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Helicopters appear to fly at relatively low altitudes compared to other aircraft.

Helicopters with turbine engines can reach altitudes of 25,000 feet during forwarding flight. However, helicopters cannot fly as high when hovering. The typical maximum height for a hovering helicopter is about 12,000. [1 ] Legendary French aviator Jean Boulet, piloting an Aerospatiale SA 315 Lama, set the world record for the highest altitude flight in a helicopter at 12,442 m (40,820 ft.). [2 ] It is a single-engine helicopter. The record stands to this day. From a practical point of view, ‘high altitude’ operations are taken to be that above FL250 (25,000 feet above mean sea level), which is the altitude at which aircraft certification requires that a passenger cabin overhead panel oxygen mask drop-down system has to be installed [3 ].

  ETYMOLOGY 

A helicopter referred as a "chopper", is a type of rotorcraft in which lift and thrust are supplied by one or more horizontally-spinning rotors. [4 ] By contrast the autogiro (or gyroplane) and gyrodyne have a free-spinning rotor for all or part of the flight envelope, relying on a separate thrust system to propel the craft forwards, so that the airflow sets the rotor spinning to provide lift. [5]The compound helicopter also has a separate thrust system, but continues to supply power to the rotor throughout normal flight.[6]

 HISTORY 

Legendary French aviator Jean Boulet, piloting an Aerospatiale SA 315 Lama, set the world record for the highest altitude flight in a helicopter at 12,442 m (40,820 ft). [7 ]According to Delsalle, the achievement planted the seed of an idea that would only be realized three decades later.[8 ]“At the time, he thought about landing on Everest. But the company was not too keen to do that, and the type of helicopter was quite sensitive to power variations at high altitude [due] to the way the engine was governed,” [9 ] Delsalle explained. “So it was not the helicopter to fly in the very turbulent environment at high altitude.”[10 ]Two years later, in 1974, Aerospatiale conducted the maiden flight of the first AS350 prototype. Over the next 30 years, the AS350 Écureuil or “Squirrel” — known in North America as the “AStar” — evolved through successively more powerful variants. In the early 2000s,[11 ] Delsalle was assigned as a test pilot for the most powerful version yet, the AS350 B3, when it occurred to him that he might have found a helicopter with the potential to land on Everest. [12 ]

 DEFINITION OF HIGH ALTITUDE 

Altitude, like elevation, is the distance above sea level. Areas are often considered "high-altitude" if they reach at least 2,400 meters (8,000 feet) into the atmosphere.[13 ] High-altitude locations are usually much colder than areas closer to sea level. This is due to the low air pressure. Air expands as it rises, and the fewer gas molecules—including nitrogen, oxygen, and carbon dioxide—have fewer chances to bump into each other.[14 ]

 ACHIEVEMENTS 

Jean Boulet, piloting an Aérospatiale SA 315 Lama, set the world record for the highest altitude flight in a helicopter at 12,442 m (40,820 ft). [15 ]

 OPERATIONAL MANAGEMENT 

There are various postulates that are laid down by the Federal Aviation for high altitude operations. Dual crews are preferred for high altitude operations especially in unpressurised cockpits as decision making capabilities vastly reduce with lower oxygen content. Oxygen carrying capacity of a helicopter though not widely accepted among organizations outside the military have the ability to enhance and improve performance at higher altitudes. Fatigue is also a common occurrence among pilots that fly at higher altitudes. For this reason pilots are generally advised not to fly when fatigued to avoid accidents [16 ]

 OPERATIONAL CHALLENGES 

High altitude operations pose a variety of challenges especially in the areas that they are used in :

 Weather Issues 

High altitude operations frequently have weather issues like downdrafts, low or high temperatures and the advent of low air density making such operations extremely risky.

 Visibility 

Visibility at high altitudes is also often a problem that many pilots face, this limits the amount of sorties that pilots can make in such environments. This situation can also arise mid-flight, seriously affecting the operability of the aircraft.

 Aircraft Efficiency and response 

The aircraft at higher altitudes will have a slower response rate and a much lower efficiency. This means that the engine control and operation must be optimized to a fair extent, most helicopters operating at high altitudes prefer not to land or switch off the engine as starting it again would be extremely difficult

 Threats 

Since a lot of high altitude operations take place in hostile territory the danger of threats has a counter effect on the efficiency as the added weight from carrying armor plating in the aircraft causes an increase in parasitic weight.

 HIGH ALTITUDE ROTORCRAFTS DESIGN AND DEVELOPMENT 

One of the most notable aircrafts that have been developed for High altitude operations has been the Aérospatiale SA 315B Lama. It had been designed and developed specifically keeping in mind areas that had a requirement of regular high altitude operations like India, Nepal and South America. The attributes that made this design a successful one were its lightweight construction and powerful engine, while being stripped off of any unnecessary weight on the aircraft.[16 ] [17 ] Other notable examples for high altitude rotorcrafts with the ability to operate above 20,000 feet are the Eurocopter AS350 Écureuil and the Mi-8 helicopters that are also regularly used by organizations around the world for high altitude operations.[18 ] [19]. Many new development have been taking place for creating higher performing helicopters at high altitudes namely :

 Aerospace Materials 

The materials are continuing to get lighter and tougher thus creating more opportunity for design flexibility. This has especially changed in the last few decades with the introduction of carbon fiber construction, fiberglass and titanium. At the same time research into the fatigue and creep failure of materials has also made the need of designing bulky structures unnecessary. [20 ][21 ]

 Alternative Engines 

With the growth and the potential of electric engines in the engineering field various advantages sprout up when coupled with the current traditional forms of internal combustion engines. The basic principle of the entire system is that internal combustion engines run efficiently at higher RPMs for long periods of time and electric motors are good at providing high torque and power for short periods of time. This is one method of effectively increasing the efficiency and safety 0f the aircraft system along with increasing the redundancy. With this rise of electrical power systems the battery technology is also increasing, paving the way for more efficient energy carriers and thus lesser take off mass.[22 ]

Optronics

Since a lot of operations take place at high altitudes the visibility of such areas is often hampered by the weather elements. With the advent of newer and lighter electronic components coming into development it is now easier to complete the sortie with relative safety as compared to before. Examples of these systems include Lidar, Instrument landing systems, Visual aids etc.[23 ]

Configurations

With the development of more effective electrical systems there has been a growing trend towards Distributed Electric propulsion, this not only helps in decreasing the complexity of transmission systems but also improves the survivability of the aircraft by increasing the redundancy. [24 ] Distributed electric propulsion also helps us to use configurations interchangeably thus helping in improving efficiency. Examples of this include Tilt wing, tilt rotor and Compound system VTOL configurations.

 OPERATIONAL MANAGEMENT 

There are various postulates that are laid down by the Federal Aviation for high altitude operations. Dual crews are preferred for high altitude operations especially in unpressurised cockpits as decision making capabilities vastly reduce with lower oxygen content. Oxygen carrying capacity of a helicopter though not widely accepted among organizations outside the military have the ability to enhance and improve performance at higher altitudes. Fatigue is also a common occurrence among pilots that fly at higher altitudes. For this reason pilots are generally advised not to fly when fatigued to avoid accidents [23 ]

 REFERENCES 

Category:Aircraft Category:Helicopters