Rough Landings in High Seas
Operating helicopters from the deck of naval vessels in poor weather is a significant challenge demanding the highest levels of skill and a number of technologies.
Small vessels like frigates, destroyers and offshore patrol vessels represent an even greater challenge, for obvious reasons.
Regardless of wave and weather helicopter operations must continue if essential activities like anti submarine warfare are required.
In addition to training and avionics a number of systems exist that allow a helicopter to be secured to the heaving, rolling and pitching flight deck and once secured, moved into the hangar.
Australia, the USA, Canada, Japan, Spain and Taiwan use the the Curtis Wright (Indal Technologies RAST (Recovery Assist, Secure and Traverse) system. US Navy ships use RAST, except the LCS, where the MacTaggart Scott TRIGON is installed. RAST is a development of a system used by the Royal Canadian Navy in the 1960s’.
RAST assists with landing and is then used to move the helicopter (or UAV) into the hangar using a rail embedded into the flight deck and hangar.
The operation sequence is as follows;
The typical operation of the RAST system begins with the pilot making a normal approach to the flight deck and establishing a hover. The messenger cable is lowered to the deck and is manually connected to the main recovery assist tethering cable, which is then hauled up to the helicopter and automatically locked into the main RAST probe. During a period of quiescence in the ship’s motion, the pilot requests the LSO to apply tension to the recovery assist cable. This tension produces a strong centering effect to stabilize the hover and directs the helicopter toward the designated landing area, as the pilot slowly flies the craft down. Immediately upon touchdown, the LSO closes the RSD’s arresting beams securing the helicopter probe. The aircraft is ready to be aligned and traversed into the hangar.
The RSD is often referred to as the ‘bear trap’
RAST requires personnel on deck to secure and position the helicopter and an operator, the aircraft pilot alone can not operate the system. It is also not suitable for helicopters with nose wheels but can be used to enable recovery in up to Sea State 5 conditions. Because on the SH60 the probe is not at the centre of rotation, once down, the RSD is released in one direction to enable the helicopter to be positioned ready for the movement.
MacTaggart Scott (another one of these brilliant UK defence manufacturers very few have heard about) pioneered helicopter recovery systems.
Their TRIGON system was originally developed for use with the Wasp helicopter and much smaller flight deck. As the Lynx replaced Wasp, the same system was developed and used.
The deck lock system requires the pilot to hover over a steel grid in order to deploy the locking ‘harpoon’.
Once engaged the hydraulic actuator system, from Claverham, pulls the helicopter onto the deck, compressing the oleo leg in conjunction with negative thrust from the rotor. This system can secure the helicopter to the deck without needing any personnel to approach it, an important safety consideration.
The deck lock grid is available from a number of manufacturers and widely used.
The video below from Prism Defence shows a deck lock system in operation
Additional securing straps are often used and the deck lock released, it is a flexible system and because the actuator sits on the centre of rotation the helicopter can be easily manoeuvred into the most favourable position for subsequent takeoff. The pilot has immediate confirmation that the helicopter is secure and is not reliant on others.
One might consider these systems strategically important, without being able to operate helicopters in high sea states surface combat vessels will be extremely vulnerable. The Japanese government recently expressed concern about the French company DCNS selling their system to China.
Once secured to the deck a means of transporting to the hangar is required and these fall into two broad types, rail assist and tug.
The RAST uses a rail system, TRIGON using a series of cables with the PRISM system on the Type 23 for Merlin, using the PRISM rail system.
The MacTaggart Scott TRIGON system is used by many operators and makes use of computer controlled steel wire ropes to secure and move helicopters. This document makes a good case for the all round superiority of TRIGON.
The PRISM system on the Type 23 Frigate was designed specifically for the Merlin helicopter and uses a three rail system instead of the TRIGON.
For large ships or small ships in calmer weather, an electric tractor unit is used to move the helicopter.
The Royal Navy use the Indal MANTIS (formerly Douglas Equipment) battery powered handle for example.
In service in the Royal Navy are a number of heavy duty tie down straps, couplers and chains from Drallim
The offshore energy industry also needs to operate helicopters in bad weather, mainly for crew transfers and aeromedical evacuation.
The Dutch company Barge Master have partnered with the aluminium helicopter deck specialists Bayards Aluminium to produce a motion compensated helicopter deck that supports large helicopters like the AgustaWestland EH101 and Sikorsky S-92.
Put simply, it expands the safe helicopter operating enevelope.
Barge Master specialise in heave compensated work platforms and ‘walk to work’ access systems for the offshore energy industry.
Does this technology have military applications?
Landing, securing and moving helicopters in poor weather is a serious business; personnel safety, operational effectiveness and avoidance of equipment damage are all important factors. The various systems all have their pros and cons and will have to be considered and tested for specific conditions, speeds, ships and helicopters.
An important element of naval operational success.