OK... Ill toss a grenade into this campfire...
FAR 23 and 25 are CERTIFICATION standards... The manufacturer (not the pilot) is responsible for certifying the aircraft therefor these regulations are applicable to someone wanting to obtain or change the type design of the aircraft.
Sec. 25.1
Applicability.
(a) This part prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for transport category airplanes.
(b) Each person who applies under Part 21 [New] for such a certificate or change must show compliance with the applicable requirements in this part.
Part 25 aircraft are not allowed the use of TR's, must have brakes worn to limits, and have an Engine Failure Recognition time built into performance calculations. Does this mean pilots are not allowed to use TRs, should always have worn brakes, and wait 2 seconds after Vef to initiate an abort? Of course not...
Remember that when the FAA determines a new aircraft complies with ALL of Part 25, it is issued a Type Certificate. As long as the aircraft continues to meet requirements of the TC, it complies with Part 25 (among others) and the Airworthiness Certificate is valid. It is not up to the pilots to ensure compliance with 25.903, 33.76 or 25.1455 any more than it is their responsibility to ensure compliance with 25.121. The aircraft met the requirements when it was certified and WAT conditions will not invalidate an Airworthiness Certificate.
Aircraft Certification and Climb requirements
During certification, Part 25 only requires that manufacturers establish weights, altitudes and temperatures (WAT) that produce particular climb gradients (see Part 25.111, 25.115, and 25.121) and publish that data in the AFM (it would require about 5 pages). However, I have hundreds of pages of WAT data for climb gradients ranging from 0.0% 2nd segment OEI to as high as 14% using weights lighter that my BEW (9% is about the most realistic number I can achieve OEI). Why are they publishing such wide ranging data? Because there is no regulatory requirement that an aircraft actually have a 2.4% climb gradient (or any other gradient) under every possible WAT combination when it comes on off the production line (its physically impossible). The minimum legal climb gradients that a pilot will be required to meet will be established by a number of variables ranging from OpSpecs to country of registration.
All US registered aircraft must meet the applicable requirements of Part 91 unless they are subject more stringent regulation. Turbojet aircraft are no exception. An interesting point is that under Part 91, there are no regulations that require climb requirements above 35' be calculated by a pilot (91.605). While WAT were established during aircraft certification that produced particular gradients, the aircraft does not have to be operated at those WATs. As long as there is approved data in the AFM (again, I have data for 0.0% 2nd segment OEI operation) and you do not exceed a published limitation in the AFM, you are legal for a Part 91 flight.
Accept a departure procedure or instrument approach (MACG), operate under 121, 135, 125 or just want to physically climb over anything taller than a lake with OEI and it gets more complicated.. Legal does not mean safe.
SIDS and DPs
I think this was in some earlier posts but if you accept a departure procedure, you are expected to maintain the appropriate climb gradient – all engines operating. Does your acft manufacturer publish all engines operating climb data? Mine does not, so (if yours does, skip ahead) how does one know if they will meet the climb requirements without a AFM data. You don’t….
You will not find anything in the regulations or AFMs that allows you to use OEI data to calculate all engine operating climb gradient. Would your Check Airman let you use a flaps 12 table to calculate a flaps 20 take off?. Of course not...
As logical as the argument may be, it isn’t legal (remember, Safe does not mean Legal).
FYI - There have been some ALJ decisions and FSIM guidance that is slowly changing this but, as of now, this is what we have.
Engine Failure
Lose an engine at V1 and what are you legally required to do under Part 91? Assuming you have decided you have an emergency? Nothing.
Hopefully you have made some calculations with regards to the laws of physics but TERPS, FARs and AFMs do not trump Emergency authority. Climb at 0.0 westbound across the Pacific until you have burned off fuel – that’s legal. Legal does not mean safe.
That should be enough dynamite in the campfire for one night.. I welcome your input and the references that go with it.