Certain significant amounts included in the condensed consolidated balance sheets as of December 31, 2021 and September 30, 2021, consisted of the following (in thousands):

	December 31,			September 30,
	2021			2021
Expendable parts and supplies, net:
Expendable parts and supplies	$	30,877		$	29,297
Less: obsolescence and other		(5,671	)		(4,830	)
	$	25,206		$	24,467

Prepaid expenses and other current assets:
Prepaid aviation insurance	$	140		$	2,171
Lease incentives		1,715			1,445
Other		2,633			3,269
	$	4,488		$	6,885
Property and equipment, net:
Aircraft and other flight equipment	$	1,637,154		$	1,611,544
Other equipment		4,971			4,934
Leasehold improvements		2,776			2,776
Vehicles		1,184			1,184
Building		699			699
Furniture and fixtures		292			300
Total property and equipment		1,647,076			1,621,437
Less: accumulated depreciation		(489,154	)		(469,546	)
	$	1,157,922		$	1,151,891
Other assets:
Investments in equity securities	$	19,005		$	25,149
Lease incentives		12,705			10,957
Other		42			15
	$	31,752		$	36,121
Other accrued expenses:
Accrued property taxes	$	8,598		$	8,783
Accrued interest		4,481			2,565
Accrued vacation		5,829			5,936
Other		17,375			16,373
	$	36,283		$	33,657
Other noncurrent liabilities:
Warrant liabilities	$	22,064		$	21,964
Lease incentive obligations		7,289			6,358
Other		4,253			6,269
	$	33,606		$	34,591

We record impairment charges on long-lived assets used in operations when events and circumstances indicate that the assets may be impaired, the undiscounted net cash flows estimated to be generated by those assets are less than the carrying amount of those assets, and the net book value of the assets exceeds their estimated fair value. We have assessed whether any impairment of our long-lived assets existed and have determined that no charges were deemed necessary under applicable accounting standards as of December 31, 2021. Our assumptions about future conditions relevant to the assessment of potential impairment of the long-lived assets are subject to uncertainty, and we will continue to monitor these conditions in future periods as new information becomes available, and will update our analyses accordingly.

Depreciation of property and equipment totaled $20.5 million and $20.2 million for the three months ended December 31, 2021 and 2020, respectively.

In connection with a negotiated forward purchase contract for electrically-powered vertical takeoff and landing aircraft (“eVTOL aircraft”) executed in February 2021, we obtained warrants giving us the right to acquire a number shares of common stock in Archer Aviation, Inc. (“Archer”). We estimated the initial equity warrant asset value to be $16.4 million based on publicly available information as of the grant date. In September 2021, the merger between Archer and a special purpose acquisition company (“SPAC”) was completed, resulting in a readily determinable fair value of our investments in Archer. Accordingly, gains and losses associated with changes in the fair value of our investments in Archer are measured in earnings, in accordance with ASC 321, Investments – Equity Securities. In connection with closing of the merger between Archer and the SPAC, we purchased 500,000 Class A common shares in Archer for $5.0 million, and obtained an additional warrant to purchase shares of Archer with a total grant date value of $5.6 million. During the three months ended December 31, 2021, all of our warrants were exercised into shares of Archer common stock.

The initial grant date values of the warrants were recognized as a vendor credit liability within other noncurrent liabilities. The liability related to the warrant assets will be settled in the future, as a reduction of the acquisition date value of the eVTOL aircraft contemplated in the related aircraft purchase agreement.

Losses on our investments in Archer totaled $6.5 million during the three months ended December 31, 2021 and are reflected in loss on investments, net in our condensed consolidated statement of operations. The total carrying amount of our investments in Archer common stock was $13.7 million as of December 31, 2021.

Our investments in Archer are classified as Level 1 within the fair value hierarchy as the values are determined using quoted prices for the equity securities.

In connection with a negotiated forward purchase contract for fully electric aircraft executed in July 2021, we obtained $5.0 million of preferred stock in Heart Aerospace Incorporated (“Heart”), a privately held company. Our investment in Heart does not have a readily determinable fair value, so we account for the investment using the measurement alternative under ASC 321 and measure the investment at initial cost less impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for identical or similar investments from the same issuer. We consider a range of factors when adjusting the fair value of these investments, including, but not limited to, the term and nature of the investment, local market conditions, values for comparable securities, current and projected operating performance, financing transactions subsequent to the acquisition of the investment, or other features that indicate a change to fair value is warranted. Any changes in fair value from the initial cost of the investment in preferred stock are recognized as increases or decreases on our balance sheet and as net gains or losses on investments in equity securities, in other income (expense), net. The initial investment in preferred stock was measured at cost of $5.0 million. There were no observable price changes or transactions as of December 31, 2021 and as such, no adjustments to the carrying amount of the equity investment have been recorded.