A common problem with the application of HLDs is how to deal with the gap between the HLD and the main wing. This gap can be either sealed or left untouched. In both cases, there are undesirable side-effects. If the gap is left open, the airflow from the downside escapes to the upper surface which in turn degrades the pressure distribution. In contrast, if the gap is sealed by means such as a diaphragm, it may be blocked by ice during flights into colder humid air. In both cases, it needs special attention as an operational problem. In the following, the technical features of various HLDs are discussed.