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10 July 2013

Asiana 777 accident investigation and the role of the autothrottle

During the Tuesday, 9 July 2013 NTSB press conference, numerous facts about the accident were revealed, including details about the training and experience of the pilots in the cockpit, and the fact that the initial impact not only tore the tail section off the aircraft, but also caused two flight attendants seated in the rear of the aircraft to be ejected out of the cabin. Both survived, but were injured.

Perhaps the most revealing information from the conference was evidence gathered from statements from the pilots, that seemed to indicate that the while the pilots had planned to use the autothrottle to control the aircraft's airspeed during landing, the autothrottle was not engaged at the time the crew was attempting to execute a go around in order to attempt another landing.

The NTSB emphasized that this was preliminary factual information that has yet to be corroborated with other data from sources such as the flight data recorder. However, it implies that one of two possible scenarios may have occurred during the latter stages of the flight:

  1. The crew intended to use the autothrottle, but did not take all the steps needed to engage the autothrottle, or
  2. The flight crew took steps to engage the autothrottle, but the autothrottle either did not engage or it disengaged at some point.

Basic autothrottle operation
In order to understand the possible significance of these preliminary NTSB findings, it helps to have a bit of background knowledge on how autothrottles are used.

In the 777, as in many modern airliners, the autothrottle allows a pilot to control the power setting of an aircraft's engines automatically rather than manually. Flight crews use the autothrottle to maintain, or try to attain a particular value for either speed or thrust without having to manually adjust throttle settings. For example, a pilot may want to maintain a specific airspeed, and would use the autothrottle to maintain that airspeed while the pilot may be manually controlling other aspects of the flight.

The autothrottle can also enhance safety by keeping the aircraft within safe operating limits. For example, if the pilot commands the autothrottle to attain a speed that is at or below a minimum safe speed or above a maximum safe speed, the autothrottl will not allow the aircraft to fly at those unsafe speeds.

Arming and engaging the autothrottle
In the 777, using the autothrottle to control airspeed is a two-step process. First, the autothrottle has to be armed using two switches (one for each engine) on the mode control panel (MCP). Being armed means the autothrottle is available to be used. The second step is to engage the autothrottle, which means it is now being used to control airspeed. The autothrottle is engaged by using an appropriate switch on the mode control panel.

Mode control panel
The cockpit of many modern airliners, including the 777, have a mode control panel (MCP), which contains the controls that the flight crew would need to automatically manage the aircraft's flight, and in the 777, the MCP controls a number of functions, including the autopilot and autothrottle.

Below are two photos depicting the 777. The first is a representation of the MCP from a NASA computer simulator, and the second is from the cockpit of a 777 and shows both the MCP and several cockpit display screens. Note the neither one may represent the exact configuration of the MCP in the Asiana accident aircraft.

(click to enlarge)

(click to enlarge)

The following video describes how a simulated version of the 777 MCP behaves. The first couple of minutes describes how the autothrottle has to be armed before it can be engaged and used to control speed.

Is the autothrottle the key to the cause of the accident?
The NTSB emphasized in their press conference that all of the information that they have released so far is factual in nature, and that they have no intention of speculating or deciding upon a cause or causes of the accident at this stage of the investigation. Also, the information gathering stage of the investigation is ongoing, and there may be other facts that the NTSB either has not yet discovered or has not yet released to the public.

Another perspective on this accident
For an excellent perspective on this accident and the revelations from the early part of the investigation, I highly recommend the Slate article of Patrick Smith, a professional airline pilot and recent guest of the podcast.

Dr. Curtis and Capt. Tom Bunn discuss the crash
The day after the crash, Dr. Curtis of and Capt. Tom Bunn of the SOAR fear of flying program, who both spent several hours on the day of the crash on cable news programs providing expert commentary, discussed the media's response to the accident and shared their thoughts on the early reports of the crash.

Additional information 13 July 2013 article 10 July 2013 article on the role of the autothrottle 8 July 2013 article on early findings of the crash investigation
Other Asiana plane crashes
Other 777 plane crashes
Accident details from Aviation Safety Network
Wikipedia page on this accident

Photos:, NASA

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