Date: 202005
Local time of day: 0601-1200
Location: ZAN.ARTCC, AK
Aircraft: B747-400
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Descent
Airspace: Class E ZAN
ATC / advisory: TRACON A11
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Confusion; Time Pressure
Second reporter function: First Officer; Relief Pilot
Anomaly: ATC Issue All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: While descending on NEELL 6 RNAV Arrival into ILS 07R at ANC with Auto Pilot engaged; ATC asked us to maintain 310 kts. or greater transition airspeed for spacing. When aircraft just passing below 11;000 ft. it began to slow to 240 kts. Then; ATC asked us to keep up the speed. Then I asked ATC more than 3 times to confirm to descend below 10;000 ft. with more than 250 kts. high speed. 310 kts. ATC's response was 'Affirmative!' Shortly afterward; ATC told us to maintain 280 kts. or greater until further advised.


Second narrative: While in cruise just before we started our descent; ATC instructed us to increase speed for separation into ANC. During the descent approaching 10;500 ft. ATC instructed us to maintain 280 kts. or better as long as practical. I don't remember the exact wording of the ATC clearance at this point; but it was obvious that ATC wanted a high speed descent because we were #1 for ANC. As we approached 10;000 ft. I realized that we were not slowing to 250 kts. I looked over to the 3rd middle observer and mentioned that our speed was too high.The issue was not brought to the operating crew's attention. As we continued our high speed descent through 9;960 ft.; I raised my voice to bring the speed to the crews attention. At this point the two operating crew-members recognized that I was advising them to slow down but claimed that ATC had given them high speed below 10;000 ft. I replied 'ATC can't instruct you to do that'. We continued a high speed descent. I did not press the issue because there were no immediate safety concerns. From the 4th observer seat; I was at a disadvantage because I had no direct link to the intercoms and raising my voice could have caused a CRM issue that was worse than the initial issue. We eventually slowed to 250 kts. at 4;500 ft. Afterwards; the flight continued normally. After we blocked in; we talked about the issue but the entire crew maintained the position that ATC had instructed them to exceed 250 kts.; therefore it was allowed. The decision for a high speed decent seemed to be from a confusion between the regulations vs. the ATC clearance. Additional confusion seemed to arise from the regulations in foreign countries; and which countries allow us to exceed the limitation.


Synopsis: B747 flight crew reported that ATC instructed them to maintain greater than 250 kts. airspeed below 10;000 ft.