PPRuNe Forums - Winds & Temps Aloft // True Airspeed

Ok, first things first! I'm a hobbyist FS pilot developing a freeware program to lighten pre-flight load with all those time-consuming course, heading, GS, TAS, time-distance, and W&B calculations... ;)

There are two questions I would like to ask to real-world pilots/FIs before I complete the section of my program dealing with... atmospheric calculations, to be very brief:

1) This is a custom flight route made up to make my point clear. As part of my pre-flight planning, I took a look at winds & temps aloft forecast data for the US Pacific West area. Below is an excerpt from this data showing figures from 4 reporting stations in Washington, as necessary for this particular flight:

Code:

FD1US1

DATA BASED ON 031200Z    

VALID 031800Z   FOR USE 1400-2100Z. TEMPS NEG ABV 24000



Alt ->        3000        6000                9000

AST        2329        2329-02                2340-07

GEG                2526-02                2532-09

SEA        2323        2423-03                2339-09

YKM        2123        2536-01                2640-08

Question: Assuming the flight will take place at 7,500MSL and NOT necessarily over the above reporting stations, how can I interpolate W&T aloft for different enroute waypoints?

2) One of the well-known methods to calculate TAS is through density altitude (DA):

Code:

TAS = IAS / (1 - 6.8755856^10-6 * DA)^2.12794

But the resulting value is inevitably prone to error since DA is (almost?) always relies on density of dry air, even that reported by ASOS/AWOS stations. One way to correct for this "error" is to use a dew-point temperature reported for that particular measuring station, or the waypoint.

Question 1: Knowing that moist air will definitely affect TAS (by increasing DA), which is mainly used for time-distance calculations, can you please advise/show any real-world example of how you do (or, do not?) consider "wet density altitude" in your TAS calculations?

Question 2: How and where can I obtain dew-point or relative humidity data for flight planning purposes (not those reported in airfield surface METARs!), i.e. for different waypoints? Is it really possible, or you just interpolate, or maybe you don't even consider the air's moisture level during pre-flight?

I kindly ask you to be as specific as possible (no rules-of-thumb, please!) - your answers will help me to write the program in a more accurate way which, by its nature, must rely on concrete figures and formulas, not approximations. :cool:

Thank you very much in advance!!!

Rustam ALHAS