def metric_clutch_torque_converter_model(y_true, y_pred, on_engine):
    """
    Metric for the `clutch_torque_converter_model`.

    :param y_true:
        Reference engine speed vector [RPM].
    :type y_true: numpy.array

    :param y_pred:
        Predicted engine speed vector [RPM].
    :type y_pred: numpy.array

    :param on_engine:
        If the engine is on [-].
    :type on_engine: numpy.array

    :return:
        Error.
    :rtype: float
    """
    from co2mpas.utils import mae
    return float(mae(y_true[on_engine], y_pred[on_engine]))
