Vibration and Buffeting

Design changes to large aeroplanes that include the installation on the fuselage of a large^* radome or antenna covered by an aerodynamic fairing must comply with CS 25.251(b), which states that each part of the aeroplane must be demonstrated in flight to be free from excessive vibration under any appropriate speed and power conditions up to V_DF/M_DF.

Such design changes must also comply with CS 25.251(d), which states that there may be no perceptible buffeting condition in the cruise configuration in straight flight at any speed up to V_MO/M_MO, except that the stall warning buffeting is allowable.

The extent of the aeroplane modifications, particularly the size and location with respect to the unmodified aeroplane, may cause significant changes in the aerodynamic flow field around the aeroplane at high speed, which may lead to excessive vibration. Potential vibration sources include unsteady flow conditions on the new or modified fuselage, tail assembly, or control surfaces arising from shocks, flow separation or other unsteadiness in the flow.

Because of these potential effects, the original demonstration of compliance with CS 25.251(b) and (d) may not be valid for the modified aeroplanes. The demonstration of compliance must be based on flight tests only.

Nevertheless, an applicant may propose an Equivalent Safety Finding (ESF) based on an acceptable method (such as a similarity analysis to other EASA approved designs, computational fluid dynamics tools, vibrations analysis, partial flight tests) to substantiate that the original demonstration of compliance with CS 25.251(b), or CS 25.251(b) and (d), at time of TC remains valid with the design change applied. This ESF provides compensating factors allowing to reach an equivalent level of safety per point 21.B.80(a)2 of Part-21 (Annex I to Regulation (EU) No 748/2012).

EASA already published an ESF on this subject in April 2016 that can be found under the following link on the EASA website: https://www.easa.europa.eu/en/document-library/product-certification-consultations/equivalent-safety-finding-cs-25251b-vibration

The present ESF aims at extending the scope of the formerly published ESF, which was limited to CS 25.251(b), to include CS 25.251(d). All changes introduced compared to the proposed ESF at Issue 1 are tracked for traceability reasons.

Considering all the above, the following Equivalent Safety Finding to CS 25.251(b), or CS 25.251(b) and (d), at Amdt 28 is defined.

^{* A definition of what constitutes a “large antenna installation” is provided in chapter 2 of Certification Memorandum ref. CM-S-013 at Issue 1 published by EASA on 15 November 2019.}