Maintenance and Manufacturing Staff Instructions, MSI 66

Subject: Number: MSI 66
Authorization for Deviation from Scheduled Maintenance Requirements Revision No: 0
  Number of Pages: 4
File No: AARP-5009-3-66 Issue Date: May 5, 2004

1. Purpose

1.1 The purpose of this staff instruction is to provide guidance to Civil Aviation Safety Inspectors (CASI) to authorize deviations from scheduled maintenance requirements. This instruction does not deal with tolerances or extensions as these are already explained in TP 13094, Maintenance Schedule Approval Policy and Procedures Manual.

2. References

2.1 Canadian Aviation Regulation CAR 605.86(3)

Canadian Aviation Regulation STD 625.86(9)

3. Background

3.1 A maintenance schedule deviation may be defined as a one-time variation from the approved interval for a specific scheduled maintenance task. This is distinct from the term 'extension,' which implies a permanent change to the interval. Deviations may be applied in addition to any tolerance described in the approved maintenance schedule. The CAR & STD references above provide the regulatory basis for the granting of deviations by delegated CASI's.

3.2 Due to unforeseen circumstances, air operators may find it necessary to request a deviation from a scheduled maintenance event. The circumstances behind these requests may vary widely and could be based on parts shortages, lack of appropriate test equipment, increase in operational workload and shortages in personnel, to name a few. It is not the intent of the regulations to approve deviations solely for reasons of financial benefit, but rather to approve deviations for those operators that have demonstrated that the deviation will not adversely affect aviation safety. Demonstrating that the deviation will not adversely affect aviation safety may require the completion of additional maintenance actions to confirm that the item is in a safe condition to operate for the period of the deviation.

3.3 Frequent requests for deviations may be an indication of either poor maintenance management or the need to review and amend the approved maintenance schedule.

4. Application Procedure

4.1 Standard 625.86(9) makes reference to a fully documented application, however no application form has been developed for this purpose. Operators may request deviations by letter, indicating the particulars of each request. This will include, where applicable, the aircraft make, model and registration, the specific task and where applicable, the part involved by part number and serial number. The request should state the proposed period for completion of the task by date, flying hours or cycles as applicable.

4.2 It has been common practice for operators to request approval from the aircraft or component manufacturer in support of requests for deviations. Manufacturers have been reluctant to grant approval, in part due to liability issues, and usually issue a letter indicating they have no technical objection. Manufacturers who do support an operator's request for a deviation may recommend the performance of additional maintenance tasks (NDT, operational checks or visual inspections at increased frequency) during the deviation period. This information is valuable and will assist the inspector towards granting the deviation. However, the regulations do not require the manufacturer's approval. In the absence of the manufacturer's support, the inspector should review other sources, including his own experience of similar requests, or the maintenance schedules of other operators. Aircraft Evaluation Division in Ottawa may be able to provide information regarding the basis of the original interval. A review of related Service Difficulty Reports may also be of value.

5. Rationale for Deviations

5.1 Deviations may be requested for a variety of reasons. Situations that would typically fall within the intent of the regulations include;

  • Shortage of parts or maintenance services;
  • Increases in operational workload due to unexpected events such as natural disasters;
  • Opportunities to maximize the effectiveness of resources by synchronizing out of phase maintenance tasks that otherwise would not quite coincide with a major maintenance event (e.g., NDT of a fuel tank interior when the normal inspection of the tank at C or D check will occur within the next few months).
  • Deferring cosmetic or long-term preventive maintenance tasks on aircraft that will be removed from service in the near future.
  • To gain service experience of operation at the increased interval, in support of an eventual permanent extension. (e.g., an operator may request a deviation to the approved engine TBO, with the aim of obtaining a satisfactory strip report after the engine completes the extended time in service).

5.2 Similarly, situations that would not normally be accepted unless additional information is offered to ensure that aviation safety was not adversely affected:

  • Deferring tasks for aircraft operating away from base to facilitate extension of a lease.
  • Deferring task for a seasonal operator simply to complete the season.
  • Deferring tasks to compensate for failures of the maintenance control system.

6. Review & Approval of Deviations

6.1 Deviations will be reviewed and approved on a case-by-case basis. A sample format is included as Appendix A. The review will consider the reason for the request and any documentation to support it. Where the task is minor, such as the extension of the annual compass swing or ELT battery replacement, a statement verifying that the item is operating normally may be sufficient. For major tasks such as component overhaul or NDT inspections, additional technical justification should be provided to ensure an adequate level of safety is maintained. The inspector should consider whether additional inspection tasks should be instituted to mitigate the risk. This could apply for example, in the case of a deviation to the interval for engine or landing gear overhaul where increased oil filter inspection or landing gear dimensional checks may be appropriate.

6.2 These examples are only a few of the many deviations that can be requested. As a minimum, inspectors should consider the following:

  • The reason for the deviation.
  • Technical justification.
  • The need for additional maintenance actions.
  • Is the maintenance control system adequately developed to ensure that deviation requests are kept to a minimum?

6.3 Once these points are reviewed the inspector can determine if the deviation can be approved.

7. Effective date

7.1 This instruction comes into effect immediately.

8. HQ Contact

8.1 The responsible officer indicated below may be contacted for information regarding this MSI:

Chief Standards and Procedures
Aircraft Maintenance & Manufacturing
Phone: (613) 952-4386
Facsimile: (613) 952-3298

D.B. Sherritt
Maintenance and Manufacturing

Appendix A

May 19, 2004

ABC Airlines 1 Aviation Way Any town, Ontario M1B 1A1

Dear Sir:

Your application requesting a **** hour/cycle deviation to the scheduled NDT inspection of (state the specific inspection task) for your Cessna 550 aircraft, registration C-GXYZ, has been reviewed and found acceptable.

Pursuant to Canadian Aviation Regulations 605.86(3), this letter is your authority to extend the NDT inspection of (restate the specific task) on your Cessna 550 aircraft, registration C-GXYZ, a maximum of **** hours/cycles. This inspection will now be due at **** hours total airframe time. Please note that this is a onetime deviation and does not constitute a permanent approval for this task..

To ensure the continuing airworthiness of your aircraft during the period of this deviation, the following additional inspections are required;

Quote any additional inspections or operational checks recommended by the manufacturer.

Yours truly

John Smith

Civil Aviation Safety Inspector,
Aircraft Maintenance & Manufacturing
Civil Aviation, Ontario Region
Transport Canada

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