Equivalent airspeed errors?

[FSIGRAD]

I was discussing equivalent airspeed errors with a student and repeating the mantra I got from all my previous instructors, that equivalent errors are negilable below 200 kt's and 10,000 feet. I understand it is due to compression of air inside the pitot tube and it makes sense that this would happen at high airspeeds but why does it increase with pressure altitude?

I'm thinking also that in practice this error is rarely calculated since at these types of speeds and altitudes most pilots would be more concerned with a Mach number....

Any ideas?

 

[TriDriver]

I was discussing equivalent airspeed errors....I'm thinking also that in practice this error is rarely calculated since at these types of speeds and altitudes most pilots would be more concerned with a Mach number....

Any ideas?

FSIGRAD

You're probably right in at least mentioning EAS and the "mantra" with students, but in practice it's all figured out by the ADC (air data computer) that drives the airspeedindicators on a high-flying transonic aircraft.

Even the old 727's generally had a TAS indicator on the panel if not on the airspeed indicator. The newer aircraft all have TAS displayed on the FMS.

Students should know the ICE=T acronym, but it really doesn't have much practical value.

From time to time, when I'm flying along at FL 370, I'll pull out my little Jepp CR-5 whiz wheel (that I've had since 1968) and do a TAS calculation. You're supposed to use CAS, but I just put IAS next to PA, use TAT or RAT (True Air Temp or Ram Air Temp) instead of OAT, and the TAS comes out within a couple knots of what the ADC or FMS TAS display says. The one thing that comes out a little wrong is the temperature rise - it's usually off by about 10 degrees on the whiz wheel.

Down at 15,000 feet in a King Air, I do the same calculation using IAS and indicated altitude (unless you have a horrendous high or low pressure altitude in the area, you don't need to reset the altimeter to 29.92 - I can't read 150 foot difference on my CR-5 anyway!).

Of course your old "rule of thumb" for TAS works well also. TAS is 2% greater than indicated for each 1000 feet MSL, i.e., if you're doing 100 knots at 10,000 feet, 2% for each 10,000 feet would be 2%x10=20%. Take 20% of 100 knots and add it to 100 knots and your TAS=120 knots.

Take a look at 737 at FL350. Scroll down to the picture of the panel and you can see an IAS of about 255 kts. Below that is a picture of what is call a PDC (Performance Data Computer) and it displays some info about the temperature and the TAS of 431 kts. If I do the calculation on my CR-5 I come up with 434 kts. If I do it with the rule-of-thumb (2%x35 is 70% so multiply IASx1.7) I come up with 433 kts.

Now the 737 is not a real fast aircraft. On the picture of the airspeed/mach indicator you can see a little window showing .755 mach. In a 747 or Citation X doing .85 or better, there might be a little more difference due to compressibility.

TriDriver Bob