It hadn't been working for weeks.
Did the pilots know this, and did it make a difference?
Experts Eyewitness News has spoken to say this instrument landing system could have been especially helpful to this pilot landing this type of jet for the first time at the San Francisco Airport.
And, Eyewitness News has learned it's not the first time a Glide slope was out of service when a Korean Airline crash landed.
In Guam, in 1997, 228 people died when Korean Airlines-Flight 801 crashed landed just a couple of miles from the runway. NTSB investigators found that a contributing factor in the crash was the flight crew's confusion about whether an instrument landing aid, known as a "Glide slope" was working. That confusion, the NTSB concluded caused the captain to lose awareness of the flight's position and improperly descend.
The Glide slope landing aid was also out of service when Asiana Airlines Flight 214 crashed on the runway in San Francisco. It's not known yet whether that played any role in the crash since it was a clear day allowing for visual landings.
"It's a very good tool to have," said Daniel Rose, a pilot and aviation attorney.
Pilot and Aviation Attorney Daniel Rose says while the Glide slope wasn't necessary, had it been operable, it would have been very helpful to the pilot who had only 43 hours experience in the Boeing 777.
"It's a good safety backup. If it's not there, it can compromise the crews' ability to get into a proper position to land the plane," Rose said.
Also, likely to be scrutinized by investigators is an increasing reliance on automated cockpits.
"It's either a pilot's best friend or the pilot's worst nightmare," said Robert Ober, a former Delta pilot.
Robert Ober, who spent decades flying commercial airlines, says pilots are increasingly reliant on computers to fly the plane reducing the hands-on skills.
"You get over-reliant on this system and when that happens, if this stuff fails, the amount of experience and training used to go back to basic fundamental flying the airplane is lacking," Ober said.
The NTSB says it will be investigating whether the Glide slope outage had any role in the crash.
Experts tell Eyewitness News it is likely to turn out to be a chain of events that led up to the crash.
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