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DANZ
16th Feb 2002, 04:48
Ok, so what's the deal with CWS? What is it's purpose and how is it used? <img src="confused.gif" border="0">

Thanks guys!. .Dan

Checkboard
16th Feb 2002, 07:34
Have a look at the <a href="http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=get_topic&f=3&t=002385&p=" target="_blank">Control Wheel Steering</a> thread from August last year.

Willit Run
16th Feb 2002, 09:07
CWS kinda makes a joke of hand flying the airplane. The plane supposedly stays in the last position you, the pilot puts it in. It stabilizes the plane while, where as, if it were not on, like me, i would be flying all over the place. Think of it as a stabized raw data hand flying plane.. .Any more questions, ask someone who really knows whats going on!

caio for now!

Slasher
19th Feb 2002, 15:11
I only use CWS in severe turbulance. Works quite well in assisting to maintain a desired attitude (up to a point of course). Cant understand why the kids insist on using it for hand-flying in perfect conditions.

I speak about Boeings. Airbus aircraft are weird and probabley have CWS as the norm.

411A
19th Feb 2002, 19:09
CWS is approved for use in the Lockheed TriStar from brakes release at takeoff to the end of the landing roll (FAA operators, UKCAA slightly different) and indeed quite useful after takeoff if low level turns close in are required by the SID, especially in suspected wind sheer.

john_tullamarine
20th Feb 2002, 04:43
... can also be quite useful to reduce the pilot's mental workload when handflying a close-in, low vis, circling approach.

crackerjack
22nd Feb 2002, 04:26
One of the things I miss about flying a Fokker 50 (!) is that with CWS you didn't have to disconnect the autopilot when you got a TCAS RA.

Also if the flight director was 'lazy' you could give it a tweak while remaining on automatics.

Good piece of kit I recon.

mono
22nd Feb 2002, 16:01
Technically CWS is an autopilot sub mode (I am talking here about Boeing/late airbus here as I can't talk about other types). It is in essence attitude hold but where the pilot can make inputs into the autopilot via the yoke. A kind of fly by wire even for a/c which have a mechanical i/p to the control surfaces.

Sensors in the yoke (called force transducers) pick up any input force that the pilot may feed into the control column and transmit the resulting electrical signal to the relevant surface actuator to deflect it to produce the desired input. When the yoke is released the autopilot maintains the attitude at which the column was released. . .It is not TCS where the pressing of a button momentarily disengages the control surface servos and them re-engages them when the button is released.

On Boeings it is selected by either a mid detent on the a/p engage paddle or a middle button on the AFCS MCP.

Of course on Airbus it is in use all the time the pilot is 'manualy' flying as it is a FBW a/c.

As for when it is used? Don't ask me, I am only an engineer!

polzin
27th Feb 2002, 05:46
I used it in 737's when descending and trying to get a some what constant feet per minute. This was in Alaska where we often descended from FL350 without any level off because we were the only traffic.

On the L1011 I never used it because their we could select the feet per minute for the descent.

I was also told in Alaska that there are those who have and those who will, land the aircraft in CWS. Not that I did it of course.........

polzin
27th Feb 2002, 05:56
I used it in 737's when descending and trying to get a some what constant feet per minute. This was in Alaska where we often descended from FL350 without any level off because we were the only traffic.

On the L1011 I never used it because their we could select the feet per minute for the descent.

I was also told in Alaska that there are those who have and those who will, land the aircraft in CWS. Not that I did it of course.........

NorthernSky
27th Feb 2002, 15:23
Now, careful with CWS chaps...

On the F50 it's a means of re-datuming the FD, and provided that you have armed the altitude capture mode, you will still level off where you should.

In the Boeing, it takes away some workload, and thus reduces arousal. So, you're climbing or descending to cleared level, in CWS. As you approach the cleared level, you'll get all the usual indications (800ft to level chime/'c' chord) and light, but the aircraft will sail through the level unless you do something. There's a danger that you won't do anything because of the relatively low arousal. You'll know about this when you get the altitude alert passing 200/300ft the wrong side of the level. By then, of course, it's too late.

So, I don't use CWS other than in significant turbulence or if hand-flying and I need to put a chart away quickly or whatever. In the latter circumstances, I never leave it in longer than a few seconds. It's also handy when you've been hand-flying with raw data, and you're about to put the automatics back in, because engaging CWS legitimises use of the MCP without having to call for everything.

Stamatis
27th Feb 2002, 17:14
In a B737, when CWS is engaged, can the FD operate in any mode selected in he MCP?

As an example, can one fly in CWS AP mode and have the FD in FL CH and LNAV modes?

Stamatis

NorthernSky
27th Feb 2002, 21:02
Yes, the FD operates normally. The aircraft does NOT follow any FD commands whilst in CWS.

Cornish Jack
27th Feb 2002, 22:52
One further point re. CWS. On the L1011, it was able to capture and maintain the dialled-up Alt/FL. That being so, it became a useful tool in turbulence for holding patterns since it had a higher bank angle limit than the A/P - (from memory) 38 degs as against 25.. .It also, of course, was part responsible for the Everglades crash, coupled with the lack of Alt deviation warning.

VSB1976
12th Oct 2006, 05:09
Dear friends

While in CWS mode, when a pitch change is commanded by the pilot, does the autopilot will trim the horizontal stabilizer or just keep the elevator on the commanded position?

Thank you!

Best regards

Vicente

Alex Whittingham
12th Oct 2006, 06:41
We used to use the roll channel in CWS with the pitch channel in CMD and ALT when refuelling fighters. It gave us smooth roll control for the formation combined with a good height lock. Don't know if it is still an approved technique.

Denti
12th Oct 2006, 08:54
In the 737 Classic the aircraft will capture any MCP selectect altitude, level out and automaticly enter ALT HLD mode out of CWS. Additionally the AT will try to maintain the selected speed as much as possible so that CWS is a nice tool to limit your pitch if you plan to release the girls early (ever tried to push a trolly through the cabin at 12° pitch up?). Apart from that i normally don't use it, flying raw data in my company means you're flying completely manual except for AT which is supposed to be in ARM.

hetfield
12th Oct 2006, 09:04
CWS - must be like cybersex.

We don't use it on A306

Wodrick
12th Oct 2006, 12:35
Post #15 "Pitch Trim in CWS"

As far as I am concerned Pitch trim is part of basic Autopilot function in All modes.
The Autopilot always trims the Stabilizer to null the standing elevator deflection.
Not being one of the Chosen few L1011 experts, that may be a bit different due to the lack of elevators.

glhcarl
12th Oct 2006, 15:57
Post #15 "Pitch Trim in CWS"

As far as I am concerned Pitch trim is part of basic Autopilot function in All modes.
The Autopilot always trims the Stabilizer to null the standing elevator deflection.
Not being one of the Chosen few L1011 experts, that may be a bit different due to the lack of elevators.
The L-1011 has elevators, and I don't mean the galley lifts! The L-1011 elevators are attached to the aft spar of the horizontal stabilizer and are positioned by movement of the horizontal stabilizer. When the horizontal stabilizer is full up (+1 degree) the elevators are faired. When the hoirzontal stabilizer is full down (-14 degree) the elevators are full up (+25 degrees).

Intruder
12th Oct 2006, 19:00
Some versions of the 747-200 had CWS instead of the "Manual" autopilot mode. I've tried it a few times, but don't like it at all. I can trim it manually better with the autopilot off, and not get the undesirable feedback from the yoke when I try to turn.

I-2021
12th Oct 2006, 20:26
Now, careful with CWS chaps...
In the Boeing, it takes away some workload, and thus reduces arousal. So, you're climbing or descending to cleared level, in CWS. As you approach the cleared level, you'll get all the usual indications (800ft to level chime/'c' chord) and light, but the aircraft will sail through the level unless you do something.

Sure about that ? If the altitude has been selected in the MCP, the aircraft will capture the altitude and level off.

Wodrick
12th Oct 2006, 20:40
GHLCARL ok I stand corrected, always thought they were missing, did say I was not one of the chosen few !

VSB1976
3rd Nov 2006, 10:26
There is something about the L-1011 horizontal stabilizer that makes it unique: In the early design stages, the airlines told Lockheed they wanted the pilots to have more power and control over the pitch system, so that in case of a jet upset, the crew could pull the aircraft out of a dive.

Lockheed designed this extra power into the tail: The L-1011 is the only commercial jet that can take off with full nose down trim. This was actually demonstrated to the airlines during the flight testing phase - with full nose down trim (which is a mis-set trim setting but has happened in the past on other aircraft and caused accidents) the pilot is still able to rotate the aircraft and climb away - it takes a lot of control wheel force to do it but it can be done - in other aircraft, the control column can be pulled full aft but with full nose down trim the nose won't come up for rotation. For those who don't know: Jet upset was an early event that happened more in the 1950's and early 1960' with the commercial jets than we see today - via rough air, the aircraft would get knocked out of it's stablized cruise condition until it was diving towards the ground out of control - the pilots would attempt to pull back on the wheel to get the nose to come up but most of the jets of that era did not have the power to overcome the excessive speed and thus they could not pull the aircraft out of the dive - when the L-1011 was being designed, the airlines wanted to make sure the L-1011 had enough pitch force to do this should it happen - to this day, a TriStar has never been lost as a result of jet upset and not being able to pull out of the resulting dive.

Now on to the technical description of the L-1011 pitch control system. The L-1011 incorporates an all-flying horizontal stabilizer to control movement in the pitch axis. This "all flying tail" is unique in the commercial aircraft industry. First of all, pitch control on most airplanes are usually controlled by elevators - on the L-1011 however, the primary part of the tail that controls pitch is the front part called the horizontal stabilizer, the leading edge of which which move up and down - the elevators are attached to the rear spar of this movable stabilizer piece - the elevators do move, yes, but not under direct pilot control. The elevators move as a direct result of stabilizer movement only via a physical mechanical link: When the stabilizer moves it's nose downward, the elevators deflect upward to increase the camber of the entire tail - this causes a downward moment and thus pulls the entire rear portion of the aircraft in a downward direction - this causes the nose of the aircraft to pitch up. When the pilot pushes the control wheel forward, this causes the stabilizer nose to move up, causing the elevators to deflect downward, increasing the camber effect to cause a lifting moment and thus pull the tail of the aircraft upward and this in turn causes the nose to pitch down.

The L-1011 has four hydraulic systems - by contrast, the DC-10 and MD-11 only has three hydraulic systems. All four of the 1011 hydraulic systems power the horizontal stabilizer, any one of which can operate it. Not all of the flight controls are powered by all of the hydraulic systems - most are powered by three of the four and only the tail is powered by all four - this is because the pitch system is considered to be the most critical. Like all widebodied commercial jets, the flight controls are only powered by the hydraulics - there is no manual cable backup system - if you lose all the hydraulics to a given flight control, you lose that flight control.

--------------------------

Much ore on this fantastic bird at FlyTriStar (http://trijets.net/tristar/)

A37575
3rd Nov 2006, 10:56
Denti. flying raw data in my company means you're flying completely manual except for AT which is supposed to be in ARM

Extract from B737 FCTM re manual flying and A/T:
"To simplify thrust setting procedures, autothrottle use is recommended during take off and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged...The autothrottle arm mode is not normally recommended because its function can be confusing."

Cough
3rd Nov 2006, 12:52
CWS useful for me in 2 ways....

When deviating around minor patches of weather. Leave AP in CMD and de-select LNAV/HDG and move the aircraft where you want in roll. Remains in CMD in the pitch mode so you won't deviate from the assigned alt.

De-selecting LVL CHNG/VS in climb and descent (again A/P in CMD) then pitch up/down for whatever you want. Autopilot will still acquire the altitude selected in the MCP so again no level busts. Useful for bunting over/under weather/traffic.

Using the CWS mode completely...Why? We are pilots, fly it!

alexban
3rd Nov 2006, 15:16
If I'm not mistaken the plane will capture the selected altitude (on 737) when climbing or descending in CWS only if the selected mode for AP is CMD,even though you're climbing in CWS Pitch (for ex if you deselected the pitch mode). If the selected mode is CWS ,you'll pass through the selected altitude,as NorthernSky said.
I-2021,am I wrong?

yrvld
3rd Nov 2006, 17:20
If I'm not mistaken the plane will capture the selected altitude (on 737) when climbing or descending in CWS only if the selected mode for AP is CMD,even though you're climbing in CWS Pitch (for ex if you deselected the pitch mode). If the selected mode is CWS ,you'll pass through the selected altitude,as NorthernSky said.
I-2021,am I wrong?

I think you are right, Alex. It captures only if the paddle is in CMD, but you are actually in CWS PITCH by either deselecting the vertical mode or using some power on the yoke.
If the AP paddle is only halfway up, therefore in CWS, it'll not capture. I might be wrong, but I think that's the case.

Denti
3rd Nov 2006, 17:20
Extract from B737 FCTM re manual flying and A/T:
"To simplify thrust setting procedures, autothrottle use is recommended during take off and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged...The autothrottle arm mode is not normally recommended because its function can be confusing."

While that may be true the autothrottle arm mode provides alpha floor protection. In my experience it isn't confusing at all although different versions of the same aircraft (all -300) behave very much differently when approaching a selected altitude in manual flight with AT in arm (descending), according to our techies it's down to different MCP and FCC versions.

Anyway, it is SOP to leave the AT in arm mode, the rest as you quote it above.