Autopilot versus (Lat Nav + V Nav + Auto Throttle)

[shon7]

What is the difference between having the autopilot on and having the VNav, L Nav and Auto Throttle on? How are the two modes different?

[NWSRG]

I'm not a professional, but as far as I know...

LNAV and VNAV are Lateral and Vertical modes for the autopilot, driven from information stored in the flight management computer. In other words, the autopilot will follow the loaded flightplan, both in direction and altitude.

The other mode of autopilot operation is where a selected heading or altitude (entered by the pilot manually) is maintained / achieved by the autopilot.

Auto-thrust is independant of the auto-pilot, but again can be driven either by the pilot selecting a particular speed, which the auto-thrust then holds, or by taking information from the FMC. This allows a flight plan to be flown to ensure the aircraft reaches a certain point at the correct time. Also, auto-thrust can deliver selected speeds / thrusts in various sections of the journey, eg. automatically providing take-off power.

[timzsta]

On the 737 with VNAV engaged the autopilot will climb or descend the aircraft in accordance the profile selected on the CLB or DES page of the FMC. On reaching the altitude selected on the MCP (mode control panel) the aircraft will level off. In a climb or descent with VNAV engaged the auto-throttle will be commanded to set the appropriate power setting to achieve the correct profile as per the FMC. You can select best angle or best rate of climb speed, or enter your own.

The auto-throttle can also set a specified N1 setting during other stages of flight, including the take off roll and a go around. During the take off roll and on initiating a go aroudn this is done by pressing the TOGA buttons. The auto-throttle will then set the N1 as programmed in the FMC for take off or go around.

If LEVEL CHANGE is used for a climb then the auto-throttle will set max continous climb power and the aircraft will climb at the speed dialled into the MCP. Rate of climb will depend on the excess thrust over that required to climb at a given speed. In a descent if LEVEL CHANGE is engaged the auto-throttle retards the thust levers to idle and the autopilot will maintain the speed selected on the MCP by adjusting the aircrafts pitch.

The final vertical mode is VS (vertical speed). Here the VS required and the airpseed required are both dialled up on the MCP and VS is engaged. The aircraft will then climb at the commanded airspeed and vertical speed - say 230kts and 2000fpm during the early part of a SID.

[jtr]

Typical Navigation modes
LNAV, VNAV, HDG SEL, ALT HOLD, V/S

These all send the FD's around the PFD trying to get the pilot, or Autopilot to follow them, and achieve the desired result. They either come from the FMC's or as a result of inputs to the MCP (the numbers and dials on the dash)

Typical flight control methods
Autopilot, manual flight

Simply put, the autopilot can generally only respond to commands from either the FMC, or the MCP. Net result is that the FD's must be on for the AP to be on.
The pilot (manual flight) can respond to commands from the FMC, or the MCP, or can disregard them, or turn them off (FD off), and do his/her own thing

Typical thrust control methods
Autothrottle, manual thrust

Autothrottle gets commands from the FMC wrt speed control
Manual thrust, pilot controls the thrust setting.

You can have almost any combination of the above, depending on A/C type.

Bear in mind there are a lot of generalisations in the above, and a very simple answer to your question.