Balance Sheet Information - Summary of Certain Significant Amounts Included in Condensed Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2025	Sep. 30, 2024
Expendable parts and supplies, net:
Expendable parts and supplies	$ 20,640	$ 39,089
Less: expendable parts warranty	(4,583)	(6,079)
Less: obsolescence	(2,408)	(4,738)
Expendable parts and supplies, net	13,649	28,272
Property and equipment, net:
Property and equipment-gross	75,247	600,924
Less: accumulated depreciation	(38,401)	(174,573)
Property and equipment-net	36,846	426,351
Property and equipment, net	36,846	426,351
Other assets:
Investments in equity securities	300	300
Lease incentives		812
Contract asset	4,948	6,081
Other	1,648	516
Other assets	6,896	7,709
Other accrued expenses:
Accrued property taxes	2,045	4,650
Accrued interest	1,071	2,997
Accrued vacation	6,450	7,421
Accrued lodging	3,855	4,433
Accrued maintenance	1,387	2,493
Accrued employee benefits	1,169	1,075
Accrued fleet operating expense	2,842	2,751
Other	9,380	6,488
Other accrued expenses	28,199	32,308
Other noncurrent liabilities:
Warrant liabilities	25,225	25,225
Lease incentive obligations		1,050
Long-term employee benefits	1,748	485
Other		1,819
Other noncurrent liabilities	26,973	28,579
Aircraft and Other Flight Equipment [Member]
Property and equipment, net:
Property and equipment-gross	65,659	591,421
Other Machinery and Equipment [Member]
Property and equipment, net:
Property and equipment-gross	$ 9,588	$ 9,503