Xjyuk

I am thinking of the driver take-over request in automated driving, when the system limits are reached and the back-up driver has to intervene / drive manually. Is there something similar in aviation, e.g. autopilot is engaged and has reached its system limits? Maybe you can give me some examples.

Edit: The reason why I am asking is, that there is a lot of concern about take-over situations in the automobile industry because cars are getting more and more automated. Since automation has a longer tradition in aviation than in automobile industry I assume there is a thing or two to learn from you guys. So basically I want to know what take-over situations there are and what is done to make take-over easy for the pilot (airplane or helicopter pilot).

I assume in your question the autopilot is engaged. When the auto-flight system reaches the limits of its capabilities it simply disconnects. On Boeing aircraft you get a

> AUTOPILOT DISC

Warning Message on the EICAS and the siren sounds. Now it is all manual flying for the pilots.

An example would be turbulence exceeding the auto-flight system capabilities to maintain the intended trajectory.

There are many possible reasons for the autopilot to disconnect. The details will depend on the aircraft, but here are the criteria for a Boeing 737 (source: 737 NG FCOMv2 4.20.2 Automatic Flight - System Description):

Autopilot Disengagement

The A/P automatically disengages when any of the following occurs:

• pushing either A/P disengage switch


• pushing either Takeoff/Go-around (TO/GA) switch with a single A/P
  engaged in CWS or CMD;
  
  
  • below 2000 feet RA or,
  
  
  • with flaps not up or,
  
  
  • G/S engaged
  
  


• [Option - A/P auto-disengages for TO/GA above 2000 feet RA]
  
  [Option - Honeywell -708 FCC and on]
  
  pushing either Takeoff/Go-around (TO/GA) switch with a single A/P
  engaged in CWS or CMD above 2000 feet RA with flaps not up or G/S
  engaged.


• [Option - Fail-Operational Autoland]
  
  pushing either TO/GA switch after touchdown with both A/Ps engaged in
  CMD (except with LAND 3 or LAND 2 annunciated)


• pushing either TO/GA switch after touchdown with both A/Ps engaged in
  CMD


• pushing an illuminated A/P ENGAGE switch


• pushing the A/P DISENGAGE bar down


• [Option - Fail-Operational Autoland] activating either pilot’s control wheel trim switch (except with LAND 3
  or LAND 2 annunciated)


• activating either pilot’s control wheel trim switch


• moving the STAB TRIM AUTOPILOT cutout switch to CUTOUT


• either left or right IRS system failure or FAULT light illuminated


• loss of electrical power or a sensor input which prevents proper operation
  of the engaged A/P and mode


• loss of respective hydraulic system pressure.

As Chris already said in his answer, the A/P disengagement will trigger a warning horn and also illuminate the Autopilot (A/P) Disengage Light (number 1 in the following image from the FCOM):

The pilot flying will immediately take over manual control of the yoke and fly the aircraft. This is typically not as time critical as in a car, where a small deviation from your lane on the highway could result in a collision within seconds. The only phase of flight where A/P disengagement is critical is during an Autoland, where the pilot flying is already fully focused on monitoring the A/P and is ready to take over at any moment.

Re-engagement of the autopilot is possible whenever the autopilot engagement criteria are fulfilled:

Each A/P can be engaged by pushing a separate CMD or CWS engage switch. A/P
engagement in CMD or CWS is inhibited unless both of the following
pilot–controlled conditions are met:

• no force is being applied to the control wheel


• the STAB TRIM AUTOPILOT cutout switch is at NORMAL.

There is also a system on some aircraft requiring some input from the flight crew after a set time, similar to car systems complaining when you drive with cruise control and lane assistant without touching the steering wheel. For example, on the Boeing 777 (source: 777 FCOMv2 15.20.11 Warning Systems - System Description):

Crew Alertness Monitor

The FMC continuously monitors switch action on the MCP, EFIS control panel, display select panel, CDUs and radio transmitter microphone switches. When a predefined time elapses after the last switch action was detected, the EICAS alert message PILOT RESPONSE is displayed.