I don't think I fully understand the implications of CWS. I do understand the a/c will hold the attitude you put it in under CWS. However I would expect the same thing with normal manual steering if the air was perfectly smooth (which I realize it never is) and the a/c in trim. So I can see the advantage of CWS in bumpy air since the a/c will automatically try to maintain the desired attitude without pilot intervention, but are there other advantages I'm missing?

Thx,

Al

 

 

but are there other advantages I'm missing?

 

For what it's worth, I seldom use it (sim-wise, not a RW pilot on the 73).  To me, it's kinda like a poor man's autopilot, which I've used a few times when I was flying a plane with an inop autopilot, solo, and needed to take care of things that required two hands (folding charts, using an iPad EFB, writing things down, and so on).  I could see CWS being useful for being able to let go of the yoke in certain situations to be able to set things yourself.

 

Note the INOP sticker on the AP and my legs being used to isolate the stick - kinda like CWS...

 

In a two person crew environment, though, I'm not quite sure I see the purpose (and you'll note that the 777 - a later evolution - does not have CWS).  Perhaps it was an attempt to emulate some of Airbus's behaviors?  It's not entirely worthless, but it seems like it would be more helpful in a single-person environment.  With two crew members, I could just as easily ask the other crew member to take the controls, or change something for me.

 

It could be nice for holding a constant pitch climb/descent, or constant radius turn, however.

Perhaps it was an attempt to emulate some of Airbus's behaviors?

From what I understand, it's been around a lot longer than those Airbus behaviors, dating back to the 737-100 with the Sperry SP-77. I suppose with a primitive autopilot it might have been more useful.

I don't think I fully understand the implications of CWS. I do understand the a/c will hold the attitude you put it in under CWS. However I would expect the same thing with normal manual steering if the air was perfectly smooth (which I realize it never is) and the a/c in trim. So I can see the advantage of CWS in bumpy air since the a/c will automatically try to maintain the desired attitude without pilot intervention, but are there other advantages I'm missing?

Thx,

Al

If you release the wheel to neutral in a turn with autopilot off, the bank angle response depends on the aircraft's spiral stability. Only a plane with neutral spiral stability would maintain bank angle as it was. Most will either increase or decrease bank angle (spirally unstable and spirally stable respectively). As the change in bank angle is slow, spiral instability is not seen as dangerous. Usually it's an acceptable aerodynamic trade off for stable Dutch Roll characteristics (you don't want an unstable Dutch Roll motion). However if left unchecked a spirally unstable aircraft will enter a spiral dive eventually. Spirally stable aircraft will tend to slowly roll wings level.

 

There is no advantage to CWS as such. Though as it's available on the 737 a pilot can choose to use it if it's convenient. It's the mode the autopilot will enter if no other FD mode is active when you engage it. Generally the pilot will have a roll or pitch mode selected. CWS is a hangover from the days where autopilot modes were less sophisticated and the basic mode was attitude hold. There was usually a manual controller which commanded roll and pitch attitude changes and CWS is a way of doing this using the control wheel itself instead.

From what I understand, it's been around a lot longer than those Airbus behaviors, dating back to the 737-100 with the Sperry SP-77. I suppose with a primitive autopilot it might have been more useful.

Exactly. Before FBW came on the scene, the A300/A310 autopilot had CWS mode like the 737. In fact in the standard aircraft (which had no Mach Hold mode) CWS was the only way to maintain a constant Mach in the climb. I don't think it was annunciated as CWS, but that's how it behaved. While the Airbus FBW Normal Law acts rather like CWS it's much more sophisticated and is of course an autopilot off mode (aka "manual flight").

From what I understand, it's been around a lot longer than those Airbus behaviors, dating back to the 737-100 with the Sperry SP-77. I suppose with a primitive autopilot it might have been more useful.

 

Good point!  Didn't think that far back.

 

 

 

Exactly. Before FBW came on the scene, the A300/A310 autopilot had CWS mode like the 737. In fact in the standard aircraft (which had no Mach Hold mode) CWS was the only way to maintain a constant Mach in the climb). I don't think it's annunciated as CWS, but that's how it behaves. While the Airbus FBW Normal Law acts rather like CWS it's much more sophisticated and is of course an autopilot off mode (aka "manual flight").

 

Learned something new today.  Thanks!

http://www.pprune.org/tech-log/263397-practical-uses-cws.html

 

 

 

Gerasimos Stamatatos

CWS actually does have a benefit. It can be useful in certain situations. When I flew DC10s, we used CWS mainly when flying a circling approach. We would leave the AP in ALT hold and select CWS for the roll mode. In this configuration, I have circling mins set in altitude hold so all I have to worry about is roll while I keep my eyes outside during the manuever. A few guys did it in heading select, but it was much easier to do it in roll CWS. In the Gulfstreams, we call it TCS(touch control steering). It's useful when you have to make a sudden manuever. With TCS, you press and hold the TCS button on the yoke and steer/pitch as needed. Once you're done, release the button and the AP will resume what ever mode it was in. It will even correct back to course/heading. A good example would be this scenario. You are climbing in flight level change while above mach change over. You hit a thermal layer and suddenly you are slow/fast. The AP will aggressively pitch to correct to speed. This is why guys will fly vert speed or vnav while above mach change over. Flight level change sucks during this time. In this scenario, you can press the TCS/CWS and smoothly correct back to speed before re-entering FLCH. You could even just select vert speed and vert it back to speed also.