A simple question has turned into a very interesting discussion. Thanks for the responses and further clarification is always welcomed.
The rules that govern a particular airplane’s airworthiness is found in multiple places. The main one is a Type Certificate Data Sheet (aircraft specifications sheet if very old). This lists the exact rules the aircraft manufacturer had to SHOW COMPLIANCE with at the time of certification. These do not change. However, AD’s (part 39) and operations (part 91, 135, 121 etc) also apply to said aircraft, and have changed over time. Even the registration markings (part 45) and registration (part 47) apply. When a shop modifies an old CAR 3 airplane 50 years after it was built, the engineering data is written to SHOW COMPLIANCE with the old CAR 3 rules on the Type Certificate Data Sheet..
One example, a CAR 3 1956 Cessna 172 vs the latest and greatest whiz bang “Carbon Piston Machine”
Seat foam replacement - Assume the foam meets all burn certification requirements. Just changing the seat foam in the “Carbon Piston Machine” needs to be treated as a Major Alteration because without taking great care to ensure the foam cushion is designed properly, it may no longer meet the very detailed dynamic seat & restraints regulation (think crashworthiness) in Part 23. In the CAR 3 172, the seat foam is not considered part of the supporting structure that was designed to SHOWED COMPLIANCE with CAR 3 50 years ago.
§23.562 Emergency landing dynamic conditions.
(a) Each seat/restraint system for use in a normal, utility, or acrobatic category airplane, or in a commuter category jet airplane, must be designed to protect each occupant during an emergency landing when—
(1) Proper use is made of seats, safety belts, and shoulder harnesses provided for in the design; and
(2) The occupant is exposed to the loads resulting from the conditions prescribed in this section.
(b) Except for those seat/restraint systems that are required to meet paragraph (d) of this section, each seat/restraint system for crew or passenger occupancy in a normal, utility, or acrobatic category airplane, or in a commuter category jet airplane, must successfully complete dynamic tests or be demonstrated by rational analysis supported by dynamic tests, in accordance with each of the following conditions. These tests must be conducted with an occupant simulated by an anthropomorphic test dummy (ATD) defined by 49 CFR part 572, subpart B, or an FAA-approved equivalent, with a nominal weight of 170 pounds and seated in the normal upright position.
(1) For the first test, the change in velocity may not be less than 31 feet per second. The seat/restraint system must be oriented in its nominal position with respect to the airplane and with the horizontal plane of the airplane pitched up 60 degrees, with no yaw, relative to the impact vector. For seat/restraint systems to be installed in the first row of the airplane, peak deceleration must occur in not more than 0.05 seconds after impact and must reach a minimum of 19g. For all other seat/restraint systems, peak deceleration must occur in not more than 0.06 seconds after impact and must reach a minimum of 15g.
(2) For the second test, the change in velocity may not be less than 42 feet per second. The seat/restraint system must be oriented in its nominal position with respect to the airplane and with the vertical plane of the airplane yawed 10 degrees, with no pitch, relative to the impact vector in a direction that results in the greatest load on the shoulder harness. For seat/restraint systems to be installed in the first row of the airplane, peak deceleration must occur in not more than 0.05 seconds after impact and must reach a minimum of 26g. For all other seat/restraint systems, peak deceleration must occur in not more than 0.06 seconds after impact and must reach a minimum of 21g.
Goes on and on……