B737 Autothrottle (A/T) - Normal and Non-Normal Operations — Flaps 2 Approach

Mode Control Panel (MCP) showing A/T on/off solenoid switch and speed window. The MCP shown is the Pro model manufactured by CP Flight in Italy

The Autothrottle (A/T) is part of the Automatic Flight System (AFS) comprising the Autopilot Flight Director System (AFDS) and the autothrottle. The A/T provides automatic thrust control through all phases of flight.

The autothrottle functionality is designed to operate in unison with the Autopilot (A/P), Nevertheless, a flight crew will not always adhere to this use, some crews preferring to fly manually or partially select either the autopilot or autothrottle.

A search on aviation forums will uncover a plethora of comments concerning the use of the autothrottle which, combined with autopilot use and non-normal procedures, can be easily be misconstrued. An interesting discussion can be read on PPRuNe.

This post will examine, in addition to normal A/T operation, some of the non-normal conditions, their advantages and possible drawbacks. Single engine operation will not be addressed as this is a separate subject.

Additional Information:

737 Automatic Systems Review (Smart Cockpit).

Autothrottle (A/T) Use

The autothrottle is engaged whenever the A/T toggle is armed and the speed annunciator is illuminated on the Mode Control Panel (MCP). Either of these two functions can be selected together or singularly.

The autothrottle is usually engaged during the takeoff roll by pressing the TO/GA buttons located under the thrust lever handles. This is done when %N1 stabilises for both engines at around 40%N1. This will engage the autothrottle in the TO/GA command mode. The reason the autothrottle is used during takeoff is to simplify thrust procedures during a busy segment of the flight.

FMA Captain-side PFD showing TO/GA annunciated during takeoff roll

Once engaged, the TO/GA command mode will control all thrust outputs to the engines until the mode is exited, either at the designated altitude set on the MCP, or by activating another automaton mode such as Level Change (LVL CHG). When TO/GA is engaged, the Flight Mode Annunciator (FMA) will announce TO/GA providing a visual cue.

The use of the autothrottle is at the discretion of the pilot flying, however, airline company policy often dictates when the crew can engage and disengage the A/T.

The Flight Crew Training Manual (FCTM) states:

‘A/T use is recommended during takeoff and climb in either automatic or manual flight, and during all other phases of flight’.

When to Engage / Disengage the Autothrottle

A question commonly asked is: ‘When is the autothrottle disengaged and in what circumstances’ Seemingly, like many aspects of flying the Boeing aircraft, there are several answers depending on who you speak to or what reference you read.

In the FCTM, Boeing recommends the autothrottle be used only when the autopilot is engaged (autopilot and autothrottle coupled).

In general, a flight crew should disengage the autothrottle system at the same time as the autopilot. This enables complete manual input to the flight controls and follows the method recommended by Boeing.

My preference during an approach is to disconnect the autothottle and autopilot no later than 1500 feet AGL. This corresponds to the altitude that the aircraft must be in landing configuration, gear down, flaps 30 and within vertical and lateral navigation constraints with landing checks completed. Disconnecting the autothrottle and autopilot earlier in the approach provides additional time to transition from automated flight to manual flight, and establish a 'feel' for the aircraft before landing.

It's not uncommon that flight crew will manually fly the aircraft, especially 'old school' pilots who are very conversant with hand flying. I know some crews that will fly from 10,000 feet to landing using the Flight Director (FD), ILS, VNAV and LNAV cues on the Primary Flight Display (PFD) for guidance and the information displayed on the Navigation Display (ND) for situational awareness. Many pilots enjoy hand-flying the aircraft during the approach phase.

Important Point:

Whenever hand flying the aircraft with the autothottle not engaged, it's very important to monitor the airspeed. This is especially so during the final approach, when thrust can easily decay to a speed very close to stall speed.

The Autothrottle is Designed to be used Coupled with the Autopilot

The autothrottle is a sophisticated automated system that will continually update thrust based on minor pitch and attitude changes, and operates exceptionally well when coupled with the autopilot. But, when the autopilot is disengaged and the autothrottle retained, its reliability can be questionable.

Some crews believe that if a landing is carried out with the autopilot off and the autothrottle engaged, and a fall in airspeed occurs, such as during the flare, then the autothrottle will apply thrust causing the potential for a tail strike. Likewise, if during the approach there are excessive wind gusts, pitch coupling (discussed below) may occur.

The advantages of using the autothrottle and autopilot together are:

(i) Speed is stabilized;

(ii) Speed floor protection is maintained;

(iii) Task loading is reduced; and,

(iv) Flight crews can concentrate on visual manoeuvring and not have to be overly concerned with wind additives

The disadvantages of using the autothrottlewithout the autopilot engaged are:

(i) Additional crew workload and possible loss of situational awareness (due to workload);

(ii) Potential excessive and unexpected throttle movement caused by pitch and attitude changes;

(iii) Potential excessive airspeed when landing in windy conditions with gusts;

(iv) The potential for pitch coupling to occur (discussed below); and,

(v) A loss of thrust awareness (out of the loop).

Important Point:

The autopilot and autothrottle should not be used independent of one another.

Boeing 737 Design

The design of the 737 airframe is prone to pitch coupling because of its under wing mounted engines. The engine position causes the thrust vector to pitch up with increasing thrust and pitch down with a reduction in thrust.

The autothrottle is designed to operate in conjunction with the autopilot, to produce a consistent aircraft pitch under normal flight conditions. If the autopilot is disengaged but the autothrottle remains engaged, pitch coupling may develop.

Pitch Coupling

Pitch coupling is when the autothrottle system actively attempts to maintain thrust based on the pitch/attitude of the aircraft. It occurs when the autopilot is not engaged and manual inputs (pitch and roll) are used to control the aircraft.

If the pitch inputs are excessive, the autothrottle will advance or retard thrust in an attempt to maintain the selected MCP speed. This coupling of pitch to thrust can be potentially hazardous when manually flying an approach, and more so in windy conditions.

Scenario - pitch coupling

For example, imagine you are in level flight with autothrottle engaged and the autopilot not engaged, and a brief wind change causes a reduction in airspeed. The autothrottle will slightly advance the throttles to maintain commanded speed. This in turn will cause the aircraft to pitch slightly upwards, triggering the autothrottle to respond to the subsequent speed loss by increasing thrust, resulting in further upward pitch. The pilot will then correct this by pushing forward on the control column to decease pitch. As airspeed increases, the autothrottle will decrease thrust causing the aircraft to decrease more in pitch.

The outcome is that a coupling between pitch and thrust will occur causing a roll-a-coaster type ride as the aircraft increases and then decreases pitch, based on pilot input and autothrottle thrust control.

A/T ARM solenoid, N1 and speed button. The N1 and speed button illuminate when either is in active mode. In the image, the A/T is armed; however, the speed option is not selected (the annunciator is extinguished). This enables thrust to be controlled manually

Autothrottle Non-Normal Operations (Arm Mode)

The primary function that the A/T ARM mode is to provide minimum speed protection. A crew can ARM the throttle but not have it linked to a speed. To configure the autothrottle in ARM mode, the A/T toggle solenoid on the MCP is set to ARM, but the SPEED button is not selected (the annunciator is not illuminated).

Scenario - speed button not selected during approach

Some flight crews prefer during an approach, to arm the autothrottle, but not have the speed option engaged (speed annunciator extinguished).

By doing this during a non-precision approach, it enables a Go-Around to be executed more expediently and with less workload (the pilot flying only has to push the TO/GA buttons on the thrust lever and the autothrottle will engage).

If the approach proceeds smoothly and a Go-Around is not required, the crew will prior to landing, disengage the A/T solenoid switch on the MCP by either manually 'throwing' the toggle or pressing the A/T buttons located on the thrust levers. Although favoured by some flight crews, this practice is not authorized by all airlines, with some company policies expressly forbidding the ARM A/T technique.

The recommendation by Boeing in the B737 Flight Crew Training Manual (FCTM) states:

‘The A/T ARM mode is not normally recommended because its function can be confusing. The primary feature the A/T ARM mode provides is minimum speed protection in the event the airplane slows to minimum maneuvering speed. Other features normally associated with the A/T, such as gust protection, are not provided’. (When the A/T is armed and the speed button option not selected).

Autothrottle Speed Protection and Vref in Windy, Gusty and Turbulent Conditions

To provide sufficient wind and gust protection, when using the autothrottle during an approach in windy conditions, the command speed should set to the correct wind additive based on wind speed, direction and gusts (between Vref+5 and Vref +20).

The use of an additive creates a safety envelope that takes into account potential changes in wind speed and minimises the chance of the autothrottle commanding a speed that falls below Vref. Remember, that as wind speed varies the autothrottle will command a thrust based on the speed.

During turbulence, the autothrottle will maintain a thrust that is higher than necessary (an average) to maintain command speed (Vref).

Important Points:

When the autothrottle is not engaged, or the speed option on the MCP deselected, minimum speed protection is lost.
Always add a wind additive to Vref based on wind strength and gusts. Doing so provides speed protection when the autothrottle is engaged.

Refer to Crosswind Landings Part 2 for additional information on Vref.

A/T disengage button on throttle thrust lever. This is an OEM throttle from a B737-300 series. The button is identical to that used in the NG with the exception that the handles are usually white and not grey in colour. Depressing this button will disengage the autothrottle and disconnect the A/T solenoid switch on the MCP

Manual Override - Engaging the Clutch Assembly

Occasionally, for any number of reasons, the flight crew may need to override the autothrottle.

The Boeing autothrottle system is fitted with a clutch assembly that enables the flight crew to either advance or retard the thrust levers whilst the autothrottle is engaged. By moving the thrust levers, the clutch assembly is engaged and the autothrottle goes offline whilst the levers are moved.

The clutch is there to enable the autothrottle to be manually overridden, such as in an emergency or for immediate thrust control.

ProSim737 does not (as at 2018) support manual autothrottle override.

Simulation Nuances

The above information primarily discusses the systems that operate in the real aircraft. Whether these systems are functional in a simulation, depends on the avionics suite used (Sim Avionics, Project Magenta, etc).

For example, the autothrottle may not maintain the speed selected in the MCP during particular circumstances (for example, turns in high winds). If this occurred in the real world, a crew would manually override the autothrottle. However, if the avionics suite does not have this functionality, then the next best option is to either:

(i) Disengage the autothrottle and manually alter thrust; or,

(ii) Deselect the speed annunciator on the MCP.

Deselecting the speed annunciator will cause the throttle automation to be disengaged; however, the autothrottle will remain in the armed mode. The second option is a good way to overcome this shortfall of not having manual override. By deselecting the speed option, the thrust levers can be jiggled forward or aft to adjust the airspeed. When the speed has been rectified by manual input, the autothrottle can be engaged again by depressing the speed button.

It's important if the autothrottle is not engaged, or is in the ARM mode, that the crew maintains vigilance on the airspeed of the aircraft. There have been several incidents in the real world whereby crews have failed to observe airspeed changes.

Manual Flying (no automation engaged)

The benefit of flying with the autothrottle and autopilot not engaged is the ease that the aircraft can be maneuvered. The crew sets the appropriate %N1 that produces the correct amount of thrust to maintain whatever airspeed is desired; gone are the thrust surges as the autothrottle attempts to maintain airspeed.

Granted, it does take considerable time and patience to become competent at flying manually in a variety of conditions, but the overall enjoyment increases three-fold.

Company Policies

Airline policies often dictate how a flight crew will fly an aircraft, and while some policies are expedient, more often than not they are based on economics (cost savings) for the company in question.

Policies vary concerning autothrottle use. For example, Ryanair has a policy to disconnect the autothrottle and autopilot simultaneously, as does Kenya Airways. Air New Zealand and QANTAS have a similar policy, however, define an altitude that disconnection must occur at or before. If an airline doesn't have a policy, then it's at the discretion of the flight crew who should follow Boeing's recommendation in the FCTM.

Confusion and Second Guessing - Vref with A/T Engaged or Disengaged

There is considerable confusion and second guessing when it comes to determining the Vref to select dependent on whether the autothrottle is engaged or disconnected at landing. To simplify,

If the autothrottle is going to be disconnected before reaching the threshold, the command speed should be adjusted to take into account winds and gusts (as discussed above and refer to Crosswind Landings Part 2). It's vital to monitor airspeed when the autothrottle is not engaged as during the approach the speed can decay close to stall speed.
If the autothrottle is to remain engaged during the landing (as in an autoland precision approach), the command speed should be set to Vref +5. This provides speed protection by keeping the engine thrust at a level that is commensurate with the Vref command speed. If wind and gust indicate a higher additive speed, then this should be added to Vref.

Refer to Wind Correction Function (WIND CORR) for information on how to use the Wind Correction function in the CDU.

Final Call

There is little argument that the use of the autothrottle is a major benefit to reduce task loading; however, as with other automated systems, the benefit can come at a cost, which has lead several airlines to introduce company policies prohibiting the use of autothrottle without the use of the autopilot; pitch coupling, excessive vertical speed, and incorrect thrust can lead to hard landings and possible nose wheel collapse, unwanted ground effec t, or a crash into terrain.

Ultimately, the decision to use or not use the autothrottle and autopilot as a coupled system is at the discretion of the pilot in command, and depends upon the experience of the crew flying the aircraft, the environmental conditions, and airline company policy. However, the recommendation made by Boeing preclude autothrottle use without the autopilot being engaged.

Disclaimer

The content in this post has been proof read for accuracy; however, explaining procedures that are convoluted and often subjective, can be challenging. Occasionally errors occur. If you observe an error, please contact me so it can be rectified.

Acronyms and Glossary

A/P – Autopilot (CMD A CMD B).
A/T – Autothrottle.
AFDS – Autopilot Flight Director System .
Command Speed - In relation to the Autothrottle, Command Speed is Vref +5 knots.
FCTM – Flight Crew Training Manual (Boeing Corporation).
FMA – Flight Mode Annunciator.
Manual Flight – Full manual flying. A/T and A/P not engaged.
MCP – Mode Control Panel.
Minimal Speed Protection – Function of the A/T when engaged. The A/T has a reversion mode which will activate according to the condition causing the reversion (placard limit). (For example, flaps, gear, etc).
Pitch Coupling – The coupling of A/T thrust to the pitch of the aircraft. A/T thrust increases/decreases as aircraft pitch and attitude changes. Pitch coupling occurs when the A/P is not engaged, but the A/T is enabled.
Selected/Designated Speed – The speed that is set in the speed window of the MCP.
Take Off/Go Around (TO/GA) – Takeoff Go-around command mode. This mode is engaged during takeoff roll by depressing one of two buttons beneath the throttle levers.
Vref – Landing reference speed.