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Bit effectiveness

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Bit effectiveness

  1. 1. BUILT IN TEST (BIT) EFFECTIVENESSBIT Effectiveness DefinedBIT Effectiveness (BITEFF) is the probability of obtaining the correct operationalstatus of the Controller using BIT during the mission phase of the aircraft. It is afunction of:a) Total Controller Failure Rateb) Fault Detection Capabilityc) False Alarm Probabilityd) The operating time (T) required to conduct BITBITEFF = e − FR1.T .(1+ FAP)  +  FDC .1 − e − FR1.T .(1+ FAP)     1 + FAP        FDC Where: Minimum (Worst Case, when T → ∞ ) BITEFF = (1 + FAP ) FDC = Fault Detection Capability FAP = False Alarm Probability FRD Where: FDC = FRD = BIT Detectable Controller Failure Rate FR1 FR 1 = Total Controller Failure Rate FRFA Where: FAP = FRFA = BIT False Alarm Rate, Note 1 FR2 FR 2 = FR1 - (Failure Rate of BIT Circuitry) Note 1: The prediction or measurement of False Alarm Rate is very difficult. Design steps have to be taken to reduce False Alarm RateFault Detection Capability (FDC) Requirement ?False Alarm Rate Requirement ? Customer SpecifiedHilaire Perera
  2. 2. The percent of false alarms is a difficult parameter to measure accuratelybecause an initial fault detection followed by an analysis indicating that no faultexists can signify several different occurrences, such as:• The BIT system erroneously detected a fault.• An intermittent out-of-tolerance condition exists ….. somewhere• A failure exists but cannot be readily reproduced in a maintenance environmentThe percent of false removals can be a more difficult problem to address. Falseremovals may be caused by:• Incorrect BIT logic.• Wiring or connection problems which manifest themselves as faulty equipment• Improper match of tolerances between the BIT and test equipment at the next higher maintenance levelThe resolution of each type of false alarm and false removal requires asubstantially different response. From a logistic viewpoint, false alarms oftenlead to false removals creating unnecessary demands on supply andmaintenance systems. Of potentially more concern is the fact that false alarmsand removals create a lack of confidence in the BIT system to the point wheremaintenance or operations personnel may ignore fault detection indications. BUILT IN TEST (BIT) EFFECTIVENESS Example −6 FR1 := 42 ⋅ 10 Total CS Failure Rate (Failures/Hour) FAP := 0.01 False Alarm Probability FDC := .99 Fault Detection Capability T := 30 Operating Time (Hours) Required to conduct BIT − [ FR1 ⋅ T ⋅ ( 1+ FAP) ]  FDC  − [ FR1 ⋅ T ⋅ ( 1+ FAP) ] BITEFF := e   +  ( 1 + FAP )  ⋅ 1 − e     BITEFF = 0.999975 −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− FAP := 0.20 False Alarm Probability − [ FR1 ⋅ T ⋅ ( 1+ FAP) ]  FDC  − [ FR1 ⋅ T ⋅ ( 1+ FAP) ] BITEFF := e   +  ( 1 + FAP )  ⋅ 1 − e     BITEFF = 0.999736Hilaire Perera

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