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How Autothrottles Work

Autothrottles are a big hurdle to overcome for new airline pilots and those upgrading to advanced aircraft. For your entire aviation career, you've been manually managing engine power and it can be difficult to get comfortable letting automation do it for you. Understanding how to use autothrottles efficiently is one of the best tools available for you in the flight deck.

If you've flown with autothrottles before, this article may refresh the basics but is primarily intended to provide a foundation to prepare for aircraft-specific training.

Live from the Flight Deck

A Fully Automated Flight

Let's take a look at what autothrottles can do at the highest level of automation. Even before the plane has left the gate, you've programmed a series of speeds into your FMC for climb, cruise, descent, and approach. Autothrottles can be thought of as a separate system from your autopilot. You can fly with A/P on and A/T off or vice versa.

From the moment you engage autothrottles on takeoff, the autothrottles can manage engine power to meet climb restrictions, your exact cruise speed, and speed/altitude restrictions on the descent. When ATC wants a new speed, you command the autothrottles to maintain a new speed via the speed window. During an autoland, the autothrottles can even bring the thrust levers to idle in the flare. All without you physically moving the thrust levers.

So how does this all come together and what are the basics of using autothrottles?

Programmed Power To Achieve Commanded Speed

During takeoff, climb, cruise, descent, and go-around, there are pre-programmed power settings for required efficiency and performance. The nice thing is that you won't have to do much math, most of the power settings are calculated without your input. On takeoff, you may perform a reduced thrust takeoff (when safety margins allow you to use more runway on a takeoff roll)... or maybe due to runway contamination, you'll be performing a max thrust takeoff.

These power settings help you achieve desired speed and performance when using autothrottles. Based on the commanded speed, climb, or descent profiles from the FMC VNAV page or your autopilot control panel, the autothrottles will power the engines up to the maximum limit for that phase of flight. So, if you want to expedite your climb, but you're already flying at max climb thrust at the programmed speed, you'll need to reduce speed to increase your rate of climb.

Let's say you're hand-flying with the autothrottles on and you've selected a VNAV climb (VNAV SPD). The programmed climb speed is 250 knots. As you pull back to increase the rate of climb, the autothrottles will increase power to meet that rate while maintaining 250 knots. When you push forward to shallow your climb, the autothrottles reduce thrust. Just like you'd do manually.

Arming Autothrottles

Autothrottle systems have an "arm" switch. You can think of this as powering the autothrottle system. When armed, the autothrottles are ready to operate. Disarmed, they cannot turn on.

Engaging Autothrottles

On the takeoff roll, as you start to bring the thrust up, you'll likely engage the autothrottles when you reach an aircraft-specific power setting. This will command a takeoff thrust setting.

During climb out, when you select a VNAV or speed setting, the autothrottles will change from takeoff performance to climb performance (indicated by reduced thrust on engine instruments). Unless a fault occurs, they may remain on for the rest of the flight.

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Disengaging Autothrottles

To disengage the autothrottles, you push a disengage button (located on the side of the throttles in Boeing aircraft). But this does not disarm the autothrottles, it only "puts them to sleep." They may still be instantly re-engaged by pushing the autothrottle button or go-around buttons in some aircraft. You should announce to your other pilot that you're disengaging autothrottles.

There are limits to how low you can fly using autothrottles, which is aircraft-dependent. Some pilots prefer clicking off the autothrottles when they turn off the autopilot for a hand-flown approach, while others leave them engaged throughout the approach.

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Disarming Autothrottles

To disarm (deactivate) the autothrottle system, you'll move the arming switch to off. In some aircraft, this is done following an engine failure if the autothrottles are not certified for single-engine operations.

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Overriding Autothrottles

You can always override the autothrottles. On Boeing aircraft, the autothrottles physically move via a small motor system. They operate based on a flight computer which may react slower than you, so many pilots will slightly override the autothrottles if the flight computer reacts too slowly to changes in commanded speed.

It is important, to not over-fight the autothrottles. It's easy to create a pilot-induced oscillation when the computer and pilot are reacting in opposition.

Airbus vs. Boeing

While there are many differences between Airbus and Boeing, one of the most noticeable is autothrottles. On Boeing aircraft, autothrottles physically move based on engine power. On Airbus aircraft, the thrust levers remain in a stationary detent and do not physically move as engine power changes.

Every Airplane Operates Differently

Each airplane you fly with autothrottles will have different system logic. The details in this article are meant to be the foundation blocks for you to learn more about your specific airplane. Keep a close eye on exactly what protections each autothrottle mode provides you.

Speed Protections

Depending on your aircraft and selected speed mode, the autothrottles can offer you speed protection to prevent stalling. As your airspeed decreases, it will reach a point (a margin figure above stall speed) where the autothrottles "wake up" and begin increasing the engine power to prevent a stall.

For example, on the 757/767 during a go-around (lots of excess power), the autothrottles will command a climb of 2,000 feet per minute to initiate a significant climb without skyrocketing back into the sky at 5,000 feet per minute and making a stable go-around difficult.

Autothrottle Mode Confusion

In the case of Asiana Flight 214, the pilots lost situational awarness of what autothrottle mode they were in until it was too late:

What Else Do You Want To Learn?

We want you to be prepared for every step of training and your career. What else would help you feel prepared? Let us know in the comments below!

Swayne Martin

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and a First Officer on the Boeing 757/767 for a Major US Carrier. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), is a former pilot for Mokulele Airlines, and flew Embraer 145s at the beginning of his airline career. Swayne is an author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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