Course/heading?
This is an interesting thread. The AP is an area I never fully understood from my ATPL studies
Would somebody mind explaining the CWS (Control Wheel Steering?) function? Is it to allow hand flying whilst the AP takes care of trim and rudder movements etc.?
Thanks
0918
Would somebody mind explaining the CWS (Control Wheel Steering?) function? Is it to allow hand flying whilst the AP takes care of trim and rudder movements etc.?
Thanks
0918
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From my limited understanding of the 737NG autopilot system, CWS mode seems to be quite fly-by-wire in it operation.
There are two modes, CWSp (Control wheel steering PITCH) and CWSr (Control wheel steering ROLL).
By having the autopilot in the normal CMDa or CMDb setting and pitiching the aircraft up (or down) with the yoke, the autopilot will automatically switch to CWSp pmodel allowing you to pass pitch commands direct to the autopilot via the yoke, while the autopilot maintains the selected heading/course/speed. It auto stabilises the aircraft for thrust induced pitch.
The same applies to CWSp if you manually roll the aircraft.
As far as I am aware it's much like the Airbus system in that you command an attitude, and when you release the yoke, the autopilot (in CWS mode) will do it's best to maintain that attitude/rate of turn.
There are two modes, CWSp (Control wheel steering PITCH) and CWSr (Control wheel steering ROLL).
By having the autopilot in the normal CMDa or CMDb setting and pitiching the aircraft up (or down) with the yoke, the autopilot will automatically switch to CWSp pmodel allowing you to pass pitch commands direct to the autopilot via the yoke, while the autopilot maintains the selected heading/course/speed. It auto stabilises the aircraft for thrust induced pitch.
The same applies to CWSp if you manually roll the aircraft.
As far as I am aware it's much like the Airbus system in that you command an attitude, and when you release the yoke, the autopilot (in CWS mode) will do it's best to maintain that attitude/rate of turn.
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“Would somebody mind explaining the CWS (Control Wheel Steering?) function? Is it to allow hand flying whilst the AP takes care of trim and rudder movements etc.?”
Dealing with the last bit first....
The rudder pedals in the 737 are used for :
Other than that the rudder pedals are rarely used because there is no need. The yaw damper takes care of everything else :
CWS has no input to the rudder.
Moving right along to CWS (control wheel steering)....
You might wonder why you would want to use CWS. I wonder that too!
CWS might be useful if you wanted the airplane to hold an attitude, for example during a turbulence encounter.
There are several ways to get into CWS mode :
There are several ways to get out of CWS mode :
In CWS pitch mode the autopilot will adjust the stabiliser trim as required. If you use the control wheel thumb switch to trim the stabiliser whilst in CWS mode the autopilot will disengage and you will no longer be in CWS mode.
Please note that if you are in CWS roll mode with an angle of bank greater than 6 degrees the autopilot will maintain attitude (eg 10 degrees left wing down) but will not maintain any particular rate of turn (eg 3 degrees a second / rate 1).
If you’re having trouble trying to make sense of this stuff and wondering why anyone would design an airplane this way don’t be too put off.
The 737 is in many respects a very old airplane. Even on the latest NG versions much of the systems design work was done by drug-smoking hippies in the early to mid 1960s.
The 737NG was not so much “designed”, rather, it “evolved”.
Gordon Bethune did a fabulous job of managing the B737X project but freely admits (even boasts) that his famous “row 5 test” put the kybosh on lots of potential niceties.
An airplane designed fresh from a clean sheet of paper would look very different. Have you ever heard of a 777....
Dealing with the last bit first....
The rudder pedals in the 737 are used for :
- Tracking the runway centreline during take-off and landing; and,
- Crosswind landings; and,
- Flight with one engine inoperative; and,
- To “lead” turns during manual reversion (loss of hydraulic systems A and B).
Other than that the rudder pedals are rarely used because there is no need. The yaw damper takes care of everything else :
Either yaw damper is capable of providing dutch roll prevention, gust damping and turn coordination. Yaw damper operation does not result in rudder pedal movement.
[this text relates specifically to B737NG ie 600/700/800/900/BBJ – the yaw damper in the 300/400/500s is similar but not identical]
[this text relates specifically to B737NG ie 600/700/800/900/BBJ – the yaw damper in the 300/400/500s is similar but not identical]
Moving right along to CWS (control wheel steering)....
With CWS engaged, the autopilot maneuvers the airplane in response to control pressures applied by either pilot. The control pressure is similar to that required for manual flight. When control pressure is released, the autopilot holds existing attitude.
If aileron pressure is released with 6 degrees or less bank, the autopilot rolls the wings level and holds existing heading.
If aileron pressure is released with 6 degrees or less bank, the autopilot rolls the wings level and holds existing heading.
CWS might be useful if you wanted the airplane to hold an attitude, for example during a turbulence encounter.
There are several ways to get into CWS mode :
- Press CWS A or CWS B button on the mode control panel.
- Press CMD A or CMD B button on the mode control panel with no pitch mode or no roll mode selected.
- Deselect the selected mode (eg press HDG button while in HDG SEL mode will deselect HDG SEL mode and result in CWS roll if you were previously in CMD mode).
- Applying pressure to the control column will result in CWS pitch (if in CMD mode).
- Applying pressure to the control wheel will result in CWS roll (if in CMD mode).
There are several ways to get out of CWS mode :
- Select a valid mode (eg press HDG button will change the autopilot roll mode from CWS roll to HDG SEL).
- Disconnect the autopilot completely to fly the airplane manually.
In CWS pitch mode the autopilot will adjust the stabiliser trim as required. If you use the control wheel thumb switch to trim the stabiliser whilst in CWS mode the autopilot will disengage and you will no longer be in CWS mode.
Please note that if you are in CWS roll mode with an angle of bank greater than 6 degrees the autopilot will maintain attitude (eg 10 degrees left wing down) but will not maintain any particular rate of turn (eg 3 degrees a second / rate 1).
If you’re having trouble trying to make sense of this stuff and wondering why anyone would design an airplane this way don’t be too put off.
The 737 is in many respects a very old airplane. Even on the latest NG versions much of the systems design work was done by drug-smoking hippies in the early to mid 1960s.
The 737NG was not so much “designed”, rather, it “evolved”.
Gordon Bethune did a fabulous job of managing the B737X project but freely admits (even boasts) that his famous “row 5 test” put the kybosh on lots of potential niceties.
An airplane designed fresh from a clean sheet of paper would look very different. Have you ever heard of a 777....
