This invention relates to improvements to suspending loads from a rotary wing aircraft (preferably a helicopter) for applications such as placing a human upon an electric transmission line, or transmission line tower, wherein the transmission line may be either energised (live) or de-energised (dead).
Constructing, maintaining and repairing electric transmission line systems are done either in an energised or de-energised state. When repairing or maintaining such systems it is generally uneconomical to de-energise the system. Hence, it is preferred that maintenance or repair be carried out upon energised conductors, Where this is not possible maintenance and repairs are required to be done as quickly as possible so that loss of revenue and possible inconvenience to users is minimised.
One approach to repair and maintenance has been that of humans climbing the large transmission system towers to access conductors, insulators, and the various pars of the towers themselves. This is a time consuming and inefficient approach in which vast amounts of revenue may be lost in cases when the lines have to be de-energised for safety reasons.
Another approach is that of using both insulated and non-insulated vehicles or towers in which when working upon energised lines such equipment must be insulated. Nevertheless, there is still a potential for fault discharges due to structural defaults or negligence of personnel which can cause damage to equipment and more seriously death to personnel. Another disadvantage is that this equipment is not suited to rugged terrain and its transportation speed is relatively slow. Consequently, when considering Australian transmission line distances and terrain, repair and maintenance may be time consuming and impractical using this approach.
Another approach is that of carrying personnel and equipment upon a helicopter wherein a human sits on a rigid platform attached to the helicopter. The helicopter manoeuvres close to a desired area (the area being either energised or de-energised) and the worker then carries out maintenance, repair, or installation whilst being supported by the helicopter. It is possible for humans to work on energised conductors using this approach as both the human and helicopter are ungrounded and therefore in an ideal situation there is no danger of damage or death. Unfortunately, this is a relatively expensive approach as there is required at least a helicopter pilot, a service and maintenance engineer, and a helicopter which must be in operation throughout the maintenance or service operation. In addition, there is a danger that the helicopter's rotor blades may cause a short between two conductors or may hit part of the transmission system which may just damage the system or could snap a rotor blade causing death to the pilot and engineer.
Another more suitable approach has been developed and published in U.S. Pat. No. 4,673,059 in which a maintenance engineer is suspended from an ungrounded support means such as a helicopter. The engineer may be suspended from a suspension means of insulated components so that the probability of shorting between conductors or a grounding discharge is further reduced. The suspension means is connected to a cargo hook attached to the helicopter and the engineer sits in a basket or seat device. There is a limited slip means to allow for a margin of error when the load (engineer) is simultaneously tethered to the helicopter and stationary component of the transmission line system. As an alternative to the limited slip device there is a common break-away link adapted to rupture when an instantaneous dynamic force of about 900 lbs is applied.
Unfortunately, the invention disclosed in U.S. Pat. No. 4,673,059 has problems associated with safety of suspended loads (especially when near live transmission lines). One such problem with the above arrangement is that in one embodiment the load is suspended from only a cargo hook in which there is no backup safety feature. Thus, the pilot may inadvertently or erroneously release the hook therefore endangering the engineer and other people.