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On some instrument approaches, the autopilot-coupled approach is not authorized or not authorized below a certain minimum. For example,

  1. Autopilot-coupled NA (MRY: ILS OR LOC RWY 10R) enter image description here
  2. Autopilot-coupled NA below a minimum (LVK: ILS RWY 25R) enter image description here

Moreover, on some approaches, the DA is lower if we have a AP/FD/HUD.

I guess the lower DA is because we can give more attention outside the window when there is AP/FD/HUD's assistance. I didn't come up with any idea or context why autopilot-coupling won't work.

If it is turbulent and the autopilot doesn't respond quickly enough to keep the needle centre, we may not use autopilot at all. If the air is calm, the autopilot may well outperform hand-flying. However, it depends on the quality of the autopilot.

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It's the fact the autopilot works more accurately than a pilot which is actually the cause of the restriction.

The decision usually comes from the certification body after the ILS inspection flight detected glideslope erratic variations. The variations are likely due to interference from the environment or the nearby activities with the runway equipment.

The ILS signal may vary quickly and if the autopilot were active it could react to the variations, while a pilot will not notice the interference, or will disregard it unless it follows a trend for a few seconds.

Using the erroneous signal from the ILS may divert the aircraft from the approach path. Being below the glideslope at a short distance of the threshold is indeed not desirable, especially for a jet aircraft for which a thrust increase is not instantaneous.

Example of glideslope reversal during an inspection:

enter image description here
(Source, page 15-97)

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  • $\begingroup$ I expect the autopilot will be enable to filter out the erroneous noise and take the average. I think the AHRS does the signal noise reduction. $\endgroup$ – skyoasis Mar 16 '16 at 7:07
  • $\begingroup$ @skyoasis: The AHRS has no ILS input. $\endgroup$ – mins Mar 16 '16 at 7:13
  • $\begingroup$ I mean since AHRS can a lot of variation filtering due to engine vibration, turbulence to make heading, pitch reading stable. In analogous to AHRS, the NAV receivers should do the same. $\endgroup$ – skyoasis Mar 16 '16 at 7:22
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    $\begingroup$ @skyoasis: Probably to some degree (albeit it should be confirmed by reading the regulation for ILS receivers), however this doesn't prevent incidents to occur, like at O'Hare in July 2012. A pilot would look at the PAPI if one is available, or to the vertical path of the navigational display, if the aircraft is fitted with one, and detect something inconsistent with the ILS path. $\endgroup$ – mins Mar 16 '16 at 7:45
  • $\begingroup$ @mins can you provide some authoritative evidence for this answer. $\endgroup$ – rbp Mar 16 '16 at 13:56

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