At 4 seconds before impact, the sound of the “stick shaker” can be heard. This is a device which gives both an aural and physical alert to the pilot that the aircraft is approaching stall speed – too slow to maintain lift and keep flying normally. In commercial aircraft, the “stick” is actually a yoke used by the pilot to adjust wing roll and nose pitch.
“The speed was significantly below 137 knots, and I’m not talking about a few knots,” Hersman said at a briefing Sunday afternoon, noting also that the throttles had been pulled back to idle as the aircraft slowed below the target speed.
At 1.5 seconds before impact, someone in the cockpit called for a “go around” – which means adding power, waving off the approach to landing, and climbing back up to an altitude necessary to fly around for another attempt to land.
The throttles were advanced and the engines responded as they should. But by then it was too late. The Boeing 777’s low altitude and sink rate were such that its tail clipped the seawall off the end of the runway, and the aircraft skidded several hundred yards to a stop as one engine and parts of the wings came off.
Asked if all of this indicated pilot error – particularly since the weather was good and there had been no reported mechanical problems – Hersman declined to answer directly, citing the need to validate the information on the recorders.
“Everything is on the table right now,” she said. “We won’t speculate; we’re just telling you what we know to be true.”
The NTSB team headed by Hersman expects to be on the scene in San Francisco for at least a week, or however long it takes to complete the initial on-scene investigation.