Why does the cabin air sometimes smell of jet fuel, although bleed air is tapped before it comes in contact with fuel ? How is this possible?

I have smelled this a couple of times in my life (twice extremely irritating), mostly during boarding/taxiing but at least once also during the flight (although in that case I don't remember if the smell was already present before take-off and merely lingered on).

Apparently a similar observation was reported in Q25405 but limited to the taxiing phase, whereas I witnessed the smell when the plane was still connected to the PBB (and also in-flight with the caveat above). The root cause may differ.

Meanwhile, I have found a satisfying answer on the net, but cannot add it anymore since the question is closed:

« You’ll occasionally notice a strong odor when the plane is on the ground – a pungent smell similar to the exhaust from an old car or bus that fills the cabin shortly after pushback. Usually this happens when exhaust gases are drawn into the air conditioning packs during engine start. The wind is often to blame, causing air to backflow or blowing fumes through the pack inlets. It normally lasts only a minute or so, until the engine is running and stabilized. It’s unpleasant but little different from the fumes you occasionally breathe in your car while stuck in traffic. » (source)

(Notice that in my cases it lasted definitely much longer than "only a minute or so".)

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    $\begingroup$ In my little plane I smell it all the time when taxiing behind a Jet-A burner and sometimes on landing and takeoff. But my aircraft isn’t sealed and pressurized. I suspect what you smell is from fresh air being circulated through the cabin and it picks up some kerosene smell from aircraft around you. $\endgroup$ – JScarry Dec 10 '18 at 18:04
  • $\begingroup$ I read elsewhere that once the engines are running, the exhaustion gases can be sucked in again as part of bleed air coming into the cabin. I think from what I've heard so far, this would be the most probable cause. Given that the aircraft is motionless at that point, the fumes will be around the aircraft. $\endgroup$ – summerrain Dec 10 '18 at 18:21
  • $\begingroup$ @JScarry: Are you sure that a passenger airliner is "sealed" ? I don't think the fumes came fleeting from elsewhere as there was no aircraft in the immediate vicinity at the gate and the smell stayed for an extended time. $\endgroup$ – summerrain Dec 10 '18 at 18:24
  • $\begingroup$ @summerrain They are not completely airtight but unlike little airplanes, they are pressurized so that the air inside doesn’t leak out the doors and windows and conversely the air outside doesn’t leak into the cabin. The air inside the cabin is being constantly recirculated and some outside air is let into the cabin. This article explains how it works. aviation.stackexchange.com/questions/43702/… $\endgroup$ – JScarry Dec 10 '18 at 18:52
  • $\begingroup$ @JScarry: Isn't "the air outside doesn't leak into the cabin" contradicted by "some outside air is let into the cabin" ? $\endgroup$ – summerrain Dec 11 '18 at 12:28

You might have been smelling cooked turbo oil. Leaking seals can potentially allow oil traces into the compressor bleed discharge, especially on auxiliary power units. The other possibility is cooked deicing fluid, which is quite common.

Turbo oil and deicing fluid are the two major sources of petrochemical cabin stink.

Another possibility would be an APU with stalling/surging problems where there can be backflow into the compressor end of the motor from the burner can, which will admit burner can gasses into the air conditioning system. If it is really kerosene you are smelling, that is the probable source.

It could be coming from the main engine but there would likely have been audible rumbling or thumping sounds if it was stalling/surging while on main engine bleed.

  • $\begingroup$ If your main engine is stalling/surging you've got bigger problems than a kerosene smell! $\endgroup$ – cbrian Dec 10 '18 at 15:09

If you're smelling it on the ground, it's because there's kerosene in the air around an airport and the plane's door(s) are open. Anything on the wind will blow in. If you don't notice the smell until you're taxiing, remember that the cabin air is recycled by using outside air bled through the engine compressor stages. If you happen to be 6th in a line of aircraft taxiing to the take off end of the runway, there's a lot of burnt kerosene headed your way for the engines to ingest & blow into the cabin of your plane.

If you're smelling it in the air, it's likely that the smell was there on the ground and you just didn't notice and the air inside the cabin hasn't been fully replaced yet.

If the airport is downwind from a paper manufacturing plant, you'll probably never notice the kerosene smell because the paper plant smell would totally overwhelm it, but it would prompt questions of "I was flying out of PAPR and the air smelled really bad, what happened at the airport to cause this?"

  • $\begingroup$ re: "there's kerosene in the air around an airport and the plane's door(s) are open" no, the doors were closed. The only open door was connected to the PBB. There were no other planes in the immediate vicinity. re: "you just didn't notice" no, this is impossible not to notice, it's very irritating. But as I wrote in my question it's possible that it already started on the ground. $\endgroup$ – summerrain Dec 10 '18 at 15:55
  • $\begingroup$ @summerrain During the pax unloading/loading process, there is at least one pax door open for you to use, and there will almost always be at least one other cabin door open for the maintenance crews, and there will be one or more cargo doors open. None of these doors are weather sealed when opened on the ground for access, and all will allow ambient air into the cabin. $\endgroup$ – FreeMan Dec 10 '18 at 15:59
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    $\begingroup$ so in short: enjoy your cancer :/ $\endgroup$ – Cloud Dec 11 '18 at 16:25

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