Originally Posted by
Smoked Toilet
I still don't quite understand aerodynamically why temperature would not influence (or for that matter, why density is not being considered) the landing distance.
OK, there's two different questions here, and it's an exercise in pedantry to separate them, but I will.
The answer is that
in the real world there
is a temperature/density effect on the actual amount of runway you need to land, but the requirement for actual landing distance to be determined is
regulatory in nature, and ever since Part 25 was first released in 1964 the relevant words have read:
Sec. 25.125
Landing.
[(a) The horizontal distance necessary to land and to come to a complete stop (or to a speed of approximately 3 knots for water landings) from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane):
(My emphasis)
So all I am required to do, as a Part 25 OEM, is determine (and publish) landing distance data for ISA. I'm allowed, legally, to ignore the second order effect on actual landing distance due to temperature.
And all an operator is required to do, legally, is operate to the approved data associated with the Part 25 aircraft.
§ 121.195 Airplanes: Turbine engine powered: Landing limitations: Destination airports.
(b) Except as provided in paragraph (c), (d), or (e) of this section, no person operating a turbine engine powered airplane may take off that airplane unless its weight on arrival, allowing for normal consumption of fuel and oil in flight (in accordance with the landing distance set forth in the Airplane Flight Manual for the elevation of the destination airport and the wind conditions anticipated there at the time of landing), would allow a full stop landing at the intended destination airport within 60 percent of the effective length of each runway described below from a point 50 feet above the intersection of the obstruction clearance plane and the runway.
(again my emphasis)
Legally, we can produce ALD and LFL excluding temperature effects, and you can use them as gospel.
And it works. This is one of those cases where the extra effort to provide the exact answer doesn't seem to make the world any more or less safe - and that 60% factor between ALD and LFL is to blame (or thank, if you like). There are so many other variables in the "landing distance of the day" compared to ALD that a bit of temperature deviation gets lost in the noise. A bit too much float, a bit late on the brakes, a bit more rubber on the runway and it's neither here nor there.
After all, your WET LFL is utter nonsense, scientifically speaking. Only 15% more distance to stop, wet vs dry? Reality is probably more like 30-40% - but if the world applied a 33% LFL wet factor, say, half the world's air routes would only be viable when the sun was shining....
just to note that J_T covered a lot of that in terms of the regulations, just didnt quote specific regs