a. Increases in engine overhaul periods may be approved in increments mutually agreed upon by the operator and the assigned principal maintenance inspector. Increase in time limitations are normally considered on the basis of satisfactory service experience and a teardown examination of one exhibit engine. The engine chosen for exhibit should have operated to within five percent of the currently approved time limitation; or
b. Any other method acceptable to the assigned principal maintenance inspector such as industry experience, etc., may also be used to increase engine overhaul periods when sufficient justification can be furnished by the operator.
c. Engine accessories may be operated to multiples of the approved engine overhaul limit if it can be shown that satisfactory service and overhaul experience justifies the increase and will not adversely affect the airworthiness of the component involved.
8. APPLIANCES - REVISION OF TIME LIMITATIONS. Increases in established times for inspection, bench test, or overhaul periods are normally based on consideration of the following factors: (1) geographical area or areas of operation; (2) number of landings, long haul vs. short haul; (3) maintenance organization and inspection procedures; (4) manufacturers' recommendations; and (5) service history.
When electrical/electronics appliances are maintained on an "on condition" basis, special consideration should be given to the continued airworthiness of mechanical components of such equipment.
9. RELIABILITY VS. HARDTIME CONVERSION. The following procedures apply to aircraft transferred from a nonoverhaul program (maintenance control by reliability methods) to a program utilizing hard times.
a. The record of last overhaul which is required by section 121.380 will be used to determine time on units which have been overhauled.
b. When a record of last overhaul is not available for units not subject to overhaul (for example, units under reliability control), the carrier will determine TSO or TSN on each unit by subtracting total time on aircraft at the time of unit installation from the total time on the aircraft at the time of change to an overhaul program. The time on each unit so determined will be used to determine time remaining to overhaul on the new
program.
c. If adequate records are not available to determine TSO or TSN, the unit involved will have to be overhauled before further use.
10. EMERGENCY EQUIPMENT. The inspection periods for first aid kits, flotation equipment, evacuation slides, and other emergency equipment should assure the continued serviceability and immediate readiness of such equipment for its intended emergency purposes. Major inspection periods are established for the purpose of determining that all components of the emergency equipment are complete and serviceable and may be expected to remain in this condition until the next major inspection or actual use under emergency conditions.
11. AVIONICS AND INSTRUMENTS. All Operations Specifications, Aircraft Maintenance, must identify the manufacturer and model number of the major components in ATA systems 22, 23, 24, 31, 33, 34, and 77. If the operator chooses, the major components and required data can be listed in any other approved manual or document provided that the manual or document is identified in the approved Maintenance Specifications, preferably in the General section.
12.-15. RESERVED.
CHAPTER 2. PRORATION
16. PURPOSE. Proration is a mathematical procedure used to determine the percent of overhaul time expended by one operator and to establish the time remaining to overhaul for the new operator. As new type equipment becomes available, many air carriers acquire this new equipment and sell or lease their older equipment to other air carriers. Since the older equipment has accumulated a certain amount of time in service, it is desirable to transfer this time as it applies to an airframe and its installed powerplants, propellers, and appliances to the buyer. This "time in service" may be phased in or prorated to the approved overhaul time of the buyer to permit further utilization of the affected items without the need for immediate overhaul.
17. APPLICATION.
a. When a buyer's approved overhaul time limits are lower than those of the seller, the buyer has two options:
(1) He (the buyer) may elect to use the proration process.
(2) The buyer may elect to use direct inclusion providing the previous operator's actual time since overhaul is less than the buyer's approved overhaul time limit.
b. When the direct inclusion option is used, the difference between the buyer's approved overhaul time limit and the previous operator's actual time since overhaul will determine the time remaining to overhaul for the buyer.
c. When the buyer's approved overhaul time limit is higher than that of the seller, proration procedures should be used to adjust the time since overhaul. However, based upon the buyer's assigned PMI's comparison of the buyer's and seller's maintenance program for similarity, direct inclusion may be used if both programs are found comparable.
18. SCOPE AND LIMITATIONS.
a. An operator's responsibility for maintaining his aircraft in an airworthy condition is in no way lessened by proration of the aircraft's time since overhaul.
b. All times obtained by use of the proration formula may be rounded out to the nearest 10-hour figure.
c. The percentage of aircraft overhaul time expended is computed on the basis of the previous operator's actual time since overhaul.
d. When block/pattern overhaul time is to be prorated, each block/pattern shall be treated separately, as though a complete aircraft were being prorated. (See Appendix 1, Figures 1 and 2.)
e. The operator will not be permitted to change aircraft records to reflect only the adjusted time since overhaul. The operator will be required to maintain adequate records showing the adjusted time since overhaul and the actual time since overhaul.
f. No partial proration will be acceptable. If a carrier elects to prorate, then the airframe and all its installed powerplants, propellers, and appliances will also be prorated. Spare engines and propellers acquired by the buyer at the time of sale or at a later date with "time in service" may also be prorated.
g. Aircraft components that are life-limited and airworthiness directives may not be prorated.
h. Amendments to "Operations Specifications - Aircraft Maintenance," which increase overhaul time limits, are applicable to all aircraft of the same type and model operated by a carrier. Such time increases would apply equally to aircraft operating on a prorated time basis as well as to the others in the fleet. Aircraft operating on prorated times will not have the time increase prorated, but will be credited with the entire increase.
i. "Operations Specifications - Aircraft Maintenance," and the applicable preface page which established prorated time limits, will be cancelled when the aircraft, powerplants, propellers, and appliances to which they relate are first overhauled. Thereafter, overhaul will be accomplished in accordance with the operator's approved overhaul time.
j. For proration purposes, aircraft obtained from military or non-air carrier civilian sources will be assigned the Standard Operations Specifications - Aircraft Maintenance or Maintenance Review Board Report times as applicable to the particular aircraft.
k. Any foreign air carrier aircraft listed in the Standard Operations Specifications - Aircraft Maintenance or an applicable Maintenance Review Board Report may be phased into a U.S. air carrier's program, through the proration formula, provided the U.S. operator presents satisfactory evidence which indicates that the program under which the aircraft was maintained is at least equivalent to a program approved by the FAA for a similar type of aircraft.
l. Aircraft acquired by either a "new operator" or "buyer" (see definitions, Chapter 1, paragraph 4) may enter service via proration or direct inclusion. The reason for the difference in terminology lies in the fact that a new operator will be limited by the maximum times set forth in the Standard Operations Specifications
- Aircraft Maintenance (Appendix 2) or the applicable MRB report. On the other hand, a "buyer" will have previously established time limits for the particular aircraft in question. For ease in presenting proration sample exercises shown in Appendix 1, the terms "new operator" and "buyer" are used interchangeably.
19. DATA REQUIRED. The following data will be submitted to the assigned air carrier maintenance inspector by the operator, when he desires to apply for approval of prorated overhaul times:
a. All "Operations Specifications - Aircraft Maintenance" containing the overhaul time limits utilized for the particular aircraft by the previous operator. If the Operations Specifications do not show hours, the buyer shall submit such other specifications or documents which will, in fact, establish the overhaul time limits in hours. If a conversion to hours is necessary, the computations used for the conversion will be included (see Appendix 1, Figure 7).
b. All "Operations Specifications, Preface Pages" pertinent to the particular aircraft. This is essential because the previous operator may have operated the subject aircraft in accordance with a preface page showing utilization of adjusted time since overhaul calculated in accordance with the proration formula.
c. A document itemizing (for airframe, powerplants, propellers, and appliances):
(1) The previous operator's:
(a) Approved overhaul time.
(b) Time since overhaul.
(c) Percent of overhaul time used.
(2) The buyer's:
(a) Approved overhaul time.
(b) Prorated time since overhaul.
(c) Prorated time remaining to overhaul.
(See Appendix 1, Figure 5.)
d. When a block/pattern or similar overhaul system was used by the previous operator, a document will be submitted showing:
(1) The time since overhaul for each block or pattern together with a list of items which are part of the block or pattern (see Appendix 1, Figure 8).
(2) The time since overhaul for each individual item on the aircraft.
e. "Operations Specifications - Aircraft Maintenance" or a "Proration Document" containing prorated times for each aircraft operating on prorated times. In either of these cases, a pertinent preface page must be prepared and submitted.
(1) In the first case where the approved prorated times are listed on "Operations Specifications
- Aircraft Maintenance" pages, a preface page such as that shown in Appendix 1, Figure 9, will be executed.
(2) Where a proration document is used to list prorated times in lieu of showing them on the "Operations Specifications - Aircraft Maintenance" pages, a document shall be prepared for each aircraft operating on prorated times. Each document shall be clearly identified and each shall be made a part of the Operations Specifications by means of a preface page such as the example shown in Appendix 1, Figure 11.
(3) The information required by item 15c will provide the supporting data for these specification pages or documents.
The aforementioned documents are essential to the application of the formula in establishing the adjusted time since overhaul on each component, area, section, etc., of the aircraft because if the proration formula has been used by the previous operator of the aircraft involved, the time since overhaul figures presented in the document will not be the same as the actual hours of time in service since the last overhaul of the component, area, etc. The proration formula for the buyer should be calculated, using the actual time since overhaul figures maintained by the previous operator.
20. PRORATION FORMULA. The simple mathematical procedure explained below will result in a figure that will be the "time remaining to overhaul" for the buyer.
KNOWN 8,000 hours --- Previous operator's approved overhaul time limit.
2,000 hours --- Previous operator's time since overhaul.
12,000 hours --- Buyer's approved overhaul time limit.
STEP I Divide the previous operator's TSO figure by the previous operator's approved overhaul time limit (carry out to three places). This division will result in a decimal which can be used to represent the percentage of approved overhaul time already used.
_____.250 --- Percent of O.H. time used by 8,000/2,000.000 previous operator.
STEP II Multiply the buyer's approved overhaul time limit figure by the decimal arrived at in STEP I. This multiplication will result in the prorated TSO to be used by the buyer.
12,000 --- Buyer's approved O.H. time limit.
x .250 ------
600.000 2400.0 -------- 3000.000 --- Buyer's prorated TSO.
STEP III Subtract the prorated TSO arrived at in STEP II from the buyer's approved overhaul time limit. The resultant figure will be number of hours remaining to overhaul for the buyer.
12,000 --- Buyer's approved O.H. time limit.
-3,000 ------ 9,000 --- Buyer's prorated time remaining to overhaul.
21.-25. RESERVED.
---------------------------------------------------------------- FIGURE 1. TRANSFER OF AIRCRAFT FROM A BLOCK/PATTERN SYSTEM
TO A ONE-TIME OVERHAUL In this type of situation, each block/pattern is prorated as though it was the overhaul time for the entire aircraft.
KNOWN
16,000 hours --- Previous operator's approved overhaul time limit which is divided into four blocks/patterns of 4,000 hours each.
3,000 hours --- Previous operator's TSO for block/pattern A.
7,000 hours --- Previous operator's TSO for block/pattern D.
11,000 hours --- Previous operator's TSO for block/pattern C.
15,000 hours --- Previous operator's TSO for block/pattern B.
12,000 hours --- Buyer's approved overhaul time limit.
COMPUTATION BLOCK/PATTERN A STEP I _____.187 --- Percent of O.H. time used by previous 16,000/3,000.000 operator.
STEP II 12,000 --- Buyer's approved O.H. time limit.
x .187 ------
84.000
960.00 1200.0 -------- 2244.000 --- Prorated TSO for buyer's block/pattern A.
STEP III 12,000 --- Buyer's approved O.H. time limit.
-2,244 ------ 9,756 --- Prorated time remaining to overhaul for buyer's block/pattern A.
COMPUTATION BLOCK/PATTERN B STEP I ______.937 --- Percent of O.H. time used by previous 16,000/15,000.000 operator.
STEP II 12,000 --- Buyer's approved O.H. time limit.
x .937 ------
84.000
360.00 10,800.0 ---------- 11,244.000 --- Prorated TSO for buyer's block/pattern B.
STEP III 12,000 --- Buyer's approved O.H. time limit.
11,244 ------ 756 --- Prorated time remaining to overhaul for buyer's block/pattern B.
COMPUTATION BLOCK/PATTERN C STEP I ______.687 --- Percent of O.H. time used by previous 16,000/11,000.000 operator.
STEP II 12,000 --- Buyer's approved overhaul time limit.
x .687 ------
84.000
960.00 7200.0 -------- 8244.000 --- Prorated TSO for buyer's block/pattern C.
STEP III 12,000 --- Buyer's approved O.H. time limit.
- 8,244 ------- 3,756 --- Prorated time remaining to overhaul for buyer's block/pattern C.
COMPUTATION BLOCK/PATTERN D STEP I _____.437 --- Percent of O.H. time used by previous 16,000/7,000.000 operator.
STEP II 12,000 --- Buyer's approved O.H. time limit.
x .437 ------
84.000
360.00 4800.0 -------- 5244.000 --- Prorated TSO for buyer's block/pattern D
STEP III 12,000 --- Buyer's approved O.H. time limit.
- 5,244 ------- 6,756 --- Prorated time remaining to overhaul for buyer's block/pattern D.
It should be recognized that when an operator elects to transfer an aircraft from a block/pattern system to a one-time overhaul, a substantial amount of "time remaining to overhaul" must be sacrificed.
In the example given here, the buyer's "time remaining to overhaul" for each block/pattern is as follows:
A - 9756 C - 3756 B - 756 D - 6756 Since the buyer's approved overhaul time is 12,000 hours, it follows that all the items in block/pattern "B" will be due for overhaul within 756 hours. To phase the aircraft into a onetime overhaul, all blocks would have to be overhauled at this time.
---------------------------------------------------------------- ---------------------------------------------------------------- FIGURE 2. TRANSFER OF AIRCRAFT FROM A BLOCK/PATTERN SYSTEM TO A
BLOCK/PATTERN SYSTEM In this type of situation, each block/pattern is prorated as though it was the overhaul time for the entire aircraft.
KNOWN (Based on the figures established in Figure 1.)
16,000 hours --- Previous operator's approved time limit which is divided into four blocks/patterns of 4,000 hours each.
.187 --- Percent of block/pattern A overhaul time used by previous operator.
.937 --- Percent of block/pattern B overhaul time used by previous operator.
.687 --- Percent of block/pattern C overhaul time used by previous operator.
.437 --- Percent of block/pattern D overhaul time used by previous operator.
12,000 hours --- Buyer's approved overhaul time limit which is to be divided into four blocks/patterns of 3,000 hours each. (Blocks/patterns I, II, III, and IV) COMPUTATION BLOCK/PATTERN NO. I STEP I 12,000 --- Buyer's approved overhaul time limit.
x .187 --- Percent of overhaul time used by previous ------ operator.
84.000
960.00 1200.0 -------- 2244.000 --- Prorated TSO for buyer's block/pattern No. I.
STEP II 12,000 --- Buyer's approved overhaul time limit.
-2,244 ------ 9,756 --- Prorated time remaining to overhaul for buyer's block/pattern No. I.
COMPUTATION BLOCK/PATTERN NO. II STEP I 12,000 X .937 ------
84.000
360.00
10,800.0 ---------- 11,244.000 --- Prorated TSO for buyer's block/pattern No. II.
STEP II 12,000 -11,244 ------- 756 --- Prorated time remaining to overhaul for buyer's block/pattern No. II.
COMPUTATION BLOCK/PATTERN NO. III STEP I 12,000 x .687 ------
84.000
960.00 7,200.0 --------- 8,244.000 --- Prorated TSO for buyer's block/pattern No. III.
STEP II 12,000 -8,244 ------ 3,756 --- Prorated time remaining to overhaul for buyer's block/pattern No. III.
COMPUTATION BLOCK/PATTERN NO. IV STEP I 12,000 x .437 ------
84.000
360.00 4,800.0 --------- 5,244.000 --- Prorated TSO for buyer's block/pattern No. IV.