Calculating TAS in the flight levels

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
For lower altitudes I’ve used those online calculators which work great, but how can this be done above 180?
For T, is the use of RAT ok, or does one still need OAT? Then I find references to SAT being used instead.
For the AltSet input, I don’t understand how the use of 29.92 would be accurate as it is an arbitrary number, not the true AltSetting. So what can be used (if looking at a photo of a panel’s display of ias, alt, etc)?
No, not all a/c have a TAS panel display, thanks.
 
Shall we review the formula for true airspeed one more time, my liege?
 
Let'sgoflying! said:
For lower altitudes I’ve used those online calculators which work great, but how can this be done above 180?
For T, is the use of RAT ok, or does one still need OAT? Then I find references to SAT being used instead.
For the AltSet input, I don’t understand how the use of 29.92 would be accurate as it is an arbitrary number, not the true AltSetting. So what can be used (if looking at a photo of a panel’s display of ias, alt, etc)?
No, not all a/c have a TAS panel display, thanks.
Click to expand...
OAT and SAT are the same thing, and unless your computer asks for RAT, don't use it.

Above 18,000, everybody's using pressure altitude, with few enough exceptions that it's probably not worth considering anything other than a 29.92 altimeter.
 
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Let'sgoflying! said:
For T, is the use of RAT ok, or does one still need OAT? Then I find references to SAT being used instead.
Click to expand...
Assuming your calculator doesn't correct RAT back to OAT then you use OAT. It's pretty much negligible for GA piston planes anyway.

Let'sgoflying! said:
For the AltSet input, I don’t understand how the use of 29.92 would be accurate as it is an arbitrary number, not the true AltSetting. So what can be used (if looking at a photo of a panel’s display of ias, alt, etc)?
Click to expand...
29.92 is somewhat arbitrary, but it *is* a standard, so if you know altitude at that setting you can easily get the atmospheric properties needed to get the density and density ratio. Most calculators and steam gauge indicators take pressure altitude (altitude with 29.92 in the Kollsman window) anyway, so in the flight levels doesn't matter. Using pressure altitude simplifies the pressure calculation used with OAT to get density and the density ratio at altitude; and as a result, true airspeed.

Nauga,
holding the KEAS
 
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Indicated + 1/2 Altitude

270 kias @ FL300 = 270 + 150 = 420 KTAS

Close enough
 
unsafervguy said:
Look at the nd screen where it says TAS.....
Click to expand...

Everyone did not read my entire original post, apparently. Seems I'm getting good at anticipating the POA smartass comments (probably because of all the ones I make!).

For your review:

Let'sGoFlying said:
No, not all a/c have a TAS panel display, thanks.
Click to expand...
 
Let'sgoflying! said:
Maybe the actual example will help.

IAS 202kts
Alt 360
RAT m52 (isa+4)

A calculator:
http://indoavis.co.id/main/tas.html
I do not think you can plug in 29.92 there.
Click to expand...
If you're at FL360 you *do* plug 29.92 in, 'cause that's what you've set to indicate that altitude. If you're flying at 36,000 with anything besides 29.92 in you're not at FL360. A TAS calculator backs out local pressure from the given altitude. To be thorough you should use OAT and calibrated airspeed but what you've given gets you to ~374 KTAS.

Look at a TAS gauge - notice the altitude scale you dial the temp to reads "Press Alt" or "P Alt" or something close to that?

True airspeed is calibrated (equivalent, really) airspeed corrected for the local density ( the density you're at). Since you can't measure the density, you calculate it from local pressure and temperature. A calculator calcs pressure from the altimeter - using pressure altitude lets you skip the step of correcting indicated altitude (@local setting) to pressure altitude (@29.92).

Nauga,
and his whiz wheel
 
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Let'sgoflying! said:
For the AltSet input, I don’t understand how the use of 29.92 would be accurate as it is an arbitrary number, not the true AltSetting. So what can be used (if looking at a photo of a panel’s display of ias, alt, etc)?
Click to expand...

It's not arbitrary. It's standard pressure. With 29.92 set in the Kollsman window, the altimeter reads pressure altitude. Pressure altitude and temperature are the quantities necessary to compute TAS.
 
374, ok thanks all - makes sense now; overthinking it. Will submit self for alzheimer's check soon.
 
At higher speeds/altitudes you have to factor in compressability, too. The flight computers designed for use in jets have that feature built in.
 
Let'sgoflying! said:
374, ok thanks all - makes sense now; overthinking it. Will submit self for alzheimer's check soon.
Click to expand...
If you ever think the FAA questions are too hard, you need to take a break and realize numbskull teenagers like me passed the exams just fine. If it seems difficult, you’re probably reading too much into it.
Going through OCS, I aced the engineering exam while almost the entirety of my class struggled and some barely passed. They were almost all engineers. There comes a point when the smart guys usually have to realize that this is all very basic, and it helps to dumb it down a little in your mind. If you try to be too precise or go beyond what they’ve explicitly taught, you’ll likely confuse yourself.
 
Now that I have more time to respond...

Your question mentions RAT. The only place I've ever seen RAT used is in the DC8 and DC9. Where are you seeing RAT?

SAT is STATIC air temperature or the actual air temperature before the fast-moving airplane came along and disturbed it. i.e. when the air was "static".

TAT is TOTAL air temperature and is SAT plus ram-rise. Ram-rise is the increase in temperature due to the fast-moving airplane coming along and disturbing the air. The air heats up due to the friction of the airplane's movement through it.

Before fast airplanes had air-data computers, to do all the math for us, we had a temperature gauge which read something between SAT and TAT. It "captured" most of the ram-rise but not all of it. The RAT in the DC8s and DC9s that I flew was such a temperature probe. There was no display of TAT or SAT in those airplanes but that was okay because all of the performance charts used RAT on their temperature scale for airborne calculations.

The engineers would know the ram coefficient (Ct) for the RAT gauge--typically between .8 and 1.0). If the Ct was 0.8 then the temp gauge was displaying SAT plus 80% of the ram-rise. This Ct could be used to more accurately calculate performance values, such as true airspeed, based on your indicated temperature on the RAT. In practice, we never used Ct, we just used performance charts that had it built in.

As far as the pressure altitude, all methods of calculating true airspeed that I'm aware of use pressure altitude. It makes no difference at what altitude you are flying because pressure altitude is pressure altitude at any altitude. Just set your altimeter to 29.92, if it isn't already there, and read P.A. for use in the calculation.

Compressability comes into play at higher speeds because, at those speeds, the air can't get out of the way of the airplane fast enough and is compressed somewhat creating a higher localized air pressure. This adds an additional error in your indicated airspeed that must be corrected in order to calculate true airspeed. IAS is corrected for installation error to get calibrated airspeed. CAS is corrected for non-standard pressure and temperature and compressability to get true airspeed. At the slower speeds typical of G.A. airplanes the compressability errors are small enough to be disregarded.

The CR-style flight computer (Jepps CR-2, CR-3, and CR-5) had provisions for calculating true airspeed, corrected for compressability, and also returned Mach and ram-rise. Here's a picture.

http://www.stefanv.com/aviation/flight_computers/cr.jpg
 
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