Much of the speculation about why Asiana Airlines flight 214 crashed-landed in San Francisco, killing two people and injuring scores more, is focusing on the pilot's experience and the equipment used. Information released by investigators thus far raises several questions, the biggest being why the Boeing 777 slowed so dramatically in the final minute of its approach. We won't have definitive answers for some time, but we can break down how Saturday's approach transpired and explain exactly how a modern airliner makes it to the ground – and what could go wrong.

There were four pilots on board the 777, which is not unusual for a transoceanic flight. Typically, one pair will sleep or relax in the crew rest bunks just behind the cockpit while the other is flying. According to the National Transportation Safety Board, the pilot at the controls was a captain in training with 43 hours in a 777 but nearly 10,000 hours in other airliners, including the Boeing 747. In addition to the flight experience, the pilot, identified as Lee Kang-kook by Asiana Airlines, also would have undergone many hours of transition training in a 777 simulator.

The "approach plate" showing the information, including the glidepath in the lower right, used during an ILS approach to runway 28L at SFO.

While it might sound unusual that a pilot with just 43 hours of experience would be landing the plane, it is normal, and of course necessary for pilots to fly an airplane new to them with relatively low experience in a particular type. It was first officer Jeff Skiles' first day in an Airbus A320 when he and Captain Sully Sullenberger performed the "Miracle on the Hudson" by successfully landing in the Hudson River in 2009.

And the captain sitting in the left seat of Flight 214 has more than 3,000 hours of experience flying 777s, and more than 12,000 hours of total experience. Crew resource management is another factor to consider, which includes the delegation of duties in the cockpit and often includes one pilot flying, while the other pilot reads out critical information including airspeed and altitude.

But under normal circumstances any pilot who has passed the exam and has a license should be able to make a visual approach and determine whether or not they are likely to land short of the runway and adjust accordingly. This will clearly be a focal point of the investigation of Asiana 214.

During a normal flight, the pilots will configure an airplane like the 777 for a stabilized approach well before landing. In a stable approach the aircraft's configuration including the flaps, power setting, speed brakes, and landing gear are selected as necessary and appropriate during the descent. The airspeed and rate of descent are ideally stable, or at least within an acceptable range that will result in the preferred speeds during the final phase of the approach. Occasionally an air traffic controller will ask for a non-standard approach. One example is referred to a "slam dunk" approach when an aircraft might be kept at a higher than normal altitude early in the approach phase, and then perform an expedited descent to the final phase before landing. These are not unusual under visual conditions, and according to many airline pilots they are common at SFO. Such an expedited descent can lead to an approach that is not a standard stabilized approach, at least during the early phase and would require extra attention.

According to the NTSB, Asiana flight 214 was cleared for a visual approach for runway 28L. This allows the pilots to fly using only their eyes to guide them to the runway. This is a normal type of approach when the skies are clear and there are no adverse weather conditions to deal with – as was the case on Saturday, a picture-perfect day in San Francisco.

In cloudy weather when the visibility is worse, pilots can use different equipment to guide them to the airport well before they can see the runway outside the window. Today, GPS is used regularly, but a common type of approach for an airliner is an instrument landing system, or ILS. An ILS approach has two principal components, a localizer transmitter which provides a radio signal guiding the aircraft laterally, and a glide slope transmitter that provides a signal that guides the aircraft vertically. These signals can provide extremely precise guidance for an airplane, and the most advanced types allow it to touchdown on the center line of the runway, in the landing zone, with zero visibility outside the window.

The glide slope transmitter for the ILS approach on runway 28L at SFO has been out of service since June 1. This means an ILS approach would not be used for that runway if the weather was bad. There are other types of instrument approaches that can be used for 28L, including a GPS-based RNAV approach that offers nearly identical "minimums," meaning it can be used in almost the same kind of conditions as the ILS.

Even on a day like Saturday, when the pilot is cleared for a visual approach, a pilot may use the ILS or other instrument approach as a source of guidance, but it is not required and all licensed pilots are capable of making a visual approach to landing without using the navigation instruments. Since they were cleared for a visual approach, the pilots of Asiana 214 would use other tools to guide them to the proper glide slope for runway 28L.

A satellite view of runway 28L and 28R at SFO. Image: DigitalGlobe/USGS/USDA/Google

For a visual approach there are several aides for pilots to guide them to touchdown on the runway. The first is simply basic pilot training where they learn to judge whether they are above or below a path that will take them to their intended touchdown point. The changing perspective of the runway during approach gives the pilot a chance to estimate whether or not they are high or low, indicating whether they will land long or short of their intended touchdown point. This technique is used commonly on smaller airports, including grass runways where there are no other tools available to the pilot.

At larger airports, there markings on the runway that indicate the touchdown zone as well as an aiming point. The touchdown zone markings are spaced every 500 feet at each end of a runway, while the aiming point markings are solid rectangles located 1,000 feet from the end of the runway. Based on the tire marks left behind on runway 28L in the picture above, it appears that airliners typically land between 1,000 to 2,500 feet down the 11,381-foot-long runway 28L at SFO.

An approach to runway 19L at San Francisco International Airport made in a 777 simulator at Boeing's flight test facility shows the airplane above the glide slope as indicated by the four horizontal white PAPI lights to the left of the runway. Image: Boeing An image from a 777 simulator flight done at Boeing's flight research facility shows a high final approach to runway 19L at SFO. The four horizontal lights to the left of the runway are the PAPI lights indicating the high approach.

There are also visual indicators located to the side of the runway to help guide pilots on the proper glide slope. Runway 28L at SFO uses a precision approach path indicator (PAPI) which consists of four bright lights that can be seen up to five miles away during the day. If all four of the horizontally placed lights are white as in the image above, the aircraft is too high and will land beyond the touchdown zone unless changes are made to the approach. If one light is red, the aircraft is slightly high. If there are two red lights and two white lights, the aircraft is on the proper three-degree glide slope and will land in the touchdown zone. Three red lights means you're slightly low, and four red lights means you're well below the glide path and will land short of the touchdown zone. Saturday's crash damaged the PAPI lights and they were subsequently placed on a Notice to Airmen, or NOTAM list for the airport and listed as out of service.

Because Asiana 214 was cleared for the visual approach, and there was an inoperable glide slope, under normal circumstances the pilots would use the runway markings as aiming points, and the PAPI lights to place them on the correct glide slope. It is not yet known why this system did not work and is a question the NTSB is seeking to answer.

The airplane was flying on autopilot during the initial part of the approach. This is a typical procedure for most airlines and the autopilot is treated as a third crew member by most airline pilots. With the autopilot engaged, the pilots are still responsible for adjusting things such as the flap settings as well as lowering the landing gear.

At 1,600 feet and 82 seconds before the crash, the autopilot was disengaged. Nine seconds later the airplane was at 1,400 feet and the airspeed was 170 knots (196 mph). The speed the pilots wanted to fly during the final portion of the approach is known as the "reference speed", or Vref. Based on the weight and configuration of the 777-200ER on Saturday, the Vref speed was 137 knots.

According to the NTSB there was no discussion of any aircraft anomalies by the pilots and the engines appeared to be working normally.

At 1,000 feet and 54 seconds before impact, the airplane had slowed to 149 knots. Even though the pilots would not be using the ILS navigation information displayed in the cockpit, they would still be using other instruments displaying the their airspeed, altitude, rate of descent as well as engine information. In addition, there is normally an audible reading of the altitude as the airplane makes the approach with a voice reading out key altitudes. The NTSB says they are still reviewing the glide slope information, and hope to release the details of when the aircraft departed the proper glide slope that caused it to impact the ground short of the touchdown zone.

At 500 feet and just 34 seconds before impact, the airspeed slowed to 134 knots, three knots below the Vref speed which is clearly indicated on the speed tape – a vertical display bar showing the airplane's airspeed on a glass display in the cockpit. There is also a visual warning on the speed tape as the airplane slows below Vref, and more visual warnings as the speed continues to decrease.

There's been a lot of talk about "stall speed" and the Vref is not the stall speed. First, a stall is an aerodynamic situation where the wing exceeds a critical angle with reference to the oncoming air, the angle of attack. When the critical angle of attack is exceeded, the airflow begins to detach from the wing which is no longer able to generate sufficient lift to keep the airplane flying. The stall speed is the airspeed based on the weight and configuration of the airplane where the critical angle of attack would occur under normal flight conditions. Vref is an approach speed above the stall speed, giving the pilots a margin of safety during the approach.

Traveling a few knots below the Vref speed, while not ideal, is not going to cause an immediate problem. It is however an indication to a pilot that it is time to change the configuration of the airplane by either pitching the nose down or adding power – or both – to get back to the Vref speed while maintaining the proper glide slope.

According to the NTSB it doesn't appear the necessary changes were made, and at 200 feet the airspeed had slowed to 118 knots. Eight seconds later the throttles began moving forward according to the NTSB. It is not clear if the pilots realized they needed to boost their speed and/or adjust their rate of descent, or if the automatic "wake up" mode that automatically adds more power as the stall speed is approached had engaged. In either case, one of the challenges of flying a jet-powered aircraft is that unlike a car or even a propeller airplane, there is a significant delay between the moment you apply power with the levers, and when the engines produce the thrust you are requesting. The throttles began moving forward just eight seconds before impact at 125 feet above the water and at an airspeed of just 112 knots.

At this point the 777 is approaching its stall speed and four seconds before impact the "stick shaker" can be heard according to the NTSB's analysis of the cockpit audio recording. The airplane has a device that measures the angle of attack of the airplane and as the critical angle of a stall nears, the control yoke shakes in the pilots hands, providing the pilots a final vibrating indicator that a stall is imminent if nothing changes.

Three seconds prior to impact the airplane reached its lowest speed of 103 knots with the engines at 50 percent power and increasing, according to the NTSB. Moments later, at 1.5 seconds before impact, the NTSB says the pilot called for a "go-around". This means the pilots wanted to abandon the approach and climb again to make another attempt. A go around in a 777 is typically executed by pushing a switch known as the TOGA (take-off, go-around) located on the throttle levers. When the switch is pushed, the airplane automatically goes to a power setting for a 2,000 foot per minute climb, and a second push provides full take off power. But again, the engines take time to "spool up" and deliver the requested thrust.

In the case of Asiana 214, the NTSB did not say whether or not the go-around function was engaged or not in the 1.5 seconds between the call for a go-around and the impact. The 777 impacted the ground at 106 knots – 122 mph – and at least several hundred feet shy of the runway touchdown zone.

Investigators in Washington D.C. are completing a more thorough examination of both the cockpit and flight data recorders. The four pilots are being also being interviewed and should be able to provide valuable information into the cause of the accident.

It deserves noting that there were a total of 307 people on board the 777, including passengers and crew. The high survival rate is a testament to the safety of modern airliners and the training of the crew. The aircraft structure is more crash-worthy than early airliner designs and the design of the passenger seats are capable of absorbing loads 16 times the force of gravity. Add in the cabin crew's ability of evacuating the passengers from the burning jumbo jet in a short period of time, and the result is hundreds of lives saved and a surprisingly low amount of injuries and – especially – fatalities.