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The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

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Journal Archive (Newest First)

Entries in Autothrottle (1)

Tuesday
Sep022014

B737 Autothrottle (A/T) - Normal and Non-Normal Operations

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

LEFT:  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 (click image to enlarge).

The A/T 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 A/P or A/T.

A search on aviation forums will uncover a plethora of comments concerning the use of the A/T which, combined with A/P use and non-normal procedures (conditions), can be easily be misconstrued.  An interesting discussion can be read here.

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

For those interested in revising the AFDS system in detail, I recommend perusing the Boeing B737 Automatic Systems Review.

When to Engage / Disengage the Autothrottle (A/T)

The A/T 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.  Likewise, during the takeoff roll the A/T can be engaged (usually after reaching 80 knots ground speed) by pressing the TO/GA buttons located under the thrust lever handles.  This will engage the A/T in the TO/GA command mode.  The main reason that TO/GA command mode is engaged to control the A/T during takeoff and climb is that it simplifies thrust procedures during a busy segment of the flight.

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.

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

A question commonly asked is: ‘When is the A/T disengaged and in what circumstances?’  Seemingly, like many aspects of flying the Boeing aircraft, there are several answers depending on who you speak to.

Ultimately, A/T use 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’. 

In the FCTM, Boeing recommends the A/T is only used when the A/P is engaged (the A/T and A/P are coupled).

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

My preference during a non-precision approach is to disconnect the A/T and A/P no later than between 1500 and 1000 feet, or for precision approaches at whatever height is stipulated on the approach chart.  It is not uncommon that a flight crew will manually fly an aircraft from 10,000 feet to landing using the ILS, VNAV and LNAV cues for guidance and situational awareness.  Many pilots enjoy hand-flying the aircraft during the approach phase.

When hand flying the aircraft with the A/T not engaged, it is very important to monitor the airspeed.  This is especially so when during the final approach as thrust can easily decay to a speed very close to stall speed. 

The Autothrottle is Designed to be used in Unison with the Autopilot

The A/T 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 A/P.  But, when the A/P is disengaged and the A/T retained, its reliability can be questionable.

Some crews believe that if a landing is carried out with the A/P off and the A/T engaged, and a fall in airspeed occurs, such as during the flare, then the A/T will engage to 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 A/T and A/P together are:

•    Speed is stabilized
•    Speed floor protection is maintained
•    Task loading is reduced
•   Flight crews can concentrate on visual manoeuvring and not have to be overly concerned with wind additives

The disadvantages of using the A/T without the A/P engaged are:

•    Additional crew workload and possible loss of situational awareness
•    Potential excessive and unexpected throttle movement caused by pith and attitude changes
•    Potential excessive airspeed when landing in windy conditions with gusts
•    The potential for pitch coupling to occur (discussed below)
•    A loss of thrust awareness (out of the loop)

Boeing 737 Design

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

LEFT:  B737 NG style thrust levers.

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

Pitch Coupling

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

If the pitch inputs are excessive, the A/T 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 A/T on, A/P off, and a brief wind change causes a reduction in airspeed. The A/T will slightly advance the throttles to maintain commanded speed. This in turn will cause the aircraft to pitch slightly upwards, triggering the A/T 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 decreasing pitch. As airspeed increases, the A/T 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 A/T thrust control.

Non-Normal Operations of the Autothrottle (speed button not engaged)

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 A/T in ARM mode, the solenoid A/T toggle is set to ARM, but the SPEED button located on the MCP is not selected (the annunciator is not illuminated).

LEFT:  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 not selected allowing thrust to be controlled manually.

Scenario - speed button not engaged during TO/GA

To facilitate a TO/GA, some flight crews arm the A/T but do not engage the speed option (speed button on MCP).  This allows a TO/GA to be accomplished more expediently and with less workload during a non-precision approach (the pilot flying only has to push the TO/GA button and the A/T will automatically become operational). 

If a Go-Around is not necessitated during the approach, the A/T solenoid switch is disengaged prior to landing, either by 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 manoeuvring 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).

A/T Speed Protection and Vref in Windy, Gusty and Turbulent Conditions

When using the A/T during an approach in windy conditions, to provide sufficient wind and gust protection, the command speed should be set to Vref +5 knots.  Variances above or below command speed will cause the A/T to either increase speed (if command speed falls) or conversely decrease speed.  The Vref +5 knots will provide a safety buffer to protect against these variances.   During turbulence, the A/T will maintain a thrust that is higher than necessary (an average) to maintain command speed (Vref).

If the A/T is disengaged or the speed option not engaged, minimum speed protection will be lost.

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

Manual Override - Engaging the Clutch Assembly

Occasionally, for any number of reasons, the flight crew may need to override the A/T automation. 

LEFT:  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 white and not grey.  Depressing this button will disengage the A/T and disconnect the A/T solenoid switch on the MCP.

The Boeing A/T is fitted with a clutch assembly that allows the flight crew to either advance or retard the thrust levers whilst the A/T is engaged.  Moving the thrust levers during automation will engage the clutch allowing the thrust to be controlled manually.

Whilst this style of flying is usually not recommended, the clutch is there to allow the A/T to be manually overridden, such as in an emergency or for immediate thrust control.

Most flight crews, will not use this override facility, preferring to dial into the speed window of the MCP an altered speed.

Currently ProSim737 does not support manual A/T override.

Simulation Nuances

This discussion relates to the flight dynamics of a real aircraft. Whether these traits present in a simulation depends on how well the flight controls are calibrated, the finesse of the avionics package used, and the accuracy of the aircraft flight model.

For example, depending on which avionics suite is used, the A/T may not maintain the speed selected in the MCP.  This is especially prevalent during extended turns such as during a procedure turn in windy conditions.  To counter against incorrect airspeed you must either, change the speed in the MCP speed window, disengage the A/T and manually alter thrust,  or press the speed annunciator on the MCP.  Pressing the speed annunciator will cause the throttle automation to be turned off; however, the A/T remains in the arm mode.

The last option is a good way to overcome this shortfall of not having manual override.  By disconnecting the speed option, the thrust levers can be jiggled forward or aft to adjust the airspeed.  When the turn has been completed with manual input to maintain the correct airspeed, the automation (A/T) can be engaged again by depressing the speed annunciation button.

It is important if the A/T is not engaged, or is in the ARM only 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 A/T and A/P not engaged is the ease that the aircraft manoeuvres.  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 A/T 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 company policies often dictate how a flight crew will fly an aircraft, and while some policies are expedient, more often they relate to cost savings for the company in question.

Policies vary concerning A/T use.  For example, Ryanair has a policy to disconnect the A/T and A/P simultaneously, as does Kenya Airways.  Air New Zealand and QANTAS have policies that encourage a crew to disengage the A/T and A/P by a certain altitude for non-precision approaches. 

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 A/T is engaged or disconnected at landing.  To simplify,

  • If the A/T is going to be disconnected before touchdown, the command speed should be adjusted to take into account winds and gusts (as discussed above and in ‘Crosswind Landings Part 2’).  It is vital that a flight crew monitor airspeed when the A/T is not engaged as during the approach the speed can decay close to stall speed.
  • If the A/T 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.

Final Call

There is little argument that the use of the A/T 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 A/T without the use of the A/P; pitch coupling, excessive vertical speed, and incorrect thrust can lead to hard landings and possible nose wheel collapse, unwanted ground effect, or a crash into terrain.

Ultimately, the decision to use or not use the A/T and A/P 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, as mentioned earlier, the recommendation made by Boeing preclude A/T use without the A/P being engaged.

Disclaimer

The content in this post has been proof read for accuracy; however, explaining procedures that are convolved and subjective can be challenging.  Errors on occasion present themselves. 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