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:

 

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):

 

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 nature, assumes relying on concrete figures and formulas, not approximations. :cool:

 

Thank you very much in advance!!!

Of all the, not-quite-true-to-life, shortcomings in desk-top flight simulation; the aspects that are 100% true-to-life, can be found in flight-planning.. so kudos for your efforts :smile:

 

Alas, the paraodx here, is that you're applying exact methodology where only inexact data is available.

 

Every pilot remembers the early training days, E6B in-hand.. analyzing each leg, going from true course, to magnetic heading, via deviation, variation, winds-aloft.. then to ground-speed via CAS to TAS accounting for headwind/tailwind components.. and so on.. At some point realizing that most of his data is forecast at best, winds-aloft direction and magnitude are always changing, and that no pilot can hold a heading +/- a degree or three, a new pilot might start questioning this effort :wink:

 

Of course I would never disuade a pilot from taking the time to flight-plan for every leg of his flight to that degree, because, at the very least, this process keeps his piloting frame of mind sharp.. but we drill this type of planning in students, so that all the concepts become instinctual. Kinda like drilling weight-n-balance sheets for a C172. Unless every clothed person steps on a scale, and every bag is weighed, you won't get the acuracy that those charts imply. The goal of these drills is to cement the theories into a new pilot's subconscious... that payload and CoG limits are an important part of flight-planning. Of course bigger and faster airplanes increase a need for accuracy, but you'd have to almost be TRYING to "mis-load" a C172, or the like :biggrin:

 

To the questions:

 

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?

 

You really can't..as noted, you're dealing with estimations (forecasts) at best.. and a dynamic, ever-changing, in-flight environment, to boot.

 

As for relativetive humidity when calculating TAS (questions 1 and 2).. ? Remember, for planning purposes, it all boils down to ground-speed... The effect of relative-humidity, is a non-factor to ground-speed, compared to head/tailwind components (which are already subject to forecast innacuracies). Even actual density-altitude is a marginable variable, save runway needed for takeoff, because once you're airborne, and in that dynamic environment, it's more about, pressure-altitude. Which brings us to that little adjustment wheel found on some airspeed indicators, where you line pressure-altitude (your altimeter reading when set to 29.92") up with OAT. Now,, even if that could give us an accurate TAS, you can't derive it until you're already in-flight... and even then, only for that place and moment.

 

So, for planning purposes, TAS is a constant after adjusted for altitude. A rule of thumb (IAS+2% per 1000ft) works quite well in the face of all the forecast-data errors, and the fact that you're not only in a dynamic enviroment, you're constantly moving to new environments.

 

The program you're working on sounds neat. You already have the awareness and state-of-mind of an experienced pilot.. writing and utilizing this application will only strengthen that. Keep us posted.

Every pilot remembers the early training days, E6B in-hand.. analyzing each leg, going from true course, to magnetic heading, via deviation, variation, winds-aloft.. then to ground-speed via CAS to TAS accounting for headwind/tailwind components.. and so on.. At some point realizing that most of his data is forecast at best, winds-aloft direction and magnitude are always changing, and that no pilot can hold a heading +/- a degree or three, a new pilot might start questioning this effort :wink:

 

Oh Brett... I just loved that apt and eloquent description of (at least my) student flight planning!

 

Just a few comments / questions:

• What do I do with that little card thingy on the compass?
  
• You mean winds aloft are a forecast??? Maybe nothing like what it will be when I get up there???
  
• "I can hold +/- 1°... I can hold +/- 1°..."
  

Another one of those things that fits in the "measure it with a micrometer, mark it with chalk and cut it with an axe".

Another one of those things that fits in the "measure it with a micrometer, mark it with chalk and cut it with an axe".

 

You owe me a new keyboard.. mine is now saturated with coffee..lol

Hi Bina,

 

About the only thing I can add is the actual flight is a continual check and recheck of your careful planning... Even the best forecasts can be wrong or things change... While I think most of us probably skip these checks for short flights... The proverbial $100.00 hamburger flight - I know I have 5 hours of fuel on board - I'm flying for one hour and consuming 10 GPH. I really don't get into the nitty gritty of calculating my fuel situation... I do have my flight timer running so i know how long I've been in the air... Fuel planning becomes infinitely more important when you area pushing your aircraft to its max endurance. On flights like these - I'm constantly checking whether I'm ahead or behind my planned flight... Start getting behind and you should start thinking about an alternate..,

 

Good luck with your project...

 

Regards,

Scott

I know I have 5 hours of fuel on board - I'm flying for one hour and consuming 10 GPH. I really don't get into the nitty gritty of calculating my fuel situation... I do have my flight timer running so i know how long I've been in the air...

 

That's how I came to do it also Scott. If 5 hrs fuel... start the engine & note time... in four hours I will be back on the ground.

 

You owe me a new keyboard.. mine is now saturated with coffee..lol

 

Sorry btw...

But the resulting value is inevitably prone to error since DA is (almost?) always relies on density of dry air,

 

According to the following density altitude calculator, the difference between wet air and dry air is only a couple of hundred feet. Unless I'm doing it wrong.

 

http://wahiduddin.net/calc/calc_da.htm

 

Hook