Winds & Temps Aloft // True Airspeed
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Joined: Jan 2011
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From: UBBB
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:
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):
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
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
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
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
Joined: May 2003
Posts: 412
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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.
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.
Last edited by hawk37; 6th December 2012 at 14:09. Reason: spelling
Thread Starter
Joined: Jan 2011
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From: UBBB
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!
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!
Last edited by simurq; 8th December 2012 at 21:27.
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From: Home soon
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!
