Reliability & Maintenance Management: Landing Gear of SAAB 340b.

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Reliability and Maintenance Management:

Landing Gear of Saab 340b:

Rishab Anand (12002640)

Department of Aircraft Engineering,

Perth College,

University of Highland and Island

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CONTENTS:- Page no.

Introduction:- 03

Background :- 03

AIM:- 03

Scope of Investment:- 03

Relevance of Argument:- 04

System definition and description:- 04

Schematic Diagram:- 05

Operational and Functional Statement:- 05

Components and there reliability and failure rates :- 06

Quality of Evidence:- 08

RBD:- 08

Pareto Analysis:- 10

Fault Tree Analysis:- 11

FMECA :- 13

Decision Tree Analysis :- 14

Conclusion:- 16

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Introduction:-

Background :-

Landing gear is the most important structure of an aircraft, as it is solely responsible for the

aircraft to do all the ground operations from taxing to take- off and landing, but it was found

that landing gear creates a massive drag when the aircraft is in air. So, in order to reduce

the drag and increase the efficiency of aircraft, landing gear is retracted when the aircraft is

in air and extended just before reaching ground. Now coming on to the other subject of this

report, Saab 340B is a twin turboprop engine aircraft, with a tricycle landing gear

configuration.

Aim :-

The main aim of this report is to see how reliable the landing gear extension and retraction

system is, for the Aircraft Saab 340B.

Scope of Investment:-

The Scope will be in the operation of extension and retraction of landing gear. MSG 3

analysis can be done on this to know about the different ways of feasible maintenance. It

will further provide ways to reduce the risk factors and failure rates. Analysis like FMECA,

Decision tree, and others gives us easy ways to fight with the risks associated with the

system.

RCM:-

RCM or Reliability Centred Maintenance is a basic process which ensures that the Landing

Gear extension and retraction is operating as perfectly as it is assigned to, and provide

answers to a few basic questions like “What is the item supposed to do and its associated

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performance standards? In what ways can it fail to provide the required functions? What are

the events that cause each failure? What happens when each failure occurs? In what way

does each failure matter? What systematic task can be performed proactively to prevent, or

to diminish to a satisfactory degree, the consequences of the failure? What must be done if

a suitable preventive task cannot be found?” (Handle Consulting and Training, 2013)

Relevance of argument:-

System definition and description:-

Saab 340B has a conventional tricycle landing gear, with each gear having duel wheels. Its

nose gear is located in the fuselage and the main gear is located in the engine nacelles.

Retraction and extensions are basic necessities of an Aircraft. When the aircraft is airborne,

the landing gear has no basic role to play. So, it was found that the landing gear was

creating an awful amount of drag. In the crisis of sustainable energy, and the demand of the

market for making money, the extension and retraction system was initiated in aircrafts. It

was also found out that if the drag can be reduced, fuel burn will get reduced. It will get

reduced because of more thrust generation. More thrust will generate more Lift and more

velocity, and this is achieved by the same amount of engine power that was used before .

This will reduce the fuel usage, and will also reduce the time required for journey. And thus

it can boost the aviation sector by bringing cheap and environmental friendly aviation, and

can make lots of money as well for the airlines. It was after that the used of Landing gear

extension and retraction being implemented in the industry in all the aircrafts. In SAAB 340

B, all of the landing gears of the aircraft retract in forward direction. The gears can be

extended and retracted in normal mode, and can also be extended in emergency mode.

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SCHEMATIC DIAGRAM : -

(SAAB340B)

Operational and Functional Statement:-

“A mechanical lock on the landing gear control handle prevents selection of gear up on

ground. In flight this lock is disengaged by a solenoid. Should the solenoid fail, the handle

can be released by pressing the down lock release button to the left of the gear handle.”

(SAAB340B) When the aircraft is in air the handle performs the working of the landing gear

extension and retraction from the cockpit. When the handle is turned up, an up signal is

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transmitted to the gear selector, with the function of hydraulic pump and the accumulator, a

hydraulic pressure is ported to release the down lock and pressurize the hydraulic

actuators. When the landing gears are fully retracted, an up lock hook engages the rollers

on the landing gears and helps the gears to stabilize in their respective positions. When the

handle is selected in down position, the gears are selected down as well, and in the same

way the hydraulic pressure is ported to disengage the up locks and the actuators. Further,

when the landing gears are fully extended, the down locks engage the landing gears in their

respective position. If the normal way of extension fails for the landing gear, it can be

extended in emergency mode as well. Normal failure can be of two types. Failure in the

hydraulic pump, with other hydraulics working or a case of complete hydraulic failure. If

there is a normal failure, the extension can be processed by selecting the hand pump

instead of hydraulic pump in the centre position, and then selecting the handle down.

In the case of complete hydraulic failure, the gear can be extended by an emergency

system. This is a complete different system for extension and has no direct links with the

normal extension system. There is an emergency handle present in the floor of the cockpit.

When this handle is pulled, the gear actuator pressure line prevents the activation or

functioning of any hydraulic locks. Emergency accumulator ports hydraulic pressure which

releases the hydraulic up locks. Once the up lock is released, the landing gears will extend

because of gravity. Since the landing gear retracts foreword, the gears are locked back

because of drag.

Components and there reliability and failure rates :-

Components Moving Parts ( High /

Low )

Failure

Rate/hour, λ

Reliability, Rt

= e- λt

ID

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Solenoid V. Low 1 x 10-8 0.99999999 1

Main Handle Low 1 x 10-6 0.999900005 2

Gear Selector High 1 x 10-4 0.9900498 3

Accumulator V. Low 1 x 10-8 0.99999999 4

Hydraulic Pump High 1 x 10-4 0.9900498 5

Hand Pump High 1 x 10-4 0.9900498 6

Down lock and

hydraulic actuator

Moderate 3 x 10-5 0.997000 7

Up locks and

hydraulic actuator

Moderate 3 x 10-5 0.997000 8

Emergency

Extension handle

Low 1 x 10-6 0.999900005 9

Emergency Gear

selector

High 1 x 10-4 0.9900498 10

Emergency

accumulator

V. Low 1 x 10-8 0.99999999 11

Emergency Pump High 1 x 10-4 0.9900498 12

Landing Gear Moderate 3 x 10-5 0.997000 13

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ID is the identification number of each component used to describe the component for

further reference.

The reliability is calculated by the formulae Reliability, Rt = e- λt

The time is 100 hours, as more hours will give a closer value of reliability.

Quality of Evidence:-

RBD:-

“A reliability block diagram (RBD) is a diagrammatic method for showing how

component reliability contributes to the success or failure of a complex system. RBD is also known

as a dependence diagram (DD).

A RBD or DD is drawn as a series of blocks connected in parallel or series configuration. Each

block represents a component of the system with afailure rate. Parallel paths are redundant,

meaning that all of the parallel paths must fail for the parallel network to fail. By contrast, any

failure along a series path causes the entire series path to fail.” (Aerospace Reserch Institute)

ULA : - Up lock and actuator.

Gear Selector

Handle

ULA

DLA

Accumulator

Hydraulic pump

Hand pump

L.Gear

Emergency Handle

Emergency Gear Selector

Emergency

Accumulator

S

Emergenc

y pump

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DLA : - Down lock and actuator.

S : - Solenoid

Resultant Reliability :-

For the two pumps in stand by redundancy net Reliability is calculated by

Rt = e- λt(1+ λt),

Rt = 0.9901488

For the Locks and actuator, the reliability is calculated by parallel formulae,

R = 1 - (1 – R1) (1 – R2)

R = 0.994000.

For the Emergency setup in series, the reliability is

R = R1 x R2 x R3 x R4

R = 0.980100581

Now the reliability of the emergency system and the normal system is taken into account as a

stand by system, and is calculated as:-

Rt = e- λt(1+ λt),

0.9900498 0.9999000

05

0.997000

0.997000

0.99999999

0.9900498

0.9900498

0.99700

0.9999000

05

0.9900498 0.99999999

0.9999

9999

0.9900498

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R = 0.981080682

Now the complete can be taken in series and can be calculated by the formulae

R = R1 x R2 x R3 x ........... x Rn.

R = 0.972268605 .

Henceforth, the net resultant reliability of the normal extension and retraction is found to be

0.972268605.

Pareto Analysis:-

It is a way of analysing the occurrence of any failure. It can be done by graphical way or by pi –

chart way. In this case the representation of the failure rate of all components is done using a pie –

chart method.

10 9 8

7 11

12

13

1

6

2

3

4

5

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The ID is mentioned from 1 to 13 in the above pie - chart and their failure rates are further

calculated in degrees and in percentage as well in the table bellow.

ID Components Angular value % value

1 Solenoid 27.81845538 7.7727348132

2 Main Handle 27.81567395 7.726575668

3 Gear Selector 27.54165646 7.650459702

4 Accumulator 27.81845538 7.7727348132

5 Hydraulic Pump 27.54165646 7.650459702

6 Hand Pump 27.54165646 7.650459702

7 Down lock and hydraulic

actuator

27.7373500029 7.704166319

8 Up locks and hydraulic actuator 27.7373500029 7.704166319

9 Emergency Extension handle 27.81567395 7.726575668

10 Emergency Gear selector 27.54165646 7.650459702

11 Emergency accumulator 27.81845538 7.7727348132

12 Emergency Pump 27.54165646 7.650459702

13 Landing Gear 27.7373500029 7.704166319

Fault Tree Analysis:-

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“Fault tree analysis (FTA) is a top down, Deductive reasoning failure analysis in which an

undesired state of a system is analyzed using boolean logic to combine a series of lower-level

events. This analysis method is mainly used in the field of safety engineering and Reliability

engineering to determine the probability of a safety accident or a particular system level

(functional) failure.

In Aerospace the more general term system Failure Condition is used for the undesired state / Top

event of the fault tree. These conditions are classified by the severity of their effects. The most

severe conditions require the most extensive fault tree analysis. These system Failure Conditions

and their classification are often previously determined in the functional Hazard analysis.

FTA can be used to:

understand the logic leading to the top event / undesired state.

show compliance with the (input) system safety / reliability requirements.

prioritize the contributors leading to the top event - Creating the Critical

Equipment/Parts/Events lists for different importance measures.

monitor and control the safety performance of the complex system (e.g., is a particular aircraft

safe to fly when fuel valve x malfunctions? For how long is it allowed to fly with the valve

malfunction?).

minimize and optimize resources.

assist in designing a system. The FTA can be used as a design tool that helps to create

(output / lower level) requirements.

function as a diagnostic tool to identify and correct causes of the top event. It can help with the

creation of diagnostic manuals / processes.” (Fault Tree Analysis )

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FMECA Proforma :-

“Failure mode effects and criticality analysis (FMECA) is an extension of failure mode and effects

analysis (FMEA). FMEA is a bottom-up, inductive analytical method which may be performed at

either the functional or piece-part level. FMECA extends FMEA by including a criticality analysis,

which is used to chart the probability of failure modes against the severity of their consequences.

The result highlights failure modes with relatively high probability and severity of consequences,

allowing remedial effort to be directed where it will produce the greatest value.” (University of

Ljubljana)

Its method Includes a few basic Steps :-

System definition

System Function

Functional Failure

Landing Gear Extension and Retraction

Lock and Hydaulic

Accumulator Emergency

Accumulator

Gear selector Hydraulic & Hand

Pumps

Emergency

Pump

Emergency

Gear selector

Emergency

handle

Handle

Solenoid

OR

And

And

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Failure Mode (Cause of Failure)

The aftereffects of failure

Failure consequences

Maintenance Approach

SYSTEM FUNCTION FUNCTIONAL FAILURE

FAILURE MODE (Cause of Failure)

EFFECT OF FAILURE

FAILURE CONSEQUENCES

MAINTENANCE APPROACH

Landing Gear Extension and retraction System

Retracts landing gear after take off to reduce drag and to extract before landing to land the aircraft safelt.

Total Failure Landing gear not gets retracted

No major effect can land back and checks for error.

No adverse consequences. No effect on flight and no safety of environment hazard

Pro active as it has the least possibility of occurrence.

Partial Failure Landing gear gets retracted but not gets extracted

Can lead to disaster. Impossible for aircraft to land without wheels.

Hazardous consequence. Can destroy the aircraft, kill lives on it, and destroy the environment around. Etc...

Proactive task is worth doing.

Decision Tree Analysis : -

“A decision tree is a decision support tool that uses a tree-like graph or model of decisions and

their possible consequences, including chance event outcomes, resource costs, and utility. It is

one way to display an algorithm. Decision trees are commonly used in operations research,

specifically in decision analysis, to help identify a strategy most likely to reach a goal.

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When dealing with information in decision analysis, there are often biases and errors in judgment,

such as the fact that people pay more attention to information that is easily available.

It is important for a decision maker to receive plenty of input from others to avoid any bias. A

visualization of a complex decision-making situation in which the possible decisions and their likely

outcomes are organized in the form of a graph that resembles a tree. In economics, utility is a

representation of preferences over some set of goods and services. Preferences have a utility

representation so long as they are transitive, complete, and continuous. Bias is an inclination

towards something; predisposition, partiality, prejudice, preference, predilection.” (Boundless)

Is the landing gear extension task technically feasible and worth doing?

NO YES

Is any other scheduled task,

with similar result, feasible

and worth doing?

Perform the task

YES

YES

NO

NO

Perform the Task

YES

Does the failure mode

have a safety hazard?

Proactive maintenance

is worth doing if it

reduces the risk of the

failure to a tolerable low

level

Does the failure mode have an

environmental hazard?

Re-design may be necessary if a

proactive task cannot be found

which reduces the risk of the failure

to a tolerably low level.

YES

NO

NO

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Conclusion : -

Evaluation of Reports Findings : -

On a simple and basic note, the objective of this report has been achieved. The Landing Gear

retraction and extension for the aircraft SAAB 340 B is highly reliable, and its reliability rate was

calculated to be 0.972268605. This also conveys about its failure rate as the failure and the

reliability adds up to 1. So the failure can be calculated by 1 – 0.972268605 = 0.027731395, and

the reliability and failure percentage being calculated as 97.2268 % and 2.7731 % respectively.

The Fault Tree Analysis explains the reason for the failure of landing gear extension and

retraction, depending on other components, and it is accomplished by use of logical gates. For

SAAB 340 B OR Gate and AND Gates are used. OR gate Fails means all of the options have

failed, whereas AND Gates fails even if one of the options have failed. The Landing Gear

Extension and Retraction fails if both the Locks and the Hydraulic Actuator fails. The Locks and

Hydraulic Actuator will fail if both the normal Accumulator and the emergency Accumulator fails,

basically it’s an OR Gate. Now the Normal Accumulator will fail, if any of the following fails : Gear

Selector; Hydraulic Pump and the Hand Pump; Handle. Further the Handle can fail if the solenoid

fails to disengage the locks. Now, Moving on to the Emergency Accumulator, it will fail if any of the

Emergency Handel or the Emergency Gear selector or the Emergency Pump fails. They are

connected via AND Gate.

FMECA is another technique use to pre determines the possible errors, the causes of it and the

after consequences. The name itself suggests about different failure modes, its effects and the

analysis of the failure, and that has been discussed in the table above. Decision Tree Analysis is

another tool which discus about the different maintenance procedures applicable for different

Failure Modes.

Maintenance Strategies : -

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Although, the fault rate is very low, and the reliability is adjacently on a higher rate, proper

maintenance strategies should be implemented on a regular basis in order to attain the reliability

for a longer period of time. The maintenance strategies should be simple and should also be done

on a regular basis. Few of the strategies include like “All maintenance tasks must address a

specific failure mode. Use the least expensive and most effective task to maintain the asset. The

maintenance task interval will be such that it addresses the failure at the optimal point in that

asset’s failure cycle. The total cost of the failure must exceed the cost of the tasks to maintain the

asset. PM should ultimately be a time-based refurbishment, not an inspection. Failures created by

operating an asset outside of capability cannot be maintained. The assets must be redesigned.”

(reliable plant)

Wider Concept of Safety and Sustainability : -

Safety and Sustainability is the next bulls eye, the whole industry is aiming at. Sustainability is

required to bloom with one of the biggest R&D in this sector. Non sustainable form of Energy in

the form of fuel and other things are being used in an around the world in the aviation Industry.

The just don’t destroys the natural resources our green earth is having, but is also responsible for

the harmful gases that are being released in the environment. The landing gear even uses many

types of hydraulic fluids and many other day to day assets that are highly not sustainable. The

extension and retraction also produces lots of noise, by the moving parts in the gear system,

which transmits noise pollution in the environment. Be it in air, or in water, in land or in space there

is only one factor that can never be compromised, and that being the safety factor. The landing

gear extension is very necessary for the aircrafts and it is virtually and practically impossible to

land an aircraft over a paved runway. This can lead to loss of lives and property. Therefore, a few

emergency systems have been equipped in the aircraft. In case of normal Failure, Hand pump can

be used instead of Hydraulic pump, and secondly there is a complete different emergency system,

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which comes into operation of hydraulic failure. The emergency system ensures a safety landing

of the aircraft in case of with passengers.