Hello All,

I was on a flight to ORD in a CO 737 on Friday. We landed towards the end of rushhour, so it was incredibly busy on the taxiways, especially near United, which is where we had to drive through to get to the CO gates. Anyway, we sat waiting for a gate to open up, and eventually another CO 737 got a pushback from the gate we were waiting for, and tried to start its engines. Eng #2 smoked like crazy for a while, then stopped. I suppose it tried to start up again, cause the smoke started billowing once more, then to my surprise, shot 30 feet of flames out like a giant blowtorch for a few seconds. It was pulled right back into the Gate. Ok, I'm writing this cause I'm kinda excited about having seen this, but I do have some questions. Firstly, I could have sworn that the flames shot out of the bottom of the engine, as though they came from the bypass section, rather than the core. Can this even be, or did it likely come out of the bottom of the core? (ok, nobody could reasonably be expected to answer that without having seen what happened). Secondly, is more or less rare to have engine fires on startup compared to takeoff or cruise? Lastly, why didn't any emergency vehicles show up besides a Ford Explorer with a mars light 10- minutes later? Anyway, thanks for any input-also wouldn't mind hearing some similar stories, if that kind of thread hasn't been done in a while.

O

Sounds like an ignition problem. Raw fuel coming out of the back of the engine appears to be smoke when it is actually a fog of atomized unburned fuel. When the engine finally lit off a fireball resulted from the ignition of the fuel built up in the engine and emerged from the tailpipe. It's possible that some of the "fog" around the engine ignited as well making it appear that the fire was coming from underneath the engine. They probably got a "hot start", that is they exceeded the max EGT on startup and shut it down so that MX could check it out.

The only way that the fire could actually have come from under the engine is if there was a fire in the accessory area around the core. This would have given a fire indication in the cockpit and the crew would have blown a fire bottle and called for a fire truck. Since only an Explorer showed up it's unlikely that this was the case.

It is also possible that the fire came from the bypass section as you surmised but this is the least likely scenario as this only happens with a severe compressor stall called an unstart. During an unstart the flow through the engine actually reverses and flames shoot out the front of the engine. The fan could then suck the flame back and send it out the coldstream. It is accompanied by a very loud explosion which you would have not only heard but would have even felt in you plane.

Of course this is all speculation. I can't say with any certainty what really happened. Hope this helps!

Dl757Md

When I do engine runs and an engine fails to light up I do a dry motoring to remove all unburned fuel from the combuster and turbine section before trying another start attempt. I work on ERJ´s and the AE3007A engines have these starting problems sometimes at the first start on a cold day after a longer shut down period.

Maybe the crew decided not to motor the engine. Instead they tried the next engine start which resulted in the fire you have observed on this 737.

Exactly.....After discontinuing the start,A Dry motoring of the Engine would have cleared the Unburned fuel & then a Fresh start should have commenced.
I wonder if the HOT start was severe,needing Borescope Inspections,Wonder what the EGT was.
regds
MEL

Thanks, folks, for your replies.



Dl757MD

your "hot start" explaination makes perfect sense given what I saw, even if we can't be completely sure.

"It's possible that some of the "fog" around the engine ignited as well making it appear that the fire was coming from underneath the engine."

Having reread my original post, and this portion of your reply, however, it's clear that I was very UNclear in my description of the flames. Seems like exuast from the core and bypass air blast out of concentric circumfrences, and what i meant to say was that, rather than blasting evenly from one of these circumfrence, the flames came from the 6 o'clock position, when viewed from the front or back. My initial, very uninformed assumption was that oil or fuel had pooled up at the bottom of the core, and thus caught fire and blown straight out the back of the engine at a point closest to the ground. But your explaination is clearly more informed. My next question is this: why does so much atomized fuel pour out of the engine? Obviously the fire was a malfunction, but I've seen numerous pics on this site of smokey (foggy?) startups, and seen a few positively enormous plumes of smoke on startup while aboard NW's A319s. Are their other reasons for the smoke, or is this wasted fuel thing a common occurance at low temperature? Seems like clouds of atomized fuel drifting around crowded airports like ORD is a serious safety hazard, though if it actually were, it wouldn't happen as often. Also, I assume startup procedure is fairly automated on new jets, but do pilots have to time the introduction of fuel into a starting engine on the older ones? Ok, that's more than one question. thanks in advance for your replies.

O

why does so much atomized fuel pour out of the engine?

During a start, when N2 reaches a certain level the fuel cutoff lever is moved from the cutoff position to the idle position. This not only starts fuel flow into the combustors but triggers ignition as well. If there is a problem with ignition the fuel flows anyway(until the pilot moves the cutoff lever to cutoff). The air going through the engine carries the raw fuel out the tailpipe.

Sorry to post such a short incomplete answer but I've got to run.
More later.

Dl757Md

The start sequence of a main engine (modern or not) goes something like this:

- engage starter
- monitor N2 (N3 on some RR engines)
- at max motor or the minimum N2 required by the manufacturer, introduce fuel via the fuel cut-off lever. At this point the ignition system begins to operate.
- engine light-up occurs within a specified time (which varies with engine type)
- flight crew monitors FF, EGT, N2 (N3) and oil pressure.
- at specified N2 (N3) starter cut-out occurs (auto or manual, depending on time).
- engine accelerates to idle and stabilizes.

The "smoke" you see is a combination of unburned fuel and condensation from a cold engine. When an engine is cold, it takes a little bit of time for the flame to propagate around the combustor. Ignition is usually introduced at one or two points around the combustor. From there the flame front must travel around the circumference of the combustor. So, whatever is not initially burned is blown out the back end of the engine.

If an engine fails to ignite, you have raw fuel blowing out the exhaust. The proper procedure to follow if ignition does not occur within the manufacturer's limit is to return the fuel cut-off to cut-off and continue motoring the engine until the tailpipe is clear (staying within starter duty limits). Then you may attempt a start again after switching ignition systems.

The problem occurs when the enigine lights-up somewhere between the normal time and the limited time. You will usually see a flame out the tail pipe. This does not always cause a high EGT event but will usually cause a touch of concern to the person on the headset (usually the flight crew is not aware of the flame, only of the delayed light-up).

This is a fairly common occurance and usually does not require AFR intervention and in fact, they are usually not even notified.

DI757md is right. But on modern jet engines the FADEC (Full Authority Digital Engine Control) introduces fuel at a certain N2 speed. During automatic starts with FADEC in operation pilots have no chance to control the time of fuel introduction. However, if the FADEC fails, the engines can be started on a standby FADEC system or on some aircrafts with manual control.

A tailpipe fire during engine start does not always mean a hot start! As the fire occurs only for a very short time period and sometimes far behind the turbine (where the EGT or ITT probes are located) the flight deck operator may not notice the fire on his engine indications. You may only notice a hollow "bang" and thats all. I had this a few times. Only the guy on the other end of the cable told me about the "beautifull afterburner".

During a hot start there was an exceedence of the maximum EGT (or ITT) allowed during engine start. But not only as a result of a tailpipe fire. Hot starts are more common with troubles in the engine fuel system or with malfunctions in the compressor control system resulting in a disturbed air flow. Both will cause an incorrect fuel to air mixture.

Air2gxs:

Good explanation. Sorry, I did not read your reply before I send my last post cause I was writing on it. Some questions are answered twice now...

Quoting Speedracer1407 (reply 0): Lastly, why didn't any emergency vehicles show up besides a Ford Explorer with a mars light 10- minutes later?

No emergency vehicles showed up because (unless the flame catches the actual aircraft on fire) It was relatively not dangerous. If there was a fire still burning in the engine then the fire sensors would have alerted the flight deck and the entire Calvary of Emergency fire trucks would have been called. Hope this helps! 

-Sam

FutureUApilot,

The flight crew have little indication of a tail pipe or of an internal engine fire. The fire detection system only detects fires and overheat conditions under the cowling and sometimes in the pylon area. The only indication of a tail pipe fire would be an increase in EGT, assuming the fire is in the immediate area of the probes. Some engines do have an internal engine overheat system that monitors bearing temp (RR Tay, #3 bearing?), but that is not pire detection. The only recourse the crew has for a tail pipe fire is to place fuel in cut-off and continue motoring. Blowing the engine fire bottles will just waste time and money.

I wonder How many times has this occured:-
An Engine fire in the Exhaust,Dry Motoring continued to blow out flame & starve the Engine of fuel,No avail,use the Fire bottles + Back up,No avail & requiring the Grd staff with Fire Extinguisher to Extinguish the Fire.
Although I feel that the latter would not be very helpful if the rest was not successfull.
regds
MEL

I dont´t think that your scenario had happened ever. There is a very good chance to extinguish tailpipe fires by dry motoring the engine. And if all the remaining fuel is burned the fire will stop that way. As Air2gxs mentioned, the fire bottles are useless in such a case. They can only extinguish a fire below the engine cowling.

But let´s hope that we will never find it out in real life...

Have seen this once with a Turkish Pegasus B737-400 in TXL.
Pilot started the engine (I think it was No.1) . Apparently N1 was sluggish on startup, so she pushed the throttles forward DURING ENGINE START! There were lots of flames coming out of the tail pipe, but she cleared off the guy on the headset. The tow truck driver called the tower over his radio and got them to stop the plane, which was already taxying with flames coming out of the engine. The tower told her to stop and that she was on fire, upon she fired both firex bottles (she didn´t have a fire warning due to the fire happening outside the cowling), shut down, with the engine still burning and ordered the evacuation of the plane, which happened ON THE SIDE OF THE BURNING ENGINE. The firefighter came (it literaly happened just outside their fire station) and extinguished it. Now this happened at a time when Kurdish PKK militants were torching Turkish owned shops in Berlin to extort money from the owners, so the police cordoned the plane off immideately, suspecting a terrorist attack. As a result the plane was sitting on the taxiway for several hours, until it was decided to tow it into the Lufthansa hangar to let the Lufthansa mechanics have a look at it.
It was found that the N1 shaft was completely seized, explaining the sluggish behaviour on startup. Now with the low pressure compressor not turning, pushing the throttle forward caused a too rich air / fuel mix, resulting in a combustion not completing inside the combustor, but until way behind the engine. The resulting flames damaged the engine pylon and the lower surface of the wing. On idle there were no visible flames yet, but they appeared when she accelerated the engines to taxy away.
The plane was repaired by Lufthansa Technik and later returned to Turkey.
The problem was that the pilot didn´t respond to the N1 indication (normally you´ll see N1 moving at N2 max motoring). If there is no visible N1 rotation you´ve got to shut down and investigate.
The next fault was that she didn´t tell the cabin crew on which side to evacuate, resulting in passengers getting off the plane on the side the fire was happening.
Contributing was the fact that the person walking the plane out was a loader with limited English and not being a mechanic, he probably didn´t know what exactly to look for, e.g. if I would see and engine lighting up without N1 rotation I would tell it to the crew.
Jan

[Edited 2005-02-16 11:53:48]

[Edited 2005-02-16 11:55:52]

Jan, very interesting story. Never came across a similar one. "Never say never" comes to mind. This is valid especially in case of aircraft system malfunctions paired with human errors.

What was the EGT gauge reading.
regds
MEL

I don´t know, the pilots (all female crew, the plane was painted in Pegasus livery, but had an N registration, the whole thing happened somewhere between 1994 and 1996) apparently didn´t watch it, else they would have noticed that something was wrong, together with the missing N1 indication. I didn´t talk to them. One female avionics technician was very p*ssed off about the "stupid cows firing both extinguisher bottles", because she had to change them.

Jan