SYS-RAMS

A condition having an effect on the aircraft and/or its occupants, either direct or consequential, which is caused or contributed to by one or more failures or errors, considering flight phase and relevant adverse operational or environmental conditions or external events.

A condition with an effect on the aircraft and its occupants, both direct and consequential, caused or contributed to by one or more failures, considering relevant adverse operation or environmental conditions.

The way in which the failure of a system or item occurs.

The consequence(s) a failure mode has on the operation, function or status of a system or an item.

Loss of function

• Total loss -> the function cannot be performed anymore
• Partial loss -> the function can still be performed somehow but:
  • reduced effectiveness
  • increased difficulty
  • using alternative means

e.g. loss of one (partial) or all (total) hydraulic systems

Malfunction (also called Erroneous Operation)

• The function is performed incorrectly

e.g. erroneous indication of airspeed

The Aircraft Functional Hazard Assessment (AFHA) is a process which allows the identification and evaluation of potential hazards related to an aircraft regardless of the details of its design.

It is performed early in the development process and is used to guide the development of the aircraft to achieve a safe design.

basically, installing a new part shall not prevent other systems or parts to work propperly.

To reduce, lessen, or diminish a hazard to the least practical amount with current technology and materials.

(concept somtimes called ALARP: as low as reosanably practicable)

The concept of DAL has been introduced to minimize the number of errors that will remain at the end of the developent of Software and Complex Electronic Hardware.

It is devided into the four following levels:

DAL A: errors are Extremely Improbable and occure once in less than 10⁹ flight hours

DAL B: errors are Extremely Remote and occure approximately once between 10⁷ and 10⁹ flight hours

DAL C: errors are Remote and occure approximately once between 10⁵ and 10⁷ flight hours

DAL D: errors are Probable and occure approximately once between 10³ and 10⁵ flight hours

The analysis is performed at functional level. (Funktionen auf kleinstem Niveau)
the functional FMEA us mostly used for electrical components, where it wouldn't make sense to analyse every single capacitor, resistor etc. 

The analysis is performed at component level. (Effektiv jedes einzelne Bauteil wird angeschaut)

Basically, only the approach is different (top down and bottom up), the results should be the same!

FMEA: what is the effect of this part failing
FHA: what component can cause this function to fail

f(t) expresses the probability of a specific failure occuring at a specific time t.

F(t) is the probability that a specific failure as occured up until a specific time t. It's derrived by integrating the PDF f(t).

\(F(t) = \int_{0}^{t} f(t)dx \\ F(0) = 0 \\ F(\infty) = 1\)

R(t) = the probability that a specific failure hasn't occured up until a specific time t.

It's derrived by subtracting F(t) from 1

\(R(t) = 1 - F(T) = \int_{t}^{\infty} f(x) dx \\R(0) = 1 \\R(\infty) = 0\)

Often it is assumed that 1 Flight Hour = 1.25 Operating Hour

By integrating the Reliability function R(t)

\(MTTF = \int_{0}^{\infty} R(t) dt\)

The term MTTF is used for non repairable items

The term MTBF is used for repairable items

It illustrates what types of failure (early life, useful life and wearout life) are caused by a time-dependant lambda(failure rate)

If the failure rate (lambda) is time-dependant and decreases with time.

If the failure rate (lambda) is approximately constant and doesn't change with time.

If the failure rate (lambda) is time-dependant and increases with time.

In a qualitative FTA  minimal combinations of components failure resulting in system
failure are analysed.

in a quantitative FTA  the probability or frequency of the specific system failure are determined

The FTAmay be used to:

• Quantify probability of occurrence
• Evaluate proposed system architecture:
  • Hardware reliability budgets
  • Development Assurance Levels (A/B/C/D)
  • etc.
• Assess the impact of a design modification
• Identify the need for a design modification
• Identify unique situations that require special attention
• Show compliance with qualitative and/or quantitative safety objectives
• Provide a visual aid to qualitatively illustrate the significance of the software with respect to the failure condition classification of a top level event.
• Establish crew and maintenance tasks and intervals

The period of time during which an item must fail in order to cause the failure effect in question. This is usually associated with the final fault in a fault sequence leading to a specific failure condition.

Time period between
Time when the item was last known to be operating properly
Time when it will be known to be operating properly again

A Minimal Cut Set is the smallest combination of basic events causing the top event:

• All the basic events need to occur to cause the top event
• A MCS is a combination of basic events
• The complete set of MCS provides the complete set of causes of the top event