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Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
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      
Number of reportable segments | Segment 1      
Outstanding letters of credit to be collateralized by amounts on deposit, classified as restricted cash $ 3,000,000 $ 3,100,000    
Asset impairment 73,709,000 54,343,000 $ 171,824,000  
Impairment loss held for sale $ 73,700,000 50,600,000 62,100,000  
Revenue recognition percentage (9.80%)      
Lease revenue $ 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] Contract revenue Contract revenue Contract revenue  
Current and non-current deferred credit balances $ 4,100,000 $ 5,100,000    
Recognized deferred credits within contract revenue 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 3,200,000 3,100,000 1,900,000  
Deferred heavy maintenance balance, net of accumulated amortization 6,400,000 8,000,000    
Engine overhaul expense 23,000,000 32,400,000 23,600,000  
Engine overhaul pass-through expense 23,500,000 31,900,000 21,700,000  
Insurance claim reimbursement net of engine overhaul expense 500,000      
Airframe check expense 22,900,000 23,400,000 22,100,000  
Airframe check pass-through expense 16,300,000 16,900,000   $ 3,200,000
Impairment loss 0 0 10,500,000  
Net (loss) income (91,015,000) (120,116,000) (182,678,000)  
Cash flow provided by operations 34,244,000 (24,091,000) 13,362,000  
Working capital deficit 113,200,000      
Accumulated deficit (162,134,000) (71,119,000)    
Cash and cash equivalents 15,621,000 32,940,000    
Property and Equipment [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Asset impairment     109,700,000  
Other Long-lived Assets [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