@FE Hoppy – I’m not sure I understand what you asked/stated but if you are not limited by the AFM or regulation, then it is legal. Data will not be in the AFM unless it is approved during the type certification process which is done in compliance with Part 23/25.
(*AFMs may actually have unapproved data but it must be clearly notated. For example, I have overweight landing Vref speeds but that does not authorize me to plan overweight landings.)
@HazelNutz39 – there is no requirement in Part 25 that a manufacturer provide operational data that exceeds any particular climb gradient. Certification rules (Part 25) say that the manufacturer will provide each WAT within the operational limits of the aircraft that produce a climb gradient of 2.4% OEI (two engine acft). In other words, it’s not contrary to Part 25 for manufacturers to publish a WAT for a 0.4% second segment OEI climb, nor is it contrary to provide data for a 14% second segment OEI climb.
I completely agree, you must comply with what is in the AFM (FAR 91.9) and MTOW, altitude limitations, temp limits (min and max), CG limits, Vne ect. Remember that performance charts are the laws of physics – simplified. If you configure an aircraft as per any performance chart, the aircraft CANT do anything else but deliver the numbers in the book.
As for 91.605, I agree you cant depart an airport that you don’t have performance data for (or is prohibited by the AFM), but Part 91 does not specify climb gradients after 35’. I other words, I am in compliance (legal), if I have a WAT in my AFM for 0.4% OEI climb, and I can legally depart. Consider the implications of 25.117.
@john_tullamarine I agree, the AFM is created for the sole purpose of ensuring that the design limitations are clearly outlined. FAR 91.9 then elevates the guidance in the AFM to that equal to a regulation. There will not be anything in an AFM that is contrary to a certification regulation (the FAA will not approve it).
AFMs do not specifically require a minimum climb gradient. Every AFM will have the data required by FAR25.121 and most will have more data but no AFM will have less. If you need a particular OEI gradient, the performance section will tell you what WAT will give you your requested climb performance. The OEI climb performance you require may be due to TERPS, OpSpecs, 121/135 regulation, DP, noise or JAA/CAA. If in the course of trying to obtain the desired climb performance, you “run off the chart” or data is not provided for your desired OEI climb then, yes, you must find another way. You are allowed to interpolate but cannot extrapolate.
I am interested as to what the lowest OEI climb gradient is published in your AFM. MGTOW, 40C and 4200 msl will result in 0.0% OEI climb in mine.
@FlightPathOBN – FAR 23 and 25 are aircraft design certification requirements and FAR 25.121 is titled Climb: One- Engine Inoperative. All most all of my climb charts are OEI (in fact all engine operating charts are the issue). There are several companies out there specializing in custom departure procedures (and they are affordable). We reference them for 90% of our take offs and brief the alternate departure procedures in about 25% of our operations. We meet the SID climbs with OEI the rest of the time. As for MACG, we don’t often calculate that (but we should) and we have procedures to adjust our mins so that we can meet a MACG in places where it becomes critical (ASE in the summer for example).