If you depreciate an asset over 5 years, it means that in five years time you reckon it has zero value. Since that is clearly not true, 5 years is not a good depreciation rate to use.
Depreciation represents the diminution in value of an asset over its useful economic life. It is based on the capital cost of the asset. Maintenance reserves represent the cost of keeping the asset.
So, how long do you plan to keep the aircraft and, with its appropriate maintenance schedule (which you accrue for and put in a reserve), what will be its value in ten years time when you plan to sell it or scrap it. Then, you take that value, say £50,000 from the origianl purchase cost (to you),say $120,000, leaving £70,000. You depreciate the helicopter over 10 years at £7,000 per annum.
I have absolutely NO experience at owning or operating a R22 and these are general accounting pineapples. If there are any particular quirks that you wish to factor in, let me know!
Cheers
Whirls