HP 15C/33s Wind Correction

Introduction

This is the code listing for a wind correction program, which is very useful when performing flight calculations. I wrote this for the HP 15C and HP 33s RPN calculators, however, it will likely work on other RPN models as well.

Code Listing

HP 33s                   HP 15C
------                   ------
F0001 LBL F              001  42,21,11   LBL A
F0002 x<>y               002  34         x<>y
F0003 R(UP)              003  43 33      R(UP)
F0004 -                  004  30         -
F0005 ENTER              005  36         ENTER
F0006 SIN                006  23         SIN
F0007 +/-                007  16         CHS
F0008 R(UP)              008  43 33      R(UP)
F0009 *                  009  20         *
F0010 R(UP)              010  43 33      R(UP)
F0011 LAST x             011  43 36      LST x
F0012 R(DN)              012  33         R(DN)
F0013 /                  013  10         /
F0014 LAST x             014  43 36      LST x
F0015 R(DN)              015  33         R(DN)
F0016 ASIN               016  43 23      SIN^-1
F0017 R(DN)              017  33         R(DN)
F0018 COS                018  24         COS
F0019 *                  019  20         *
F0020 R(DN)              020  33         R(DN)
F0021 x<>y               021  34         x<>y
F0022 COS                022  24         COS
F0023 *                  023  20         *
F0024 R(UP)              024  43 33      R(UP)
F0025 -                  025  30         -
F0026 RTN                026  43 32      RTN

CK=34B9
LN=78

Usage

The program takes the arguments from the following registers:

X: True Airspeed
Y: True Course
Z: Wind Speed
T: Wind Direction

and outputs the following:

X: Ground Speed
Y: Wind Correction Angle

---

Example: Your true course is 240 degrees and your true airspeed is 110 knots. The wind is 182@17.

Ensure that your calculator is in DEG mode (15C: 43 7 DEG; 33s: MODES 1).

Use the following key sequence to compute the wind correction angle and ground speed on the 15C:

182 ENTER
17 ENTER
240 ENTER
110
GSB A

The ground speed is displayed in X (100.0425). Pressing x<>y will show the wind correction angle in Y (-7.5309). You may want to add an x<>y (34) instruction at the end of the 15C code for reverse output (as is more convenient for filling out navigation logs). Adding a PSE (42 31) instruction after line 16 is also convenient for this purpose.

Use the following key sequence to compute the wind correction angle and ground speed on the 33s:

182 ENTER
17 ENTER
240 ENTER
110
XEQ F

The ground speed is displayed in X (100.0425) and the wind correction angle is displayed in Y (-7.5309).

Equations

This program implements the following equations, which were obtained by simply sketching out the vectors and using some basic trigonometry to obtain the desired angles and lengths.

WCA = asin(ws * -sin(th - wd) / ts)

GS = ts * cos(WCA) - ws * cos(th - wd)