3a Maintenance Schedule

Prepared by Associate Professor Sallehuddin Muhamad UTM City Campus

MAINTENANCE PLANNING AND SCHEDULE

1. Aircraft Reliability

Aircraft operators face with situation that demands priority of safety and the pressure of economy.

These demands lead to a very reliable aircraft. Aircraft reliability or sometime refers as schedule

reliability is defined as:

Other word, Rair is the probability of starting and completing a scheduled flight without interruption.

An aircraft with Rair = 1 has the ideal reliability.

Example:

An aircraft is fitted with two same components. After 100,000-flight hour and 80,000 departures:

Activity Times

Total removal 100

Unscheduled removal (failed or suspected) 80

Failures 50

Inoperative and caused system failure 26

Caused schedule interruption 4

Unjustified removal = Unscheduled removal – failures

= 80 – 50 = 30

The parts Mean Time Between Failures, MTBF

Mean Time Between Unscheduled removal, MTBUR

Rair = 1 - Schedule interruptions (delay/cancellation)

Total departure

MTBF = (no of part) (flight hour)

failures

= (2) (100,000)

50

= 4,000 flight hour

MTBUR = (no of part) (flight hour)

Unscheduled removal

= (2) (100,000)

80

= 2,500 flight hour


Therefore;

Ideally, maintenance efficiency of 100% is what the operator aimed for.

2. Maintenance Concepts

There are four maintenance concepts that are used in maintaining aircraft. They are:

a) Fixed When It Breaks

b) Hard Time

c) On Condition

d) Condition Monitoring

Usage of these concepts varies with its importance to flight safety. The concepts (b) and (c) are

useful in maintenance schedule.

a) Fix When It Breaks

Using this concept aircraft parts are fixed or replaced when they failed. Unless their failure

periods are known, aircraft operators have to make unscheduled removal and this can reduce

aircraft reliability. Therefore, this concept can only be applied to unessential parts for flight

operation and safety such as cabin lighting, cabin seats, and toilet.

b) Hard Time

This concept is defined as a preventive process in which known deterioration of an item is

limited to an acceptable level. Maintenance actions are carried out at a period related to time in

the service scheduled maintenance control specified by:

i) calendar time

ii) number of cycles (takeoffs and landings)

iii) flight hours

The prescribed actions normally include servicing, fully or partial overhaul, replacement

according to instruction relevant documentation so that the part is restored to suitable condition

for further use for a specified period.

Maintenance efficiency = MTBUR

MTBF

= 2,500

4,000

= 63%


Performance

Replace parts

limit

tlife month, flight hr, or cycles

Although this concept is convenient for scheduling, it is expensive and can be used on familiar

items. It however reduces schedule interruption and thus increase aircraft reliability.

c) On Condition

It is also a preventive process. However, item is inspected or tested at specific periods against

standards to determine whether it can continue in service. Thus, it requires collection of data at

regular intervals. These data are analyzed and evaluated to ascertain airworthiness.

Performance

Replace parts

limit

month, flight hr, or cycles

periodic inspections

This maintenance concept can extend the life of aircraft parts, thus reduces maintenance cost.

However, it cost saving benefit may be overwhelmed by additional man-hour requirement to

perform periodic inspections. It also causes schedule interruption due to part replacement.

Therefore, this concept is used on parts that are easy to replace such as aircraft tires or parts

with redundancy.

d) Condition Monitoring/Health Monitoring

It is not a preventive process, which causes unscheduled removals. It is a process in which

information on aircraft parts gained from operational experience is collected, analyzed, and

interpreted on a continuous basis as a mean of implementing corrective procedures.

Instruments are placed to monitor part performance.


Performance

Replace parts

limit

continuous inspections month, flight hr, or cycles

Since it causes unscheduled interruption, it reduces aircraft reliability. It also requires regular

inspection and thus expensive to apply. Usually, condition monitoring is used for new part that

its hard time is not known yet.

3. Maintenance Program

The Maintenance Program development methods have been revised through out the years in an

effort to achieve higher levels of safety and cost-effectiveness. The initial philosophy of

maintenance program was "to overhaul everything at a specific time" which was based on concept

that primary failure mode was wear out. Since then, several philosophies have materialized and

each superseding or evolving from its predecessors.

Historical Background

i. In 1961 the Federal Aviation Authority (FAA)/Industry Reliability Program was developed

aimed solely at the propulsion system reliability.

ii. In 1968, the aviation industries and FAA developed Maintenance Steering Group Revision 1

(MSG-1) Handbook, Maintenance Evaluation and Program Development, based on new

analytic approach to maintenance of wide-body aircraft. The handbook was used developed

the B747 schedule maintenance program.

iii. In 1970, MSG-2: Airline/Manufacturer Maintenance Planning Document was developed based

on experiences gained with MSG-1. Amendments were made to exclude any specific B747

information to arrive at a universal document that could be used on subsequent new aircraft.

Through this maintenance program, the concept of Hard Time, On Condition, and Condition

Monitoring were introduced.

iv. In 1972, The Association of European Airlines developed EMSG-1 (European Maintenance

System Guide) to overcome deficiencies in MSG-2. It was used to develop British Airline (BA)/

Aerospatiale Concorde and A300. An improved version EMSG-2 was introduced later.


v. In 1978, the concept of Reliability-Centered Maintenance (RCM) was published by United

Airlines under contract with the US Department of Defense. This concept contained many new

ideas notably maintenance process on hidden functions and generally provide through logic

analysis of maintenance program development process.

vi. With new regulation such as the structural damage tolerance rules (FAR 25.571) and new

development and ideas in maintenance program, aviation industry felt that MSG-3 must be

evolved. A task force was formed by Air Transport Association (ATA) in 1979 to produce

MSG-3. In 1980, MSG-3 produced Airline/Manufacturer Maintenance Program Development

Document and this document was used on new aircraft such as B767, A310, and F50.

The objectives of an optimal scheduled maintenance program are:

i. To ensure that equipment is maintained to the designed level of safety for aircraft safety.

ii. To ensure the equipment is maintained to the inherent level of reliability for aircraft operating

efficiency.

iii. To restore any deterioration of inherent levels of safety and reliability of equipment.

iv. To prevent or minimize on-aircraft failures especially those resulting in delays, cancellation of

services or loss of passenger good will.

v. To identify deficiencies in the inherent levels of safety and reliability of equipment and to seek

and initiate product improvement through modification or redesign.

vi. To accomplish the above objectives at minimal total costs, inclusive of maintenance costs and

the subsequent costs of residual failures.

There are two types of maintenance tasks contained within a maintenance program. They are:

a) Scheduled maintenance tasks

The tasks are performed at specified intervals. Its objectives are to prevent deterioration of the

inherent levels of safety and reliability. Tasks in the scheduled maintenance are:

- Lubrication / servicing - Restoration

- Operating crew monitoring - Discard

- Operational check - Inspection / functional checks

- Combination of above work


b) Unscheduled maintenance task

The tasks arise on ad-hoc basis because of the following:

- Report of malfunction by operating crew or others (PIREP)

- Data analysis findings

- Scheduled task

- Accidental damage

The objective of these tasks is to restore the equipment to an acceptable standard.

4. AirAsia Maintenance - maintenance is sustenance

"Getting a rich partner is easy.

Keeping a rich partner is

tricky." AirAsia acknowledges

maintenance is sustenance.

AirAsia may be Asia's first and

only low fare no frills budget

air carrier. But in no way does

AirAsia penny-pinch on caring for its fleets AirAsia conducts line checks at transits, night-stops,

and "A" checks, low level maintenance checks that are conducted once a month. Hangar checks

are "C" and "D" checks conducted once yearly for between 10 to 21 days. The line check is

conducted by the AirAsia's maintenance crew, and the hangar check is conducted at the

Singapore Airlines Engineering Company. The engineering team attained JAR OPS1 or more

commonly referred to as M1 status. AirAsia is so well maintained; it has become more an

expectation than a requirement.

5. Planning and Control

Planning and control is important to determine duration, tasks, and tools required during

maintenance. Planning is a process that requires extensive analysis on problems so that plans can

be executed properly. The objectives of planning are:

a. to forecast the usage of resources,

b. to forecast cost for comparing with the expected income,

c. to prepare instructions and tasks for workers.

The outcome of planning is normally prepared in term of schedule. Resources are capital, workers,

parts, and equipment. An effective planning must include short-term, mid-term and long term plans

of an organization.

i. Long-term Planning


An airline must prepare a long-term maintenance plan for at least five years. The plan is used

to prepare the airline fleet size, utilization of aircraft, number of workers, hangar size, tools,

and capital. With large number of workers, aircraft time in hangar is reduced. Thus, increases

flying hours at a higher maintenance cost.

ii. Mid-term planning

The mid-term planning covers 12 to 18 month of the airline operation. To ease scheduling,

maintenance plan and flight plan are prepared simultaneously. In mid-term planning,

maintenance dates for an aircraft is set.

iii. Short-term planning

The short-term planning is prepared for 30 to 60 day operations. The plan covers types of

aircraft, workload, hangar utilization and replacement parts requirement are prepared for daily

schedule. The plan is constantly updated to fit the rate of failure and traffic delays.

6. Critical Path Method (CPM)

Various methods were introduced in order to improve the effectiveness of planning and control.

In the 50s, a work schedule system was developed to enable a project to be completed at a shorter

time than previously required.

E.I. Du Pont de Nemours and Rand Corporation developed the Critical Path Method in 1956 to

control the construction and maintenance of chemical plants. The method involves time calculation

of an activity from the start to the end, based on the activity duration. It also analyzes float time in

the planning so that other activities do not interrupt the whole project. In 1970s, this method is

accepted and widely used.

The Critical Path Method is a management techniques in a project where its covers planning,

scheduling of activities and controlling. The method is divided into three phases, which are:

a. Planning Phase

It lists the fundamental objective of a project. It also identifies appropriate activities for the

project.

b. Scheduling of Activities Phase

It is the continuation of the Planning Phase. In this phase, all plans are scheduled according to

activity time with analysis on optimal resources requirements such as time, workload, and

equipment.

c. Controlling Phase


In this phase, all activities for the project are controlled and updated so that they meet the

plans.

7. Gantt Chart

A Gantt chart is a horizontal bar chart developed as a production control tool in 1917 by Henry L.

Gantt, an American engineer and social scientist. Frequently used in project management, a Gantt

chart provides a graphical illustration of a schedule that helps to plan, coordinate, and track

specific tasks in a project. Gantt charts may be simple versions created on graph paper or more

complex automated versions created using project management applications such as Microsoft

Project or Excel.

A Gantt chart is constructed with a horizontal axis representing the total time span of the project,

broken down into increments (for example, days, weeks, or months) and a vertical axis

representing the tasks that make up the project. Horizontal bars of varying lengths represent the

sequences, timing, and time span for each task. A bar on the graph represents the amount of time

expected to spend on a task. Other tasks are added below the first one and representative bars at

the points in time they were undertaken. The bar spans may overlap if more than one task is

conducted during the same time span. As the project progresses, secondary bars, arrowheads, or

darkened bars may be added to indicate completed tasks, or the portions of tasks that have been

completed. A vertical line is used to represent the report date.

Gantt charts can be used:


a. To schedule and monitor tasks within a project.

b. To assess time characteristics of a project.

c. To show links between scheduled tasks.

d. To give a clear illustration of a project status.

The advantages of Gantt charts are:

a. A wide audience can understand it.

b. It is easily comprehended for a small project.

c. It can coordinate activities to avoid ‘inferences’.

d. It provides a graphic schedule for the planning and controlling of work, and recording progress

towards stages of a project.

However, Gantt charts have these limitations:

a. It is not suitable for big activities (more than 30 activities).

b. It communicates relatively little information per unit area of display.

c. It does not represent the size of work elements; therefore, the magnitude of a behind-schedule

condition is easily misunderstood.

d. All activities show planned workload as constant. In practice, many activities have front-loaded

or back-loaded work plans.

The PERT chart, another popular project management charting method, is designed to do this.

Automated Gantt charts store more information about tasks, such as the individuals assigned to

specific tasks, and notes about the procedures. They also offer the benefit of being easy to change,

which is helpful. Charts may be adjusted frequently to reflect the actual status of project tasks as,

almost inevitably; they diverge from the original plan.

An example of an overhaul process flow chart for a PT6A engine with its Gantt chart is shown in

the class.

8. Maintenance Scheduling

Maintenance schedule lists works required on an aircraft through out its operational life. There are

two ways to prepare the schedule. They are:

a) Pyramid system

b) Progressive system

a. Pyramid System

In the Pyramid system, a set of works is grouped on a basic period, for an example every

200-flight hour. Another set of work will be performed at double period (i.e. 400, 800 and 1600


flight hours).

The advantages of this system are:

ii. It reduces number of long inspection.

iii. Preparation and completion of job at each inspection.

iv. Optimize utilization of workers.

v. Planning and controlling the work is easy since each set of work has its own tasks.

Nevertheless, this system has the following disadvantages:

i. Workers idle time between inspections is large for air operators with small number of

aircraft.

ii. Long down time for inspection especially in the later period.

b. Progressive System

In the Progressive system, works are distributed through out the aircraft operational life so that

aircraft inspections and repairs can be performed at short intervals, thus reducing aircraft down

time. Factors affecting this system are:

i. Aircraft flight pattern and schedule.

ii. Number of aircraft per type.

iii. Aircraft reliability and failure rate.

iv. Number of workers.

v. Reliability to get spares.

vi. Maintenance cost

The aircraft Maintenance Schedule are divided into three categories, which are:

a) Line maintenance

b) Periodic maintenance

c) Heavy maintenance (overhaul)

a) Line Maintenance

Line maintenance is conducted daily and at every stopover. It is conducted prior to fly as

preflight check, correcting aircrew write-up (post-flight check) and routine airworthiness

inspections.

In civil aircraft, these checks are called transit check, stayover check, and supplement

stayover check. The Malaysia Airlines (MAS) line maintenance is given as follows:


In military aircraft, these checks are labeled alphabetically.

Check Description of Check

A Preflight check (prior to the first flight of a day)

B End of day check (before night stop)

C Turn around check (between stops/ transit)

D Additional check (every 25 hr for Alluette III B helicopter)

E Cleaning (weekly-normal or daily for sea operation)

S Standby check (armament check)

b) Periodic Maintenance

The periodic maintenance is performed at specific intervals (flight hours, cycles, or days) as

prepared in maintenance program. These inspections determine the condition of an aircraft

and the maintenance required to return the aircraft to an acceptable airworthiness condition.

Functional checks and repairs are performed.

i) Small aircraft (less than 12,500 lb)

Small aircraft undergoes its annual and 100-h inspection to determine its airworthiness.

Checks are performed according to manufacturer checklists. However, aircraft operator

can prepare its own checklists but the checklists must be approved by the relevant

authority.

Example - Pyramid system

- Progressive system

ii) For civil aircraft larger than 12500lb or turbine-power multiengine aircraft, its periodic

checks are divided into four series of checks with specific recurring frequency. The checks

are:

A-check: primary inspection to disclose general condition

Aircraft Transit Stayover Supplement Stayover

B747-200/300/400 Every stop 6 hr planned 12 hr unplanned

-

DC10 Every stop 6 hr planned 12 hr unplanned

-

A300 Every stop 6 hr planned 12 hr unplanned

7 days

B737-400 Every stop Daily 18 days

B737-200 Every stop Daily -

F50 Every stop Daily 7 days

DHC6 Every stop Every night stop 125 hr or 75 days


B-check: intermediate check

C-check: system and component check, airworthiness evaluation

D-check: structural inspection, determine airworthiness

An example of the periodic checks for large civil aircraft is shown in the following table.

Aircraft A-check B-check C-check D-check

B747-400 500FH or 7 weeks

B1 to B2: 1000FH C1 to C2: 5,000 FH 25,000 FH or 6 yrs on1st cycle. 20000 FH or 5yrs on subsequent cycle

B2 to B1: 1000FH or 14 wk

C2 to C1: 5,000 FH or 18 mth

B1 to B1: 2000FH or 28 wk

B737-400 375FH B1 to B2: 750FH C1 to C2: 3,000 FH H1 to H2: 10,000FH

B2 to B1: 750FH or 5 mth

C2 to C1: 3,000 FH H2 to H1: 10,000FH

H1 to H1: 20,000FH

F50 500FH or 3 month

1,000FH or 6 month C1 to C2: 3,000 FH D1 to D2: 12,000FH

C2 to C1: 3,000 FH or 18 mth

D2 to D1: 12,000FH or 6 yrs

iii) For military aircraft, the periodic checks are numbered from 1 to 5. Check 1 and check 2

are called Ordinary Level Maintenance (OLM) while Check 3 and Check 4 are called

Intermediate Level Maintenance (ILM). The final check, Check 5 is Depot Level

Maintenance (DLM). An example of the Periodic Maintenance for military aircraft is as

follow:

Check Type of Maintenance S61A-Nuri Alluette III B

1 Ordinary Level Maintenance

60 hr 150 hr (6mth)

2 180 hr 300 hr (12 mth)

3 Intermediate Level Maintenance

540 hr 900 hr (36 mth)

4 1080 hr 1800 hr (72 mth)

5 Depot Level Maintenance 4320 hr 3600 hr (108 mth)

c) Heavy Maintenance

Heavy Maintenance or generally known as overhaul involve major structural inspection. It involves

dismantling the aircraft, inspect all components, repair, and rebuild it required; systems and parts

are tested and repaired or replaced if necessary. Some modifications are made during heavy

maintenance.

9. Computerized Aircraft Maintenance Management

Maintenance of an aircraft is a complicated task since it involves pilots, mechanics, engineers,


inspectors, and aircraft owners. In addition, the amount of information involved the process is very

large. This information is typically multi-modal and available in multiple storage media. Commonly,

this information is also geographically distributed. A major goal of the maintenance process is to

perform the most appropriate repairs on the most efficient way in the short time. Since

maintenance-related process relies on relevant information, comprehensive and timely information

delivery to the individuals involved in the maintenance can be significantly benefit the process.

Computerized Aircraft Maintenance Management (CAMM) system ease the task of keep tracking

of aircraft maintenance schedule. At the end of a flight or the day, pilot enters all of the recorded

flight data such as flight time, number of engine and airframe cycles, and over limit events in the

CAMM system. For each part, the cumulative number of hours of flight time is compared with the

life-cycle data specified in the maintenance documentation. These reference parameters are

initialized when the CAMM system is installed and updated whenever the aircraft manufacturer

issues modifications to the maintenance documentation. The results of the comparison are used to

define what maintenance work needs to be carried out on the aircraft, and allows maintenance

schedules to be drawn up.

Being able to plan maintenance work gives the operator a better overview of inventory movements

and allows new stocks to be ordered at the appropriate time. This enhanced predictability also

improves the management of invoicing procedures and the qualification management of pilots,

crews, and engineers.

Documents

3a Maintenance Schedule