George Neuner  <gneuner2@comcast.net> wrote:
+---------------
| GP lisper <spambait@CloudDancer.com> wrote:
| But you do adjust trim as you're descending.
| Trim control - the poor man's autopilot.
+---------------

Not really. You adjust trim to set airspeed [which you normally
decrease once or twice prior to landing, true]. But whether you
go up or down actually depends on the power setting. Trim just sets
the *airspeed* at which you'll descend or climb [depending on power].

+---------------
| With a real 3-axis autopilot (there are 2-axis models as well), it's
| typical for pilots to let it fly the approach ... if the autopilot is
| integrated with ILS, it can follow the ILS glide path right down to
| 50ft above the runway. Even ILS free, it's easy enough to program the
| descent glide from the marker.  
+---------------

Even more fun is full Category IIIc ILS Autoland, wherein the autopilot
controls the plane all the way to touchdown:

    http://en.wikipedia.org/wiki/Instrument_Landing_System#ILS_categories
    ...
    Category III C - A precision instrument approach and landing
      with no decision height and no runway visual range limitations.
      A Category III C system is capable of using an aircraft's autopilot
      to land the aircraft and can also provide guidance along the
      runway surface.

Also see:

    http://en.wikipedia.org/wiki/Autoland
    ...
    Autoland systems were designed to make landing possible in visibility
    too poor to permit any form of visual landing, although they can be
    used at any level of visibility. They are usually used when visibility
    is less than 600 meters RVR and/or in adverse weather conditions,
    although limitations do apply for most aircraft -- for example, for
    a B747-400 the limitations are a maximum headwind of 25 kts, a
    maximum tailwind of 10 kts, a maximum crosswind component of 25 kts,
    and a maximum crosswind with one engine inoperative of five knots.
    They may also include automatic braking to a full stop once the
    aircraft is on the ground, in conjunction with the autobrake system,
    and sometimes auto deployment of spoilers and thrust reversers.
    ...

There's extensive discussion later in that page on failure modes
during autoland.

And guess what?!? The Dutch Safety Board says that the Turkish Airlines
plane *was* on an autoland approach when the altimeter failed!

     Accidents
     On February 25, 2009, a Turkish Airlines Boeing 737-800 crashed
     about a mile (1500m) short of the runway at Amsterdam Schiphol
     Airport. The Dutch Safety Board published preliminary findings
     only one week after the crash, suggesting the autoland played a
     key role in downing the plane. According to the Flight Data
     Recorder, the airplane was on a full autoland approach at a
     height of 1950 ft / 595 m when the left Radio Altimeter suddenly
     misreported a height of -8 ft. The autoland system responded
     accordingly and configured the plane for touchdown, idling the
     engines. This made the plane lose speed and stall. When the
     flight crew received stall-warnings, they were already too low
     and too slow to pull up again. As a secondary factor, the Safety
     Board suggested the crew did not have a visual ground reference
     because of foggy conditions.

     The final investigation report will be published later this year.


-Rob

-----
Rob Warnock			<rpw3@rpw3.org>
627 26th Avenue			<URL:http://rpw3.org/>
San Mateo, CA 94403		(650)572-2607