You may wish to review the restrictions on the use of Power by the Hour components and engines in external load operations. Many helicopters have STC's permitting even greater external load capacity than those specified by the OEM - the limitation is the combined weight of the equipped helicopter and external load, and the certified ambient performance. If these factors are combined it is possible to lift loads in excess of the certified hook weight with a light aircraft, at dawn, flown by racing snake flight crews and very little gas and still be within the certified external gross weight limitation for the aircraft. The increased gross weights permitted for external loads with helicopters are a function of the ability to instantly jettison the load and immediately reduce gross weight and perform all kinds of recovery manoeuvres in the event of a power loss in a multi-engine aircraft.