Question on TAS/EAS/CAS?

[lukeajd]

Not sure if this is the right place to post but here goes; i need help with the solution to the following question, any help is much appreciated!

An airliner undergoing certification tests and the manufacturer wishes to evaluate its performance under 'hot and high' conditions. A point of interest is the landing speed at a typical airport at high elevation, and the test location is chosen to be Denver, Colorado (elevation 1655m).

(i) The aircraft has a predicted approach airspeed of 150 KEAS. Find the corresponding values of CAS and TAS in the ISA +25 degrees C atmosphere at a height of 345m. Take MSL pressure to be the ISA value.

(ii) Calculate the pressure altitude corresponding to the air pressure at the point described in part (i).


Again any help much appreciated!

[MarkerInbound]

Equivalent A/S is CAS corrected for compressibility issues. Compressibility doesn't show up until about 180 knots so at 150 EAS=CAS.

Pressure altitude is what you read when the altimeter is set to ISA standard.

This information is worth what you paid for it.

[B737900er]

Remember ICE-TG

IAS = read off the instrument panel

IAS corrected for instrument and position error = CAS

CAS corrected for ISA conditions = TAS

TAS corrected for wind = GS

[keith williams]

If you follow the link below you will find a calculator which will solve most of your questions.

Aviation Calculator (http://www.hochwarth.com/misc/AviationCalculator.html)

It does not provide step-by-step calculations, but it does provide the answers. Judging by the questions which you have asked it looks like you are doing something other than the EASA ATPL theory course. If this is the case then you should go back to your course text books and use the methods provided.

[oggers]

Hi Luke. Not such a simple question after all.

(i) The aircraft has a predicted approach airspeed of 150 KEAS. Find the corresponding values of CAS and TAS in the ISA +25 degrees C atmosphere at a height of 345m. Take MSL pressure to be the ISA value.

The height is 345m. But that is above the airport elevation of 1655m. That's a potential gotcha. The true altitude you are calculating for is 1655 + 345 = 2000m.

To find CAS you need to remove the compressibility error correction from the EAS. As mentioned, at 150KEAS it is negligible. There is a graph if you must do it (I never have nor have I used the equation).

To find TAS you apply density error correction to the EAS. The correct equation is TAS = EAS/√Relative Density. However, the easy way to do it is to put the pressure altitude and temperature into a flight computer and read off the TAS for any given CAS (or EAS to be more correct). There is also a rule of thumb out there.

(ii) Calculate the pressure altitude corresponding to the air pressure at the point described in part (i).

And there is the problem with this question imo. The pressure altitude is what the altimeter would read with standard pressure setting on the altimeter subscale. But it won't be 2000m because the temperature is 25º hotter than standard, therefore the instrument will be under-reading due to temperature error. It is calibrated for standard temperature and therefore the error is proportional to the temperature deviation and height above datum. There is an equation to bowl that out but I couldn't quote it. ATC use it I believe. There is a rule of thumb for pilots which is to correct by 4 feet per each 1000' above datum per each ºC of ISA deviation. In this case you would subtract that amount - once you have figured it out - and that would give you the nominal pressure altitude that an altimeter would indicate. There is a table for temperature error correction but it's only for low temps (because it is intended for ensuring terrain clearance during approaches in very cold weather).

The other complication is that the "ISA value" quoted is MSL. But pilots and ATC use a setting that has already been corrected for temperature error at the airport elevation. That is the "altimeter setting" and becomes the datum. So usually the aircraft altimeter would only be in error proportional to its 345m height above the airport (and the +25ºC ISA deviation). But the airport altimeter setting is not given in the question, only the MSL pressure. Therefore the question implies that the datum is sea level and that means the temperature error of the altimeter will be proportional to its 2000m true altitude. I think.

Sorry if your eyes are glazing over at this point.