Winds & Temps Aloft // True Airspeed

[simurq]

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.

Thank you very much in advance!!!

Rustam ALHAS

[AerocatS2A]

Don't worry about dew point for the TAS calculation.

[hawk37]

Re interpolation, simple to just use an easy linear relationship.

Usually aircraft performance figures are based on a certain TAS or Mach number. Use this rather than trying to factor in the IAS. Then, no dew point nor DA considerations.

[simurq]

Thanks for quick replies!!!

I figured it out. Also, I made a couple of tests which show that an increase of density altitude in the range of several hundreds feet factoring in humidity does not seriously affect true airspeed, well... less than 3-4 knots. So, I can live with that!

[de facto]

I figured it out. Also, I made a couple of tests which show that an increase of density altitude in the range of several hundreds feet factoring in humidity does not seriously affect true airspeed, well... less than 3-4 knots. So, I can live with that!

The B737 perf graph in the AFM section 4 shows speed corrections to be made with change of density altitude.