Crash landings of helicopters Classified into three categories
I. A first category is an uncontrolled crash wherein the helicopter impacts the ground at a high velocity. Helicopter pilots and passengers are not expected to survive such a crash.
Altitude range from 40 ft to 400 ft - No technology
II. A second category is a "hard landing" wherein the pilot is in virtually complete control of the helicopter and wherein there is little or no physical damage to the helicopter upon impact with the ground. Pilots and passengers can be expected to survive a hard landing without injury.
Altitude > 400 ft – Autorotation
III. A third category of crash landing, a "survivable crash," has an impact, which is not as severe as an uncontrolled crash, but is more severe than a hard landing. A survivable crash results in physical damage to the helicopter and may result in injury to the helicopter pilot or passengers. Survivable crashes can be defined as crashes wherein the helicopter impacts the ground with sufficient velocity to cause physical damage to the helicopter, and wherein the helicopter impacts the ground with a vertical velocity less than or equal to 41 feet per second. Often, although the helicopter pilot and passengers can be expected to survive such an impact, they are likely to sustain injuries, such as spinal injuries, due to forces transferred to their buttocks through the helicopter fuselage and helicopter seats upon impact of the helicopter with the ground.
Altitude ≤ 40 ft – crashworthy seat, airframe, landing gear
Initial accident investigations show that in spite of using restraints, fatal or severe injuries caused by preventable head strikes and associated neck injuries mean that existing safety equipment in helicopters is not sufficient for occupant protection during a crash. It is expected that helicopter-related crash fatalities could be reduced by 30 to 50% if effective protective devices for occupants could be installed in helicopters.
Previous Work
Martin-Baker had developed a practical helicopter crew ejection seats escape system over 25 years ago and delivered two such systems for NASA research helicopter. Russian designers also had developed a rocket/parachute enforced escape system for combat helicopters.
I. Operational ejection seat
The rotor blades are fastened to the hub by means of explosive bolts. Prior to ejection , the blades are jettisoned. The jettisoned blades pose serious threat to nearby objects, and therefore a certain amount of separation is required while flying in formation.
II. Rocket/parachute enforced escape system
The pilot is extracted from the helicopter by means of a solid-propellant rocket motor attached to a strong, but light, cable, once the rotor blades have been jettisoned to facilitate unhindered egress. The seat is more of a tractor rocket system than a full ejection seat, but it does provide for escape from the helicopter. The pilot is also able to bail out of the helicopter manually.
The system first fires explosive bolts to jettison the rotor blades then jettisons the canopies and finally tractors the crewmembers out. Their parachutes automatically deploy seconds later. In both case the jettisoned blades pose serious threat to nearby objects, and therefore a certain amount of separation is required while flying in formation . This innovation did not find widespread of acceptance.
Present Work
The modern helicopter structures are designed to absorb some impact forces and prevent collapse of the cabin. Statistics show that the vast majority of accidents the helicopter impacts but with high a vertical descent rate. Today, there are two kinds of techniques available to protect the pilot and the passenger in the modern helicopter, which is;
Ø Crashworthy seat and Crashworthy airframe
Ø Autorotation
I. Crashworthy
The crashworthy technical discipline known as crash dynamics focuses on technologies to improve the structural crashworthiness of helicopter and the potential survivability of occupants. The scope of interests includes the measurement and understanding of structural and passenger loads experienced during crashes, studies of the energy-absorbing characteristics of new helicopter materials and assembled components such as sub-floors and seats, the development and validation of analytical design methods, and the impact of crashes on special aircraft equipment such as emergency locator transmitters. A key goal in this area of research is to provide enhanced survivability with little or no increase in aircraft weight or cost.
II. Crashworthy Seat
Modern helicopters with robust structures designed to survive the crash. However, the human body cannot tolerate the crash force and acceleration. The back and neck are particularly vulnerable to major and other fatal injuries. The crashworthy seat technologies offer reliable and proven solution to this problem by attenuating crash energy so that force and accelerations imposed on the seat occupant are below injury thresholds. In this process of attenuating energy, the seat stokes (moves downward) in a controlled fashion. The energy is attenuated by the seat structure over a longer time period and keeps it well below injury thresholds.
III. Autorotation
During helicopter-powered flight, the rotor drag is overcome with engine power. When the engine fails, or is deliberately disengaged from the rotor system, some other force must be used to sustain rotor RPM so controlled flight can be continued to the ground. This force is generated by adjusting the collective pitch to allow a controlled descent. Airflow during helicopter descent provides the energy to overcome blade drag and turn the rotor. When the helicopter is descending in this manner, it is said to be in a state of autorotation. In effect, the pilot gives up altitude at a controlled rate in return for energy to turn the rotor at an RPM which provides aircraft control. Stated another way, the helicopter has potential energy by virtue of its altitude. As altitude decreases, potential energy is converted to kinetic energy and stored in the turning rotor. The pilot uses this kinetic energy to cushion the touchdown when near the ground.
Most autorotations are performed with forward airspeed. For simplicity, the following aerodynamic explanation is based on a vertical autorotative descent (no forward airspeed) in still air. Under these conditions, the forces that cause the blades to turn are similar for all blades regardless of their position in the plane of rotation. Dissymmetry of lift resulting from helicopter airspeed is therefore not a factor.
To maintain a protective, crashworthy shell around crew and passenger compartments, airframe components such as engines and transmissions must be anchored to be able to withstand high G loads under the stress of crash impact. Unrestrained, massive components become crushing projectiles under the force of impact with the ground. Crash attenuating seats that "stroke" or compress during impact, limit G loads on helicopter occupants. Containment of fuel is the only practical method of preventing post-crash, helicopter fires. Impacts resistant fuel tanks that are positioned away from ignition sources and penetrating objects are basic considerations.
Many helicopters today are built with self-sealing fuel tanks and breakaway fuel line connection valves. Fuel is contained upon crash impact, reducing the possibility of fire, which gives the passengers and crew a chance to successfully egress the aircraft. Inertial reel, five-point safety belts with a single-point release mechanism, allow for rapid helicopter egress. Doors and windows that jettison further enhance egress requirements. Emergency breathing devices provide oxygen in the event of forced water landing that submerges the crew. Ejection seats are feasible, but the problem of interfacing seat ejection with main rotor clearance requirements has proven to be too complicated and cumbersome to incorporate into system design.
