Safety analysis of aircraft systems

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Safety analysis of aircraft systems

• In aviation, safety is defined as the absence of
  accidents and incidents.
• JAR 25 treats systems as a whole.
• Acceptable accident rates must be established,
  100 safety can never be guaranteed.
• A relationship must be established between
  severity of effect and probability of occurrence.

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Probability versus severity of effect
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The principle of graceful degradation

• In any system the failure of a single element,
  component or connection should not prevent
  continued safe flight and landing.
• This single failure should also not lead to an
  unacceptable workload for the operating crew.

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Types of failure to be considered

• Single active failure
• Passive and undetected (dormant) failures
• Combinations of independent failures
• Common-mode failures
• Cascade failures
• Failures produced by the environment

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Errors

• Design errors
• Manufacturing errors
• Maintenance errors
• Pilot mismanagement
• Errors in manuals or checklists

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Dormant failure

• Reverser is deployed!
• Lauda Air B767 , 26-5-1991
• Design errors in the thrust reverser electric
  systems led to unobserved deterioration of the
  HIV valve

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Common-mode failure

• Whatever you do, keep us away from the city!
• UA 232, 19-6-1989, Sioux City, Iowa.
• No. 2 engine fan disc disintegration severed all
  3 hydraulic lines in the tail area.
• Exceptional flying by the crew led to a landing
  at Sioux airport

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Cascade failure

• THY 981, 3-7-1974, Paris
• Inadequately closed lower deck door opened,
  causing floor collapse
• This blocked the flying control runs under the
  floor, causing catastrophic failure

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Failure rates in light single engined aircraft

• Engine failure. A minimum demonstrated flying
  speed must be 61 kts or below, to enable a
  succesful off-airport landing.
• Instrument systems for IFR operations must be
  dual and independent. Vacuum pump MTBF 700 hrs.
• Prevention of flap asymmetry must be adequate

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A few examples

• Cessna 172. Seat rails, flap system, elevator
  control
• Piper PA 28 wing attachment
• Robinson R22 helicopter, mast bumping

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Current concerns for GA

• Inadequate training
• Inadequate currency
• Insufficient pilot ability
• Lack of familiarity with the full flight envelope
• Inadequate understanding of increasingly complex
  systems

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