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dcoded
11th Aug 2023, 12:02
Hi,

I recently discovered in the Lido Cold weather calculator that the gradient needs to be corrected too. I have never heard about that. Can some one care to explain why the gradient is affected?

I am thinking about if there is a specific minima which requires a higher climb gradient,for example 5%.

we dont have anything in our operation manuals that we need to correct the gradient.

but if you play around with the lido calculator the 5% gradient will be corrected to a larger value with temperature below ISA.

Luc Lion
11th Aug 2023, 13:29
I understand that the climb gradient capability of the airplane is not negatively affected by cold temperature, for the excess thrust is improved by the cold temperatures.
However, if flying a SID with a minimum climb gradient, any gradient monitoring tool that uses barometric pressure will under-read the actually achieved gradient.
If I am not mistaken, this will be valid for both a climb in HDG-V/S mode or in TRK/FPA mode.
So, if a SID stipulate a minimum 5% climb gradient and if the temperature is ISA minus 20C, it is sensible to add 8% to the targeted gradient (ie: 5.4%) and thus to the computed minimum vertical speed target or to increase the FPA angle from 3.0 to 3.2.
(Note: 3.0 and 3.2 are conservative; the angles for gradients 5.0% and 5.4% are precisely 2.86 and 3.09)

Note: not an airline pilot here but a modest private pilot. So, I expect corrections coming from seasoned airline pilots.