Revenue Recognition Leasing revenue Revenue from leasing of engines, aircraft and related parts and equipment is recognized as operating lease revenue on a straight-line basis over the terms of the applicable lease agreements. Revenue is not recognized when cash collection is not reasonably assured. When collectability is not reasonably assured, the customer is placed on non-accrual status and revenue is recognized when cash payments are received. Under the terms of some of the Companys leases, the lessees pay use fees (also known as maintenance reserves) to the Company based on usage of the leased asset, which are designed to cover expected future maintenance costs. Some of these amounts are reimbursable to the lessee if they make specifically defined maintenance expenditures. Use fees received are recognized in revenue as maintenance reserve revenue if they are not reimbursable to the lessee. Use fees that are reimbursable are recorded as a maintenance reserve liability until they are reimbursed to the lessee, the lease terminates, or the obligation to reimburse the lessee for such reserves ceases to exist, at which time they are recognized in revenue as maintenance reserve revenue. Certain lessees may be significantly delinquent in their rental payments and may default on their lease obligations. As of December 31, 2019 , the Company had an aggregate of approximately $2.7 million in lease rent and $3.8 million in maintenance reserve receivables more than 30 days past due. Inability to collect receivables or to repossess engines or other leased equipment in the event of a default by a lessee could have a material adverse effect on the Company. The Company estimates an allowance for doubtful accounts for receivables it does not consider fully collectible. The allowance for doubtful accounts includes the following: (1) specific reserves for receivables which are impaired for which management believes full collection is doubtful; and (2) a general reserve for estimated losses based on historical experience. No customer accounted for greater than 10% of total lease rent revenue in 2019 and 2018 . Gain on sale of leased equipment The Company regularly sells equipment from its lease portfolio. This equipment may or may not be subject to a lease at the time of sale. The net gain or loss on such sales is recognized as revenue and consists of proceeds associated with the sale less the net book value of the asset sold and any direct costs associated with the sale. To the extent that deposits associated with the equipment are not included in the sale, any such amount is included in the calculation of gain or loss. Spare parts sales The Spare Parts Sales reportable segment primarily engages in the sale of aircraft engine parts and materials through the acquisition or consignment of engines from third parties or the Companys leasing operations. The parts are sold at a fixed price with no right of return. In determining the performance obligation, management has identified the promise in the contract to be the shipment of the spare parts to the customer. Title passes to the buyer when the goods are shipped, and the buyer is responsible for any loss in transit, and the Company has a legal right to payment for the spare parts. Management has determined that physical acceptance of the spare parts to be a formality in accordance with Accounting Standards Codification (ASC) 606-10-5-86. The spare parts transaction price is a fixed dollar amount and is stated on each purchase order for a fixed amount by total number of parts. Spare parts revenue is based on a set price for a set number of parts as defined in the purchase order. The performance obligation is completed once the parts have shipped and, as a result, all of the transaction price is allocated to that performance obligation. Management has determined that it is appropriate for the Company to recognize spare parts sales at a point in time (i.e., the date the parts are shipped) under ASC 606. Additionally, there is no impact to the timing and amounts of revenue recognized for spare parts sales related to the implementation of ASC 606 upon adoption on January 1, 2018. Equipment Sales Equipment sales represent the selective purchase and resale of commercial aircraft engines and other aircraft equipment. The Company and customer enter into an agreement which outlines the place and date of sale, purchase price, payment terms, condition of the asset, bill of sale, and the assignment of rights and warranties from the Company to the customer. Management has identified the promise in the equipment sale contract to be the transfer of ownership of the asset. Management believes the asset holds standalone value to the customer as it is not dependent on any other services for functionality purposes and therefore is distinct within the context of the contract and as described in ASC 606-10. As such, management has identified the transfer of the asset as the performance obligation. The transaction price is set at a fixed dollar amount per fixed quantity (number of assets) and is explicitly stated in each contract. Equipment sales revenue is based on a set price for a set number of assets, which is allocated to the performance obligation discussed above, in its entirety. The Company has determined the date of transfer to the customer to be the date the customer obtains control and title over the asset and the date which revenue is to be recognized and payment is due. Therefore, there is no impact to the timing and amounts of revenue recognized for equipment sales related to the implementation of ASC 606 upon adoption on January 1, 2018. Managed Services Managed services revenue predominantly represents fleet management and engine storage services which may be combined on a single contract with a customer. Fleet management services are performed for a stated fixed fee as agreed upon in the services agreement. Engine storage services are for a fixed monthly fee. For a contract containing more than one performance obligation, the allocation of the transaction price is generally performed on the basis of the relative stand-alone selling price of each distinct good or service in the contract. As each of the services provided within the contract have separate prices, the Company allocates the price to its related performance obligation described above. Management has determined each of the revenue elements contain performance obligations that are satisfied over time and therefore recognizes revenue over time in accordance with ASC 606-10-25-27. The Company utilizes the percentage-of-completion method (input method) for recognizing fleet management services and will calculate revenues based on labor hours incurred. Additionally, as is required by ASC 606-10-25-35, as circumstances change over time, the Company will update its measure of progress to reflect any changes in the outcome of the performance obligation. Engine storage services are recognized on a monthly basis utilizing the input method of days passed. Therefore, there is no impact to the timing and amounts of revenue recognized for managed services related to the implementation of ASC 606 upon adoption on January 1, 2018. Amounts owed for managed services are typically billed upon contract completion. At December 31, 2019 , unbilled revenue was $0.8 million and the Company expects it to be fully recognized by June 30, 2020. Additionally, managed services are presented within the Other revenue line in the Consolidated Statements of Income. Other Revenue Other revenue consists primarily of management fee income, lease administration fees, third party consignment commissions earned, service fee revenue, interest revenue, and other discrete revenue items.