B777 Undifferentiated or Other Model
Situational Awareness; Confusion; Human-Machine Interface; Communication Breakdown
ATC Issue All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural FAR
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Chart Or Publication; Airspace Structure; Procedure
I was the IRO on [this flight]. Arriving into KIAH; the controller cleared our flight to descend via the MSCOT2 RNAV arrival. He subsequently assigned us to fly at 300 KIAS which is 20 knots above the published speed of 280 KIAS on the STAR. The crew flew the arrival in the LNAV/VNAV mode. The controller asked the crew to keep our speed up as long as possible. At MSCOT the aircraft leveled off at 10;000 feet and began slowing to 250 KIAS. The controller immediately queried the crew as to why we were slowing. He stated that he intended for us to keep up our speed till DOMNO due to following traffic. He immediately issued S turn to following traffic. The flying pilot then disconnected the autopilot and continued to descend though 10;000 feet increasing our speed. We were flying 290 down to 8000 feet when we began to slow to 250. There was a lack of communication between ATC and the crew. The ATC controller assumed we would continue to DOMNO before we descended through 10;000 feet and slowed. The crew left the aircraft in VNAV and allowed it to begin slowing at MSCOT. The clearance from ATC to keep our speed up was not definitive. The confusion led to our exceeding 250 knots below 10;000 MSL.
The Captain reminded the controller that we were below 10;000 and needed to be at 250 kts. The controller said; he didn't care and to increase the airspeed. I flew 290 kts; at 9;000 ft. As we were given a lower altitude the controller than slowed my speed below 250kts.
A VHF debate erupted between Approach Control and the flight crew of a B-777 when the controller's request to keep their speed up on the MSCOT RNAV STAR to IAH conflicted with the flight crew's desire to be below 10;000 feet and; thus; at 250 KTS as they approached DOMNO; which is required to be crossed between 10;000 feet and 8;000 feet.