Date: 201904
Local time of day: 0001-0600
Location: ZGZU.ARTCC, FO
Light: Night
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Initial Approach
ATC / advisory: Center ZGZU
Aircraft component: ILS/VOR
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Distraction; Confusion; Time Pressure; Workload; Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Requested ATC Assistance / Clarification; Flight Crew FLC Overrode Automation; Flight Crew Became Reoriented
Contributing factors: Aircraft; Human Factors
Primary problem: Ambiguous

Narrative: During our ILS approach to ZGGG; we had a GPS Loss of Signal/GPS Signal Interference for about 30 seconds.We were given a vector heading to intercept the ILS to Runway 01 about 20 miles from the runway. Just as we were intercepting the localizer; 'ADS-B OUT L' EICAS (Engine Indicating and Crew Alerting System) alert appeared and the NAV display changed from GPS to Inertial. I was about to switch the transponder to the R side when Approach Control advised us that they had lost our transponder signal. After switching to the R transponder; Approach confirmed that they were receiving our signal. The EICAS alert blanked and the NAV display showed GPS about 30 seconds after the initial signal loss.After landing; we asked Ground Control to advise Approach Control of the GPS interference on the final approach. We had experienced our first GPS jamming/interference event on our previous flight. Due to our short layover in ZZZ and much needed rest; I chose to wait until our longer Guangzhou layover to submit an [ASRS] report per the FOM X.XX Mandatory Report table. The table shows only an [Safety Report] and Security reports are mandatory; the AML entry box has no 'x'. In the research for the report; I reread the entire Bulletin XXX-XX-XX GPS; 'Loss of Signal / GPS Signal Interference.' It was at this time that I discovered that I had misread the Bulletin and neglected to make an AML (Aircraft Maintenance Logbook) entry regarding the GPS interference. I have submitted a Security Report for both flights.As stated above; this [ASRS] report is essentially applicable to this flight and our previous flight. The same causes apply to both.The cause of the loss of GPS was most likely due to jamming and/or interference.Neglecting to properly document and communicate the GPS issue was primarily due to my misreading of the GPS bulletin. In a rush to reference the technical part of the bulletin to assist in troubleshooting; I skimmed the administrative paragraphs and missed the 'A detailed AML write up is required for any GPS discrepancies even if the GPS function returns to normal.' I made the mistake of assuming that; because the event was not caused by our equipment; an AML entry would not be required. The lack of an AML entry requirement in the FOM (Flight Operations Manual) X.XX table contributed to my error. The bulletin IS controlling as it was issued subsequent to the current FOM. Additionally; this was our first GPS event experience.Our improper assumptions were additional causes. We assumed that a notification to ATC or [Operations Center] was not necessary because we never had a loss of navigation capability and we had no aircraft system malfunctions.In the future; I will notify ATC; [Operations Center]; document with an AML entry; file a [Safety Report] and a Security Report for a GPS jamming/interference event.Going forward; after a 'Non - Normal' event is complete; stabilized or we are blocked in; a detailed review of the QRH; Bulletin or other source document would be helpful. A slower review during a low workload time would help to avoid this type of mistake. A more detailed review would have led me to make an AML entry; notify ATC and [Operations Center].Additionally; an update to the FOM Table X.XX for the GPS events would help to prevent this type of error.




Synopsis: Air carrier Captain reported possible GPS jamming event during an ILS approach to Guangzhou Baiyun International Airport.