Date: 201910
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Descent; Takeoff / Launch
Aircraft component: Air Conditioning Compressor
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While operating Aircraft X; left seat Captain and I detected a 'dirty socks' odor (non-visible) emitting from the conditioned air system. The odor was present from takeoff through level off; and also during the descent through taxi phase. The odor was most intense during the descent phase but varied in strength and seemed to be strongest during large thrust changes. The flight was conducted safely and we arrived to [destination] with no other issues. He notated the issue in the maintenance logbook as suspected aerosolized engine oil in the bleed air system.


Second narrative: While operating Aircraft X; the right seat Captain and I detected a 'dirty socks' odor (non-visible) emitting from the conditioned air system. The odor was present from takeoff through level off; and also during the decent through taxi phase. The odor was most intense during the decent phase but varied in strength and seemed to be strongest during large thrust changes. On the previous flight the other operating Captain; with myself acting as PM (pilot monitoring)/right seat pilot; had notated the same discrepancy. He documented it in the maintenance logbook as suspected aerosolized engine oil in the bleed air system. The mechanics at ZZZ investigated the issue and conducted an engine run-up; but was unable to duplicate the odor. We discussed the issue at length with Dispatch; Maintenance Control; a Duty Pilot and a Chief Pilot. During that discussion it was made known that this aircraft had at least two previous write-ups in regard to this type of odor; one of which was by myself. Eventually we decided that the aircraft was airworthy but if the odor was encountered again; O2 could be used to protect against the odor; which I suspect may be toxic. Unfortunately; the fumes reappeared on the next flight; and were strong enough that; during descent; supplemental O2 was periodically used by myself; but the right seat Captain elected to forgo supplemental O2. The flight was conducted safely and we arrived to [destination] with no other issues. We notated the issue and verbally discussed the matter again with Dispatch. To further elaborate why I fear the odor may be toxic is the multitude of documented odor/fumes events that have had detrimental effects on flight crew members; with [another airline flight number] being the most dramatic. In that event both crew members became incapacitated during flight. From further research; the specific 'dirty socks' smell is attributed to a detergent used in jet turbine engine oil. The detergent (commonly called ToP) is considered very toxic. That said; I cannot determine if the odor encountered on our flight is in fact the same; but the similarities are striking and disconcerting. Perhaps there is another; non-toxic; reason for these fumes/odors; but will not know until the source is found. Furthermore; it has been noted that this issue hasn't been documented in the maintenance logbook since August; but in my experience the vast majority of pilots are unaware of the possible connection with aerosolized engine oil; with most stating it 'must be dirty packs' or something similar. In-fact the other operating Captain; the duty pilot; and Chief Pilot all had the same response on our initial conversations of the topic. There are several sources online but would like some official clarity of the issue.


Synopsis: B767-300ER flight crew reported 'dirty socks' odor present from takeoff; level off; and descent of varying intensities. Anomaly was documented in logbook and assigned to Maintenance.