If HELIPAD is coded as JGNKRCF, then how will BUY be coded as?

Let's analyze the pattern by comparing letters of the word to its code:
* $\text{H}(8)\stackrel{+2}{\to }\text{J}(10)$
* $\text{E}(5)\stackrel{+2}{\to }\text{G}(7)$
* $\text{L}(12)\stackrel{+2}{\to }\text{N}(14)$
* $\text{I}(9)\stackrel{+2}{\to }\text{K}(11)$
* $\text{P}(16)\stackrel{+2}{\to }\text{R}(18)$
* $\text{A}(1)\stackrel{+2}{\to }\text{C}(3)$
* $\text{D}(4)\stackrel{+2}{\to }\text{F}(6)$

The pattern is a constant positional shift of $+2$ for every letter. Applying this same pattern to BUY:

* $\text{B}(2)\stackrel{+2}{\to }\text{D}(4)$
* $\text{U}(21)\stackrel{+2}{\to }\text{W}(23)$
* $\text{Y}(25)\stackrel{+2}{\to }\text{A}(1)$ (wrapping around the alphabet: $25+2=27\to 27-26=1$)

Hence, BUY is coded as DWA.