Date: 201503
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Embraer Jet Undifferentiated or Other Model
Operator: Fractional
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Waste Water Disposal System
Component problem: Improperly Operated
Reporter organization: Fractional
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution
Contributing factors: Aircraft; Human Factors
Primary problem: Ambiguous

Narrative: We departed with passengers on board. Climbing out from 9;000 feet we heard a very noticeable harmonic vibration throughout the cabin that sounded as if it was coming from the airframe. My partner was flying asked if I heard it; too which I said yes. We leveled out at 10;000 feet and advised ATC that we needed to stay there in order to assess the situation.I had heard a similar noise before on another flight several months ago that caused a divert; and we had found that the lavatory service port was loose and dangling. We had a theory after the last divert that a false latch on the port was creating a scenario that caused the cap to pop when pressure differential increased and bouncing around in a closed door created a harmonic vibration echo in the lavatory itself. However; I never got direct confirmation of this being the issue. I transferred the radio to my Second In command (SIC) who was pilot flying and went back in the cabin to better hear the noise. The passengers asked me if I heard the noise to which I advised them I did; and had an idea what the issue was and that safety was not going to be a concern; but as there is no checklist for this a conservative protocol dictated we return to know for sure. I did go back in the Lavatory compartment and lifted the toilet seat and the noise increased dramatically; confirming my suspicion. We returned uneventfully and landed. Upon shutdown we went back and found the service port cap dangling from it's chain in the lav service area which had been closed. The Line Tech who had earlier serviced it inspected it with me and we saw that it could easily have a false latch issue. There was no damage to the inside or outside of the aircraft. We contacted the Chief Pilot; and escalated it to the Fleet Chief Pilot.  At this point we discussed whether or not we could continue flying the passengers with a new release legally without a maintenance write up as we had diagnosed and resolved the issue; and that no damage to the aircraft had occurred. The Chief Pilot emailed us a letter documenting that this is a known issue and that safe and legal flight can continue if the cap is on properly. This has happened twice to me now; and in my discussions with the Chief Pilot I am not the only one. Both times the lav had been serviced prior. The first time it had been by company Personnel. Each time I had performed required preflight items as well as final walk around duties. The culprit of these diverts is seemingly a false latch of a cap that is hidden in a latched cover on the UNDERSIDE of the airframe. I really don't foresee a realistic way that the pilots can prevent this. There needs to be better training; placarding; and perhaps re-design of the cap in order to prevent this scenario. There also needs to be more information put out to the pilot group so they are aware of the potential for this problem.




Synopsis: An Embraer Captain experiences a strange vibration climbing out of 9;000 feet and levels at 10;000 feet. Previous experience indicates that the lavatory service port cap may be loose. The Captain elects to return to the departure airport where the suspicion is proved correct.