Unlike heat and smoke, detection of fumes is not automated and there is consequently considerable variation both in their detection and the description of fumes by air crew. Also, with the imposition of the locked flight deck door, the language of descriptive communication from cabin crew to flight crew about detected fumes in the passenger cabin has become particularly important.
Fumes may be visible in the form of a haze but detection of fumes is usually by smell, irritation of the eyes or skin, or effects such as headaches or dizziness.
Origin of Fumes
The origin of fumes may be local to the source, as in the case of faulty electrical equipment, the overheated contents of a galley oven, or a failed cabin fluorescent light fitting; or it may be distributed from the source through the cabin air conditioning system. In the latter case, it can be extremely difficult to be sure of the cause, or whether the fumes could be in any way be related to a fire hazard.
While investigating the source of any fumes, it is important to consider that the fumes themselves may be hazardous to crew or passengers regardless of whether there is any immediate irritation. It is be prudent for one or both pilots to fit oxygen masks (100% Oxygen) as soon as there is any indication of fumes.
On the Ground
If the source of the fumes is external to the aircraft while the aircraft is parked, such as ground equipment or jet exhaust from another aircraft, consider switching off the air conditioning, closing doors, or switching off any offending ground equipment. If the effects are particularly bad then an evacuation of the aircraft may be prudent.
In the Air
The first consideration should always be to land the aircraft as soon as possible (see In-Flight Fire: Guidance for Flight Crews) because, if the source/cause of the fumes is not known then it is not possible to judge how extensive or life threatening the situation may become.
As with smoke, the fumes may be cleared by depressurising the aircraft. Descent will be a pre-depressurisation requirement and before such action, the consequences need to be considered such as the need for crew and passengers to be on oxygen and the impact that will have on the crew communication and the ability of the cabin crew to isolate the source of the fumes.
The Airbus philosophy on smoke (and by extension, fumes) in flight is that if it is detected by the aircraft - i.e., a "cargo smoke" or an "avionics smoke" automated warning is generated - the appropriate action is to follow the ECAM actions. If it is detected by the crew (with no associated warning), the appropriate course of action is to follow the paper checklist in the QRH. The paper checklist systematically isolates potential sources of the smoke or fumes going through avioncs, air conditioning (one pack at a time) etc. If the source is determined, it is isolated and the checklist stops at that point. If the source is not determined, the checklist final actions result in the aircraft in the emergency electrical configuration as the pilots have been unable to determine a specific electrical fault, and individual isolation of the packs has determined that the source is not bleed air related. The critical overall guidance of the checklist is - if the smoke becomes the greatest threat, the checklist to determine the source is stopped and the smoke removal checklist is actioned. It is at this point that depressurisation is a part of the actions.