gistfile1.aw

<?php

function filter_term_archive( $query ) {
   if( is_tax( some_custom_post_type_tax_slug() ) ) {
		$query->set( 'tax_query', array(
			'relation' => 'AND', // Change to OR if we only want to filter one type.
			array(
				'taxonomy' => some_custom_tax_slug(),
				'field' =>  'slug',
				'terms' => $array_of_terms
			),
			array(
				'taxonomy' => some_custom_post_type_tax_slug(),
				'field' =>  'slug',
				'terms' => $current_archive_term
			)
	    ));
	}
    return $query;
}
add_filter( 'pre_get_posts', 'my_get_posts' );

// Or this

function filter_term_archive( $query ) {
   if( is_tax( some_custom_post_type_tax_slug() ) ) {
		$query->set( 'tax_query', array(
			array(
				'taxonomy' => some_custom_tax_slug(),
				'field' =>  'slug',
				'terms' => $array_of_terms
			)
	    ));
	}
    return $query;
}
add_filter( 'pre_get_posts', 'filter_term_archive' );

You should be running is_tax() off of the query object you're passing in, not the global $wp_query object, which the helper function is_tax() is aliased to.

function filter_term_archive( $query ) {
    if ( $query->is_tax('your_taxonomy') ) {
        // do your stuff
    }
}
add_action( 'pre_get_posts', 'filter_term_archive' );

Also, pre_get_posts is an action, not a filter. It amounts to the same thing in this case, but WP's not looking for a return value.