Bird Strike of Propeller

Posted on Twitter dated 4th January 2022 at 10:49 pm under the Twitter account Fabricio Darosci Jr, A SA Airlink Jetstream JS-41, reg ZS-NRJ performing a charter flight from Johannesburg to Venetia Mine (SA), was on approach when a bird impacted the right-hand propeller causing one of the blades to separate and penetrate the cabin. While the aircraft sustained substantial damage, none of the passengers or crew were injured. The aircraft continued for a safe landing.

This is the case of a “serious incident”, which the ICAO defined as “An incident involving circumstances indicating that there was a high probability of an accident “An accident is defined as “An occurrence associated with the operation of an aircraft ..., in which:

a) a person is fatally or seriously injured …

b) the aircraft sustains damage or structural failure

c) the aircraft is missing or is completely inaccessible

For this case, no serious injury or fatality, however, there was significant structure damage, where the blade penetrated the cabin when the propeller blade bokeh off after being impacted by birds.

This propeller and aircraft have been designed and certified to the technical standards in accordance with the ICAO Annex 8 and these technical standards have been translated into the country regulations i.e FAR Part 35 or CS P for Propeller and the aircraft has been designed to FAR Part 25 or EASA CS 25 or JAR 25 Large Aeroplane.

Let's review the design and certification requirement for the propeller and aircraft in regards to the “Bird Impact”. For the propeller, the EASA CS-P, Subpart C paragraph 360, requires the propeller to withstand bird impact specified in the aircraft design and certification requirement and the bird shall not exceed 1.8 Kg. The test may use natural birds or synthetic birds. However, on the Jetstream JS-41, reg ZS-NRJ one of the propellers separated and went into the cabin. The accident investigation report may explain the cause of the blade separation. Looking at the design requirement, the blade shall withstand the bird impact. One of the other possible reasons was the size of the bird.

? The aircraft design and certification standard or airworthiness requirements published in FAA FAR Part 25 published three paragraphs related to propeller failures 25.631, and 25.875. The condition in FAR Part 25, the propeller must have a “Type Certificate (TC)”, refer to the FAR Part 25.905. It means the design and certification requirements must have been fulfilled, and the propeller must be able to withstand the “Bird Impact”.

The FAR Part 25.631 has required the empennage structure to withstand 8 Lbs bird impact which is a 3.6 Kg bird. FAR Part 25.875 “no windows near propeller”, for ice impact.

The INTERNATIONAL CIVIL AVIATION ORGANIZATION, Annex 8 - Airworthiness of Aircraft, Twelfth Edition, July 2018 does not provide specific requirements on bird strike for the propeller but it has requirements for the protection of occupants from bird strike when designing the fuselage structure.

There was no injury to the occupant because the aircraft is designed with the requirement of installation the minimize the hazards to the airplane when the propeller failed, refer 25.905.

Risked based design approach may be able to address these issues in detail.?