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Was wondering if ATC can detect if aircraft is being flown manually vs. autopilot and if that affects their clearances given out for surrounding aircraft?

More specifically, can they tell through ADSB signal or just by observation (holding flight level, turn rate, correction for wind, etc)?

I would imagine that in high winds/bad weather non-AP operation would need to be better isolated from other aircraft?

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    $\begingroup$ No, the autopilot state is not part of transponder (mode A/C/S) or ADS-B data. In what way would you suspect an ATC clearance might depend on whether an autopilot was in use? If you can elaborate on that part of the question I'll write up a full answer for you. $\endgroup$
    – TypeIA
    Apr 9, 2014 at 13:00

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No, the air traffic controller cannot see the whether the autopilot is used or not.

However, in the Mode S radar and ADS-B protocols there are messages defined that contain state of the autopilot. Currently there is no requirement that this data is provided by the aircraft, but some aircraft/transponders do provide the information nevertheless. The data is not displayed to the controller though.

In some control centres the aircraft's Mode Control Panel / Flight Control Unit Selected Altitude can be displayed to the air traffic controller. The information can be used to reduce Level Busts.

From observations of the radar track it is difficult to tell. A competent pilot will be able to maintain altitude and heading in such a way that it is difficult to see the difference from an autopilot at the resolution of the ATC display.

There are circumstances where the pilot is required to use the autopilot, especially to keep separation from other aircraft. In Reduced Vertical Separation Minima (RVSM) airspace the vertical separation is 1000 ft between Flight Level 290 and FL410. The aircraft must be flown using the autopilot in this airspace to ensure the flight levels are adhered to strictly.

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The autoPILOT is not for ATC's convenience. They neither know nor care if it's on, off, or trailing behind the airplane on it's cables. All ATC cares about is that the transponder works, and even that isn't a requirement.

ATC may suspect something is amiss if the aircraft is not holding heading or altitude very well, but if ATC notices this we are well into a major problem. Either there's a major malfunction onboard or the plane is no longer under the control of a competent pilot. Any pilot who passed their commercial can hold heading and altitude just as well as the box - I have zero formal instruction and I can hold a Cessna on course to the point where no one will notice.

In cases where the plane is fine but the crew has a major malfunction (like dying, à la Payne Stewart) then ATC will consider what mode the autopilot is likely in, as that will tell them where the plane will probably be when it runs out of fuel. (If it's turned off, the discussion will never get to this point on account of the plane already turning into a lawn dart)

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    $\begingroup$ A working transponder is a requirement in many airspace classes. Also, try manually holding altitude at 36,000 feet in a transonic jet aircraft for an hour. It's not easy at all even for a professional pilot. The autopilot will definitely do a better job. Finally, ATC is generally not educated about autopilot modes or operation. In a Payne Stewart situation, chances are they will be scrambling to find a qualified instructor to assist (unless, by good fortune, the controller happens to be an experienced pilot/instructor). $\endgroup$
    – TypeIA
    Apr 9, 2014 at 13:38
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    $\begingroup$ Throw turbulence into the mix and this all goes out the window. $\endgroup$
    – egid
    Apr 9, 2014 at 13:46
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    $\begingroup$ Holding altitude at FL360 manually can perhaps be done, but it is not allowed. Without operable autopilot, the aircraft is required to descend below FL290 (out of RVSM airspace). The ATC does not know, though; it's responsibility of the pilots. $\endgroup$
    – Jan Hudec
    Apr 10, 2014 at 7:19
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    $\begingroup$ So if my autopilot fails at FL360 over the Atlantic I have to descend to a denser altitude, burn more fuel and possibly ditch offshore when I run out? And I have to land with 300 passengers at a (probably remote) diversion field with no immigration facilities? Don't see either one happening, esp. after uttering the magic word "Mayday" and thus nullifying all those annoying regulations. $\endgroup$
    – paul
    Apr 10, 2014 at 10:49
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    $\begingroup$ @paul: If descending to a denser altitude will make you ditch offshore, you left home with too little fuel on board. What would you do with a pressurization failure at the same place? $\endgroup$
    – hmakholm left over Monica
    Apr 10, 2014 at 14:04

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