Design for reliability and safety

Question 1

Why is reliability important

Answer 1

people (customers) expect high reliability from products

Question 2

Discuss the reliability of commercial aircraft

Answer 2

in 2017 there was no deaths from commercial air flight despite a huge number of flights occurring

The high reliability of aircraft is partly the result of reliability being designed into aircraft systems

Question 3

Discuss the reliability of cars

Answer 3

A typical car engine lasts over 100,000 miles

Reliability has to be designed into the product

Question 4

Give three examples of where reliability has been designed into a product

Answer 4

1. aircraft
2. cars
3. washing machines

Question 5

Defines single point failure

Answer 5

single failure leading to catastrophic failure

Question 6

Define common mode failure

Answer 6

single effect that causes several sub systems to fail

Question 7

Define cascade failure

Answer 7

one failure that leads to another etc

Question 8

Define fault intolerant system

Answer 8

single failure causes the system to fail

Question 9

Define fail safe

Answer 9

fail in a non-catastrophic way

Question 10

Define safe life

Answer 10

period of safe operation

Question 11

What does MTBF stand for

Answer 11

mean time between failure

Question 12

How is mean time between failure (MTBF) calculated

Answer 12

MTBF = 1/failure rate (gamma)

Question 13

What is the failure rate (gamma)

Answer 13

number of failures per time period t

Question 14

At t = MTBF, the reliability = ____

and the failure = ____

Answer 14

reliability = 0.37
failure = 0.63

Question 15

What are the three types of failure distribution (sketch them)

Answer 15

1. Exponential
2. normal
3. weibull

Question 16

What is component reliability R(t) defined as

Answer 16

R(t) = probability of survival as a fraction

R(t) = number of surviving parts after time t/(Total number)

Question 17

For a constant failure rate what is the equation for component reliability

Answer 17

R(t) = e ^((-gamma)t)

Question 18

What does R(t) stand for

Answer 18

probability of component surviving after time t

Question 19

what does gamma stand for

Answer 19

failure rate (per hours)

Question 20

How do you calculate gamma (failure rate)

Answer 20

gamma = -(lnR)/t

Question 21

What does t stand for

Answer 21

time (hours)

Question 22

for a constant failure rate: R(t) = e^((-gamma)t

When t=0 R=1 meaning..

Answer 22

no possibility of failure

Question 23

for a constant failure rate: R(t) = e^((-gamma)t

When t=infinity R=0 meaning..

Answer 23

certainty of failure

Question 24

What is the equation for component failure after time t (F(t))

Answer 24

F(t) = 1 - R(t)

Question 25

Sketch the profiles of reliability and failure probabilities with respect to time

Answer 25

SKETCH

Question 26

Draw a representation of a product that consists of a number of components that must all work for the product to function. How is the reliability of this system calculated

Answer 26

SKETCH Rtotal = R1 x R2 x R3 x R4

Question 27

Draw a diagram for a fault intolerant system and describe how it can fail

Answer 27

failure of any component will cause failure of the system SKETCH

Question 28

Draw a representation of a product that has a 'one in two' component redundancy How is the reliability of this system calculated

Answer 28

SKETCH Rtotal = R1 x R4 x R2+3 where R2+3 = (R2+R3)-(R2xR3)

Question 29

Draw a diagram for a fault tolerant system and describe how it can fail

Answer 29

there is some tolerance to failure of some components. The system will still function if only components 2 or 3 fail - same as 'one in two' redundancy SKETCH

Question 30

Give examples of two component redundancy systems

Answer 30

1. Two computers on an aircraft 2. Two tape players on a hi-fi 3. Hospital generator for power supply

Question 31

Draw a representation of a product that has a 'one in three' component redundancy How is the reliability of this system calculated

Answer 31

SKETCH Ra+b+c = 1-(1-Ra)(1-Rb)(1-Rc)

Question 32

Is it better to have four small or two big engines on an aircraft

Answer 32

Four small engines are more reliable than two big engines But it is often more expensive to have a larger number of smaller engines

Question 33

What are the 6 steps for reliability modelling

Answer 33

1. Identify components 2. Calculate reliability of each component for time t from given failure rate (gamma) 3. Formulate block diagram 4. Calculate reliability 5. Identify areas of low reliability 6. Devise methods for improving reliability

Question 34

Give examples of fail safe systems

Answer 34

1. Overheating in an electrical product causes a fuse to fail and disconnects the electrical supply 2. A ductile failure in a bicycle wheel rim prevents catastrophic loss of structural support 3. most modern cars are fitted with twin hydraulic brake circuits, with two mast cylinders in tandem, in case one should fail

Question 35

Give examples of design features that ensure fail safe

Answer 35

1. Electrical fuses 2. electrical circuit breakers 3. kill cord on speed boats that switches acceleration off 4. protect kevlar liners in aircraft fuel tanks to stop or reduce fuel leakage in the event of impact 5. shields around aircraft engines to prevent/reduce debris flying out in the event of engine failure

Question 36

Give the case study of the padstow speed boat disaster as an example of the importance of a fail safe system

Answer 36

A family were on a speed boat and no one was attached to the kill cord. The kill cord turns the accelerator off if the cord is pulled from the controls, where as a car requires constant force to keep the accelerator down, a boat often has a fixed accelerator due to the motion of the boat preventing an accelerator similar to a car The boat did a sharp turn and everyone fell off board and the engine kept going resulting in serious injuries and death

Question 37

Give the case study of the Concorde disaster

Answer 37

The concorde aircraft went over debris on the runway at take off which resulted in a large fire, stopping two engines from working and the aircraft crashing. Investigators concluded that failure (fuel leakage) would have been reduced if the aircraft had adequate shielding

Question 38

Describe how the concorde disaster could be described as a cascade failure

Answer 38

A cascade failure involves one initial failure leading to another, then another and so on with these subsequent failures getting more and more out of control 1. an engine wear strip had not been installed or manufactured properly which led to it falling of an aircraft on the runway 2. the concorde ran over the engine strip and burst a tyre 3. debris from the tyre hits the fuel tank 4. pressure wave ruptures the weakest part of the tank 5. fuel catches fire due to hot engines 6. two engines stop worked 7. plane turns and pilots reduce power on the two other engines 8. plane stalls and crashes

Question 39

What does FMECA stand for

Answer 39

failure modes effects and Criticality Analysis

Question 40

What are the measures in a FMECA table

Answer 40

1. Occurence 2. Severity 3. Detectability

Question 41

What does RPN stand for and how is it calculated

Answer 41

Risk Priority Number RPN = O x S x D

Question 42

What would an FMECA table tell you

Answer 42

It tells you what would be the worst type of failure so that it can be mitigated

Question 43

What does FTA stand for

Answer 43

Fault tree analysis

Question 44

What is a fault tree analysis (FTA) and why is it used

Answer 44

The objective of a FTA analysis is to model or record how a failure or disaster can occur based on a series of events Events can either be combined with an AND box or specified as independent events with an OR box. FTA is used to help understand a past failure or predict how a future failure might occur