Interval Based Maintenance Programs

Introduction

Interval based maintenance involves the recurring maintenance event set by the intervals given on a maintenance program.

If the functionality for access control is used in Fleet and Asset Management, the maintenance program and belonging serials will be access-controlled. Only users that have specified access to the serials or maintenance program can view or update the information. For more information on access control in Fleet and Asset Management, refer the online help file Access Control. 

The procedure for interval-based maintenance program is as follows:

Define Maintenance Programs

This functionality allows control of revisions for a maintenance program. Different maintenance programs may be required for a certain part number due to different local, national or legal requirements where the part number is operated. Another reason for requiring different maintenance programs for a certain part number is when there are several different manufacturers of the same part. The different manufacturers can have different maintenance requirements which requires usage of different maintenance programs. 

Within each maintenance program one or more maintenance groups can be assigned for each part. Each maintenance group reflects different operational conditions, such as long or short distance, cold or warm weather, etc.

A maintenance program is the core for an interval-based maintenance program. 

Define Maintenance Plans

When the valid combination of serial parts and maintenance groups are entered for a maintenance program you can create a maintenance plan to be used for the maintenance program. As mentioned above, a maintenance plan must be connected to a maintenance program before the maintenance program can be set active and further work on the interval-based maintenance program can be carried out.

Here, you can connect or create intervals for the maintenance plan that is to be used in the maintenance program. Intervals defined per operational parameter (time, miles, number, etc.), together with calendar time for each part and maintenance group, determines the due dates for preventive maintenance. 

In addition to the primary operational parameter, which together with the calendar interval generates interval-based preventive maintenance, any number of other operational parameters may be added. These are registered by the serial part and maintenance group.

For each serial part and maintenance group, the maintenance codes (inspections, overhauls, checks) are then defined. Any number of maintenance codes may be defined with connected intervals (operational parameter and/or calendar). 

The maintenance codes that will be valid to connect to the maintenance plan used in a maintenance program are defined as basic data. When you have completed the creation of a maintenance plan these maintenance codes will be valid for all revisions of the serial part and can be connected to a task card.

Task cards can either be created or an existing task card can be connected for the maintenance code that is assigned to a maintenance plan. When you define task cards per part revision for the maintenance plan, you will be defining the work to be executed on the selected serial parts that are to be maintained.

Phased inspections, meaning inspections which are dependent on each other, can be defined in the maintenance plan. By entering preceding inspections, you can form a sequence or a cycle of phased inspections.

Calculation Rules

The Calculation Rule determines how calculations are performed on each maintenance code connected to the maintenance plan. The table below describes each of these rules.

Phased Inspections

Phased inspections should be used when one or more inspections are dependent on each other. In the maintenance plan, phased inspection information can be entered to indicate which inspection should be performed prior to the next inspection etc. By entering preceding inspections, you can form a sequence or a cycle of inspections. All maintenance codes that should be handled as phased inspections, must be marked with calculation rule Phased Inspection and the Phased Inspection Start check box must be selected for the maintenance code being the starting point of the sequence or cycle of inspections. If the last maintenance code in a phased inspection sequence is not used as a predecessor for other maintenance codes, it can be defined as recurring.

A typical example of a phased inspection cycle can be an A-check on a commercial airliner which is broken into A1- through A4-checks, where the A1 is done first after e.g. 400 hours, then A2 is done 400 hours after the A1, A3 is done 400 hours after the A2 and A4 is done 400 hours after the A3. When the A4 is done, the cycle restarts with the A1 which is done 400 hours after the A4. The cycle definition of the phased inspections in the maintenance plan, will then look like:

The same A-check can be used in an example of a phased inspection sequence, where the A1 is done first after e.g. 400 hours, then A2 is done 400 hours after the A1, A3 is done 400 hours after the A2 and A4 is done 400 hours after the A3. When the A4 is done, no further occurrences of A1 or A2 will be created. The sequence definition of the phased inspections in the maintenance plan, will look like this:

Rules When Entering a Maintenance Plan

The maintenance codes in the maintenance plan are defined by the number of times they will be executed within the cycle (controlled by the maintenance code set as Cycle).

If a maintenance code is defined several times within the same maintenance plan, several occurrences of the same interval are not allowed. Furthermore, a maintenance code can use only one calculation rule within the same maintenance plan. You cannot indicate the calendar interval and unit for the Special Case calculation rule when the due date for this rule is based on the serial's total operational value.

The Replacement calculation rule is used when a serial is replaced before an action is made, e.g., when an engine is replaced before an overhaul. You define the replacement of the serial in the maintenance plan. The replacement must be done independent of the event that is the cause of the replacement. When you calculate the due date of the replacement, the event is due for the vehicle on the top level.

Internal dependencies in the maintenance plan indicate that the accomplishment of an event will lead to the accomplishment of dependent events. For example, the accomplishment of Event E2 will also accomplish Event E1. As mentioned earlier, the calculation rule Replacement must indicate the dependency to the event that is the cause of the replacement. For other calculation rules, internal dependencies are an option. Note: You cannot define internal dependencies with circular references. The graphic and table below show the allowed connections of internal dependencies according to the calculation rule of the events.

An example of a maintenance plan for a vehicle is shown below. For simplicity, only the operation intervals are shown.

An example of a maintenance plan for a serial is shown below. For simplicity, only the operation intervals are shown.

For a due calculation of this maintenance plan for a serial, accomplished maintenance, existing operational values, and intervals in the maintenance plan are considered. If the due dates for two maintenance codes are identical, e.g., OH and BY occurs on the same time at the operational value 600, the due date for both maintenance codes will occur in the Serial Maintenance/Interval Maint. tab. If a maintenance code with the Replacement calculation rule is due in the Serial Maintenance/Interval Maint. tab, the due date is presented for the vehicle tagged with the serial article and serial number. An example for serial FL02 is shown below: 

Recurring Calculation

To ease the registration of maintenance plans where maintenance codes repeat themselves in equal intervals, recurring calculation can be used. A basic maintenance interval or calendar interval and a repetitive interval or calendar interval is stated. The due calculation will lay out the correct number of due dates within the cycle (controlled by the Cycle field). 

For the Inspection, Special Case, and Replacement calculation rules, recurring calculation can be used freely. For the Overhaul calculation rule, recurring calculation is allowed as far as maintenance does not control the cycle. For the Manufactured Date and Installed Date calculation rules, recurring calculation is not allowed.

When you use recurring calculation for the Special Case calculation rule, two due dates (next + 1) are always calculated for the maintenance code. This is because this calculation rule is controlled by the total operational value and does not follow the normal cycle or no cycle. It will also be like this for other calculation rules with recurring calculation where the maintenance plan lacks a cycle.  

An example of a maintenance plan with recurring calculation for a vehicle is shown below (corresponding to the plan above, but the maintenance code E1 is controlled with recurring calculation).  For simplicity, only the operational intervals are shown:

A due calculation of this maintenance plan for one serial, considers accomplished maintenance, existing operational values, and intervals in the maintenance plan. An example of due dates for the serial FL02 used in the maintenance plan above is shown below:

An example of a maintenance plan with reiterating calculation for a vehicle is shown below.  The maintenance plan lacks a cycle. For simplicity, only the operational intervals are shown:

An example of due dates for the serial FL02 used in the maintenance plan is shown below:

Use of Tolerance

For interval-based maintenance, tolerances can be stated for each maintenance code. If you are outside the tolerance, the use of tolerance ensures that the next maintenance with an identical maintenance code is transferred in time. 

The main rules for using tolerance are shown in the table below:

Use of Based on Performed Date

When the Based on Performed Date check box is selected for a maintenance code, all subsequent work in the maintenance plan will be based on the sign off date of the last performed maintenance (based on performed date) regardless of the maintenance code. Therefore when this feature is used, the maintenance due dates will be calculated as follows:

Next Interval Due Date = Sign Off Date of the event + Duration in Workshop of last performed maintenance code + Interval

Following Interval Due Date = Previous Due Date + Interval

If this check box is not selected for the interval maintenance, the maintenance due dates will be calculated as follows:

Next Interval Due Date = Sign Off Date of the event + Interval

Following Interval Due Date = Previous Due Date + Interval

Adjusting Calendar Based Maintenance

The default behavior for calendar based maintenance is:

• Whenever a maintenance is performed too early, the next occurrence will be due equally early (the Cal. Int. - Adjust if done Early check box will not be selected in the maintenance plan).
• Whenever a maintenance is performed too late you should not gain anything from that, and the next occurrence will be due based on the due date of the previous occurrence (the Cal. Int. - Adjust if done Late check box will be selected in the maintenance plan).

Note: The setting for Cal. Int. - Adjust if done Early and Cal. Int. Adjust if done Late will not have any effect if the Based on Performed Date check box is selected on the maintenance plan.

Whenever an interval maintenance is signed off, the Maint Date field in Serial Maintenance/Last Maint will display the actual end date of the last execution for each maintenance code. This also applies for adjusted calendar based intervals. In addition to the execution date the Calendar Correction field (in Serial Maintenance/Last Maint) keeps track of how many days the next occurrence should be shifted backward (negative value) or forward (positive value). The value in the Calendar Correction field is calculated when a calendar based interval maintenance is signed off.

The different settings in the maintenance plan affects how the Calendar Correction is calculated. These are the rules for the different settings of the Cal. Int. - Adjust if done Early and Cal. Int. - Adjust if done Late in the maintenance plan:

• If the Cal. Int. - Adjust if done Late check box is selected, and the maintenance is performed later than the due date, the Calendar Correction field is populated with the (negative) difference between due date and actual end. Next occurrence of the maintenance code will take the value of Calendar Correction into account so it will be due based on due date for the previous occurrence.
• If the Cal. Int. - Adjust if done Late check box is not selected, and the maintenance is performed later than the due date, any value in the Calendar Correction field will be cleared.
• If the Cal. Int. - Adjust if done Early check box is not selected, and the maintenance is performed earlier than the due date, any value in the Calendar Correction field will be cleared.
• If the Cal. Int. - Adjust if done Early check box is selected, and the maintenance is performed earlier than the due date, the Calendar Correction field is populated with the (positive) difference between due date and actual end. Next occurrence of the maintenance code will take the value of Calendar Correction into account so it will be due based on due date for the previous occurrence.

When the due calculation is executed for a calendar based maintenance the value of the Calendar Correction field is taken into account.

Note: The suggested Calendar Correction can be changed in the Component Life/Interval Maint tab. This is useful when you are entering start values for tolerance enabled calendar plans that require a different correction value in order to calculate the correct start or due date. The different settings in the maintenance plan affects how Calendar Correction is entered in the Interval Maint tab:

• If Cal. Int. - Adjust if done Late is selected, only negative values can be entered in the Calendar Correction field.
• If Cal. Int. - Adjust if done Early is selected, only positive values can be entered in the Calendar Correction field.
• If both Cal. Int. - Adjust if done Late and Cal. Int. - Adjust if done Early are not selected, a value cannot be entered in the Calendar Correction field.

Object Properties that affect the calculation

The object properties that affect the calculation of events and due dates are as follows:

• TASK_CALC_EVENTS: The purpose of this property is to control the behavior when you have a maintenance plan defined with an overhaul calculation rule and cycle is specified. Setting the value of this property to FIRST will calculate only the first occurrence (due date) for each of the maintenance codes in the maintenance plan. This could be useful if you for instance also have a "daily inspection" in your maintenance plan that will occur many times within the entire plan. Setting the value of this property to SEVERAL will calculate all occurrences (due dates) for all maintenance codes in the maintenance plan.
• TASK_CALC_HORIZON: The purpose of this property is to limit, in number of years, how many events that are calculated. Events with a due date later than the specified number of years from today will not be presented. Observe that maintenance codes with overhaul calculation rule and cycle specified will always be calculated even if the due date falls after the specified number of years.