1. MILITARY AIRWORTHINESS ACCEPTABLE LEVEL of SAFETY
Bob Wojcik
Air and Naval Technology
General Dynamics Canada
Ottawa, Ontario, Canada
26-29 September 2005

2. -Wilbur Wright, 18 September 1901
“If you are looking for perfect safety, you will do well to sit on a fence and watch the birds; but if you really wish to learn, you must mount a machine and become acquainted with its tricks by actual trial.”
-Wilbur Wright, 18 September 1901

3. -Alexander Graham Bell, 1906
“There are two critical points in every aerial flight – its beginning and its end.”
-Alexander Graham Bell, 1906

4. Outline Background Safety Definitions
What is an Acceptable Level of Safety?
How Safe is Safe?
Acceptable Level of Safety – Civil Aircraft
Acceptable Level of Safety – Military Aircraft
Conclusion/Recommendation

5. Background
Military authorities have always been interested in aviation safety
Many military authorities are introducing formal Airworthiness Programs
International Military Aviation Authority Conference June 2004
Common theme – need for military airworthiness regulatory authority
Many programs are modeled on civil aviation safety programs
Lack of military airworthiness standards has led to reliance on civil airworthiness standards

6. Safety Definitions
Concise Oxford Dictionary - Being safe, freedom from danger
MIL-STD 882 – Freedom from those conditions that can cause death, injury, occupational illness, damage to or loss of equipment or property, or damage to the environment
FAA System Safety Handbook – Freedom from all forms of harm.
British Standard 4778 – The freedom from unacceptable risks of personal harm

7. What is an Acceptable Level of Safety?
A relative concept based on freedom from danger or risk
Involves consideration of:
Severity of the effect
Certainty of the occurrence
Reversibility of the effect
Knowledge or familiarity of the risks
Voluntary acceptance of the risk
Compensation for the risk
Advantages of the activity
Risks and advantages of the alternatives

8. Safety Targets - Determination
Consider all consequences including both risks and benefits
Acceptance by both individuals and societies in general
Precedent of other regulatory organizations
What is reasonable and practical
An acceptable level of safety could be defined as the point when the benefits outweigh the risks from either an individual or a society perspective.

9. Regulatory Authorities
Airworthiness Regulatory Authorities conduct risk-benefit tradeoffs and decide what level would be acceptable considering:
Requirements
Impact on industry
Technology available
Input from stakeholders including public interest organizations
Action by other regulatory agencies

10. What Is Safety?
From a technical perspective Safety is a design attribute which is part of the overall development process.
Safety properties:
Safety has no absolutes
Safety is non deterministic
Accident rates are generally very small

11. Probability of Deaths per year
How Safe Is Safe?
Probability of Deaths per year
Activity
1 in 100
five hours of solo rock climbing every weekend
1 in 5,000
work in the UK coal mining industry
1 in 50,000
taking the contraceptive pill
1 in 500,000
passenger in a scheduled airline
1 in 1 million
electrocution in the home
1 in 10 million
Lightning in the UK
Source: UK MOD “What is safety”

12. How Safe Is Safe? -Transport
Mode of Transport
Accident rate per 100,000 hours
Fatality Rate
Civil aircraft – airline1
0.7
0.1
Civil aircraft - commuter1
3.7
0.4
Rail Travel2
0.06
0.02
Marine2
1.9
0.08
Motor Vehicles2
0.53
0.01
Source:
1 – Transport Canada 5 year average (1993 – 1997)
2 – DND/DGAEPM Airworthiness Risk Assessment Report (1996)

13. Aircraft Accident Cause Factors
Technical Causes
Airframe structural failure
Landing gear failure
Fire
Engine failure
System failure
Operational Causes
Weather
Controlled Flight Into Terrain
Undershoot
Overshoot

14. Prevention – Technical Causes
Structural Failure (including landing gear) – safe life, fail safe, damage tolerance
Fire – fire prevention and control technology
Engine Failure - safe life, fail safe, damage tolerance, health monitoring
System Failure – fail safe, system safety assessment process

15. Prevention – System Failures
Largest technical cause of aircraft accidents
Prevention of accidents due to system failures is one of the primary concerns of civil airworthiness regulatory authorities (FARs , & )
Severity Categories
Catastrophic
Hazardous
Major
Minor
No Effect

16. Acceptable Level of Safety – Civil
The accident rate for large civil transport aircraft has been steadily declining since the early 60’s
Generally an accident rate of 1 per million flight hours has been considered acceptable for large civil passenger transport aircraft
Therefore the probability of a serious accident should be not greater than one per million flight hours (1 x 10-6)

17. Acceptable Level of Safety – Civil
System failures account for 10% of accidents (probability of occurrence of 1 x 10-7)
100 potential failure conditions that could have a Catastrophic effect
Target average probability of occurrence established as 1 x 10-9 for each failure condition with a Catastrophic effect
General principle - inverse relationship should exist between a failure condition probability of occurrence and severity

18. Acceptable Level of Safety – Civil
Note: Civil Transport Category
Individual System
10-9
10-8
Probability
Acceptable
10-7
10-6
10-5
Unacceptable
10-4
10-3
10-2
10-1 1
Negligible
Catastrophic
Severity

19. Acceptable Level of Safety – Military
Most military airworthiness authorities have not published military airworthiness design standards
Reliance on civil regulatory material for military type certification and design change certification programs
Airworthiness design standards (FARs, CARs, JARs, etc)
Associated advisory material (FAA Advisory Circulars, RTCA DO-178B, RTCA DO-254, SAE ARP4754, SAE ARP4761, etc)
Acceptable levels of safety have generally not been established for military aircraft types by military airworthiness authorities. However, studies indicate that a higher risk level is considered acceptable for military aviation…..
and a factor of 10 is often used when comparing acceptable accident rates for equivalent military and civilian aircraft types. Therefore, a probability of occurrence of 10-8 per hour for catastrophic effects on a military transport category aircraft type (equivalent to a civil aircraft type) should be considered reasonable…..
Since helicopters and fighters are considered higher risk than transport aircraft, as demonstrated by their accepted higher accident rates, a greater probability of occurrence for catastrophic effects can be expected.

20. Acceptable Level of Safety – Military
Civil processes provides an excellent basis for military aircraft programs
Civil target levels may be problematic for military aircraft, equipment or missions
Military/Civil Gaps
Handling qualities
Weapons and stores
Self defence suites
Wartime operations
Military role/mission/task - operational necessity
Operational and usage environment
Rapid advances of military technology

21. Handling Qualities
Civil aircraft handling quality requirements do not adequately address military tactical role/mission/task requirements in the intended operating environment

22. Weapons and Stores
Civil airworthiness standards have no equivalent to military weapons and stores

23. Self Defence Suites
Military aircraft operate in a hostile environment requiring the use of chaff, flares and other self defence technology

24. Wartime Operations
Military wartime operations include extremely hazardous missions under conditions of operational necessity

25. Military Roles/Missions/Tasks
Many military roles/missions/tasks are unique and have no civil equivalent

26. Environment and Usage
Military aircraft often operate in a harsh environment which is more severe than equivalent civil aircraft types

27. Military Technology
Military performance requirements demand rapid advances in technology which may often be implemented before they are mature

28. Acceptable Level of Safety – Military
Application of civil standards must be done with judgment, care and forethought
Difficult to separate military mission and airworthiness requirements
Traditionally military equipment qualified to performance requirements rather than certified to minimum essential safety requirements
No equivalent civil standards exist for military unique equipment

29. Acceptable Level of Safety – Military
Civil airworthiness design standards are generally based on a specific aircraft category intended for use within a defined operational environment
A higher accident rate should be considered acceptable for military aircraft
Factor of 10 is often used in comparing a military aircraft type with an equivalent civil aircraft type

30. Acceptable Level of Safety – Military
Note: Transport Category
Individual System
Military Transports
10-9
Probability
Acceptable
10-7
Civil Transports
10-5
Unacceptable
10-3
10-1
Negligible
Catastrophic
Severity

31. Acceptable Level of Safety – Military
Note: Military Aircraft Types
Individual System
Military Transports
10-9
Acceptable
Probability
Military Helicopters
10-7
Military Jets
A comparison of military aircraft types is depicted in this slide.
10-5
Unacceptable
10-3
10-1
Catastrophic
Negligible
Severity

32. Acceptable Level of Safety – Military
More flexibility required for military aviation than just defining level of safety as acceptable / unacceptable
UK MOD - As Low As Reasonably Possible (ALARP)
MIL-STD Risk Index (defined as a function of severity and probability of occurrence)
DND/CF TAM Risk definitions
Extremely High Risk - Normally unacceptable
High Risk - May be acceptable
Medium Risk - Should be acceptable
Low Risk - Acceptable
Military aviation requires more flexibility than just defining a level of safety as acceptable / unacceptable. Recall from Airworthiness Fundamentals a military aviation safety program must account for both peacetime and wartime operations. In addition it must cover a more hostile environment and the use of weapons. Furthermore as an armed instrument of the government mission accomplishment for a military may on occasion take precedence and thus create conflicts with aviation safety objectives…...
To account for these factors Risk or the chance of injury or loss is measured and a Risk Index assigned.
Risk Index is defined as a function of consequence severity and probability of occurrence. (I.e. Catastrophic consequence severity and an improbable probability of occurrence.)
Technical Airworthiness Program defines four Risk Indices as follows:
Extremely High Risk - Normally unacceptable
High Risk - May be acceptable
Medium Risk - Should be acceptable
Low Risk - Acceptable

33. Acceptable Level of Safety – Military
10-8
Low Risk
(acceptable)
Probability
Medium Risk
(should be acceptable)
10-6
High Risk
(may be acceptable)
10-4
This slide shows the risk categories for a typical military transport type aircraft. If you recall on a previous slide for a civil transport aircraft the division between acceptable and unacceptable for a catastrophic event was 10-9.
This slide only demonstrates that the different risk categories reflect different levels of safety. Do not pay attention to the intersection of these curves with the vertical axis - this is not an exact science.
Extremely High Risk
(normally unacceptable)
10-2 1
Catastrophic
Negligible
Severity

34. Conclusion
Acceptable Level of Safety is generally based on an acceptable accident rate
The associated probability of occurrence for military aircraft types should be higher than the equivalent civil aircraft type
Acceptable Level of Safety for military aircraft types may be based on a risk assessment process

35. Recommendation
Need a forum for military aviation authorities to discuss airworthiness for military aircraft types
Defence industries need to present the problems associated with the application of civil standards on military aircraft programs
Closer cooperation/liaison between civil and military airworthiness authorities

36. “If we die, we want people to accept it
“If we die, we want people to accept it. We are in a risky business, and we hope that if anything happens to us it will not delay the program. The conquest of space is worth the risk of life.”
-Astronaut Virgil I. Grissom, 27 January 1967
Paraphrased: If we die, we want people to accept it. We are in a risky business, and we hope that if anything happens to us it will not delay the program. The need for military aviation is worth the risk of life.

37. QUESTIONS