As of March 31, 2022, we leased 18 aircraft, airport facilities, office space, and other property and equipment under non-cancelable operating leases. The leases generally require us to pay all taxes, maintenance, insurance, and other operating expenses. Rental expense is recognized on a straight-line basis over the lease term, net of lessor rebates and other incentives. We expect that, in the normal course of business, such operating leases that expire will be renewed or replaced by other leases, or the property may be purchased rather than leased.

Aggregate rental expense under all operating aircraft, equipment and facility leases totaled approximately $11.0 million and $11.3 million for the three months ended March 31, 2022 and 2021, respectively, and $22.2 million and $22.7 million for the six months ended March 31, 2022 and 2021, respectively.

	Three Months Ended March 31,				Six Months Ended March 31,
	2022		2021		2022		2021
Operating lease costs	$	9,663	$	9,697	$	19,144	$	19,405
Variable and short-term lease costs		1,346		1,646		3,081		3,335
Total lease costs	$	11,009	$	11,343	$	22,225	$	22,740

As of March 31, 2022, our operating leases have a remaining weighted average lease term of 3.7 years and our operating lease liabilities were measured using a weighted average discount rate of 4.4%.

During the three and six months ended March 31, 2022, we recorded a $0.4 million impairment of certain operating lease ROU assets associated with the abandonment of a leased facility.