I am not sure that the statement in the first line of the thread (…the biggest single cost of heli operation is depreciation') is necessarily correct. Insurance and staff could well be the biggest single cost; it depends on where you operate and who you employ.
Depreciation can be used in many ways - depends how it best benefits your organization.
Examples:
Buy a machine at say USD 1 million and depreciate it to zero over 3 years (USD 333,333 per year). Then sell it for USD 700,000 and you have a USD 700,000 windfall profit. Depends on the corporate and tax environment you live and work in.
Also consider that Bell 206 with 26,000 hours, which was probably manufactured in the mid 70s. Today it is has almost the same earning power as a machine half its age but is worth maybe twice as much as it cost in dollar terms.
Depends on how your accountant manages the figures as to how much depreciation that represents.