Originally Posted by
43Inches
.... Goes back to the rule you can interpolate between lines, but not extrapolate beyond them.
It is a good principle but not a rule. One rule is Regulation 91.095 which requires the pilot to operate in accordance with the 'aircraft flight manual instructions'. (I only discuss small single engine airplanes, incidentally.)
Consider an example of an LSA where the POH only has take-off and landing data for ISA SL, nil wind and a sealed runway. Just one number for take-off and another for landing. AC 91-02 provides guidance on how to extrapolate that data, conservatively, to get performance data at a higher elevation on a hot day etc. Performance tables for my airplane go to 6,000 ft pressure altitude and
40°C. The latter is not a limitation per the manual and AC 91-02 provides guidance on using density altitude to get performance at higher ambient temperatures - that is not extrapolating, is it, because I am still working within the range of density altitudes derived from the data.
Originally Posted by
43Inches
.... I mean you could say it's not a hard limitation as such, but similar to oil grade in your engine. Run the wrong SAE grade of oil intentionally the engine will damage and fail,.
Oils is an interesting discussion. Mechanics get to comply with info like Lycoming's Service Instruction on Lubricating Oil Recommendations which states, amongst other things, "The ambient ground air temperatures listed in Table 1 are meant only as a guide. A great deal of personal judgement must be used when selecting the seasonal grade of oil to put into the engines." That Table 1 is generally reproduced in the AFM/POH with little or no further guidance. My Piper Archer POH simply states average ambient temperature for starting. Pilots must follow the AFM/POH which is black and white on the subject wrt which grade of oil for a particular temperature. Most pilots ignore it anyway as they get handed a bottle of oil by someone behind a desk.