B747-400
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Confusion
ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Became Reoriented
Environment - Non Weather Related; Human Factors; Airport
We arrived at the VHHK FIR from the northeast. Guangzhou Control had cleared us via en route descent from FL410 to cross SIERA at FL230. The PF (pilot flying) set the MCP altitude to 23000 ft and the PM (pilot monitoring) entered a hard altitude restriction in the CDU (Control Display Panel) Legs page.Based on ATIS; we had preloaded and briefed the SIERA 7C arrival to Runway 07L. We pre-contacted VHHK Radar 3 min prior to SIERA and they said to expect the SIERA 7A arrival; a shorter routing. The PM reloaded the FMC accordingly and briefed the changes.As we neared SIERA; Guangzhou Control handed us off to VHHK Radar; who said 'Cleared the SIERA 7A arrival; Runway 07L'; with no further altitude instructions. The PF then stated we could descend to FL110 per the lowest altitude depicted on the STAR; at MURRY; an intermediate point on the arrival; and set the MCP (Mode Control Panel) altitude to 11000 ft.The PM checked the pEFB (Jepps page 10-2N; SIERA 7A RWYS 07L/R) for a lower altitude at LIMES; the last point on the STAR and IAF for the ILS Runway 07L approach; but no altitude was depicted there. Realizing 'something's not right'; the PM intended to query VHHK Radar about our altitude clearance before reaching TOD (Top of descent) at SIERA.But by then the aircraft had already started to descend. The PM noticed this; asked the PF why we were descending; and stated we still needed to honor the assigned FL230 restriction at SIERA. The PF quickly selected ALT HOLD on the MCP and the aircraft leveled off at [approximately] FL227; now past SIERA.Before we could clarify the clearance with VHHK Radar; they queried us: 'What altitude were you assigned?' The PM replied 'We were cleared the SIERA 7A arrival' so ATC could instruct us whether to climb back up or continue our descent. At this point they directed us to descend to FL220; after which they vectored us off the arrival.We were later assigned a different STAR and landed on Runway 07L without further incident.Our first mistake was initially interpreting the arrival clearance as a 'descend-via'--but we hadn't heard that phrase. The combination of VHHK Radar not restating the altitude assigned by Guangzhou; and the PF setting the MCP to 11000 ft; led the PM to temporarily accept this interpretation.However; feeling an intermediate altitude didn't make sense for a descend-via procedure; and not finding the expected lowest altitude depicted at LIMES (3000-6000 ft per Jepps page 11-2; ILS Rwy 07L); the PM began to recognize our first error.Unfortunately; we'd already made a second mistake when the PF; unnoticed by the PM; had inadvertently pressed the MCP altitude select knob after setting 11000 ft. This deleted the programmed restriction at SIERA and caused the aircraft to descend before we could discuss; query and/or correct the misinterpreted clearance.At some point prior to or during these events; the [relief pilot] had left the flight deck after the descent checklist. Thus he wasn't available to help monitor and trap these errors.Crews should carefully pre-study arrival procedures; including all applicable routing; altitudes; and notes. In our case this would have revealed that the SIERA 7A/C STAR is not designed for a descend-via clearance; both lacking a depicted final descent altitude at LIMES AND featuring a boxed warning 'DO NOT DESCEND WITHOUT ATC CLEARANCE'. While we initially believed we HAD been cleared to descend; 'connecting all the dots' beforehand would have helped avoid this Expectation/Confirmation Bias related error.Crews must remain aware of the difference between 'descend-via' and routing-only arrival clearances; listen carefully to the verbiage; and query ATC for clarification if necessary BEFORE leaving an assigned altitude--especially following a handoff to a new controlling agency.PFs must remain aware of the effects of pressing the MCP altitude select knob; and both PF and PM must verify after setting it (viaPFD; FMA; CDU; etc) that (only) the intended effect was achieved.[Relief pilots] should plan ahead for the descent; complete all duties; and remain on the flight deck from TOD through the shutdown checklist; per the FOM. In the case of an extended cruise/en-route descent; where final TOD is less clearly defined; a good rule-of-thumb would be to man all duty stations from descent checklist through shutdown.
B747 Captain reported an altitude deviation following confusion regarding the ATC clearance.