Thursday, July 14, 2016

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. 





Saturday, December 26, 2015

FAA Small Unmanned Aircraft Registration

1. Registration Begins from 21 Dec 2015

         The Aviation Rulemaking Committee's (ARC) Unmanned Aircraft System Registration Task Force (RTF) constituted by the Federal Aviation Administration (FAA) released the Task Force Recommendation Final Report on 21 Nov 2015. [Ref 1]. 

         Based on the RTF report, FAA has introduced statutory requirement that all small Unmanned Aircraft System (sUAS) weighing more than 0.55 pounds (250 grams) and less than 55 pounds (approx. 25 kilograms), including payloads such as on-board cameras, must be registered if it is to be operated outdoor in the National Air Space (NAS). The registry is live from the day on December 21. Registration is free for the first 30 days (up to 19 Feb 2016) and then $5.

       

2. What the Registration Mean 

           The registration could be web based or paper based. In preparation for registering online, each owner must provide his or her name, home address and e-mail address. Owners wishing to Register themselves as sUAS operator must be at least 13 years of age. Information on US citizenship or residence status is not asked. Upon completion of registration, the web application will generate a Certificate of Aircraft Registration/Proof of Ownership that will include a unique identification number for the UAS owner, which must be marked on the aircraft. 

          Owners using model aircraft or sUAS for hobby or recreation will only have to register once and may use the same identification number for all their model aircraft. The registration is valid for three years. Owners of any other UAS purchased after December 21 must register before the first flight outdoor. The same rule apply for model aircraft assembled from purchased kits. The registration is for each operator and not for each small UAS they possess.  

          The above statute indicates that a model aircraft or sUAS need not be registered at the Point of Sale (POS), however the operator must register himself if he wants to fly the aircraft.


3. Operating Rules

        Operator must mark his registration number or the serial number on all the aircraft he wishes to operate. If he has provided the aircraft serial number during registration process, he may display the same number on the aircraft. FAA has partnered with several industry associations to educate the public about using unmanned aircraft and model aircraft safely and responsively. The general operating and flight safety rules are:

    a)  Flight altitude limited to 400 feet above ground level (AGL).
    b)  Visual flight rules: Keep your unmanned aircraft always in sight. 
    c)  Never fly near manned aircraft, especially near airports.
    d) Never fly over group of people, stadium or sporting events. 
    e) Never fly near emergency response/evacuation efforts. 


4. FAA Rule Making Effort

           Department of Transportation (DoT), FAA's Federal Register Vol. 80 Sl. No. 35, No. 9544  'Operation and Certification of small UAS' was released on 23 Feb 2015 as notice to public for rule making (NPRM) [Ref 2]. The above proposed rule addresses operation of UAS, certification of their operator, registration and display of the registration marking on the aircraft.  

          The proposed rule would also find that airworthiness certification of sUAS is not required. The proposed rule would also prohibit model aircraft from endangering the safety of the National Air Space. 


5.  ARC UAS Registration Task Force (RTF) Recommendation

          FAA charted the UAS Registration Task Force of Aviation Rulemaking Committee to provide recommendation to FAA "On Registration Requirement and Process for small UAS including those used for commercial purposes, and all model aircraft". The stated objectives of the Task Force was to develop recommendations which ultimately would contribute to an enforceable rule imposed by FAA.

        The task force was comprised of individuals from a diverse group of aviation and non aviation perspectives (25 different groups from industry, commercial and business sectors, aircraft operators, aircraft pilots, academia, regulatory as well as government officials). The task forces met to discuss the three main objectives as the terms of reference set forth by FAA during 03 to 05 November 2015.  From these discussions, the task force developed high level recommendations for sUAS registration requirements that addresses the questions posed by FAA.

5.1 Min Weight UAS that Need to be Registered

         The safety of non-flying public and other users of the NAS was central to the task force's determination of what category of sUAS be excluded form the registration requirements. The formula considered was a standard aviation risk assessment formula used in consideration in manned aircraft. It was noted from a United Kingdom Ministry of Defence 2010 study that an object with kinetic energy level of 80 Joules (approximately 59 foot pound) has a 30% probability of being lethal when striking a person on the head. 

       The terminal velocity of an object falling from a height 400 feet can be estimated assuming that the net acceleration of the body will be due to acceleration due to gravity reduced by the body drag. The drag force acting on the body during the fall of the body will be a function of shape and velocity of the body at the instant. The task force considered a typical sUAS of projected area of 0.02 m >2 and and drag coefficient of 0.3. Solving for mass and velocity for a body to impact with a Kinetic Energy of 80 Joules, works out weight 250 grams and velocity of 25 m/sec or approximately 57 miles/hour. 

       The probability of such a lethal occurrences per UAS flight hours were estimated by the task force assuming the MTBF of UAS as 100 hours and population density of 10000 people per square miles. The probability of a catastrophic event was found to be 4.7 x 10 E-8 or less than 1 ground fatality for every 20,000,000 flight hours of UAS. 

        Considering that the acceptable risk levels for commercial air transportation are on the order of 1 x 10 E-9 and therefore this risk level of 4.7 x 10 E -8 seem acceptable to the task force. 


6. The UAS Certification 

In summary, it can be concluded that 
a) Unmanned aircraft weighing less than 250 gms requires neither certification of the product nor  registration of the operator. 
b) Unmanned aircraft weighing above 250 gms but less than 25 Kg needs no product certification but the operator who wishes to fly these aircraft must get himself registered with FAA.  
c) Unmanned aircraft weighing above 25 Kgs will be treated like manned aircraft as far as their airworthiness certification is concerned.

References:

1) http://www.faa/uas/publications/media/RTFARCFinalReport_11-21-15.pdf: "Unmanned Aircraft System (UAS) Registration Task Forces (RTF) Aviation Rulemaking Committee (ARC) -Task Force Recommendation Final Report"; November 21, 2015.

2) 80 Federal Register 9544 dated 23 Feb 2015, "Operation and Certification of Small UAS"; DoT, FAA.

Friday, November 27, 2015

Phases of Aircraft Certification

1.  Certification

           Certification is defined as a process that provides the possibility of making certain that any aircraft, whether civil or military, has acceptable levels of safety for a given future utilization within the defined flight load spectrum for a specified period.

2.   Phases of Certification of an Aircraft

          Certification is normally carried out in four levels as shown in Figure - 1 below:


Figure -1: Aircraft Certification Phases

a) Level 1: Requirement Finalization - The requirement of the aircraft or the technical specification, the design standards (like FAR, MIL, etc.), and the materials to be used (existing approved materials or new materials to be developed) are decided in this phase. 

b)  Equipment (LRU) and Component Qualification 

c) System-level Qualification, and 

d) Aircraft level functionality and operability assessment.  

3.  LRU Qualification 

     The LRU (Line Replaceable Units) development and Qualifications are carried out in phases as follows:

a) Development of F-F-F Model (Form, Fit and Function) - It is a model unit and used for checking the installation, functioning, and access for maintenance. 

b) Prototype Model - Based on the F-F-F model experience, a prototype model is built with the engineering material to check proper functioning in the full environment. 

c) SOFT Model - SOFT (Safety of Flight Test) model is a model that is subjected to all those exposure tests that are felt in each flight. SOFT is a subset of QT (Qualification Test), and SOFT-cleared items can be released for developmental flight trials. In parallel, full QT is carried out on other models. 

d) Qualification Test  (QT) Model - The QT model is subjected to qualification tests as follows;

     - Functional test on the test rig
     - Structural test as proof of structural integrity
     - System Integration and ground evaluation 
     - Environmental test 
     - Flight and ground test 
     - Endurance test for life cycle environment. 

Note: Qualification tests may be spread across different prototype samples as shown below: 
        1) Sample 1 - Full Range of Structural Integrity and functional tests.
        2) Sample 2 - After the quality acceptance test, the prototype is subjected to the full life or                            Endurance test. 
       3) Sample  3 - Full exposure to environmental tests (Salt, fog, fungus growth, etc.)

4.  System Level Certification 

An aircraft comprises many systems like airframe structure, mechanical, electrical, powerplant, and avionics. The configuration design is based on aerodynamic considerations. The system-level certification will consist of the evaluation of the system: 

  - System-level functional requirements under all environments
  -  System safety and hazard requirements as per airworthiness standards
  - System Reliability (Redundancy or Fault tolerance) requirements 
  -  Maintainability requirements and 
  -  Specified life 

System evaluations go in phases
 - Design evaluation as per airworthiness standards/regulations
 - Functional evaluation on the system integration rig/box
 - Performance evaluation on "Aircraft on Ground" test (initially with ground power supply and later         with aircraft's own power (engine running)) 
 - Performance evaluation on aircraft during flight.

5. Aircraft Level Evaluation 

Aircraft-level tests to verify each system's functioning at various stages:  
  - System evaluation during engine ground run (EGR)
  - System evaluation during Low Speed Taxi Test (LSTT) (speed limited to 50 knots) 
  - System evaluation during High Speed Taxi Test (HSTT) (speed limited below to Vrotation speed)
  - Flight test evaluation 

6. Flight Test and Evaluation 

The principal purpose of flight testing is to determine if the complex machine designed can safely accomplish the designated task with the pilot workload kept within the human limitations and natural instinct of human beings.

The flight test includes handling quality assessment and evaluation of airworthiness performance, as well as all mission performance requirements. The flight tests are also to establish the operational boundary of the aircraft, aero-elastic and other boundaries. The all weather assessment will involve tests and evaluations under various airport altitude and ambient conditions. 

All weather clearance tests include high altitude, operations under extreme temperatures, as well as rainwater leak tests, and night flying tests.  

However, there are some hazardous tests like determination of stall and spin behaviours, engine relight in air, safety assessment during simulated system failures, etc., which are normally carried out only after gathering enough data on the aircraft behaviour through flight tests. 

Military aircraft also need to go through weapon firing trials. 


7. Certification 

Based on the satisfactory tests and evaluation aircraft is issued with 
  - Type Certification (Civil Aircraft) 
  - Military Type Certificate (MTC) and Induction to service (Military aircraft). For the induction to
    service, an RSD (Release to Service Document), approved by CEMILAC, is issued. 


Associated activity during the certification stage is the preparation of ODM (Operational Data Manual), AFM (Aircraft Flight Manual) and MM (Maintenance manuals).  

Tuesday, October 27, 2015

DDPMAS - Indian Military Airworthiness and Certification Document?




1.  DDPMAS
            ‘Procedure for Design, Development and Production of Military Aircraft and Airborne Stores’ (DDPMAS) is a document released by Secretary, Defence Production, Ministry of Defence, Government of India on 30th October 1975. The document was issued to define the procedure to be followed by various agencies involved in the design, development and production of military aircraft and airborne stores. The document was reviewed and reissued in 2002 as DDPMAS-2002.

2. Contents of DDPMAS
            The document is divided into five sections as follows:
           Section I:      Definitions
           Section II:     Process of Development
           Section III:    Design, Development and Production of Aircraft, Aero Engines and
                                Major Airborne Equipment
                                Chapter 1: Prototype and Development Phase
                                Chapter 2: Pre-Production, Production and In-service Phase
                                Chapter 3: License Projects
                                Chapter 4: Bought –out Aircraft
           Section IV:    Design, Development and Production of Airborne Equipment, Raw Material and AGS Parts. 
           Section V:     Procedure for Flight Testing of Experimental and Prototype Equipment.

3. Omissions and Exclusions
1)  Design, Development and Production of Airborne electronic equipment /stores are   excluded from the scope of DDPMAS and will be carried out as per DDPIL-2000  Procedures. Installation of such equipment on military aircraft would follow JSG: 755:2001 and procedures stipulated in DDPMAS -2002.

2)      DDPMAS – 2002 is not applicable to UAV and missiles unless they are carried on manned aircraft.


4. DDPMAS -2002
            DDPMAS -2002 was issued to incorporate various organizational changes in the Ministry of Defence, e.g. DTD&P (Air) renamed as DGAQA; airworthiness functions changed from Directorate of Aeronautics (through CREs) to CEMILAC (through RCMAs). In addition to Air Force, the Army and Navy came in a big way in aviation wing, thus the terms ASR was replaced by ASR/NSQR/GSQR and CSDO by CSDO/NSDO/MAG (AVN) etc. In addition to these changes, the regulatory bodies also added certain additional requirements over and above DDPMAS-1975 requirements.  


5. Changes Incorporated in DDPMAS 2002
1) General content - General content of DDPMAS -2002 is similar to DDPMAS -75; however certain changes have been incorporated. 


2)    ‘Section – V: Procedure for Flight Testing of Experimental and Prototype Equipment’ has been made in two chapters as, ‘Chapter -1: Development Flight Testing’ and Chapter -2: Flight Testing by User Services’.

3)    Annexures: DDPMAS -2002 has added five more annexures compared to DDPMAS -75 (all of them added in Section 3, Chapter-1). Two annexures have been split in to two     parts. The new annexures added are shown below:
     1)    Annexure –R: Declaration of Design and Performance
     2)    Annexure –S: Request For change in configuration
     3)    Annexure –T: Hardware/Software Delivery Note
    4)    Annexure –U: Responsibilities of CEMILAC
   5)    Annexure –V: Requirement to be submitted by Developing Agency for             clearance of Airborne Equipment imported from abroad.  
 6)    Annexure – G is split into two annexures as ‘G1: Certificate o Flight Trials –          Aircraft’ and ‘G2: Certificate of flight trials – Helicopters’.
 7)  Annexure – K (annexure J in DDPMAS-75) split into two; ‘K(i): Local                   Modification Committee -Details of Modification Proposed’ and ‘K(ii): Advance        Modification Information’.


6. Changes Related to the Procedure for Ab-initio Development
            Section – III, Chapter -1: ‘Prototype and Development Phase’ has been substantially changed, the discussion on the changes are shown below:

    1) Progression of certification – A new para (para 3) has been added indicating two certification routes viz., ‘certification commences after completion of all design activities’ and ‘concurrent certification approach’ could be followed. However, DDPMAS 2002 supports the concurrent certification approach only.

2)    New Concepts brought in – Formation of ‘Airworthiness Group’, ‘Configuration Control’, ‘Technical Reviews’ and ‘Bought out Items’. Necessary details however has not been provided.

3)    Design Standard of Preparation – A new para (para 17) added in addition to the     original para of ‘Design Standard of Prototype Aircraft’ appearing at Para 68. Both the     paragraphs discussing ‘Design Standard of Prototype Aircraft’ create contradictions and        confusion.  
  
4)    Certificate of Design – In DDPMAS -75, ‘Certificate of Design’ (COD) was part of the   Type Record (Para 18(a)). In DDPMAS -2002, in addition to the above requirement,  ‘COD’ has been made a requirement from the vendor before issue of first flight    clearance. CEMILAC participates in Design Reviews, evaluates Design Reports, approved test schedules, participates in test and verification trials and finally accepts test reports. So, the requirement of ‘COD’ for each system for issue of flight clearance  does not appear justified. 

5)     Section III, Chap – 2: describes the activities to be performed during ‘Pre-Production   and Production Phase’. What is ‘Pre-Production’ phase and what are the activities to be     performed during this phase has not made clear.


7.  Discussion on the Changes of DDPMAS-2002 from DDPMAS-75

   a) Annexures – Though five annexures have been added, the annexures are not sequentially arranged in the text.

    b)    Declaration of Design and Performance (DDP) – DDP concept has been introduced in DDPMAS-2002 under configuration control. The DDP route is followed in Civil Aviation Procedures. An aircraft development house accepts equipment (parts & appliances) for installation on prototype aircraft on the basis of the DDP issued by  equipment developer, who is a DOA (Design Organisation Approval) holder from Civil Aviation Authority. As and when the prototype aircraft is certified by the CAA, the equipment also gets certified and only then, the equipment manufacture can issue  ‘Airworthiness Release Certificate’ for the equipment. As CEMILAC itself is certifying the LRU (parts and appliances), DDP may not be applicable. 

   c)    Design Approval of Firm and Airworthiness Group – The design approval of firm is to be carried out as per CEMILAC/5342/1 dated Jun 1999. The document does not indicate any privilege or delegated authority to the approved organization. The procedure for approval of airworthiness group is to be carried out as per CEMILAC/TC/03 dated Sep 2000.  

  d)    Bought Out Item – Appendix ‘V’ to DDPMAS – 2002 includes information to be   provided by vendors on bought out items. It is understood that ‘RCMA’ is to clear the BOI for installation on Prototype aircraft based on the annexure –‘V’ checklist and DGAQA is to comment on the QA process of manufacture. 
                 For ‘Of the shelf’ items, the procurement should be supported by ‘Authorized Release Certificates’ or ‘DDP’ from the vendor or ‘TSO Authorisation’ for TSO items.During prototype development stage only a very limited quantities are procured and the supplier may not provide the QTR, FMEA, MTBF/MTBR, Reliability/maintainability reports. Requirements for these documents will lead to great bottleneck and delay to aircraft design and development project. The appendix ‘V’ is abnormally big and does not provide any value addition to safety and airworthiness of the aircraft or aero-engine being developed. 


8.  The Document DDPMAS
    a)    Purpose of DDPMAS - The purpose of DDPMAS as indicated in its ‘foreword’ and ‘preface’, is to evolve an optimum coordination among various agencies involved in the design, development and production of military aircraft and airborne stores. The document was written with concurrent engineering concept with the assumption that the design and manufacturing house (HAL), the regulatory bodies (Chief Resident Engineer and Chief Resident Inspector) and user (Air Force Liaison Office) are co- located.

  b)    Certification Procedure Document – DDPMAS is not a certification procedural documents like CAR 21 (DGCA), FAA order 8110.4c, ‘Type Certification Procedures‘ or USAF Policy Directive AFPD-62-6:’USAF Airworthiness (Jun 2010)’ and USAF Instruction AFI 62-601: ‘USAF Airworthiness (May 2011)’. DDPMAS does not define the procedure, responsibilities and delegated authorities vested on approved design and production houses.

 c)    Development of Electronics Item - Development of electronic items is not covered under DDPMAS documents. An extract of DDPIL may be included to give a general overview of the concept and to appreciate whether it significantly differs from development concepts of other airborne equipment.

  d)    Approval of Design Organisations - ‘Design Organisation Approval’ (DOA) is an important step toward design and airworthiness control. The DOA recognizes the   capability of the firm and define responsibilities of the design signatories and delegates necessary authority to them. The Design Assurance System (DAS) of the DOA carries out control and supervision over design and design changes.The airworthiness authorities monitor the functioning of the DAS and accepts the design and test reports by the authorized signatories. The ‘Design Approval’ (DA) by CEMILAC is similar to  DOA and the ‘Airworthiness Group’ of CEMILAC DA is similar to DAS of DOA as per CAR (Civil Aviation Requirement). If the above design approval procedure is followed no separate approval will be required for airworthiness Group.  

  e)    Approval of Quality Management System – In the lines of the Production   Organisation Approval (POA) in CAR, DGAQA confers Approval of Firms Quality Management System (AFQMS). Here, AFQMS does not delegate any authority to the  firm and DGAQA resorts to spot check to detect quality deficiency. DGAQA have restricted the AFQMS limited to Government and Public Sector Undertakings only.


   9.  General Comment  
   a)    It is essential to have a separate document bought out by CEMILAC detailing the procedure for design certification starting from the establishment of certification basis to the culmination of issuance of type certification.

   b)    A separate document should be brought out by DGAQA detailing the procedure for Quality Assurances through AFQMS during manufacture of the prototype as well as series production.

    c)    The procedural documents released by both CEMILAC and DGAQA should clearly define the responsibilities and authorities delegated to the approved signatories so as to cater for situations where the vendors and regulatory bodies are not co-located.

  d)    The procedural documents should be made available in the electronic media so that it become accessible to the vendors, development houses and the users.    

  

Monday, September 7, 2015

Airworthy Materials.? What are they?

Airworthy Material ..!

There is nothing like airworthy material either by definition or by name. However, the aircraft design have to meet the safety and airworthiness requirements, therefore the aerospace designer chooses materials with due care to meet the material property behaviour, process of manufacturing  and qualification.  


Design Structural Weight 

Structural weight is an extremely sensitive issue in aircraft design. Increase in structural weight in air frame, aero engines or guided weapons have snowballing effect. An extra kg of weight may cause an extra kg of fuel burn for each flight and thus reduce range or payload, for an aero engine, over weight by one kg may cause a loss of as high as 30 hp engine power and 1 kg extra weight for an guided weapon may require additional 6- 7 kg of propellant to be burnt. Aerospace designers therefore choose material with higher specific strength (UTS/Density)  and specific stiffness (Young's modulus/density). 

Scatter in Material Property

Scatter in material property is unavoidable and the designers have to use the minimum of the test values while estimating the specific strength or specific stiffness. The effect of this scatter on short term property (static strength or UTS) is usually low. For example UTS of duralumin may be 40-44 kg/mm^2, the designer is required to use 40 kg/mm^2 for his analysis.

The scatter in the dynamic properties like LCF, and HCF (low cycle and high cycle fatigue) are usually high. This is reflected in the upper and lower bound of the S-N curve shown in the figure. The scatter or the width of the band is usually due to reasons which are both internal and external to the test specimen or the components. The external factors include dimensional accuracy, surface finish and presence of any stress concentration. The internal factors which give rise to the high scatter bandwidth are the presence of unwanted impurity element that provide nucleation points for initiation of crack and help in crack propagation. 

Aerospace Grade Material  

 Aerospace metallurgists have devised methods for controlling these impurities to ppm (parts per million) level. Residual gases (O, H, N, Ar, He), non metallic S, P and metalloids impurities of Pb, Bi, Sb, As, Ag, Cu, Ti, Te are controlled to extremely low levels ( 0.3 ppm) where their deleterious effects are tolerable. These activities are normally not done for general engineering material processing. The trace elements in the general engineering materials are up to 500 ppm (0.05%). SAE Aerospace Material Specification (AMS) indicate the level of controlling the trace elements and impurities.

Bandwidth of S-N Curve

Bandwidth of S-N curve is significantly lower for Aerospace Materials compared to the General Engineering materials due to the improved metallurgical processing. Thus for the same deign fatigue life, an aerospace material can be stressed to much higher level compared to an general engineering material as shown in the figure. This gives the aerospace grade material an extra advantage to reduce the structural weights of the stressed components.    

Categorisation of Parts

As per the airworthiness standard Def Stan 00 970, a part can be categorised into different grades taking cognizance of the strength and stiffness requirement of the part as shown below: 

Grade A

A part shall be graded as 'A' if the deformation or failure of the part would result in one or more of the following:

  - Structural Collapse
  - Loss of Control
  - Failure of Motive power
  - Inability to operate or unintentional operation of any system or equipment essential to the     safety or operational function of the aeroplane.
  - Incapacitating injury to any occupant
  - Unacceptable unserviceability or maintainability. 

Grade B

All other parts not covered under category 'A'. 

Selection of Material Based on Component Grade 

For grade 'A' Components 

Material conforming to aerospace grades

For grade 'B' Components 

They may be manufactured by aerospace grade material or less controlled material like general engineering grades or proprietary material at the discretion of the designer. 




Reference: 

For details please see: 

Balram Gupta, et al., 'Aerospace Materials with general Metallurgy for Engineers', Volume I, AR&DB, 1996, Printed by S Chand & CompanyLtd, New Delhi.