While I'm not a 737 pilot, I do know about the MCP pitch and roll modes, so I can answer as follows:
LNAV - is for lateral navigation, i.e a roll mode, and obviously cannot be used for vertical manoeuvres.
VNAV - can only be used if the FMC has been fully programmed, and a vertical profile has been calculated. This would be the most common mode used by airlines, and is typically engaged at 400ft after takeoff. Depends on company SOP's etc.
LVL CHG - Is typically used when ATC instruct an A/C to change to a different flight level, (climb or descent). It will use the current A/C speed, or the speed set on the MCP as the reference speed for completing the level change.
CWS- Is a degraded auto-pilot system. It is possible to make manual inputs using the control wheel and when the required roll/attitude is obtained you can release the wheel and the AP will hold that attitude until you make another input, or engage a CMD AP. I'm not aware of any UK airlines that use CWS as part of their SOP's.
I used to like CWS on the 737. It's like a super stable aeroplane- roll it into a 25 degree bank and it holds that bank and attitude perfectly. The 747 doesn't have it, and its use was not encouraged on our 737s. Don't know why.
Im not a pilot (at the moment ) but I believe that using VNAV will control the VS to climb to an alt and also watching the speed.
So say you were at FL160 at 300 kts. ATC tells you to climb to FL230, once 230 is displayed in the MCP and you have pressed VNAV again, the aircraft will keep the speed that is selected in the FMC, so lets say your in ECON. If the speed was to start reducing, the VNAV will lower the VS to enable the speed to pick back up to the ECON speed
VNAV SPD climb in ECON mode may feel like a smoother ride as it will use FMC information as well as inputs from air data computers etc to provide an optimum climb speed at rated climb power. This generally works out at about 2000-2500fpm up to about FL300.
LVL CHG is a good mode to use if required to make a more drastic change in altitude due ATC requirements (or stuffing up a descent!!) as it will fly the MCP-commanded speed at rated thrust to climb, or maintain the MCP speed with the thrust levers closed to descend.
Selecting LVL CHG during a VNAV SPD climb and selecting a lower speed in the MCP window (eg selecting 250kt during an optimal 295kt climb) will temporarily result in a large change in attitude as the autopliot tries to reach the commanded speed at normal climb power. Rates of climb can periodically go off the scale (ie >6000fpm) in extreme cases, therefore the climb will appear to be less smooth....
Otherwise I can't say offhand if it is any smoother over longer periods as the automatics are programmed to make pitch and roll changes within a specified (very low) G-tolerance.
7p3......<<Our airline doesn't have the VNAV function or autothrottles enabled and hence most of our autopilot climbs/descents are made in either VS (vertical speed) or MCP SPD (LVL CHG), or heaven help us, manual control!
Our FOM states that VS is preferred over LVL CHG because the autopilot "g" authority is lower in VS than in LVL CHG resulting in a smoother ride for the folks in back.>>
This needs investigating. I think it is fairly standard Boeing philosphy of the 737/767/747-400 era that VS mode will NOT give you minimum speed protection. So select a 1500 fpm climb and sit back and get on with something else, and the thing will maintain 1500 fpm whatever happens to speed. LVL CHG mode will hold current speed and let the climb rate fall- much safer.
B757 has no spd prot in Vs i.e. the autopilot will fly to the stick shaker or to Vmo. However, the 737 has loads of protection in VS mode, as long as the autothrottle and autopilot are engaged. In Vs, if you try and climb too fast, e.g. 4000fpm at high alt, the autothrottles will advance to attempt to achieve the demanded r.o.c. Speed will be traded off for height up to a certain point, and when speed decays to approaching minimum clean spd, then the VS mode will change to LVL CHG which opens the speed window on the MCP panel. The 737 will then maintain whatever ROC it can given max N1 thrust and the limit speed of min clean. The last resort is for the auto pilot to take over and pitch the nose down to maintain a margin above the stall. The 737 has excellent speed/stall protection. However, if the Aircraft has gone into Alt hold mode, it WILL fly to the stall if the throttles are closed, as the autopilot will prioritise holding an altitude against speed. This would only be possible if the autothrottles were also disengaged, which would be unusual under most circumstances.
Yes I do fly for SWA. I think and perhaps TR4A can correct me if I am, as is often the case, wrong. The autothrottles were not originally activated when we started the 737-300 program as the launch customer. The reasons were explained to me that coming from the 737-200 without FMS, LNAV, Autothrottles etc the jump to the -300 would be a big enough challenge without complicating our procedures. The systems differences coupled with new ops procedures would have been a big union issue as well.
I believe USAir was the launch customer for the -300. When we bought the -300 (almost ten years before I was hired) they wanted a common type rating and minimal differences. No glass, no LNAV, no VNAV, no Auto Throttles and no Auto Brakes. The philosophy was "Fly it like a 200". About 6 years ago we got LNAV.