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Friday
Nov062015

Control Wheel Steering (CWS) Explained

CWS is the acumen for Control Wheel Steering.  Broadly speaking, it is a sub-set of the autopilot (A/P) system which can used on either System A or B.  When engaged, CWS maneuvers the aircraft in response to control pressures applied to the control wheel or column.

LEFT:  B737 Mode Control Panel (MCP) showing location of CWS buttons on Collins unit .  The CMD and CWS buttons are located on the First Officer side of the MCP.  Each of the four press to engage buttons has a green annunciator which illuminates when the mode is engaged.

The control pressure is similar to that required for manual flight. When control pressure is released, the autopilot holds the existing attitude until CWS is disengaged, or the autopilot is engaged. 

The Flight Crew Training Manual (FCTM) states:

‘Control Wheel Steering (CWS) may be used to reduce pilot workload. Follow the manually flown procedure but instead of disengaging the autopilot, engage CWS.’

CWS is a similar system to the ‘Fly By Wire’ system utilized by Airbus.

CWS Benefits

The obvious advantage in using CWS is that you do not have to continually apply positive pressure on the yoke and control column to maintain a set pitch or roll.  The control pressures on the flight controls are in the order of 37 pounds push/pull value +- 3 pounds and continually applying this pressure for a protracted period of time can be tiring.

Additionally, CWS enables you to fly the aircraft using the flight controls, rather than turning the heading knob on the Mode Control Panel (MCP) or configuring other automated processes such as Level Change, Vertical Speed, VNAV, etc.  Being able to ‘feel’ the control surfaces through the yoke and column has obvious benefits that flying using the MCP cannot convey.

CWS is also advantageous when flying in turbulent conditions (additional information below) as it results in smoother transitions than when the autopilot is used.  CWS also allows for greater control of the aircraft when performing touch and goes and circuits at lower altitudes.

Practical Example

CWS is often used during the climb to altitude, with the A/P being engaged at 10,000 feet.  

LEFT: PFD with CWS engaged during climb following flaps retraction.  FMA displays CWS R & CWS P, vertical speed is 2650 and pitch mode is V/S after changing from TOGA thrust following climb out.  Pitch and roll follows the FD bars and speed is 240 with altitude set to flight level 20900.  If CWS remains engaged, the aircraft will continue at this attitude.  Of importance is that the airspeed is NOT protected in certain modes.  In other words, if the attitude is altered, the airs[peed may increase or decrease accordingly without autothrottle intervention.

Following rotation, the Flight Director (FD) bars will be followed maintaining V2+15/20 until Acceleration Height (AH) is reached.  At AH, the MCP speed will be increased to climb speed, or to a constraint if required by Air Traffic Control.  As airspeed increases the flaps will be retracted.  When the flaps are retracted, the control column will be placed in a position that correlates to the Flight Director bars and CWS A or B will be engaged – the attitude of the aircraft will now be fixed.  

The aircraft, in TOGA thrust, will maintain the established pitch as it ascends to the altitude set on the MCP.  TOGA thrust is speed protected; therefore, as long as the FD bars are followed there will not be a speed incursion.  If a roll mode is selected, the navigational data provided by this mode is also supplied to the Flight Director.  Once the desired altitude has been reached, LNAV / VNAV is engaged.

Whether a flight crew used CWS is personal preference and some flight crews use it regularly while others have never used it.

Turbulence (autopilot or CWS)

The Flight Crew Training Manual (FCTM) states:

‘That during times of turbulence the A/P system (CMD A/B) should be disengaged.’

The reason that CWS is beneficial when flying in turbulence is that the A/P (unless it was engaged in CWS) is attempting to maintain an attitude (pitch) that is based upon a predefined barometric pocket of air that is at your altitude or flight level.   In severe turbulence, this pocket of air may not be stable which will result in the autopilot seeking to change altitude or, at its worse disconnecting.

In turbulence, this is exactly what you want to try and avoid and is the reason engaging CWS is important.  If CWS is engaged, it will maintain an attitude rather than the A/P attempting to match the attitude to the possible changing barometric pressure.

Flight Crew Training Manuals differ in their content as each manual has been written with a particular airline in mind.  Many virtual flyers duplicate the procedures followed by Ryanair.  Documentation for Ryanair is relatively easy to find and the policy of this airline is reasonably conservative.  As such, I have transcribed from the Ryanair FCTM the segment on the use of CWS during turbulence.

The Ryanair FCTM states:

‘Flight through severe turbulence should be avoided, if possible.  When flying at 30,000 feet or higher, it is not advisable to avoid a turbulent area by climbing over it unless it is obvious that it can be overflown well in the clear.  For turbulence of the same intensity, greater buffet margins are achieved by flying the recommended speeds at reduced altitudes.  Selection of the autopilot Control Wheel Steering (CWS) is recommended for operation in severe turbulence’.

The recommended Ryanair procedures for flight in severe turbulence are:

•    Do not use Altitude Hold (ALT HLD) mode.

•    Airspeed - Target airspeed should be approximately 280 KIAS or 0.76 MACH, whichever is lower.

•    Severe turbulence will cause large and often rapid variations in indicated airspeed.  Do not chase the airspeed.

•    Yaw Damper – Engaged.

•    Autopilot - Optional - If the autopilot is engaged, use CWS position, do not use ALT HLD mode.

•    Autothrottle – Disengage.

•    Attitude - Maintain wings level and the desired pitch attitude. Use the attitude indicator as the primary instrument. In extreme drafts, large attitude changes may occur.  Do not use sudden large control inputs.  After establishing the trim setting for penetration speed, do not change the stabilizer trim.

Technical Data (general)

The Mode Control Panel (MCP) has two CWS buttons located on the First Officer side of the MCP beside the CMD A and CMD B buttons.  Like the autopilot, CWS has a redundancy system (system A or system B).  By default CWS A and CWS B buttons are off and must be depressed to engage either system.  

When engaged the annunciator will illuminate green and the Flight Mode Annunciator (FMA) on the Primary Flight Display (PFD) will annunciate CWS P and CWS R.

CWS can only be engaged when there is no pressure on the flight controls.  Therefore; the method to engage the system is to depress one of the two CWS buttons and then position the flight controls where you want them.  The system will then maintain the pitch/roll attitude until either the flight controls are moved, the autopilot is engaged (CMD A or CMD B), or CWS is disengaged by depressing the CWS button.

CWS cannot be engaged when any of the following conditions are met:

•    Below 400 feet.
•    Below 150 feet RA with the landing gear in the down position.
•    After VOR capture with TAS 250 kt or less.
•    After LOC capture in the APP mode.

The Flight Crew Training Manual states:

‘After autopilot engagement, the airplane may be manoeuvred using the control wheel steering (CWS) pitch mode, roll mode, or both using the control wheel and column. Manual inputs by the pilot using CWS are the same as those required for manual flight. Climbs and descents may be made using CWS pitch while the roll mode is in HDG SEL, LNAV or VOR/LOC. Autopilot system feel control is designed to simulate control input resistance similar to manual flight.

Methods of Operation

There are three main methods of operation; however, the detail can blend and easily become confusing.  The following information has been edited from documentation acquired from Smart Cockpit Airline Training.

1:  CWS A/B Engaged or CWS A/B Annunciator Illuminated.

•    Depressing the CWS A/B button on the MCP engages the A/P pitch and roll axes in the CWS mode and displays CWS P and CWS R on the FMA on both the Captain and First officer Primary Flight Displays (PFD).

•    With CWS engaged, the A/P maneuvers the aircraft in response to control pressures applied to the control wheel or column. The control pressure is similar to that required for manual flight. When control pressure is released, the A/P holds existing attitude.
•    Smartcockpit.com BOEING 737 SYSTEMS REVIEW Page 4
•    If column pressure is released with bank angle 6 degrees or less, the A/P rolls the wings level and holds existing heading. This feature is inhibited when any of the following conditions are met:

1.    Below 150 ft RA with the landing gear down.
2.    After F/D VOR capture with TAS 250 kt or less.
3.    After F/D LOC capture in the APP mode.

2:  CWS Pitch on FMA in CMD A/B or CMD A/B Annunciator Illuminated.

The pitch axis engages in CWS while the roll axis is in CMD when:

•    A/P pitch has been manually overridden with control column force. The force required for override is greater than normal CWS control column force. This manual pitch override is inhibited in the APP mode with both A/Ps engaged.

•    A command pitch mode has not been selected or was deselected.

•    Command pitch modes can then be selected and CWS P is annunciated on the Flight Mode Annunciators while this mode is engaged.

•    When approaching a selected altitude in CWS P with the A/P in CMD, CWS P changes to ALT ACQ and, when at the selected altitude, ALT HOLD engages.

•    If pitch is manually overridden while in ALT HOLD at the selected altitude, ALT HOLD changes to CWS R If control force is released within 250 ft of the selected altitude, CWS P changes to ALT ACQ and the A/P returns to the selected altitude and ALT HOLD engages.  If the elevator force is held until more than 250 ft from the selected altitude, pitch remains in CWS PITCH.

3:  CWS Roll on FMA in CMD A/B or CMD A/B Annunciator Illuminated.

The roll axis engages in CWS while the pitch axis is in CMD when:

•    A/P pitch has been manually overridden with control column force. The force required for override is greater than normal CWS control column force.  
•    A command roll mode has not been selected or was deselected.

•    Command roll modes can then be selected and CWS R is annunciated on the Flight Mode Annunciators while this mode is engaged.

•    CWS R with an A/P engaged in CMD can be used to capture a selected radio course while the VOR/LOC or APP mode is armed. Upon intercepting the radial or localizer, the F/D and A/P annunciation changes from CWS R to VOR/LOC engaged and the A/P tracks the selected course.

Final Call

The use of CWS is very much underused and underappreciated.  Although surface control loading in a simulator rarely matches that of a real aircraft, the use of CWS in a simulator environment can still have positive benefits equating to better aircraft handling, especially when flying circuits and flying in turbulence.

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Reader Comments (1)

Thanks for another great bit of info.

Yes - the CWS is really not getting the right attention is deserves.

The whole concept is brilliant. . . I wish we could have this on on the GA small birds :-)

All the best and keep up the good work.

December 3, 2015 | Unregistered CommenterIsaac Neuman
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