Airworthiness Notice - B060, Edition 1 - 27 December 2000

Extending the Time Between Overhaul or Restoration of Aeronautical Products Operated on Hard Time Maintenance Programs

Purpose

This notice provides information on extending the time between overhaul (TBO) of aeronautical products operated on hard-time maintenance programs. The principles outlined apply to complete engines (reciprocating or gas turbine), engine modules, propellers, auxiliary power units, appliances and accessories.

Introduction

The traditional concept of overhaul (the complete disassembly of an entire aeronautical product for inspection, repair or replacement of parts) is now being replaced with separate restoration tasks, under which only the work necessary to return the product to an airworthy condition is done.  Instead of all elements of, say, an entire engine being reworked at the same time, the engine will undergo a series of “shop visits” during which individual components undergo maintenance as required. Restoring only those items that require attention eliminates unnecessary maintenance and allows each part of the engine to benefit from optimum scheduling. The modular design of modern turbine engines is particularly suited to this new approach. By contrast, complete overhaul remains the preferred method for reciprocating engines, due to their type of construction. The procedures described in this document apply equally to overhaul and restoration.

Terminology

Overhaul and restoration tasks may be scheduled on either a hard-time (HT) or on-condition (OC) basis. A product’s eligibility for OC maintenance depends on several factors, the primary one being the existence of a reliable means of assessing the product’s condition by in-service tests. This document is primarily concerned with HT scheduling, under which overhauls are performed at predetermined intervals. Various kinds of TBO must be considered. To differentiate between them, the following terms will be used:

1. “Basic TBO” refers to the recommended overhaul interval established by the type certificate holder. Basic TBO are published in the Instructions for Continued Airworthiness (ICA) that are evaluated by Transport Canada during the product’s type certification. Where the recommendations of an engine or propeller type certificate holder differ from those of the aircraft type certificate holder, the basic TBO is the interval recommended by the aircraft type certificate holder.
2. “Operator’s TBOs” refers to the overhaul interval approved by Transport Canada for a specified product, when used by a particular operator. Operator’s TBOs are specified in the operator’s maintenance schedule, approved pursuant to Canadian Aviation Regulations (CAR) 605.86. Due to differences in aircraft role, configuration, environment, operating technique and experience, TBO may vary significantly between different operators of the same aircraft type. Also, differences in operating environment or duty cycle may result in different TBO for the same product when installed in different aircraft types, even when used by the same operator.

Operator’s TBO

Operator’s TBOs are approved by the Transport Canada Principle Maintenance Inspector (PMI) as part of the operator’s approved maintenance schedule for the aircraft type in which the product is installed. Usually, the operator’s initial TBO will be the same as the basic TBO. Intervals greater than the basic TBO may be considered where the operator can demonstrate satisfactory experience at higher TBO with the same or similar products in other aircraft types.

In the case of very mature products whose reliability has been demonstrated over a wide range of operators, Transport Canada may authorize new operators to use initial intervals in excess of the basic TBO, subject to certain conditions. Approval for a new operator to use an initial TBO higher than the basic TBO will depend on several factors, the primary ones being commonality of operating environment and aircraft role. Extended initial intervals approved in this manner will not be greater than the contemporary mean of all TBOs approved for Canadian operators of the aircraft type. They will only apply to mature products of proven reliability, where the basic TBO no longer reflects the actual experience of the Canadian fleet. To assist in standardization of service, Transport Canada will identify extended initial TBO for particular products and installations, and make these available to PMIs by means of staff instructions.  Staff instructions may be viewed on the Transport Canada Civil Aviation web site at http://www.tc.gc.ca/aviation/mainten/aarpc/msi/index.htm.

Operators who wish to take advantage of extended initial intervals should make application for an amendment to their maintenance schedules in the usual manner. Extended initial TBO will only be approved for operators whose role, operating environment, stage length, etc. are typical of the Canadian fleet in general, and whose maintenance schedules include appropriate means to detect any decrease in failure resistance (routine spectromatic oil analysis, borescope checks, power checks or trend monitoring, etc.)

Operator TBO Extensions

Transport Canada authorizes extensions to operator’s TBOs by approving amendments to the operator’s aircraft maintenance schedules. Approval will be based on a review of data collected by the operator and submitted to the applicable Transport Canada Centre. The usual criterion for approval is the demonstrated reliability at the increased interval. Demonstration is usually by means of a trial program.

Generally, TBO increments will be 10% of the established interval or 500 operating hours. Smaller or larger increments may be approved where circumstances warrant. If larger increments are requested, the onus is on the operator to make a credible case that no deterioration in safety will result. Obtaining a TBO extension usually begins with collecting evidence to demonstrate satisfactory reliability at the current TBO. The PMI may then authorize a trial program pursuant to CAR 605.86(3), allowing specified units to be operated beyond their established TBO for the collection of data. Temporary approvals of this nature are usually granted by letter, specifying the aircraft and/or components to which the approval applies, and any conditions applicable. Components approved for trial extensions may be identified by serial number. The intent of the trial is to demonstrate that the product is capable of remaining in service for the proposed new interval without any deterioration in safety. If the operator wishes the trial to apply not only to the core unit, but also to sub‑assemblies such as magnetos, carburetors, etc. the sub-assemblies must be installed on the trial unit for the full period of the trial, or alternatively they must have completed a comparable amount of time in service installed on other units. Should this requirement not be met, the trial will be valid for the core unit only.

The number of units submitted for trial should normally be between 10% and 25% of the total unit population, varying inversely with operator fleet size. Thus, an operator of two twin-engine aircraft would be required to provide a single engine sample (25% of the population), while an operator with a fleet of ten four-engine aircraft, would be required to provide four samples (10% of the population). Determination of the number of units required to conduct the trial may be based on several factors, including the operator’s fleet size, previous interval extensions granted to similar units, type of operation, etc.

When sample units complete the trial period and are subsequently overhauled, evaluation of the resulting strip reports will indicate the feasibility of incorporating the extension into the maintenance schedule. Analysis of the data is the applicant’s responsibility. It should include a thorough review of overhaul findings, service bulletin status, etc. Unrelated failures resulting from flawed parts or improper operating technique, need not in themselves invalidate the trials, although if the failures occur before an appreciable proportion of the trial extension has been completed, they may necessitate the use of additional sample units. The analysis and findings arising from the review must then be submitted to Transport Canada with the proposed schedule amendment. If the trial was satisfactory, a maintenance schedule amendment will be approved. If desired, the operator may then submit a new application to begin another trial, aimed at obtaining a new extension to the next level. This process may continue until either the strip reports or reliability data indicate that further extensions are inadvisable.

Reliability Monitoring

Operators are responsible for monitoring the reliability of their aircraft and the effectiveness of their maintenance schedules. This is particularly important in the case of operators who have obtained approval for extended TBO. If actual in-service experience should indicate that the current approved TBO is too long, the operator should notify the PMI and submit an application to amend the maintenance schedule accordingly.

Transfer of Products Between Schedules

The preceding information relates to products that are operated continuously by the same operator. Whenever a product is transferred between schedules (such as occurs when an aircraft changes ownership and the tasks or intervals on the new operator’s maintenance schedule differ from those on the earlier schedule) the times remaining to the next maintenance task must be recalculated. The standards for recalculating the times remaining to maintenance tasks are outlined in Appendix F to CAR STD 625, General Operating and FlightRules.