With the successful test flight and the achievement of the STC (Supplemental Type Certificate), has been completed the certification process for the cockpit floor window kit developed by SHE, Swiss Helicopter Engineering (www.she-ag.ch), for the AS350/H125 helicopter. This major modification improves pilot field of view and enhances flight safety during sling load and fire-fighting operations. It was conceived with the specific goal of offering not only excellent outward vision, but also an alternative system to those already available on the market—one that integrates seamlessly with the existing structure, comes at an affordable cost, and can be installed by any Part 145 organization.
Within a 12-month timeframe, concept was defined, designed, numerically pre-validated, optimized, manufactured, installed, flight tested and approved by EASA. During the entire process, Vicoter (www.vicoter.it) collaborated closely with SHE to ensure immediate compliance with stringent certification requirements, thereby minimizing the risk of redesign activities and avoiding costly aircraft groundings.
The kit design was developed in close cooperation with the installation mechanics to ensure minimal impact on the primary cabin structure, ease of installation, and reduced helicopter downtime. The solution involves creating an opening in the cabin floor and removing the outer portions of two transverse frames located beneath it. A new external window and structural reinforcements are riveted to the existing structures to restore the local stiffness of the modified area. A foldable cover made of machine part is installed to ease boarding the rear seat occupants and to ensure compliance with safety requirements ensuring a good access to the emergency exit handle in the event of an emergency evacuation. Finally, a transparent close-out is installed in the additional opening made in the external cowling, in order to avoid FODs to go under the cabin floor.
A comparative strategy was adopted to verify that the kit complied with all stringent requirements imposed by the certification authority, as the modification also involved the primary structure. A finite element analysis was performed on the critical portion of the cabin, both in the baseline and in the modified configuration. In the absence of actual or original design loads, various fictitious ones were applied to both models, and the assessment was carried out by evaluating the differences between the two cases. The results showed that the global and local stiffness of the cabin remained essentially unchanged, with no abnormal stress concentrations or increases that could lead to fatigue problems. The load paths in the original and modified configurations are identical and no significant variation is noticed in connections analysis. Particular attention was paid to the area beneath the pilot’s seat too, to ensure that its effectiveness in the event of a crash was not compromised, for this reason these analyses are valid for AS 350 B3 helicopter equipped with pilot seat type Sicma 159 and Sicma 198 installed on short, extended or long seat tracks. Comparison of the stress field for a fictitious load. Original (left) configuration and modified (right) configuration.Comparison of the stress field for a fictitious load. Original (left) configuration and modified (right) configuration.
Modal behaviour was analyzed too, to avoid future vibration issues.
At the end of the design phase, a trial installation of a first prototype of the cockpit floor window was conducted on a retired AS350 helicopter. This allowed all installation details and component designs to be verified and, if necessary, optimized to achieve the goal of easy installation without impacting the downtime of the helicopter to be used for flight testing.
As the area modified by the installation of the modification is also used as a step area by the occupants to embark / disembark from the helicopter, even eventual abuse loads have been verified to maintain the walkability of the area. Numerical evaluation of the capability of the part to sustain about 2700 N at the edge of the floor was realized at first, using the already developed finite element model, showing a high safety margin related to the yield stress of the part. Nevertheless, a static test was performed and successfully passed on the dummy installation, experimentally validating the results.
Flight test activities were carried out to demonstrate compliance with FAR 27.251 “Vibration” and FAR 27.1321 “Arrangement and visibility.” To verify that the installation of the cockpit floor window did not affect the NVH performance of the helicopter, both quantitative and qualitative approaches were adopted.
For the evaluation of the overall vibration level, accelerometers were installed in the cabin and data were recorded during flights performed immediately before and after the window kit installation. Various stationary and non-stationary manoeuvres were executed in category 2 flight, and through comparison of the acquired signals—in terms of overall RMS, presence of resonance peaks, and amplitudes of rotor-excited frequencies—it was possible to assess the impact of the modification as negligible.
For the readability of the cockpit instrument even a qualitative evaluation was made by the pilot during the same flight, using the standard vibration scale.
Vicoter sincerely thanks Matteo Confalonieri , Julien Blanc , and Andrea Ferrario for everything they have taught over these months regarding helicopter certification, for the trust placed, and for the friendship shown.
