Matt2218 wrote on Dec 14^th, 2009 at 12:25am:

Regarding what Richard said, I always make sure I'm below MLW so that may explain why I've been able to capture the localizer ok.   Even when I get the localizer, I'm still hit or miss on the glideslope...it seems like if I do anything out of the ordinary the AP forgets what it's supposed to do.

Please, will someone explain to me what the aircraft weight (at approach time) can possibly have to do with the inability of the AP to properly capture and fly an ILS? Even the old Sperry AP in this aircraft should be capable of intercepting and flying an ILS down to DH or lower irrespective of aircraft weight!

This old airplane does not have a FMS, fuel-burn calculators or any other sophisticated automation. Just an old Sperry autopilot and steam gauges. There is no way that this autopilot even "knows" what the aircraft weight is at any given point in a flight. How, then, could aircraft weight at any point in any flight have any effect whatsoever on autopilot operation?

Imagine taking off in a full-gross-weight 727-200, losing an engine, deciding to immediately land back at the departure airport and finding out that your AP doesn't work except at certain weights... the concept is absurd.

This AP has only 2 axes: pitch and roll. These are aerodynamic parameters, measured by spinning gyros, not weight parameters. Yes, I know that pitch angle to maintain any particular vertical flight path (level, climb, descent) at any given speed is dependent on weight, but the variations are small, fractions of degrees of pitch to 2-3 degrees or so, and even more telling, THAT IS COMPLETELY BESIDE THE POINT!

In approach mode (Auto/GS), this AP should be able to "see" the ILS needles come alive and anticipate (based on intercept angles, simple basic math) how soon BEFORE the actual intercept point it needs to begin turning or pitching to capture the correct path... mine never anticipates... it waits until the intercept point has come and gone, then makes gross overcorrections back toward the LOC and/or GS.

I submitted a ticket on this, and the response was "we do not experience this problem." This is not helpful. I am doing everything "by the book" as far as AP setup and aircraft configuration are concerned. I have read all 3 manuals cover-to-cover several times, and I am following the procedures there TO THE LETTER. Based on postings here and elsewhere, I am not the only one experiencing autopilot "wonkiness" on ILS's.

Since CS apparently won't or can't help, I am hoping that someone here on the forum can help me to understand what I am doing wrong, or at least what needs to be done to make the AP capture and fly an ILS. However, I am BAFFLED by the references to aircraft landing weight in connection with autopilot function.

Please, someone help me understand what aircraft weight has to do with AP function. This (or any other approach-capable) AP should capture and fly the needles much better than I can AT ANY AIRCRAFT WEIGHT INCLUDING 5000 LBS OVER GROSS TAKEOFF WEIGHT!

Thanks in advance to anyone who can help me to understand the bizarre concept of aircraft weight preventing correct AP function.