Airworthiness and System Safety

Airworthiness and Safety

The basic concern of airworthiness is to ensure acceptable level of safety during design, operation and maintenance of an airborne store. This 'acceptable level of safety' is a complex concern while formulating regulations and requirements Further, it greatly varies depending on whether the aircraft being considered is for civil or military application.

‘Safety means freedom from death, injury or damage to people on board as well as collateral damage to human life and property in the ground (accident)’. Every activity has a risk threshold beyond which accident will be inevitable. Airworthiness control is to minimise the risk and maximise the effectiveness. All the airworthiness standards, military or civil, whether that of USA, Europe or Russia, have a common point of reference which is that an inverse relation should exist between probability of occurrence of an event and the degree of hazard inherent in its effect.

Damage Acceptance Criteria

System safety requirements are defined in SAE ARP (Aerospace Recommended Practice) 4761 (for civil aircraft) and in US Department of Defence document Mil–STD 882 D (for military aircraft). 

A ’catastrophic damage’ indicates loss life and property. It is assigned highest severity factor of '5'. The lowest severity of damage i.e., severity factor '1' indicates minor inconvenience. Frequency of occurrences are grouped under ‘frequent or probable’ (1 in 10 hours of flight) to ‘extremely improbable’ (1 in 10⁷ in military and 1 in 10⁹ hours of flight in civil aircraft design). The frequency of occurrences are also assigned digital values as very likely is given highest numerical value of 5 while lowest numerical value '1' is assigned to the cases of extremely improbable conditions. In both these standards a hazard index is defined as the product of ‘damage severity’ and the ‘frequency of occurrence’ of the event. The acceptance criteria of design safety is based on this hazard index. In civil aircraft design  hazard index of 4 is acceptable, hazard index 5 to 10 design improvement is recommended and any design with hazard index above 10 is rejected. The design damage acceptance criteria for civil aircraft is shown in figure 1.  

Figure -1: Damage Acceptance Criteria - Civil Aircraft

The damage acceptance criteria in military aircraft design is as per Mil-Std-882 D. The damage acceptance matrix (hazard index) is shown in figure -2. 

Figure - 2: Damage acceptance Criteria - Military Aircraft 

Flight Safety and Airworthiness

Risk required to be taken for completions of a complex task and the safety considerations associated with the risk are the two extremities and we have to strike a balance between these two. Safety of any flying effort or machine would depend primarily upon, whether we are operating below or above the ‘Risk Threshold’. It also must be appreciated that this ‘Risk Threshold’ is not a stationary one and it keeps varying based on the role, function and a host of other associated factors. It needs therefore to be reassessed under each changing scenario. 

The risk threshold would vary depending upon the accuracy and resolution levels of the various instruments and other associated systems used for the purpose of flying. For example, if an aircraft depends only on barometric altimeters, the accuracy of altitude prediction may be in the range of ± 500 feet. This has an effect on the vertical separation requirement of two aircraft flying in the same flight corridor. With INSGPS the altitude could be measured at much more accurately and this has facilitated ICAO (International Civil Aviation Organisation) to reduce the requirement of RVSM (Relative Vertical Separation Minima) from 2000 to 1000 feet and ease the traffic congestion.

Thus, it could be appreciated that the safety level would mainly depend on the:

a)      Airworthiness status of the aircraft    

b)   Cost and time of development and the implement of the maintenance      procedures

c)      Operating crew and their skill

d)     Maintainability and the maintenance crew skills

e)      Air traffic control system and its effectiveness

f)       Effectiveness of the navigational aids

g)      Effectiveness of weather forecast

The ‘Flight Safety Directorate’ is primarily responsible to estimate the risk threshold under all dynamic condition and take appropriate measures. The basic purpose of flight safety studies is to ensure that the chances of achieving the tasks should be optimal while risks are minimal. The flight safety directorate has a very complex duty to perform. On one hand, the military training must give a high level of exposures to possible war scenario and threats. The directorate has to ensure that in peace time high level of risks are to be avoided within the stated training or operational tasks. This is because the accidents have very deleterious effects on the morale of the flier. During the war time, however, task achievement is paramount and hence risks are to be accepted even at high degrees if the operational requirements dictate.

Conclusion 

Airworthiness and Flight safety are very closely connected. They both have the same objectives or goals which safety o flight operations. Airworthiness tries to improve flight safety from engineering point of view by taking suitable engineering judgments during design, manufacture and maintenance. Flight Safety is concerned with the operational aspect taking cognizance of the practical limitation of deign and engineering issues.  The purpose of the flight safety studies can be summarized as follows:

a)      Identify and minimize those risks which may contribute accidents

b)      Avoid very high cost of losses and damages

c)      Identify all risk hazards real or potential at all levels and all phases of      flying

d)     Risk thresholds are dynamic and they need to be reassessed

e)      Continuously reassess the risks and make necessary readjustments.