An extra set of hands

Leonard Greene understood that nearly 70 years ago when he created the first commercial autothrottle system. Greene founded the New York-based Safe Flight Instrument Company, also known for its angle-of-attack and stall warning systems.

A few definitions first. Boeing calls its installed system an autothrottle, while a similar system on an Airbus is known as autothrust. Autothrottles typically synchronize with the autopilot and operate in either speed or thrust mode and create many practical hands-free benefits, like flight envelope over- and under-speed protection. Typically connected to the aircraft through the flight management system (FMS) computer and an outside air temperature sensor, the autothrottle calculates engine power more accurately than any human. During an instrument approach or on a standard terminal arrival route (STAR), for example, the autothrottles relieve the pilot of the throttle-jockeying work during required speed changes. If the autothrottles are switched off or become inoperative, the flying pilot can easily revert to flying the aircraft by adjusting the throttles manually.

During an engine failure aboard a multiengine airplane, the autothrottle automatically sets the best power on the good engine. Many modern aircraft also offer an additional power boost in case of an engine failure during takeoff known as reserve thrust. This system provides a boost to the good engine at takeoff or go-around when it senses a difference between both engine low-speed fan (N1) values of more than 15 percent.

Today, most transport category aircraft from Boeing, Airbus, and Embraer, and most major business jets produced by Cessna, Bombardier, Dassault, Gulfstream, and Honda are equipped with autothrottles. Some single-engine jets like the Cirrus Vision Jet and turboprops like the Pilatus PC–12 and the Daher TBM 900 series are also autothrottle equipped. General aviation aircraft that use Garmin’s Autoland system, in fact, require Garmin’s autothrottles installed.

Operationally, as a pilot takes the runway for departure, they arm the autothrottles. Then, as the pilot manually increases thrust for takeoff, an electromechanical system within the throttle quadrant takes over at a predesignated thrust lever angle and automatically sets the final engine power. As the aircraft enters the climb mode, the autothrottle again sets the appropriate power setting or climb airspeed the pilot selects on the flight control panel (FCP). The autothrottles now take their commands from the FCP and adjust pitch and power to maintain the selected speed. In a descent, autothrottle reduces power to flight idle, maintaining the airspeed by varying the aircraft’s pitch attitude.

Not all autothrottle systems operate in the same manner. On a Boeing, in most cases, the thrust levers move as the engine power varies offering a subtle clue to cockpit crewmembers. Coincidentally, the control columns on Boeings also move in harmony to input from either pilot. Once autothrust is set on an Airbus, however, the thrust levers do not move despite changes to engine power, nor do the Airbus sidesticks.

While cockpit technology, including autothrottles, has proven extremely reliable over the past few decades, it is critical that the cockpit crew operate that technology precisely as the manufacturer recommends. Confusion and chaos can rule the day for any crewmembers whose training might be lacking.

On July 6, 2013, an Asiana Airlines Boeing 777-200 struck a seawall on approach to Runway 28L at San Francisco International Airport (SFO) during a daytime visual approach in good VFR weather. The Boeing was destroyed by impact forces and a post-crash fire. Three of the 291 passengers were fatally injured; 40 passengers, eight of the 12 flight attendants, and one of the four flight crewmembers received serious injuries. According to the NTSB, one of the contributing factors to the crash was the crew’s unfamiliarity with the operation of the Boeing’s autothrottles. Additional factors included fatigue following a 10 and a half hour flight from South Korea and that the Runway 28L glideslope was inoperative.

The aircraft was being flown from the left seat by a seasoned captain receiving his initial operating experience (IOE) training on the triple-7 from a check pilot captain who occupied the right seat. An observer pilot sat in the cockpit jump seat. As the 777 neared SFO, ATC vectored it to a 14-mile straight-in approach. For reasons not entirely clear, the flying pilot mishandled the Boeing in a series of unstable flight conditions. First, the aircraft was too high, then too low on the approach, and then as much as 30 knots too slow. Eventually the pilot disconnected the autopilot and pulled the thrust levers to idle. What he did not understand was that disconnecting the autopilot placed the autothrottles in “Hold” mode. At 500 feet agl, the aircraft was slow and descending at 1,200 fpm. At no time did any of the pilots ever suggest adding power which could have alerted them the autothrottles were not working as they assumed they would. When they realized the aircraft’s vulnerable state and called for a go-around, about four seconds before impact, it proved too little, too late. During a post-accident interview, the pilot flying said he believed the autothrottles would always provide under-speed protection if needed, which they did not.

The NTSB listed the probable causes of the crash as the flight crew’s mismanagement of the airplane’s descent during the visual approach, the pilot’s unintended deactivation of the autothrottle, the crew’s inadequate monitoring of airspeed, and the crew’s delayed execution of a go-around. Contributing to the accident was the pilot’s confusion about and the complexities of the autothrottle and its interaction with the autopilot’s flight director system, which the NTSB said were also inadequately described in both Boeing’s and Asiana’s pilot training materials.

Rob Mark is a journalist, business jet pilot, and flight instructor. He publishes the industry blog Jetwhine.com.