A possible inconsistency in the currently accepted (but still heavily discussed) Aeroelastic stability ASTM industry standard F3093, Paragraph 4.1.2; The natural frequencies of main structural components must be determined by vibration tests or other approved methods. This determination is not required for Level 1 airplanes with VD up to 260 kph [140 knots] CAS and maximum gross weight up to 750 kg [1650 lbm]. The above paragraph 4.1.2 was adopted from the old Title 14 CFR 23.629 Rule, but modified adding the last statement This determination is not required for Level 1 airplanes with VD up to 260 kph [140 knots] CAS and maximum gross weight up to 750 kg [1650 lbm]. For Harmonization reasons with CS-VLA. However current FAA and EASA Rules Title 14 23.2245 and CS 23.2245 Aeroelasticity, state: (a) The airplane must be free from flutter, control reversal, and divergence - (1) At all speeds within and sufficiently beyond the structural design envelope; (2) For any configuration and condition of operation; (3) Accounting for critical degrees of freedom; and (This means must determine natural frequencies of main structural components for all airplanes) (4) Accounting for any critical failures or malfunctions. (b) The applicant must establish tolerances for all quantities that affect flutter. The possible concern is, if an exception to the rule is warranted, it should be at the rule level, and not at the ASTM standard level, as in this case. This may cause a flutter seasoned Regulatory compliance engineer or delegated DER to reject this exception anytime in future sighting that it is not in compliance with the current FAA/EASA rules.
Date Initiated: 05-14-2019
Technical Contact: Wael Nour