Takeoff Performance

[Driver 170]

Performance Margins with ATM (assumed temperature method)

True airspeed effect (TAS EFFECT)

TAS is affected by OAT. IAS is based on air pressure difference in the pitot static system. Due to a higher air density at low OAT, a slower airflow is sufficient to generate the pressure difference needed to reach a given IAS, resulting in a
lower TAS. The opposite is true for a high OAT.

-> When assuming a higher than actual OAT, the resulting takeoff performance is based on a higher TAS, but the actual TAS will be lower (because the actual OAT is lower).

Can we say GS = TAS

[Driver 170]

Found my own answer

Density Altitude – pressure altitude corrected for ambient temperature. Density altitude increases with an increase in ambient temperature.

IAS versus TAS

To convert indicated airspeed (IAS) to true airspeed (TAS) one must take into account factors such as compressibility, type specific sensor positioning error, altitude and temperature. In simplistic terms however, at sea level under ISA conditions the two speeds are virtually equivalent. Again, in fairly simplistic terms, the difference between IAS and TAS is approximately equal to 2% IAS/1000 feet AMSL. Thus, using this rule of thumb, an aircraft in flight or during takeoff or landing with an indicated airspeed of 150 kts at a density altitude of 8000 feet would have a TAS of approximately 175 kts (the actual value is 169.5 kts). This will result in a correspondingly higher ground speed in all phases of flight.

[+TSRA]

This will result in a correspondingly higher ground speed in all phases of flight.

Nope.

GS accounts for the effect of wind. A headwind will result in a GS that is lower than TAS while a tailwind will result in a GS that is higher than TAS.

Plus, you are correct that TAS is IAS corrected for all of that, but each of the corrections have specific terms - Calibrated Airspeed (CAS) and Equivalent Airspeed (EAS). So while the method is correct, it is technically incorrect to say that TAS is IAS corrected for all of that.

The technically correct method is to convert IAS to CAS to EAS to TAS then adjusted for wind to GS - although, granted you can get away with IAS to TAS for most general aviation and commuter aircraft and that is how many a POM or AFM is written.

In saying all of that - when flying under perfect ISA conditions, CAS is the same as EAS and TAS and only in nil wind conditions is it the same as GS.

[Driver 170]

Maybe it would help if i knew the definitions well.

Could we say EAS is the same as TAS?

It mentions above -

a slower airflow is sufficient to generate the pressure difference needed to reach a given IAS

Does this mean because of more air particles (denser air at lower OAT)

[PDR1]

EAS refers airspeed to the equivilent at ISO sea level.

In absolute still air at all altitudes TAS = ground speed, but EAS becomes progressively less than ground speed as you climb higher and the air gets less dense. It would also reduce as humidity increases for the same reason.


IAS is what the instrument tells you - the number the needle on the dial points at.

TAS corrects for the fact that the barometric instrument reads lower (for a given airspeed) as air density reduces for a given airspeed.

EAS is the number that gives you the "equivalent airspeed" in terms of aerodynamic forces (lift & drag).

[Driver 170]

Sorry guys i should of mentioned - lets keep the subject on jet performance or the 737 800W thats what i'm studying.

[Driver 170]

Still finding it hard to understand EAS...

[Chesty Morgan]

http://home.anadolu.edu.tr/~mcavcar/common/Airspeeds.pdf

[Driver 170]

Great link CM, so how come at the bottom table, IAS remains the same but EAS drops with altitude? Because both are subjected to compressible flow..

Sorry if i have got that wrong thats how i see it

[vh-foobar]

Because at altitude if the IAS remained the same you would be travelling faster than at sea level, and thus the compression effect is greater.

[Driver 170]

Can we say EAS is basically the flow it is experiencing or the mass of air its passing through for a given speed?

[+TSRA]

Sorry guys i should of mentioned - lets keep the subject on jet performance or the 737 800W thats what i'm studying.

This applies to all aircraft types, so if you learn it right now you'll be set for no matter what you fly down the road. The only difference is that some types do not account for compressibility because the effect is too small to make a noticeable impact.

Think about compressibility like this, although I'm sure someone has a way better explanation.

Anyways..remember that the airspeed indicator is displaying the difference between static and dynamic pressure, calibrated in whatever unit you want displayed - knots is the industry standard. Eventually a point is reached where you have to start taking the compression of air into account (+250 kts). In effect the ASI begins reading "high." We have to account for this compressibility by removing that increase in dynamic pressure. This is why EAS is less than IAS on those charts.

So take that reasoning. If we maintain a steady IAS as we climb, the compression will increase because we have to travel faster (TAS) to maintain that steady IAS. That means the level of compression will continually increase, with the the correction becoming ever larger.

I hope that makes sense and maybe someone has a slightly more technical answer, but that should get you started.

[Driver 170]

Ah right so EAS is basically corrected of the compression or in my own words - the flow of air at higher speeds entering the pitot tubes?

[Driver 170]

Or it corrects the venturi effect in the pitot tubes?

[Driver 170]

So on the 737 PFD and on the airspeed tape that shows IAS. Why wouldn't it show EAS instead as this is corrected for compression?

[+TSRA]

Yeah, in your words due to the flow of air at higher speeds. I've been thinking through the venturi effect this afternoon and I would think that would act opposite to compression, as in the venturi would create a localized area of low pressure, so would possibly reduce compressibility but would also reduce the dynamic pressure - depending on where a venturi was in the system of course. Interesting discussion point though.


I can't speak to a 737 PFD, but the rationale for using IAS instead of EAS would be that IAS would remain a constant indication throughout all flight regimes while EAS would continually update. Imagine trying to fly where your reference target is always changing.

[Driver 170]

Well we could consider the first part of the venturi effect where the air first enters. But High pressure low velocity is the initial part of the tube, so its wrong thinking of the venturi effect!

Anyway i have a basic understanding of what EAS is.

So cruising along at FL370 your EAS would constantly change because of flying through different pressure systems or isobars?

[de facto]

When do you expect to enroll and do your ATPL?

[Driver 170]

Big commitment! But i'm happy flying my fixed based 737 800 simulator.

[+TSRA]

So cruising along at FL370 your EAS would constantly change because of flying through different pressure systems or isobars?

Wrong pressure. EAS accounts for the dynamic pressure increase within the pitot tube itself, not for a meteorological pressure system.

The effect isobars create is changes in wind speed, which would affect GS, not EAS. The only way that EAS would be affected by flying into a new pressure area would be if the IAS changed. Now, IAS may change due to temperature changes when flying with reference to a specific Mach number, but saying that EAS changes because of that is akin to putting the cart before the horse.

[Chesty Morgan]

But your altitude would vary slightly due to changing environmental pressure.

[+TSRA]

True. So there may be a small change in IAS with the True Altitude changing as you fly through a pressure system with a corresponding change to EAS. But where I was going was that the pressure system itself was not the pressure that EAS is accounting for.

[Chesty Morgan]

Agreed, I just chucked that in for extra confusion :}

[+TSRA]

love it :)