Air Carrier B777 flight crew reported a speed deviation and stick shaker occurred on approach to HKG in gusty conditions. Wind shear and/or wake turbulence were cited as possible contributors.
Synopsis
Air Carrier B777 flight crew reported a speed deviation and stick shaker occurred on approach to HKG in gusty conditions. Wind shear and/or wake turbulence were cited as possible contributors.
Narrative
While descending downwind into HKG airport; ATC issued descent clearance from 9000 ft. to 6000 [ft.]; and slow to minimum clean speed which was 220 kts for today's weight. There was a heavy B777 in front of us about 5 miles; there were low level cloud build ups; heavy rain; turbulence in and around airport. While descending approximately between 7000 and 6000 ft.; airplane was about 220 kts; we hit what was either wake turbulence or possibly wind shear; the speed started to decrease below 0 flap maneuver speed. FO (First Officer) and Observer simultaneously called 'Airspeed.' Captain aggressively advanced thrust levers to recover airspeed. Aircraft was descending to assigned altitude; slowing to 0 flap maneuver speed; we got a slight upset in aircraft; AP (Autopilot) engaged; ATS (Air Turbine Starter) engaged; on heading 250 in FLCH; we got momentary stick shaker 1 to 2 seconds. After thrust levers manually advanced; airspeed settled back down; we had a momentary climb. However; clearance was to descend to 6000 ft.; and we hadn't gotten there yet. Aircraft was returned to stable; no deviations from clearance; no course deviation; ATC was advised we flew through possible wind shear. Aircraft was landed with no further incident.Cause to be determined. [Possible] heavy gross weight aircraft operating at 0 flap maneuver speed; in trail of another Heavy [aircraft]; flying in or around moderate rain showers; gusty conditions; possibly wind shear; possible wake turbulence. ATC slowing all traffic due to heavy rain on airport. Possibly too slow for weight of aircraft with the current conditions. ATC held our aircraft high on downwind due departing traffic; then basically wanted us to expedite descent but also slow to 0 flap speed.Captain could have requested higher speed; but unlikely due to following other heavy. Captain could have extended flaps to increase stall margin. However; 30 to 40 miles from airport is not typically where flaps are extended. Maybe Captain could have been less aggressive with adding thrust. However since descending; in Hold; FLCH; ATS was too slow in respond. Since it was possibly wake turbulence; or wind shear; [had to] perform recovery; which Captain did; correct any flight path deviations; which were minimal; notify ATC as Pilot Monitoring did; remain calm; USE specific training; continue calm; land safely; fill out report.
Second reporter narrative
Heading into HKG in gusty and rainy conditions; we had been cleared and were descending from 9000 to 6000 [ft.]; airspeed reduction to 220 kts; so the PF (Pilot Flying) had the speedbrake engaged and we experienced some kind of windshear; possibly combined with wake turbulence from an aircraft we were following which caused a quick momentary drop in our airspeed and a brief stall warning of about a second or two; which the PF corrected for with thrust levers forward and parked the speedbrake. The PF then got us back on a descending path towards assigned altitude. The PM (Pilot Monitoring) notified ATC that we had a slight windshear and temporary climb during the correction because of the increased power; but we were already continuing our assigned descent altitude of 6000 ft. We were eventually cleared for an approach to a normal landing; no further incidents or problems.To increase the stall margin in the existing conditions; suggestion would be that the PF could have extended flaps; even though we were still quite a distance from where we would normally configure; or possibly we could have requested a higher descent speed.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.