Date: 201812
Local time of day: 1801-2400
Location: ZZZZ.ARTCC, FO
Flight conditions: VMC
Light: Night
Aircraft: Citation III; VI; VII (C650)
Operating under: Part 135
Flight plan: IFR
Flight phase: Descent
ATC / advisory: Center ZZZZ
Aircraft component: Waste Water Disposal System
Component problem: Failed
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Confusion
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Contributing factors: Company Policy; Aircraft; Human Factors; Procedure
Primary problem: Aircraft

Narrative: PIC and I operated Aircraft X; a CL-650; as Aircraft X on an international night ferry flight from ZZZZ to ZZZZ1. We had earlier arrived at ZZZZ from ZZZ2 and once the passengers had deplaned we proceeded with servicing and turning the aircraft in a non-rushed and routine manner. Once complete; final walk arounds were commenced and we began our checklist procedures for engine start. All checklists; flows etc were accomplished in a normal and routine manner and we soon found ourselves holding short of the departure runway awaiting our IFR clearance to ZZZZ1. Once received; we commenced the take-off roll; initial climb; climb to assigned cruise altitude; cruise portion and initial descend for our arrival into ZZZZ1. Approximately half way into our descent from FL300; to the newly assigned altitude of FL140; the white 'EXTERNAL DOOR OPEN' status message illuminated; which was followed by the slightest of momentary yawing motion. The external door annunciator remained illuminated through the remainder of the flight; approach; landing; taxi-in and shutdown. I am estimating the remainder of the flight to have taken approximately 15 minutes and it was entirely normal; routine and uneventful; meaning no additional yaw; shimmy; vibration was observed. The post flight walk around revealed that the lavatory service access door had departed the airplane in its entirety. No visible damage to the surrounding structure; or any other structure of the aircraft was noted. As indicated at the beginning of this report; both pilots complied with the final walk around requirement before commencing flight and the access door was locked and appeared secure. In addition; the PIC; upon closing and locking the main cabin door checked the 'door page' on the galley CES panel which also showed all access doors closed and secured. Upon the Status Message annunciation; I consulted the applicable section of QRH2 which basically stated that the message means exactly what it states; meaning that not all external doors are closed; yet provided no further guidance whatsoever. All SOP and Best Practices were followed in fact exceeded; with PIC checking the internal CES panel and still the door departed. My guess would be that one of the snap latches failed; thusly creating a crack which the 400 MPH wind exploited.


Second narrative: Although both pilots appropriately completed a final walk around the plane before closing the cabin door and starting engines; insuring all access doors were closed; securely latched; and key locked; the lavatory access door opened in flight and separated from the fuselage during descent from FL300 to 14;000 feet over the water. The opening of the door was noted on the EICAS screen and the appropriate checklist was completed. Other than the loss of the lavatory door; no other damage was observed on post flight inspection. After closing the cabin door prior to starting engines; the captain also observed that as usual; all doors were indicating closed on the galley CES screen.


Synopsis: CL-650 flight crew reported losing their Lav service door while they were in flight.