1. BUILT IN TEST (BIT) EFFECTIVENESS
BIT Effectiveness Defined
BIT Effectiveness (BITEFF) is the probability of obtaining the correct operational
status of the Controller using BIT during the mission phase of the aircraft. It is a
function of:
a) Total Controller Failure Rate
b) Fault Detection Capability
c) False Alarm Probability
d) The operating time (T) required to conduct BIT
BITEFF = 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
Rate
Fault Detection Capability (FDC) Requirement ?
False Alarm Rate Requirement ?
Customer
Specified
Hilaire Perera

2. The percent of false alarms is a difficult parameter to measure accurately
because an initial fault detection followed by an analysis indicating that no fault
exists 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
environment
The percent of false removals can be a more difficult problem to address. False
removals 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 level
The resolution of each type of false alarm and false removal requires a
substantially different response. From a logistic viewpoint, false alarms often
lead to false removals creating unnecessary demands on supply and
maintenance systems. Of potentially more concern is the fact that false alarms
and removals create a lack of confidence in the BIT system to the point where
maintenance 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 = 0.999736
Hilaire Perera