combustor?

Obi

I'm not really sure, but there's a computer that controls the engine start sequence and EGT red-line limiting. I guess that's just the way it's programmed - But I agree that I can't see why it just couldn't run a pre-programmed EGT for engine acceleration during start anyway.

Never heard of fuel pulsing on start-up.
FCUs in my day. None of this computer
controlled MECs rubbish.
I can probably ASSUME that the fuel pulsing
has something to do with the 'Burn rate" of
kerosine.
Don't want to bung in too much motion lotion
whilst the engine establishes itself at idle RPM?
Otherwise the candle goes out.

No, you can see the fuel-flow pulsing regularly and the EGT follows this. It's all part of the plan.
When I flew the even-more rattly POS Metro 2's that didn't have any electronics on the engine for starting or limiting the EGT, I had to push the 'ENRICH' button myself, and I too would often end up pumping it in a similar manner to what the computer does on the later engines.

Ok, I've done a bit more searching (thank you Google) and it seems that two reasons for the pulsing are that its a method of controlling the combustor/turbine temperature as 18-Wheeler mentioned. Also the pulsing programme is a very easy to impliment open loop controller.

Obi

I've just discovered the following video on youtube. Its from the external camera in the vertical fin of a B-777, showing the turbine glow from both GE90 engines: YouTube - GE90-115B

Obi

I couldn't decipher what on earth the view was or if the description was real.

Daytime view.

YouTube - Contrails

Some not so great memories of used and abused engines here but the fact still stands, under normal ops modern final stage turbine exaust may glow due to temp/metallic alloy.

The only reason otherwise would be excess fuel in the turbine section during start, engine failure or after burners installed in mostly military aircraft.

Another note..EGT is only relevant to the position the engine manufaturer decided to install the "rakes" or sensors at. The exaust gas temperature varies throughout the T-stages and tailcone.

St Elmos Fire?