Practical uses for CWS

[Superpilot]

I'm currently studying Autoflight and have come across CWS (Control Wheel Steering). On a standard flight from A to B when and why would it be used?

[Sky Wave]

There is no use for CWS on a standard flight. I doubt there are any operators that ever use it.

SW

[Lost at fl345]

control wheel steering is the general mode of manual flying in the airbus

fly by wire family series as in the A300 its a choice..... im not sure if other

aircraft models are the same....:}

[Clandestino]

control wheel steering is the general mode of manual flying in the airbus fly by wire family series

There´s no control wheels on FBW airbi and there are even less CWS, I´m afraid. Methinks questions refers to Boeing´s CWS which is almost completely different compared to Airboos FBW.

[Re-entry]

Normally never used.

Only conceivable uses:-

1. In turbulence mode, A/P reverts to CWS.
2. To increase bank angle in a hold when command mode doesn't give enough.
3. To increase pitch angle when command mode in V/S max cannot contain the airspeed.

It just provides stabilisation to manual flight. Same as airbus in 'manual'.

[hetfield]

It's like having sex using a condom, I guess.

[Intruder]

Some of our 747-200s are equipped with CWS. I've tried it a few times just to see what it could do. I was not impressed. There was a significant perceivable lag in control response, compared with manual flight. If the airplane trims up well, there is little use for CWS.

[SiClick]

Flight with unreliable airspeed, you can use CWS to maintain the desired attitudes rather than doing it yourself

[airbond]

2 good uses for CWS,

in turbulance,

and for unrealible airspeed and/or altitude situations.

CWS and be used in the pitch mode or the hold mode or in both pitch and roll modes together.
Its a great thing, there to be used, if the situation warrents it.

[FCS Explorer]

no use! (after 5 years boeing)
only thing i like to do: off-set non-prec app using LNAV-VNAV:
disengage LNAV -> a/p reverts to CWS-roll: line up with centerline and keep vnav-FD-bar for vertical guidance.
but as i said: use of CWS is VERY limited.

[411A]

CWS.
Useful in the L1011.
On all L10 aircraft (except those that were on the British register, or those that were used by GF) CWS can be selected on the ground, and is useful for low level turns immediately after takeoff, where maximum allowed bank angle is desired, due to noise abatement procedures, or obstacles in the takeoff flight path.
Very smooth operation on the 'ole Lockheed tri-motor.

[teamilk&sugar]

Ummm....some rather interesting and wrong statements being made here!

airbond is on the money...

Boeings' recommended autopilot mode in severe turbulence is CWS when installed.

If you understood how it works compared to the autopilot, you would understand Boeings' FCTM statement.

...maybe get the books out guys and read a bit!

[Intruder]

For what airplane is that "Boeing" recommendation? In what document?

How many of us have ever flown in severe turbulence? I haven't. I suspect more THINK they have, than actually have done so... Also, in 8+ years in the 747, I have had to disconnect the autopilot only once because it could not hold attitude in [moderate] turbulence.

[teamilk&sugar]

For what airplane is that "Boeing" recommendation? In what document?

B757 & B767 and as far as I am aware, all Boeing types that have CWS installed.

Direct quote from the FCTM - Flight Crew Training Manual.

I have had to disconnect the autopilot only once because it could not hold attitude in [moderate] turbulence

...and that is exactly my point. Your autopilot (unless it was engaged in CWS) was not trying to maintain an "attitude" in the first place - it was trying to maintain a barometric pocket, ie: your altitude or flight level. In severe turbulence that is exactly what you want to try and avoid and why engaging CWS is so important as it will maintain an attitude instead.

[BAe 146-100]

B757 & B767 and as far as I am aware, all Boeing types that have CWS installed.

Quite a lot of 757s/767s in operation have blanking plates over the CWS, not sure if that is a delivery option or not.

146

[Kit d'Rection KG]

Well, I use it on the B737-NG for various purposes, often transitioning from raw data hand-flown to automatic flight, it allows me to select an autopilot and then programme the FDs (I could, of course, ask my co-pilot for the selections, but it's just a simple to do it oneself). Also, if hand-flying and needing a moment to check a chart, for example, or for flying visual circuits or circling approaches.

On light twins with more basic autopilots and flight directors, it's more useful, and has many purposes, but I guess the original question refers to 'heavy' aircraft.

[Futterman]

Hi all,

I've been told that if you hold the yoke in a position for more than 2 seconds, CWS will switch in and automatically override the A/P until you disengage it.

Also, I know of specific CWS switches on the 73NG MCP, but how do you select it on the 744?

Brian

[BEagle]

Useful in airliner-derivatives with split-axis autopilots during military AAR operations as it allows full manual control of the angle of bank and roll rate during turns on the AARA.

But more modern aircraft have smoother AFS response in HDG mode, so that is used by some operators - NAV mode giving little warning of the onset of a turn.

The A330MRTT may have a modified HDG mode to reduce rate of change of bank angle when entering the turn, approaching the desired bank angle, rolling out of the turn and approaching wings level again. This is intended to make the task of receiver pilots easier when remaining in contact on wing hoses in the turn.

In the VC10 tanker, we had the 'MAN' mode for the autopilot turn rate, this ensured that the pilot had direct control of the bank angle. But this feature wasn't fitted to later generation airliners - it being assumed that HDG or NAV would be sufficient.

[Old Smokey]

CWS had it's uses in earlier generation aircraft such as the A300-B4, which did not have a Mach Hold. CWS was useful to periodically adjust pitch attitude to maintain the desired Mach Number.

Other than that, on more modern types with almost every mode controllable through a basic or more advanced mode, I can't think of much use for it. If I had it, (maybe I do) I doubt that I could think of much practical application for it.

Useful in it's time, a bit out-moded now :ok:

Regards,

Old Smokey

[Ka8 Flyer]

The B744 doesn't have CWS, neither do most 757/767 (it used to be a company option). Don't know about the 777 or the 717 but doubt it. So the only "modern" Boeing that has CWS is the 737. There you can override the AP and force it into CWS P or CWS R by applying a specific force to the control wheel. That way you can have the AP keep altitude and have manual control the roll axis (or vice versa).
Note, the B744 and the B757/767's with updated MCP's and AP's will hold current ATTitude if the AP is engaged with no F/D mode active (ie. both FD's off). As has been stated, FBW Airbi don't have CWS - the "manual control" is basically something like CWS but it doesn't hold pitch but vertical acceleration when the stick is centered.

[A Very Civil Pilot]

The ATR72 has a sort of CWS mode as I seem to remember. I think the term was 'basic mode' when it was used. In an a/p climb, a second push on the IAS button, and it disconnects your pitch mode (and the pitch F/D bar disappears), leaving pitch controlled by the 'pitch trim wheel' (same as the v/s wheel in Boeings). Pitch can now be controlled by moving the wheel up and down. Tended to use it quite alot, as during the climb the a/p was not very good at holding the selected IAS, and tended to keep pitching up and down to catch it. Use basic mode to hold an attitude and let the speed settle down to roughly what it should be. Gives a smother ride for the pax

[command]

Control wheel steering, also known as touch control steering , i think its great. There is only one situation i have used it. most MDA's are not even or odd hundreds eg 700 800 etc... most are say, 740 870, and all alt alert selectors (well the ones ive seen) only round to the nearest hundred feet. so what i do is, just say the MDA was 860 ft, set 900 in the alt alert selector and then just use the TCS(cws) to go down to the MDA, then release it and the A/P will maintain the MDA. great if the WX is right on the Minima.:ok:

[xetroV]

In the MD-11, "manual flight" normally meant controlling the aircraft in roll-CWS and LSAS (longitudinal stability augmentation system, which, like pitch-CWS in the DC-10, maintains the pitch attitude when no force is exerted on the control column). And autothrottle engaged.

Certainly the strangest aircraft I know... and the most beautiful too. :)

[Clandestino]

W in CWS actually refers to control wheel, more widely known as yoke and not that small pitch thumb wheel on AP control panel. ATR´s TCS worked a bit differently compared to Boeing CWS. When you pressed and held it, it would disconnect AP servos, leaving you in manual control of the aircraft. When released, vertical AP mode (pitch, VS or IAS) would reset to current value.

Except for practice, I cant recall using it in my 2500 hrs on ATR. Also where I fly, level flight on MDA is a big no-no. We´re fans of continuous descents and VDPs.

[command]

hey clandestino,
you say level flight at the MDA is a big no no. I was just wondering what else can you do at MDA, after all the only thing you can do at MDA is fly level until you get visual or MAP. Even with a continuous descent profile you still need to level out at MDA if your still in IMC.:confused:

[Clandestino]

Actually I don´t. When we hit minimum it´s either we see the approach lights/runway or we don´t. If we don´t, we go-around. My company´s ops manual prohibits level flight at MDA.

[Shiny side down]

<you say level flight at the MDA is a big no no. I was just wondering what else can you do at MDA, after all the only thing you can do at MDA is fly level until you get visual or MAP. Even with a continuous descent profile you still need to level out at MDA if your still in IMC>

It's something practiced in the airlines I have worked for, but not in corporate.

NPA vertical profile is flown as per the chart, but with a corrected MDA (+50ft). There is no level segment until MAP. Company policy overides this to either go miss approach at cMDA, and carry out the missed approach procedure, or be visual and land.

One argument is that this avoids having a heavy jet manoeuvring at low level. Maybe European airspace makes this a more attractive option, for safety/congestion/noise/comfort factors

In corporate, in a light jet, we just flew along until the MAP, as per published procedure.

[command]

Actually I don´t. When we hit minimum it´s either we see the approach lights/runway or we don´t. If we don´t, we go-around. My company´s ops manual prohibits level flight at MDA.

Interesting, so do you sort of treat it like a DA, you get to it and if not visual then commence Missed approach (obviously without going below it). Also do you plan the continuous descent profile so as to get to MDA at the MAP point or some distance before the RWY that would just allow you to make your final approach.
Do you do circling, and what type of aircraft are you flying.

Sorry to ask so many questions, just interested.:)

[A Very Civil Pilot]

Command

Generally for a non-precision approach we plan a constant descent 3 degree approach, using the DME/altitude points as a reference, aiming to hit MDA at the MAPt. From there it's either continue visually or go-around - no level flight.

The only time you would continue with level flight is for a circling approach. However you would be visual by then, otherwise it is a go-around. This has been for all the UK companies I've flown with, in both turbo-prop and 737.

Haven't done a level flight MDA since flying a light twin.

[ahramin]

A Very Civil Pilot:

What guidance do you have for the three degree slope? Do you have some kind of psuedo Glideslope on your screen?

When you get to cMDA, how close are you usually to where you planned on being? I am curious if a big headwind combined with rough weather could have you end up at cMDA a mile too soon. I assume it would be missed approach anyway?

command:

More information on Constant Descent Angle NPAs: http://www.tc.gc.ca/CivilAviation/commerce/circulars/AC0238.htm

[A Very Civil Pilot]

ahramin

Just a simple 3 x table, and the DME/ht checks on the approach plate. A 3-degree slope equates roughly to 330' per nm.

Desc rate is calculated from the groundspeed / rate of descent table, also on the app plate. If your ground speed is 120kt, ROD may be about 650' fpm. Lower ground speed due to head wind, ROD may be 550-600' fpm.

The idea is to get to MDA and the MAPt at the same time.

[ahramin]

Thanks. So do you typically get to MDA and MAP at the same time? What is the error usually?

My FMS will draw me a g/s to whatever slope i want. I am currently rewriting our procedures to take advantage of this fact. I am surprised that you do a constant descent angle approach using only numbers in your head, but if it works for you guys, i can't argue with success.

[A Very Civil Pilot]

You go-around at whichever comes first. If your ROD is slightly greater than needed, you'll be at MDA before the MAPt, so go around (as we don't do the level flight bit). Same deal if it's the other way round, and you reach the MAPt at some height above MDA.

This does all assume that you are not visual. Had to do one the other day, and came out at MDA showing 3 reds on the PAPIs i.e we were a mile or so short. But as we were then visual, we could continue level for a bit to get back on the profile. The three other a/c that followed us in all came out to MDA at a similar position, so it seems that it wasn't my cr@p flying that got us there!

[captjns]

I have not flown the old -100 or -200 series in some 16 years which had the SP-77 autopilot system. Aileron and Pitch engagement paddles. Now correct me if I'm wrong, but I seem to recall that in the loss of hydraulics, you could still use the pitch channel in CWS and the trim motor would actuate as approriate to relieve pitch forces. Like I said its been many years and I don't have my -200 manuals with me.

[ahramin]

A Very Civil Pilot:

One more question on CDA MDAs, where is your MAPt? Obviously it is not the threshold where most MAPts begin. How do you select your MAPt?

[whogivesa????]

CDPs seem to becoming the norm for most operators of larger turboprops and jet equipment in Australia.

[A Very Civil Pilot]

The MAPt point is the one that is on the approach plate. The approach is devised so that no matter what height you are at the end of the approach, or how long you've been at that height, you go-around at a specific point - DME distance, overhead a beacon etc. It's usually marked on there somewhere as MAPt.

[ahramin]

A Very Civil Pilot:
"If your ROD is slightly greater than needed, you'll be at MDA before the MAPt"
"The MAPt point is the one that is on the approach plate."

So when planning your rate of descent, you are aiming to be at MDA+50 feet over the MAPt??? Is this correct?

I must be missing something here. In Canada, most of our MAPts are at the runway threshold. Arriving at this point at MDA+50 is a sure way to require a MA. Are your MAPts a mile or two back from the runway? What would you do if the MAPt was at the runway?

[Fokker28]

For what it's worth, in the CRJ I am leaving, we still perform the 'dive and drive' style of non-precision approach. That is to say, at each step down, we descend, and if applicable, level-off until the next step down. We descend so as to be level at MDA well ahead of the MAP, so as to be as low as permitted at the VDP. In the Q400 into which I am moving, we perform coupled VNAV non-precision approaches, which allow us to treat the MDA as a DA, including allowing us to descend slightly below in the course of transitioning to a missed approach.

[Empty Cruise]

arahamin,

Not quite - we aim to follow the procedure design gradient, typically 5,2% or thereabouts, in order to pop out on the required visual approach slope. The only reason we fly CANPA is to achieve a stabilised approach, and if we were aiming for MDA at the MAPt we'd typically be way too high, in which case we might as well dive'n'drive.

The idea is that if we follow the procdure design gradient all the way down to MDA - typically plus some increment - and either pop out with 2 white+ 2 red or initiate the GA. In respect of initiating the go-around, the MAPt is irrelevant, we only use it as a limit for when we may initiate any turns.

Empty

[A Very Civil Pilot]

ahramin

Perhaps I didn't explain it too clearly, but empty cruise seems to have cleared up the confusion. I was working on the assumption that the MDA and MAPt would coincide at a point on your nominal 3 degree glideslope. In reality you're going to reach MDA before the MAPt point. If, as you say in
Canada, the MAPt is the threshold, you're not aiming to get to MDA, of say 500' agl, at that point and try to pull off a succsesful landing from there!

[Kit d'Rection KG]

Fokker 28,

You wrote allow us to treat the MDA as a DA, including allowing us to descend slightly below in the course of transitioning to a missed approach.

Which authority, I wonder, is content for you not to apply an increment to protect against descent below the obstacle clearance altitude (or system minimum)? Are you sure that you won't be using a Company Decision Altitude, slightly above the MDA?

[gimmesumviagra]

In my humble opinion:

1. A 3 degree slope approx 320ft/NM which we expect our pilots to calculate based on DME (VOR or ILS) distance to touchdown;

2. We use MDA + 50ft as our Visual Descent Point (VDP) equiv to DA. At this altitude, if insufficient visual cues, commence GoAround, climbing & TRACK to appropriate turning point on the published Missed Approach Procedure.

This way, if you get visual you will be in the slot with no pitch changes required for the continuation to landing. This also complies with the FSF Stabilised Approach Criteria. (wheras Dive&Drive may not).