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Summary of Significant Accounting Policies - Additional Information (Detail)
 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2025
USD ($)
Jun. 30, 2024
USD ($)
Jun. 30, 2025
USD ($)
AIRCRAFT
Jun. 30, 2024
USD ($)
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     111,786,000 $ 50,923,000 73,709,000 54,343,000 $ 171,824,000  
Impairment loss held for sale 100,000       $ 73,700,000 50,600,000 62,100,000  
Revenue recognition percentage         (9.80%)      
Lease revenue 200,000 $ 29,500,000 $ 26,100,000 98,100,000 $ 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  
Current and non-current deferred credit contract liability balances 0   $ 0   $ 4,100,000 $ 5,100,000    
Recognized deferred credits within contract revenue 0 300,000 4,100,000 1,500,000 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 0 800,000 1,300,000 2,300,000 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 5,500,000 6,800,000 24,800,000 18,100,000 23,000,000 32,400,000 23,600,000  
Engine overhaul pass-through expense 5,500,000 6,800,000 24,700,000 18,100,000 23,500,000 31,900,000 21,700,000  
Insurance claim reimbursement net of engine overhaul expense         500,000      
Airframe check expense 5,100,000 3,600,000 21,000,000 14,700,000 22,900,000 23,400,000 22,100,000  
Airframe check pass-through expense 3,800,000 $ 2,100,000 17,800,000 9,100,000 16,300,000 16,900,000   $ 3,200,000
Impairment loss         0 0 10,500,000  
Cash flow provided by operations     (43,346,000) $ 19,596,000 34,244,000 (24,091,000) 13,362,000  
Working capital deficit         113,200,000      
Accumulated deficit (314,467,000)   (314,467,000)   (162,134,000) (71,119,000)    
Cash and cash equivalents 42,472,000   42,472,000   15,621,000 32,940,000    
Deferred revenue $ 1,300,000   $ 3,700,000   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