A young engineer at a client sent me a question about bird strike requirements and the speed of the aircraft to consider for the bird strike condition. Explaining the rabbit warren of part 23 certification was interesting – so I have reproduced my response here :

Hey (Name Redacted), sadly the wording of the (new) regulations (23.2320) can be ignored in favor of the ‘standards’ (the old regulations). This makes it seem like the regulations are meaningless and the advisory standards carry more weight than the regulations themselves. This is the case, and in this recent and innovative inversion of reality you can ignore the wording of the regulations.

Even the FAA does not understand how the new regulations and standards work together (despite them driving the creation process and approving them) and we will have to lead the FAA through the process of interpreting their own mess.

Just to be clear, this aspect of the ASTM only applies to level 4 aircraft (4.7.6) and we are not a level 4 aircraft (kind of the old part 23 commuter category aircraft, but not really) so this will not apply to us, and in theory we do not have to consider bird strike in this manner in order to show compliance to FAA part 23 (although we have to consider it from a composite damage tolerance point of view for the airframe structures, but that is another thing altogether and there are no explicit means of compliance for that, but ignore that for now because it will get even more confusing if we don’t.)

To make things more exciting, the ASTM standard only applies to the windshield and no other parts of the aircraft. However EASA will not allow our aircraft to be certificated in Europe until we comply with a CRI (the EASA equivalent of a FAA special condition) which is usually applied to ‘high performance’ CS 23 aircraft – this “High Performance” category is something that EASA have created that they like to apply to a lot of part 23 aircraft and has nothing to do with pre amendment 64 commuter category aircraft or post amendment 64 level 4 aircraft. So that’s good.

This CRI contains a set of specific safety standards and compliance with the CRI will automatically allow us to comply with that part of ASTM F3114. To make things even more exciting we will not know the exact wording of the CRI until we start discussions with EASA, but they have applied the same CRI with the same wording to every relevant program in the last few years so we can be relatively confident about the wording (but not completely certain):

For all CS-23 high performance aeroplanes the following applies:

• Windshield panes directly in front of the pilot(s) in the normal conduct of their duties, and the supporting structures for these panes must withstand, without penetration, the impact of a 0·91 kg (2 lb) bird when the velocity of the aeroplane relative to the bird along the aeroplane’s flight path is equal to the aeroplane’s maximum approach flap speed.
• The windshield panels in front of the pilot(s) must be arranged so that, assuming the loss of vision through any one panel, one or more panels remain available for use by a pilot seated at a pilot station to permit continued safe flight and landing.
• Continued safe flight and landing is required after impact of a 0·91 kg (2 lb) bird when the velocity of the aeroplane relative to the bird along the aeroplane’s flight path is equal to the aeroplane’s maximum approach flap speed. This must be shown for any location prone to bird strike where a safety assessment reveals a vulnerable item of concern (e.g. cockpit, fuel tanks, empennage attachments, critical flight systems in the nose or canopy area) either through direct impact or shock wave effects.

You will notice that item (1) from the CRI mirrors the wording of the ASTM. Item (2) adds another visibility criteria – we can comply with this by design because we have separate LH and RH windshields.

(As a side note, this requirement (2) for visibility was applied to commuter category part 23 aircraft and was in amendment 63, and prior amendments, of the FAA regulations as 23.775 (h)(2). For very clever ‘reasons’ this was usefully moved into F3117-23a 4.3 so unless you are psychic or an obsessive anal retentive and search all of the ASTM standards to see if it was deleted or not you would not know it still existed. Thankfully it lives on in the CRI in the original comprehensible form as a traditional sequentially read paragraph of coherent prose – very old school)

Item (3) is the fun requirement to show compliance with – and this mirrors the same wording “after impact of a 0·91 kg (2 lb) bird when the velocity of the aeroplane relative to the bird along the aeroplane’s flight path is equal to the aeroplane’s maximum approach flap speed”

To summarize:

• The wording of the regulations has been very carefully ruined by amendment 64 and can be ignored as they are now vague, confusing and not fit for purpose.
• The partially incoherent ASTMs can be treated as if they are the regulations and must be followed (Despite this being technically illegal – long story – the FAA does not understand the legal issues despite being the government entity created with the statutory authority to enact the law so, errrr, meh)
• In this instance we can ignore the regulations (because they are rubbish) and the ASTM standards because the EASA CRI envelopes the ASTM requirements and exceeds them. We will show compliance to the standards defined by the CRI. (even though a similar CRI has not been formally applied to our program but we can safely assume that it will be at some point. Probably.)
• The likely wording of the probable CRI and the ASTM standards (and the original pre amendment 64 set of regulations) mirror each other. The same wording regarding the relative speed of the bird to the aircraft is repeated more than once in the CRI making this the ‘definitively definitive definition’ of the speed of impact of the bird.
• The people in charge of the regulations and standards might not actually be intellectually subnormal but they have a lot of training and practice and by working very hard together they emulate idiocy with unnerving accuracy.

Welcome to the wonderful world of compliance.