FAR Performance Question
 

Having a discussion elsewhere about the usage of FAR 23 performance charts. The PA31-350 Piper Chieftain for example the charts only go to 35°C for a normal take off, where I live the temperature is always in the range of 34-37 (tropics), other areas of Oz can reach a high of 49° (I've experienced it), so the question is how do you legally address performance if the temperature is beyond the chart limit? Some in the aviation community consider the the charted temperature to be a limit, so the Chieftain can't legally take off if the temperature is greater than 35°C.

I've sent a message to Piper asking the question but yet to receive an answer.

Cessna tells me the following approach is acceptable, they regard the charted maximum temperature as being what the aircraft was tested to and is not a limit, and regard the density altitude calculation below as acceptable.

Calculate the density altitude and then extract from the performance graph a pressure altitude and temperature that gives the same density altitude.

For example: 50°C at zero pressure altitude gives density altitude of 4,200 feet. 30°C at pressure altitude of 2,000 feet gives a density altitude of 4,275 feet.

50°C doesn't fall within the Chieftains performance take off chart, but 30°C does.

An interpretation of (b)(2) talking about limitations is difficult to understand, given no limitations listed in that section of the flight manual. Interesting to note the lack of consistency in the manuals of various manufacturers, Chieftain 35°C, Cessna Titan 40°C (but engine out climb chart goes to 50°C), Beech 58 52°C..
Many thanks All. :ok:

I'm with Cessna on this, "maximum tested to" in civil aircraft is only a limit if it's explicitly stated to be.

Creation of a justified (which probably means a bit conservative, but not too much) extrapolation off existing performance data, and working to that seems justifiable, although if you're operating in a regulations (e.g. AOC) environment, I'd suggest putting together a robust justification of the approach being used and run it past your local regulator for approval. There are plenty of standard factors available for extrapolation of aeroplane performance data that can be used for the purpose, such as some of the UK Pink AICs.

At first stab, I think that I might run the performance calcs for 15, 25 and 35°C (ISA , ISA+10, ISA+20) assuming they're on the charts, fit a quadratic to that, and then extrapolate the quadratic out to ISA+30 and ISA+40, then re-do those calculations using the standard factors in UK Pink AIC 127(P 110)/2006 as a sanity check.

https://nats-uk.ead-it.com/cms-nats/...6_P_127_en.pdf

G

I don't have a problem with a little extrapolation and G has noted the need to make sure that you have a handle on the shape of the curve.

My concern would be that I probably would have no visibility of any other limitations or problems which might cause one some difficulty - temperature concerns (eg cooling), discontinuities in the engine output which just might bite in that extrapolated region. For this reason, I would incline to the airline NTO approach so we have at least some feedback from the OEM.

Would I have a concern, generally, with a bit of extrapolation ? Probably not. For example, I routinely used to takeoff nil flap in light twins, having done some sums re distance penalties.

For aircraft operations, you may have applicable limitations, you may have approved data, or you could be in circumstances where you have neither. So, in the absence of limitations, nothing from the airplane certification* says that you may not operate in those conditions (*though an operating approval other than aircraft certification based may (AOC?) may). If the approved limitations say you may not operate, you may not. If you don't have data to operate, maybe you can operate, if you can figure out how, and no other (non aircraft) consideration prohibits it. If something goes wrong when you're operating outside the data, but inside limitations, you may have some explaining to do, but you were not outside the aircraft limitations.

A very common example of this is the "demonstrated crosswind value", which is required to be 0.2 of the stall speed for certification. So, during certification flight testing, 0.2 Vs was demonstrated, and that was enough. For most types, as this value is not presented in the flight manual as a limitation, you can fly in whatever wind you want, though at some point, if you go off the side of the runway, you may have to explain to the insurer. A very few types actually present this as a limitation, which means that you are bound by the number (so don't attempt to land, just fly until the wind changes there), and a few airports seem to apply the non limitation as a local restriction - though I don't know how they would enforce it!

The advice above may help you "create" some data for yourself, which is what I would do, if operation in that environment is needed, and there is no limitation to not do so...