As usual jetlagged
post is very accurate.
On the classics there are two types of A/T: Approach A/T System & Full Flight Regime A/T System.
Approach A/T moves the thrust levers to acquire/maintain the preselected airspeed
, only during approach & landing phases. Whenever more than one autopilot channel is engaged it will also retard the levers at 30 ft AGL
|Quoting jetlagged (Reply 1):|
These were not very sophisticated and held airspeed to the value set on the MCP.
..the selection range is 101 to 259 knots. When the selected speed is more than 259 the first two digits are covered and only the third one remains. However the Command Airspeed Bug in the ASI
will continue to indicate until the upper stop (395 kt)
No protection whatsoever is provided by this system. Engine/airframe limitations have to be carefully monitored.
On airframes equipped with FFRATS things are a bit different.
The A/T acquire/maintain a computed max EPR
, selected airspeed
or a selected Mach number
In this case the selected range of speeds is 100 to 400 knots and the system continuously calculates the minimum safe airspeed based on the existing flap position.
On the ground, selection of Mach & speed mode is inhibited and only EPR mode is available. prior to engagement, EPR Limit for take off should be selected from dry or wet rating (on engines -7Q wet rating is not available) and then derated if necessary.
The system drives all thrust levers forward until the engine with the highest EPR reaches the calculated limit, throttle hold activated at 80 knots and from that point on the A/T remains engaged but doesn't move the levers. (Which is why the F/E always retains positive control of the engines as he still have to trim/set TO
thrust on the other three engines, before reaching the 80 knots call out)
At the climb thrust call, A/T is set to climb and drive the levers to the climb EPR limit based on TAT.
Reaching cruise altitude, cruise mode is selected until the airplane accelerates to the mach number calculated. In cruise the A/T can be used in Mach or Airspeed mode.
Speed mode is normally used for descent, approach/landing & holding.
Once G/S is captured and flaps are in beyond 23 degrees G/A mode (EPR limit) is armed and the system will automatically retard the levers at 30 ft during the flare (Flare Mode) and disengage when the thrust reversers are deployed.
Switches in levers 1 & 4 are for disengage the A/T and in levers 2 & 3 to activate the G/A Mode (if it was armed)
On airplanes with PMS, the A/T + PMS + A/P combo provides pitch/throttle control with improved fuel efficiency for the operation. (too bad we can't use PMS vertical mode anymore)
A truly remarkable design although very outdated by today standards. Nevertheless, a joy to use. (When it works properly!)
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"