The Flight Level Change Trap: NTSB Cites Boeing Design as Possible Cause of Asiana 214 Crash


By: William Ezzell 
Boeing’s 777 is regarded by many as one of, if not the safest commercial airliner ever produced. Up until the tragic accident of Asiana 214 at San Francisco last year, it’s safety record was nearly spotless. On that unfortunate day, a Boeing 777-200ER crashed short of the runway.  181 were injured.  Three lost their lives.  Last month, the NTSB released its Accident Investigation Submission and attributed the crash in part to an autoflight (aka the autopilot) mode known as Flight Level Change. The 777 offers pilots many ways to fly – including descend – the plane. One common mode is LNAV/VNAV – the 777 descends pursuant to the plane’s flight management computers, calculating and executing a descent rate and speed to hold in order to hit certain waypoints at speeds and altitudes input by pilots.  Another way is called Flight Level Change, or FLCH. More on that in a minute.
Commercial airliners’ autoflight systems are made up of two basic components: the autopilot and the autothrottle. Although they are used in tandem, they are two separate systems. Autothrottle does exactly what it says, providing automated control of the engines and throttles to maintain desired speeds and most airlines require that their 777’s have the autothrottle armed at all times. So if the pilot is flying manually, the autothrottle system remains engaged and the 777 in effect tells the pilots “you handle the yoke, I’ll handle the throttle.” Keeping the autothrottle engaged reduces workloads for pilots, lowers fuel costs, and provides an envelope of protection from stalls.  In most of the autoflight modes, if the speed of the 777 drops too low, the autothrottle will “wake up” and through a series of motors (“servos”) and push the throttles forward, increasing thrust and helping the airplane recover. However, there is one mode that does not offer stall protection: FLCH. In FLCH, the 777 will not “wake up” from a slow speed. FLCH is used in the initial descent phase or at times where the plane’s altitude is high. It is an idle thrust pitch mode, generally used to change the 777’s altitude while in cruise, not approach, and is the recommended technique for rapid descents.
Enter Asiana 214. That day, the 777 was a little high above its desired altitude just before entering final approach. The flight data recorder showed that the FLCH mode was then activated. The appropriateness of this action aside, the mode caused the engines to idle and allowed the 777 to descend as quickly as possible. The pilots believed the autothrottle would function through the entire approach to touchdown, meaning that once the 777 reached its desired altitude path (glideslope), the throttles would reengage and provide enough thrust to maintain the landing speed. This is commonly referred to as at “FLCH trap.” When a plane is descending in FLCH mode, if the throttle levers are moved to idle by pilots or computer, the autothrottle goes into a “HOLD” mode and will not move the throttles from the idle position – the servos are disconnected from the throttle levers. So when Asiana 214 approached the runway, its speed kept decreasing, as the throttles were trapped in idle. By the time the pilots realized the problem (or as many argue, their blunder), it was too late and the plane collided with the sea wall.
Make no mistake, the claims against Boeing fall squarely within product liability and this is therefore a case we are following closely here at FMG. There are many questions, for example, is there a better design alternative to this mode? Did Boeing adequately warn pilots and airlines about the potential danger? These questions alone have already garnered significant debate in the aviation industry and will only become more heated as the lawsuits progress. The NTSB report lends significant credence to the core of some of plaintiffs’ claims: design defect and failure to warn. Boeing, rightly so, has strongly denied the claims and placed the blame solely on the shoulders of Asiana’s 214 crew, a subject for a post some other day. In the meantime, this case raises a number of product issues and reignites the ever-present debate human control versus automation.