How to turn on/off airplane mode, even on new Android versions (and even with root)?

Starting with Android 4.2 , turning on/off airplane mode isn't supported using normal APIs.

It should probably work when WRITE_SECURE_SETTINGS permission is granted, but that's only for system apps (as I've read).

What should be done in order to do it on devices with root?

Should a system app also require root in order to toggle airplane mode?

To toggle Airplane / Flight mode on and off on an Android rooted device (phone, tablet, note), you can do the following:

private final String COMMAND_FLIGHT_MODE_1 = "settings put global airplane_mode_on";
private final String COMMAND_FLIGHT_MODE_2 = "am broadcast -a android.intent.action.AIRPLANE_MODE --ez state";

@SuppressLint("NewApi")
@SuppressWarnings("deprecation")
public void setFlightMode(Context context) {
    if (Build.VERSION.SDK_INT > Build.VERSION_CODES.JELLY_BEAN) {
        // API 17 onwards.
        if (isRooted(context)) {            
            int enabled = isFlightModeEnabled(context) ? 0 : 1;
            // Set Airplane / Flight mode using su commands.
            String command = COMMAND_FLIGHT_MODE_1 + " " + enabled;
            executeCommandWithoutWait(context, "-c", command);
            command = COMMAND_FLIGHT_MODE_2 + " " + enabled;
            executeCommandWithoutWait(context, "-c", command);
        } else {                
            try {
               // No root permission, just show Airplane / Flight mode setting screen.
               Intent intent = new Intent(Settings.ACTION_AIRPLANE_MODE_SETTINGS);
               intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
               context.startActivity(intent);
            } catch (ActivityNotFoundException e) {
               Log.e(TAG, "Setting screen not found due to: " + e.fillInStackTrace());
            }
        }
    } else {
        // API 16 and earlier.
        boolean enabled = isFlightModeEnabled(context);
        Settings.System.putInt(context.getContentResolver(), Settings.System.AIRPLANE_MODE_ON, enabled ? 0 : 1);
        Intent intent = new Intent(Intent.ACTION_AIRPLANE_MODE_CHANGED);
        intent.putExtra("state", !enabled);
        context.sendBroadcast(intent);
    }

To check whether Airplane / Flight mode is already on and off, do the following:

@SuppressLint("NewApi")
@SuppressWarnings("deprecation")
private boolean isFlightModeEnabled(Context context) {
    boolean mode = false;
    if (Build.VERSION.SDK_INT > Build.VERSION_CODES.JELLY_BEAN) {
        // API 17 onwards 
        mode = Settings.Global.getInt(context.getContentResolver(), Settings.Global.AIRPLANE_MODE_ON, 0) == 1;
    } else {
        // API 16 and earlier.
        mode = Settings.System.getInt(context.getContentResolver(), Settings.System.AIRPLANE_MODE_ON, 0) == 1;
    }
    return mode;
}

To execute su command, do the following:

private void executeCommandWithoutWait(Context context, String option, String command) {
    boolean success = false;
    String su = "su";
    for (int i=0; i < 3; i++) {
        // "su" command executed successfully.
        if (success) {
            // Stop executing alternative su commands below. 
            break;
        }
        if (i == 1) {
            su = "/system/xbin/su";
        } else if (i == 2) {
            su = "/system/bin/su";
        }       
        try {
            // execute command
            Runtime.getRuntime().exec(new String[]{su, option, command});
        } catch (IOException e) {
            Log.e(TAG, "su command has failed due to: " + e.fillInStackTrace());
        }   
    }
}

Alternatively, if your app:

1. Was signed with an Android framework's certificate; and
2. Was installed to the /system/app/ directory; and
3. Have the relevant tags declared in AndroidManifest.xml file (e.g. WRITE_SECURE_SETTINGS, etc).

then you can just do this:

Settings.Global.putInt(context.getContentResolver(), Settings.Global.AIRPLANE_MODE_ON, isEnabled ? 0 : 1);

Since anything defined in Settings.Global can be read-write by system apps - even third-party app created as a system app.