In light of Saturday's Asiana Airlines flight 214 crash at SFO—killing two teenage girls and injuring 49 other passengers—possible clues begin to surface. Why, just after moving over the seawall, did the plane slam into the ground, bounce in the air, and then crash back into the ground?

National Transportation Safety Board Chairman Deborah Hersman told Face The Nation that a critical system for plane landing at SFO had been turned off—turned off since June, in fact. "What we do know is that there was a notice to airmen that indicated that the 'glide slope' was out," Hersman said.

Capt. Chesley Sullenberger, who made a name for himself after landing the "Miracle on the Hudson" flight in NYC, also noted that the "glide scope" had been turned off for most of the summer due to construction. That could've played a role. Also, Sullenberger goes on to point out that, like other airports with mountainous terrain, SFO is considered a "special airport" by the FAA.

Sully said to CBS Evening News:

"In fact, the FAA classified it as a special airport, along with other airports worldwide that involve mountainous terrain or other special challenges," he said. "It is surrounded by water, and of course water is a featureless terrain where depth perception can sometimes difficult be. There are shifting winds, low visibilities, so there are several things that make it special, plus high terrain just past it."

(Anyone who regularly flies into SFO can agree that the thought of the plane clipping the water and/or hitting the seawall, just before landing on the runway, almost always springs to mind.)

Below: Sully speaks to CBS about the crash:

As it stands, there's no clear cause for yesterday's harrowing crash. NTSB's Hersman goes on to point out that, with flight voice and data recorders currently under investigation, "[e]verything is on the table right now."

All previous Asiana Airlines crash coverage on SFist.