Investigation on the chinook operations with an external slung load after cable failure

Within The Royal Netherlands Air Force (RNLAF), the Chinook tandem helicopter with external slung load is one of the most active operating helicopters, being employed in both military and humanitarian actions. Up to the present, during RNLAF operations with Chinooks, every large external load was underslung by means of a two-strop suspension system backed up by a third point of suspension called 'redundant HUSLE' (i.e. a redundant set of slings which come into action if one of the normal strops fails). The redundant HUSLE is known to be quite expensive in terms of time and operating costs. Therefore the question arose whether it would be safely enough to replace the three-point suspension system by a two-point suspension system, eliminating the redundant HUSLE. The goal of the present paper is to investigate the behaviour of a Chinook helicopter with external slung load following the premature breakdown of one of its cables sustaining the slung load and check whether the three-point suspension system can be safely replaced by a two-point suspension. For this, a simulation model representing the flight dynamics of a Chinook CH47-B helicopter carrying a suspended load in inverted V-suspension was developed and used to fly different failure scenarios in two-and-three-point suspension. The paper will show that although in general flying with the redundant HUSLE results in less violent helicopter reactions in case of cable failure, redundant HUSLE does not necessarily mean safer. It could be concluded that flying with loads up to at least 2000 kg could be safely done in a two-point suspension system, eliminating thus the redundant HUSLE.