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Reliability block diagram templates

RBD — Fly-by-Wire Pitch Channel

Triplex air data, duplex flight control computers and two independent hydraulic-plus-actuator lanes over a year of utilisation, ranking which redundancy actually moves the loss-of-pitch-control number.

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Fly-by-wire pitch control channelMission 5 000 h · 208 d · 10 blocks · 3 redundant arrangements · R 0.9525502 of 3 · R 0.999999Air data computer 1R 0.999400Air data computer 2R 0.999400Air data computer 3R 0.999400ARINC 429 databusR 0.999775parallel · R 0.999982Flight control computer AR 0.995759Flight control computer BR 0.995759parallel · R 0.952783Hydraulic system greenR 0.818731Elevator actuator leftR 0.955997Hydraulic system yellowR 0.818731Elevator actuator rightR 0.955997System reliability0.952550Unreliability0.0474System MTTF30 556 h · 3.49 yrAvailabilityno mttr givenReliability importance — Birnbaum ∂R/∂Rᵢ, with criticality share1ARINC 429 databus0.952765crit 0.45%2Hydraulic system green0.207683crit 79%3Hydraulic system yellow0.207683crit 79%4Elevator actuator left0.177863crit 16%5Elevator actuator right0.177863crit 16%6Flight control computer A0.004040crit 0.04%7Flight control computer B0.004040crit 0.04%8Air data computer 10.001142crit 1.44e-3%+2 more blocks not shownWeakest link: ARINC 429 databus — R 0.999775, Birnbaum 0.952765, 0.45% of system failure. Improving this one block moves the system number further than improving any other.Redundancy check: removing any one of the 7 branches makes its own group measurably more likely to fail — the least of them by 360%. Every one of them is earning its place.

Make it your own.

title "Fly-by-wire pitch control channel"
mission 5000h                # one year of airline utilisation

# Avionics rates in FIT (failures per 10^9 hours), as the vendor data sheets
# state them; the engine converts to a mission reliability.
block adc1  "Air data computer 1"       rate: 120 fit
block adc2  "Air data computer 2"       rate: 120 fit
block adc3  "Air data computer 3"       rate: 120 fit
block bus   "ARINC 429 databus"         rate: 45 fit
block flccA "Flight control computer A" rate: 850 fit
block flccB "Flight control computer B" rate: 850 fit
block hydG  "Hydraulic system green"    rate: 4.0e-5 /h
block hydY  "Hydraulic system yellow"   rate: 4.0e-5 /h
block actL  "Elevator actuator left"    rate: 9.0e-6 /h
block actR  "Elevator actuator right"   rate: 9.0e-6 /h

series {
  kofn 2 of 3 { adc1; adc2; adc3 }
  bus
  parallel { flccA; flccB }
  parallel {
    series { hydG; actL }
    series { hydY; actR }
  }
}