June 11, 2021, ©. Leeham News: Last week, we scaled a nine-seat aircraft to a 19-seat aircraft and examined some of the pros and cons of such a change. The aircraft are certified to the 14 CFR Part 23 rules in the US, labeled “Normal Category Aircraft“.
This week we scale the aircraft up one step further to understand product certification and operation rules for the larger Transport Category Aircraft (14 CFR Part 25) class.
Having less than 19 passenger seats, as we have previously discussed, does not automatically mean that an aircraft falls under Part 23 (“Normal Category Airplanes” in FAA speak). Many business jets, such as the Citation XLS+ or the Global 7500, are Part 25 aircraft (“Transport Category Aircraft”) since they exceed the 19,000 lb maximum certified takeoff weight threshold. On the other hand, having more than 19 passenger seats always puts the aircraft in the Part 25 category. So, what are the major differences between a Part 23 and Part 25 aircraft?
As previously discussed, as the size and passenger count of an aircraft go up, the expected level of safety goes upwards as well. As a result, Part 25 contains a number of additional and more stringent rules. These differences can be about the capabilities of the airplane or the method in which it must be demonstrated. We give a couple of examples. For simplicity of comparison language, Part 23 texts are drawn from the prescriptive version instead of the new performance-based version.
Under 23.775 for Commuter Category Airplanes, you’ll find the following requirements for airplane integrity as a result of a bird strike: “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.” (So in the speed bracket of 120-180kts, our comment).
In contrast, under EASA CS-25 (the EU Transport Category rules):
25.631: “The aeroplane must be designed to assure capability of continued safe flight and landing of the aeroplane after impact with a 4 lb bird when the velocity of the aeroplane (relative to the bird along the aeroplane’s flight path) is equal to Vc (Vc is the design cruise speed, our comment) at sea-level or 0·85 Vc at 2438 m (8000 ft), whichever is the more critical.“
25.775(b): “Windshield panes directly in front of the pilots in the normal conduct of their duties, and the supporting structures for these panes, must withstand, without penetration, the bird impact conditions specified in CS 25.631.”
And interestingly, this is even more constrained when we add in the US 14 CFR 25.631 from the FAA: “The empennage structure (the tail of the aircraft, our comment) must be designed to assure capability of continued safe flight and landing of the airplane after impact with an 8-pound bird when the velocity of the airplane (relative to the bird along the airplane’s flight path) is equal to Vc (design cruise speed) at sea level..“
So, under Part 23 we’re primarily concerned with a 2 lb bird impacting the windshield at landing speeds, while under Part 25 we must design for a 4 lb bird hitting a larger frontal area at up to cruise speed. In addition, this is an example of a non-harmonized regulation, where if we want to sell the aircraft in the US we must design for an 8 lb bird hitting the empennage structure at up to cruise speed. Part 25 rules add significant additional weight to an aircraft to achieve the needed impact resistance, more so for the US market than Europe. It also adds development and testing costs for the analysis on how to design for and later demonstrate compliance.
Next week we continue with more examples of the differences between Normal (Part 23) and Transport Category (Part 25) Aircraft.