Revenue Recognition. As a result of our diverse product and service mix and customer base, we use multiple revenue recognition practices. We recognize sales for product or services in contracts within the scope of Staff Accounting Bulletin ("SAB") Topic 13, Revenue Recognition. Products and services included within the scope of this SAB Topic include heating, ventilating, air-conditioning and refrigeration systems, non-complex alarm and fire detection and suppression systems, commercially funded research and development contracts and non-complex aerospace components. Sales within the scope of this SAB Topic are recognized when persuasive evidence of an arrangement exists, product delivery has occurred or services have been rendered, pricing is fixed or determinable and collectability is reasonably assured. Subsequent changes in service contracts are accounted for prospectively.
Contract Accounting and Separately Priced Maintenance and Extended Warranty Aftermarket Contracts: For our construction-type and certain production-type contracts, sales are recognized on a percentage-of-completion basis following contract accounting methods. Contracts consist of enforceable agreements which form the basis of our unit of accounting for measuring sales, accumulating costs and recording loss provisions as necessary. Contract accounting requires estimates of award fees and other sources of variable consideration as well as future costs over the performance period of the contract. Cost estimates also include the estimated cost of satisfying our offset obligations required under certain contracts. Cost estimates are subject to change and result in adjustments to margins on contracts in progress. The extent of progress toward completion on our long-term commercial aerospace equipment and production-type helicopter contracts is measured using units of delivery or other contractual milestones. The extent of progress towards completion on our development and other cost reimbursement contracts in our aerospace businesses and elevator and escalator sales, installation, modernization and other construction contracts in our commercial businesses is measured using cost-to-cost based input measures. Contract costs include estimated inventoriable manufacturing, engineering, product warranty and product performance guarantee costs, as appropriate. Contract costs are primarily averaged until actual costs approximate the average for those units of accounting which use output measures when measuring progress towards completion.
For separately priced product maintenance and extended warranty aftermarket contracts, sales are recognized over the contract period. In the commercial businesses, sales are primarily recognized on a straight-line basis. In the aerospace businesses, sales are primarily recognized in proportion to cost as sufficient historical evidence indicates that costs of performing services under the contract are incurred on an other than straight-line basis.
Losses, if any, on long-term contracts are provided for when evident. Loss provisions on original equipment contracts are recognized to the extent estimated contract costs exceed the estimated consideration from the products contemplated under the contractual arrangement. For new commitments, we generally record loss provisions at the earlier of contract announcement or contract signing except for certain requirements contracts under which losses are recorded upon receipt of the purchase order which obligates us to perform. For existing commitments, anticipated losses on contracts are recognized in the period in which losses become evident. Products contemplated under contractual arrangement include firm quantities of products purchased under contract and, in the large commercial engine and wheels and brakes businesses, future highly probable sales of replacement parts required by regulation that are expected to be purchased subsequently for incorporation into the original equipment. In the large commercial engine and wheels and brakes businesses, when the combined original equipment and aftermarket arrangements for each individual sales campaign are profitable, we record original equipment product losses, as applicable, at the time of delivery.
We review our cost estimates on significant contracts on a quarterly basis, and for others, no less frequently than annually or when circumstances change and warrant a modification to a previous estimate. We record changes in contract estimates using the cumulative catch-up method in accordance with the "Revenue Recognition" Topic of the FASB ASC. The net increase in operating profits as a result of significant changes in aerospace contract estimates was $167 million in 2013 driven by several favorable contract adjustments recorded throughout the year largely at the Pratt & Whitney segment. The impact of these adjustments was not considered significant to either the sales or operating profits of the segment in the quarter in which they were recorded other than the impact of a contract termination which was disclosed in the Pratt & Whitney segment results in the first quarter of 2013. The net change in contract estimates also includes the $27 million and $157 million adverse impacts revisions in estimate on the CH-148 Canadian Maritime Helicopter program resulting from ongoing program delays in 2013 and 2012, respectively.
Collaborations: Sales generated from engine programs, spare parts sales, and aftermarket business under collaboration arrangements are recorded consistent with our revenue recognition policies in our consolidated financial statements. Amounts attributable to our collaborators for their share of sales are recorded as an expense in our financial statements based upon the terms and nature of the arrangement. Costs associated with engine programs under collaborative arrangements are expensed as incurred. Under these arrangements, collaborators contribute their program share of engine parts, incur their own production costs and make certain payments to Pratt & Whitney for shared or joint program costs. The reimbursement of a collaborator's share of program costs is recorded as a reduction of the related expense item at that time.
