An alternative is to develop major (or “heavy”) inspections at certain intervals to check the structure and replace suspect components.
At Cirrus we chose to test for a useful life of 12,000 flight hours – about 60 years of average use. Most of our major structures, however, have been tested for twice this lifetime. At Cirrus we also chose to demonstrate that the structure is good for this design life without the need for any interim, heavy inspections – with their associated cost and inconvenience.
At Cirrus we expect that, as real-life aircraft approach 12,000 hours, a further round of testing, analysis, and inspections will determine how to extend the useful life.
Other manufacturers use different combinations of life and inspections. For example:
- Cessna (formerly Columbia) 350/400 have a lifetime of over 25,000 hours but with an extensive (bonded seams, checks for de-lamination, thirty wing bolt replacement, flight control removals, etc.) airframe inspection every 3,000 hours.
These aircraft are certified for utility category (0.6G additional structural substantiation over the normal 3.8G) but are significantly compromised: landing weight reduced 180 lbs from maximum takeoff weight; Vne (Never Exceed Speed) reduces with altitude to under 175 knots (KIAS); and advertised speed at 25,000 feet is about 20 knots above the corresponding Vno (maximum structural cruising speed).
- Diamond DA-40 training airplanes have no published useful life. Those airplanes are, somewhat similar to older designs, dependent upon regular “heavy” inspections to identify structural issues arising from routine use.
These airplanes have an extensive “wings-off” inspection every 1,000 hours and even more intrusive structural inspection every 2,000 hours.
All of these approaches produce sound, strong airplanes – probably the best general aviation airplanes ever produced. Different engineering approaches, though, produce different trade-offs. At Cirrus we prefer the simple, uncomplicated approach – which we believe offers the highest level of convenience and economy for our customers.
