Tas

[Pilot16]

I always use 90 knots TAS in my flight planning, as I was taught by my instructor. Recently another instructor had showd me that the TAS can differ quite noticably due to the temperature changes at altitude.

I fly the Cessna 152... so do you work out the actual TAS at your altitude before working out the GS? or assume 90 knots through out (for C152 pilots) ?

[Dude~]

When I used to fly that noble steed, I infact used 85kts but that's by the by. In my experince, the difference between IAS and TAS at a typical cruise of 2500ft is so small its hardly worth bothering with Of course, there is often a differnece and therefore if you want to be very acurate you should work it out on your flight computer before hand by looking at the temperature and winds aloft chart. Use your CRP to input altitude, temp and IAS to get TAS, then adjust it for wind as usual and get an accurate groundspeed.

In standard conditions when temperature is 15C at sea level, TAS is going to be approx 7% faster at 5000ft. so at 2500ft with IAS 85kt your TAS will be very roughly 88kts (i dont have my CRP here), not a lot in it is there


I guess to answer the question simply, i wither work out TAS correctly, or use my IAS cruise speed for 'bimbling'

I now fly an arrow at 130kts, and the TAS is often 10 or more kts different so worth considering, and if ever you fly in the states, where high crusie levels are common, its worth working out TAS accurately.

[Aerobatic Flyer]

Try playing with this link.

There are all sorts of converters where you can play with numbers and see the effect. At low speeds, the effect of temperature on TAS isn't very significant for flight planning purposes, especially given that on a hot day at low altitude your speed is likely to be fluctuating as you fly through thermals.

[FlyingForFun]

For most private flight, it really doesn't matter. Even in the States, where high-level cruising is more common.

When I was in the States I was flying a Super Cub, often at 8,000-10,000', and I always used 85kt (100mph) as my "TAS". What I found in practice was that once I got up to a few thousand feet, each leg would take a little less time than I'd planned. So my in-flight thoughts would be "I planned on being over this mountain at 43 minutes, it's now 39 minutes and I've reached the mountain a little early, as expected because my TAS is slightly low." Simple, huh?

Then, as you climb further, you find that the engine starts loosing power (assuming a normally-aspirated engine). So your IAS starts dropping off. I found that at 10,000' and with the throttle wide open, I'd have an IAS of around 85-90mph, and a TAS of around 105-110mph - still close enough to the planned 100mph for my purposes.

It's worth knowing how to find the TAS. It's even worth calculating the TAS for the kind of temperatures and altutides you're likely to use, just to see how different to the IAS it is. Once you know the ballpark figures, it's up to you whether you use them or not, but personally I wouldn't bother.

Just my opinion, of course. And I'm sure I'll be expected to calculate the TAS when I do my CPL, so I don't claim that what I'm suggesting is actually "correct", just convenient!

FFF
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[Pilot16]

Thanks for the replies. Im gona try finding the TAS for a while, and get an idea at what difference it makes as FFF suggested.

[FormationFlyer]

If you have an exam (flight test or ground exam) to do - then please work out TAS accurately - what you do once you have the licence is up to you - except that be aware that TAS is typically required for flight plans - not IAS....

BTW If you correct IAS to CAS then TAS It willl be more than 3 kts normally - more like 4-6 - why? Because most a/c have a 1-3kt error in the IAS to start with - check the POH.

[Tinstaafl]

I always check actual TAS once established in the cruise. It's a fundamental part of navigating to me. Allied with groundspeed it enables me to get some idea of headwind/tailwind + trend. It's also an a/c health monitoring system. If achieved TAS doesn't match what the power tables say you should get then time to look for a culprit: Have you misplanned? How will that affect the rest of the flight? Is the weight different from expected? Leaned correctly? Does the engine have problem leaving it down on power eg compression low?

[Pilot16]

To work out the true TAS, i need to put in the 'pressure atitude' on the airspeed window...

Now, im pretty sure it will not work if I just bang in my cruising altitude and temperature (at that altitude)..because it says 'pressure altitude' and the sea lever QNH is most likely not 1013 millibars on that day ... now can some one knowledgabe and who actually works out the true TAS for flight planning confirm that i am doing the correct calculation to find pressure altitude...

Say my planned cruise altitude was 3000ft... if the QNH was 1030mb... i would do 1030-1013=17mb

17 X 30 (as 1mb=30ft) =510 feet

3000ft - 510ft = 2490ft, or 2500 rounded up (pressure altitude to be used to calculate true TAS)

...if QNH was less than the standard sea level pressure of 1013mb...

eg. say 1009mb....1013-1009=4mb

4 X 30 = 120ft

here we ADD the answer to our altitude to get the 'pressure alitude' .... 3000ft + 120ft = 3120ft

Have I got everything right?
Thanks

[Pilot16]

anybody? i need to know this!! Would greatly apprciate if someone confirmed what i did was correct thanks

[Wireless]

If you are at the airfield you could just wind the subscale around to 1013. That will show you the difference required for press alt.

[bluskis]

My ASI has an adjustable dial, and at higher altitudes, say over 3000ft, I align my height as a FL, ie1013 mb set, with the OAT on the 'small' window, and read TAS off the now shifted ASI dial.

This can be 20 mph different to previous reading of the IAS.

This leads me to believe temperature should be taken into account in your calc.

Somebody suggested ASIs such as mine are dangerous, I find it simple and invaluable.

[rustle]

Somebody suggested ASIs such as mine are dangerous...

Some of the older ones moved the whole range of TAS against IAS -- so someone misreading the instrument at low speed could inadvertantly be below Vmca or other critical IAS (misreading TAS for IAS -- the important one for aerodynamics)

Most "newer" ASIs with adjustable TAS calibrator only move the TAS in the region safely above Vyse/stall etc. so are less of a "danger" even if you read off the wrong scale...

[georgetg]

CAS to TAS

Rule of thumb:

Add 2% of your CAS for every 1000 ft of pressure atitude


Example:

CAS: 100 Kts
Pressure Altitude: 10000 ft

2% * 10 = 20%

100 +20% = 120

TAS = 120 Kts

Cheers
George

[Pilot16]

Thanks for the replies and the rule of thumbs...

Can some one just confirm that WHAT I DID above is correct?

thanks

[On Track]

Pilot 16:

Yes, that's the way I was taught to work it out.

[foxmoth]

Pilot16 - Yes you ARE correct, but you are talking 1% (see georgetg's post) of your airspeed, ie. LESS THAN 1kt on a C152 for your 17mb difference, if you can fly that accurately you are a much better pilot than me, I would suggest this bit is not really worth bothering with.

[Tinstaafl]

Forget the maths. If you're in the a/c (bit of a 'duhh' if you're checking achieved TAS...) then set the altimeter to 1013 & read pressure altitude directly. After that set the Press. Alt. on the ASI TAS ring (or whiz wheel scale) to OAT and read the TAS.

Wish our a/c had TAS rings.