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Section 1  Safety Assurance System: Determining the Prorated Time for an Item


A.    Program Tracking and Reporting Subsystem(s) (PTRS). Activity codes: 3316 and 5316.

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B.    Safety Assurance System (SAS). Use SAS automation and the associated Data Collection Tools (DCT).

3-617    OBJECTIVE. This chapter provides guidance in determining the prorated time for an item.

3-618    GENERAL.

A.    Definition of Proration. Proration is a procedure to determine the time consumed under one maintenance system, and to establish the remaining time under a new system.

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B.    How Proration is Used. Certificate holders often sell or lease their equipment to other certificate holders. This “used” equipment will have accumulated a certain amount of time in service. This time is transferred to the new certificate holder and may be phased in or prorated to the new certificate holder’s approved time limitations.

C.    Buyer Options for Prorated Time. When a buyer’s approved overhaul time limits are lower than those of the seller, the buyer has two options:

1)    Elect to use the proration process.
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2)    Elect to use direct inclusion, providing the previous certificate holder’s actual time since overhaul (TSO) is less than the buyer’s approved overhaul time limit.
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a)    When using the direct inclusion option, the difference between the buyer’s approved overhaul time limit and the previous certificate holder’s actual TSO will determine the time remaining to the overhaul for the buyer.
b)    When the buyer’s approved overhaul time limit is higher than that of the seller, proration procedures should be used to adjust the TSO. However, based upon the buyer’s assigned principal maintenance inspector’s (PMI) comparison of the buyer’s and seller’s maintenance programs for similarity, direct inclusion may be used if both programs are found comparable.

D.    Scope and Limitations of Proration.

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1)    Proration does not lessen a certificate holder’s responsibility to maintain the aircraft in an Airworthy condition.
2)    Proration is optional.
3)    Life-limited components may not be prorated.
4)    Proration may not be applied to times specified in Airworthiness Directives (AD).
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5)    Certificate holders who have been operating equipment under Title 14 of the Code of Federal Regulations (14 CFR) parts 121 and 135 may use proration.
6)    Both adjusted and actual times must be shown on the proration document and the aircraft records.
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7)    When an item is inspected or overhauled as appropriate, the applicable prorated time limits will be canceled. Thereafter, the item will be handled according to the certificate holder’s approved program.
8)    Partial proration is not acceptable. A certificate holder electing proration must prorate the airframe and all of its installed powerplants, propellers, and appliances. Spare engines and propellers acquired at the time of sale or at a later date with “time in service” may be prorated.
9)    If an increase in a time limitation is approved for a certificate holder operating on prorated times, that increase will be credited to the prorated item(s).
10)    Amendments to a certificate holder’s operations specifications (OpSpecs) that increase time limits apply to all aircraft of the same type and model operated by a carrier. Such time increases apply to aircraft operating on a prorated time basis, as well as to the other aircraft in the fleet.
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E.    Phasing in Foreign Aircraft with U.S. Type Certificates (TC). Foreign air carrier aircraft for which there is a U.S. TC may be phased into a U.S. air carrier’s program via proration. However, the U.S. certificate holder must first present satisfactory evidence that the program under which the aircraft was maintained is at least equivalent to the new certificate holder’s program for a similar type of aircraft.


A.    Computing Prorated Time. Prorated time remaining can be determined by using the following mathematical procedures:

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1)    Divide the actual time used by the previous certificate holder’s approved time limit under which the aircraft has been operated. The result, carried to two decimal places, will represent the percentage of approved time already used.
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2)    Multiply the new certificate holder’s time limit by the percentage of time used. This will result in the prorated time to be used under the new program.
3)    Subtract the prorated time from the time limit approved in the new program. The result will represent the number of hours remaining under the new program. (See Figure 3-66, Proration Formula Example.)

B.    Block/Pattern Time Limitation.

1)    When block/pattern time is to be prorated, each block/pattern shall be treated as though a complete aircraft were being prorated.
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2)    When the previous certificate holder used a block/pattern system, a document must be submitted showing the following:

    Time limitation for each block or pattern, together with a list of items that are part of the block or pattern; and

    Time since accomplishment for each individual item on the aircraft.

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3-620    COORDINATION REQUIREMENTS. This task requires coordination between the inspector and the certificate holder.


A.    References (current editions):

    Advisory Circular (AC) 120-17, Maintenance Control by Reliability Methods.

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    Certificate holder’s documentation, including OpSpecs, for previous and new certificate holder.

    Volume 3, Chapter 18, Section 6, Parts D and E Maintenance OpSpecs/MSpecs/LOAs.

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    Volume 10, Safety Assurance System Policy and Procedures.

B.    Forms. None.

C.    Job Aids. Figure 3-66.

3-622    PROCEDURES.

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A.    Receive Data From Applicant. The certificate holder must submit required information to the district office in which the certificate holder’s principal base of operation is located.

1)    The certificate holder must submit all OpSpecs containing the time limits utilized for the particular aircraft by the previous certificate holder.
a)    If the OpSpecs do not show hours, the certificate holder must submit other documentation that will establish the time limits.
b)    If conversion to hours is necessary, the computations used for the conversion should be included.
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2)    The certificate holder must provide OpSpecs pertinent to the particular aircraft.
3)    The certificate holder must submit documents itemizing the following:
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    Engines, propellers, and appliances that have different time limitations than the previous certificate holder and are to be prorated. These will be listed by Airlines for America (A4A) chapter numbering system, showing the name, part number, serial number, and position;

    The approved time under which the aircraft has been operated;

    The actual time since last accomplishment;

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    The percent of time used by the previous certificate holder; and

    The approved time limitation for the new certificate holder.

4)    When the previous certificate holder used a block/pattern system, a document must be submitted showing the following:

    Time limitation for each block or pattern, together with a list of items that are part of the block or pattern; and

    Time since accomplishment for each individual item on the aircraft.

B.    Determine Eligibility. Determine that the aircraft and/or components are eligible for proration.

C.    Check the Prorated Time Computation. Times obtained via proration may be rounded to the nearest 10-hour figure. (See Figure 3-66.)


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A.    Complete the PTRS Record. For 14 CFR part 125, complete the required PTRS record. For parts 121 and 135, follow SAS guidance Modules 4 and 5 for data collection and reporting.

B.    Complete the Task. Successful completion of this task will result in continuation of the certification task in accordance with the appropriate certification process.

C.    Document the Task. File all supporting paperwork in the applicant’s office file.

3-624    FUTURE ACTIVITIES. Normal surveillance.

Figure 3-66.  Proration Formula Example

The example below demonstrates the simple steps involved in determining a buyer’s time remaining to overhaul.


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Previous certificate holder’s approved overhaul time limit = 8,000 hours

Previous certificate holder’s time since overhaul (TSO) = 2,000 hours

Buyer’s approved overhaul time limit = 12,000 hours

Step One

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Divide the previous certificate holder’s TSO figure by the previous certificate holder’s approved overhaul time limit. Carry this out to two places. The result represents the percentage of approved overhaul time already used.

2,000 8,000 = .25

In this example, 25 percent is the result.

Step Two

Multiply the buyer’s approved overhaul time limit figure by the decimal arrived at in Step One. The result is the prorated TSO to be used by the buyer.

12,000 x .25 = 3,000

In this example, 3,000 is the prorated TSO to be used by the buyer.

Step Three

Subtract the prorated TSO arrived at in Step Two from the buyer’s approved overhaul time limit. The resulting figure will be the number of hours remaining to overhaul for the buyer.

12,000 - 3,000 = 9,000

In this example, the buyer’s prorated time remaining to overhaul is 9,000 hours.

RESERVED. Paragraphs 3-625 through 3-640.