Dealing adequately with technical uncertainties

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FMEA Mil-STD-1629 Task 102 Criticality Analysis
  FMEA, Introduction
  Example 1: Mil-STD-1629 Piece Part FMEA
  Example 2: Mil-STD-1629 Criticality Analysis
  Example 3: Automotive FMEA
  General concerns with FMEA

The following example, Mil-STD-1620 criticality analysis, is a quantitative FMEA type.
Mil-Std-1629 Task 102

The main goal of this FMEA type is the number of failures to be
expected during a mission.



Column title
Description
1
Identification Number A unique Identifier of the item. In most cases, but not necessarily, a number.
2
Item / Functional Identification (Nomenclature) The name of the item.
3
Function
A function of the item.
There may be more than one function per item (2)
4
Failure Modes and Causes Potential failure modes of the function and potential causes.
Note: Most FMEA types offer separate columns for failure modes and causes.
There may be more than one failure mode or cause per function (3), and there may be more than one cause per failure mode.
5
Mission Phase / Operational Mode The mission phase or operational mode in which the failure mode may occur.
Note: Mission phases can be very different depending on the nature of the system in which the item is built in (e.g. aircraft, ground vehicle, submarine,...)
6
Severity Classification Classification of the severity of the failure mode or its effects. Military FMEAs use 4 or 5 classification levels. 
7
Failure Probability /
Failure Rate Data Source
Failure Probability: This is misleading and should be the failure rate of the item, because it would be more in line with the following columns. 
There are different ways to determine item failure rates. In particular for electronic equipment, there are some calculation standards available.
Failure Rate Data Source: The source or the method how the failure rate was determined (e.g. by engineering judgment, Mil-217, etc.).
The failure rate as well as the following columns make this FMEA type quantitative. 
8
Beta, Failure Effect Probability
In most cases = 1. The probability that the failure mode is accountable.
Example: Electrical connector with 10 pins. The failure modes for this connector shall be "open", "short" and "intermittent contact".
If the failure mode, for example, applies only for 2 out of 10 pins, then beta would be 0,2.
9
Alpha, Failure Mode Ratio
Percentage of the failure rate, that applies for the specific failure mode.
Example: Electrical connector with 10 pins. The failure modes and percentages for this connector shall be
Failure Mode
Alpha
Open
50%
Short
20%
Intermittent contact
30 %
 
10
Lambda, Failure Rate
This is again misleading and should be the mode failure rate. Typically expressed in failures per million hours (fpmh).
11
t, Operating Time
A characteristic time, typically a mission time or the time between two preventive maintenance actions.
12
Failure Mode Criticality,
Beta x Alpha x Lambda x t
The product of the previous four columns: Beta x Alpha x Lambda x t.
The result is "absolute expected frequency of this specific failure mode per time t"
13
Item Criticality
Sum of all failure mode criticalities of the item (column 2).
The result is "absolute expected frequency of all failure modes of the item per time t."
14
Remarks
A very important column. Put here everything that doesn't fit into the other columns, or which is important for other reasons.


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