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Control wheel steering ...again

This has been discussed before, but it would be interesting to get a range of opinions. I would put the case for control wheel steering using the following arguments.

Firstly, control wheel steering or CWS, is an autopilot mode where the pilot takes control, through the yoke, and by keeping depressed the CWS button, changes the attitude of the aircraft – for example establishing a descent, and then hands control back to the autopilot by releasing the CWS button.

The following arguments might be suggested in favour of CWS:

In line with ‘three way passage of control’ philosophy. The US advanced training academies are keen on the three way passage of control ‘you have control – I have control – you have control’. Perhaps simplistic, but if the flight deck crew in AF447 had been hard wired this way where passing control is carefully formalised, and the person passing or taking control, carefully checks that it has occurred, one link in the error chain might have been broken. CWS combined with this philosophy means the pilot takes back control, and is part of the physical interface of changing the AP mode, not just by pushing buttons, with the risk of ‘finger trouble’.

Sometimes ATC uses the word ‘now’, for obvious threat reasons. Flight deck crews used to just pushing mode buttons, may be too slow to avoid the threat, while crews habitually using CWS might be quicker. This point might be less relevant in an ACAS/GWPS environment, but what about military/GA collision risk?

Reduced risk of ‘finger trouble’ – typically a descent would involve setting a new altitude bug, changing the attitude with CWS (the autopilot will typically show the VS the pilot is manoeuvring the aircraft to achieve), setting descent power (if no auto throttle), and then ‘handing back control’ to the A/P, confirming it is doing what is expected of it. The A/P would then carry out the new FL/Altitude capture. Seeing students, including revalidation students, make persistent A/P mode selection errors, suggests that over reliance on automatics is an issue.

CWS crew are mentally prepared to take control efficiently when there is an A/P trim or loss of control event. Unfortunately quite a few examples of LOC accidents where the crew tried to trouble shoot the automatics, rather than ‘firmly take back control’.

Having come up with these arguments I am told that hardly any airline uses CWS outside of the US, with the possible exception of Ryanair?

Enstone (EGTN), Oxford (EGTK)

My advantage as a student pilot on EuroGA is that I get a free pass at asking stupid questions, so here goes: What is the difference to just pressing the A/P disconnect button, taking control and then re-engage the AP as needed?

Novice pilot
EDVM Hildesheim

With the autopilot engaged, control wheel steering (CWS) allows the pilot to maneuver the aircraft without disengaging the autopilot.

To use control wheel steering, depress the CWS button on the yoke. This releases the autopilot servos and allows you to assume manual control while autopilot control functions are placed in a synchronization state. This means that when you release the CWS button, the autopilot will smoothly resume control of the aircraft and fly it to the lateral command you were using prior to engaging CWS.

The vertical command used by the autopilot will be the one existing when CWS is released.

EDLN, Germany

With AP disconnect you lose the AP mode you were in, with CWS not. You let the button go and the AP flies back to the original course and altitude.

Alexis wrote:

With AP disconnect you lose the AP mode you were in, with CWS not. You let the button go and the AP flies back to the original course and altitude.

Or stays there if you are in pitch and roll mode (I think in that mode – which I essentially never use – it’s much easier to make adjustments than controlling and then reconnecting at the right time)

With AP disconnect you lose the AP mode you were in, with CWS not. You let the button go and the AP flies back to the original course and altitude

I see. My mistake was thinking that the AP mode and settings are “saved”. Of course I have yet to see an autopilot for real.

Novice pilot
EDVM Hildesheim

Interestingly, I never use it. If I flew more OCTA on autopilot I might well.

EGTK Oxford

RobertL18C wrote:

…three way passage of control ‘you have control – I have control – you have control’.

In my time training and flying and instructing in the multi-pilot environment this is the first time I hear from the “three way” philosophy. And I even got a multi-pilot type rating in the USA and go to an American training provider twice per year (but to their training centre in the UK). To me it seems that the good old “two way” system of " you have control – I have control " is plainly sufficient for the job. Mind you, one could extend this to 5-way or 7-way and it wouldn’t get any safer…

So I can’t see how using the CWS button (which Honeywell calls “TCS” for (I believe) “touch control steering” in the planes I fly) to establish something akin to that “three way” analogy should improve safety. And, most importantly, it is not designed to be used in this way. My personal experience is that a system works best if one uses it the way it is supposed to be used. E.g. to change altitude, (in the system I mainly use) one is supposed to dial in the new altitude first which is confirmed by the pilot monitoring and (silently) the controller who can see it on his screen courtesy of mode Sierra. Next one selects the preferred vertical mode for the altitude change, which can be “pitch”, “vertical speed” or “flight level change” (i.e. constant airspeed/Mach number) and and appropriate rate and verifies that the mode anunciators show the selected option. And of course the autopilot does what it is supposed to do: change altitude.

If I would do all those settings whilst having the hand on the controls with the CWS/TCS button pressed – which is manual steering really – my attention would be divided between the tasks of changing autopilot mode and steering the aircraft and neither task would get the attention it requires. By our SOPs this would be forbidden anyway, because when flying manually one is not supposed to do anything else. So in that case I would have to command the pilot monitoring to change the modes on the autopilot while I fly in CWS/TCS mode, thereby introducing a third person between me and the autopilot, and take away his monitoring role from him.

In real life I do use the CWS/TCS button occasionally. Usually because under certain circumstances the autopilot can get into unpleasant oscillations, e.g. when intercepting a weak glideslope signal from a greater than usual distance or at an unusually high speed. Holding the TCS for a moment will usually dampen the oscillation. Sometimes the autopilot can be slow to capture a localiser – again usually from a larger than usual distance – so a little yank on the yoke with the button depressed will point it in the right direction. This almost always happens at Le Bourget when coming from the south east. They will vector you onto the localiser 25 miles out and an overshoot will take you right into the approaches of Charles de Gaulle. TCS always saves one’s day.

EDDS - Stuttgart

Thank you w_n, very interesting. On the three way passing of control, it was part of my two ATP check ride briefs (multi and single engine), so I assumed it may be institutionalised in the US. One ride was in Illinois, the other in Florida.

I was trying to come up with reasons why the designers had thought of CWS, and to understand whether any SOPs actually use it.

Have flown systems with speed mode, but not with FL change mode and auto throttle. You would dial in the new altitude as you pitch in CWS, but can understand why some SOPs might not allow this multi tasking.

Will check with some airline training captains and see if they have input to share.

Enstone (EGTN), Oxford (EGTK)

My advantage as a student pilot on EuroGA is that I get a free pass at asking stupid questions,

There are no stupid questions

What is the difference to just pressing the A/P disconnect button, taking control and then re-engage the AP as needed?

The CWS button disengages the autopilot servos so you can fly the plane, and upon its release it re-engages it but with the various target values restored to what they were when CWS was released – with some exceptions e.g. in HDG mode it will revert to the heading bug AFAIK and not to the new heading.

It’s quite a useful thing when you have to do something quickly. It is particularly useful in under-powered planes (all of light GA basically) when trying to reach a high altitude, near the aircraft ceiling, when CWS can be used to fine-tune the pitch attitude which will get you there.

Administrator
Shoreham EGKA, United Kingdom
27 Posts
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