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Summary of Significant Accounting Policies - Additional Information (Detail) | 3 Months Ended | 6 Months Ended | 12 Months Ended
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Mar. 31, 2025 USD ($) AIRCRAFT | Dec. 31, 2024 | Mar. 31, 2024 USD ($) | Mar. 31, 2025 USD ($) AIRCRAFT | Mar. 31, 2024 USD ($) | Sep. 30, 2024 USD ($) SEGMENT | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($) | Sep. 30, 2021 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Number of operating segments | SEGMENT |  |  |  |  |  | 1 |  |  |
Outstanding letters of credit to be collateralized by amounts on deposit, classified as restricted cash | $ 3,000,000 |  |  | $ 3,000,000 |  | $ 3,000,000 | $ 3,100,000 |  |
Asset impairment |  |  |  | 111,838,000 | $ 43,043,000 | 73,709,000 | 54,343,000 | $ 171,824,000 |
Impairment loss held for sale | 12,400,000 |  |  |  |  | $ 73,700,000 | 50,600,000 | 62,100,000 |
Revenue recognition percentage |  |  |  |  |  | (9.80%) |  |  |
Lease revenue | 5,700,000 |  | $ 33,700,000 | 25,900,000 | $ 68,600,000 | $ 123,000,000 | $ 144,700,000 | $ 158,400,000 |
Deferred revenue |  |  |  |  |  | $ 9,600,000 |  |  |
Operating Lease, Lease Income, Statement of Income or Comprehensive Income [Extensible Enumeration] |  | Revenue from Contract with Customer, Excluding Assessed Tax |  |  | Revenue from Contract with Customer, Excluding Assessed Tax | Revenue from Contract with Customer, Excluding Assessed Tax | Revenue from Contract with Customer, Excluding Assessed Tax | Revenue from Contract with Customer, Excluding Assessed Tax |
Current and non-current deferred credit contract liability balances | 0 |  |  | 0 |  | $ 4,100,000 | $ 5,100,000 |  |
Recognized deferred credits within contract revenue | 3,200,000 |  | 300,000 | 4,100,000 | $ 1,300,000 | 1,800,000 | 1,700,000 | $ 900,000 |
Contract assets balances |  |  |  |  |  | 6,100,000 | 8,800,000 |  |
Contract cost amortization |  |  |  |  |  | 2,700,000 | 1,000,000 | 0 |
Amortization of heavy maintenance and major overhaul costs charged to depreciation and amortization expense | 200,000 |  | 700,000 | 1,300,000 | 1,500,000 | 3,200,000 | 3,100,000 | 1,900,000 |
Deferred heavy maintenance balance, net of accumulated amortization | 0 |  |  | 0 |  | 6,400,000 | 8,000,000 |  |
Engine overhaul expense | 9,200,000 |  | 5,500,000 | 19,300,000 | 11,300,000 | 23,000,000 | 32,400,000 | 23,600,000 |
Engine overhaul pass-through expense | 9,100,000 |  | 5,500,000 | 19,300,000 | 11,200,000 | 23,500,000 | 31,900,000 | 21,700,000 |
Insurance claim reimbursement net of engine overhaul expense |  |  |  |  |  | 500,000 |  |  |
Airframe check expense | 7,600,000 |  | 4,600,000 | 15,900,000 | 11,000,000 | 22,900,000 | 23,400,000 | 22,100,000 |
Airframe check pass-through expense | 6,200,000 |  | $ 3,200,000 | 14,000,000 | 7,000,000 | 16,300,000 | 16,900,000 |  | $ 3,200,000
Impairment loss |  |  |  |  |  | 0 | 0 | 10,500,000 |
Cash flow provided by operations |  |  |  | (33,951,000) | $ 8,388,000 | 34,244,000 | (24,091,000) | 13,362,000 |
Working capital deficit |  |  |  |  |  | 113,200,000 |  |  |
Accumulated deficit | (335,323,000) |  |  | (335,323,000) |  | (162,134,000) | (71,119,000) |  |
Cash and cash equivalents | 54,116,000 |  |  | 54,116,000 |  | 15,621,000 | 32,940,000 |  |
Deferred revenue | $ 700,000 |  |  | $ 5,000,000 |  | 11,400,000 | 3,000,000 |  |
Number of air craft leased from third parties | AIRCRAFT |  |  |  | 0 |  |  |  |  |
E-175 Aircraft [Member] |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Number of aircraft sold | AIRCRAFT | 18 |  |  |  |  |  |  |  |
Property and Equipment [Member] |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Asset impairment |  |  |  |  |  |  |  | $ 109,700,000 |
Long Lived Assets [Member] |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Asset impairment |  |  |  |  |  | 0 | 0 |  |
Aircraft [Member] |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Allowance for obsolete expendable parts and supplies |  |  |  |  |  | $ 4,700,000 | $ 4,100,000 |  |
Percentage of short term leases |  |  |  |  |  | 7.00% |  |  |
Aircraft and Rotable Spare Parts [Member] |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Property and equipment, salvage value, percentage |  |  |  |  |  | 20.00% |  |  |
Letter of Credit [Member] |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Letter of credit facility, amount |  |  |  |  |  | $ 6,000,000 |  |  |

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