Good Point
Jeff is correct KKS. Remember TAS is you ground speed with out wind. You want to know how fast you are going. IAS goes down about 2% for every thousand feet. So if you indicating 100 MPH with no wind, on the deck, your ground speed is 100 mph. If you are indicating 100 mph at 8,000 feet with no wind you are doing 116 mph over the ground (8 x 2%=16%), which is also about what your true airspeed is. So we use TAS for performance because that is what counts. If you are indicating 250 mph at 41,000 feet you are doing 455 mph over the ground (no wind). 250mph means nothing with out knowing altitude (temp and baro setting). To get density altitude in flight you can read "pressure altitude" (altitude indicated with the BARO set to 29.92 in-hg) and measure the OAT. OAT should be SAT or static air temp. The temp rise may be a few degree from "RAM" rise or air friction. You want to fly at the pressure altitude that will give you the correct density altitude. If it is a totally standard day (15C, 29.92 at sea level and standard lapse rate) than the field BARO indicated, density and Mean Sea Level (MSl) will all be the same.
The only time indicated airspeed comes into use is comparing performance of two aircraft flying side by side at the same time, same altitude. In that case the difference of IAS means something. It means how much the airspeed indicators are different (if flying side-by-side), or how much faster one airplane is faster if the instruments are matched.
We use IAS, however CAS or calibrated airspeed is better. CAS is IAS corrected for instrument and installation error. To calibrate the airspeed gauge and installation takes some effort to do well. Most report indicated which could be off several MPH.
Now with GPS we can get accurate ground speed but it requires you to fly three legs about 90-120 degrees apart and use a little program to calculate the true average ground speed with the affect of the wind subtracted out. Ground speed and true airspeed are the same with out wind, Right. Doing four legs 90 degrees apart and averaging the 4 legs works but can be in error by 1-2%. Using GPS, we don't even use the A/S gauge.
However, if you want to use your airspeed indicator (A/S) for performance there is a "standard" to go by. The "standard" is reporting TAS at specific altitudes and power settings. If we all use this standard (which most manufactures like Van's aircraft do) with can compare speeds easily.
TAS: True airspeed is the most accurate way to compare airspeeds. TAS is CAS (not IAS) corrected for altitude and temp.
Density altitude is indicated altitude corrected for temp and BARO so it is easy to report density altitude. Density altitude is "performance altitude". It is the used to compare performance. For example taking off at an airport with 1,000 elevation at 100 degrees and low pressure is not the same as taking off with the temp at 48 degrees and high pressure. Taking off at 100F with high humidity and low pressure can be like taking off from an airport at an altitude over 4,000 FEET!! If the temps are 48F, low dew point and high pressure the field elevation is below sea level!! As we know the thinner the air the lower the performance. Therefore "density" altitude gives you a basis to compare performance. This applies for flight speed comparisons, so correct your indicated altitude for pressure and temp and dew point. Recall ATIS reports density altitude. To get density altitude read pressure altitude (indicated on altimeter when set to 29.92) and OAT. Remember OAT is SAT, or static outside air temp. When the aircraft is moving friction raises (RAM rise) the temp several degrees. You want SAT or true outside air temp. If it is pretty "standard" day you can just use the field BARO and fly indicated altitude but it is not accurate in hot, cold, high / low pressure, wet days. You can see why using the GPS is the best method for determining TAS.
Standard density altitudes are usually sea level (zero feet) and 8,000 feet. Also what power you are at is needed. Knowing the engine manifold pressure (MAP) and RPM is needed. The standard power settings are usually 100% or 75% power.
The "Standard" for comparing airspeeds: Two altitudes are used sea level and 8,000 feet. When I say altitude I mean density altitude, which is indicated altitude corrected for temp and pressure. When you report TAS at these "standard" altitudes with WOT (wide-open throttle) than you are talking apples and apples. You don't need to report anything but the TAS as long as you are: WOT at either a density altitude of MSL=0' (for top speed runs) or MSL=8,000' (for 75% cruise performance).
Top speed: is TAS, at sea level (density altitude), wide-open throttle (WOT) or about 100% power. Many can't fly at sea level density altitudes, so you may need to correct for standard conditions. However if you can fly say over water or lower terrain, in the colder months you can get sea level conditions and still have a safe altitude to fly at. Other wise you can only fly as low as safety will allow and correct the airspeed for sea level (remember the 2% rule). Flying low and fast is dangerous and should never be taken lightly.
When flying at density altitude of sea level, the only difference in IAS and TAS, which should be the same in theory, is the error of the A/S indicator and installation, in other words calibrated airspeed indication (CAS).
Cruise Speed: "Standard" for reporting cruise is TAS at 8,000 feet density altitude (density altitude is indicated altitude corrected for temp and pressure). The standard power setting is assumed 75% power. The 8,000' altitude is about where most engines (without a turbo charger) are making 75% power with wide-open throttle (WOT). This is most useful because this how we fly most of the time and is easier to measure speed at a high altitude in smoother air away from the ground. Top speed performance at sea level is not always possible.
The problem you have SSK is that people do not report enough info or accurate info for you to compare. IAS with out knowing the power setting, altitude, temp and BARO, is useless as Jeff said. Of course knowing IAS (or CAS) and all the other factors you can calculate TAS.
If you know the pilot is at the "standard" altitude, power (100% or 75%) and the reported airspeed is TAS, you can compare the speeds directly.
The other frustrations are airspeed indicators are not accurate or their installations calibrated. Many time?s pilots use outside air temps (OAT) that are in error. Altimeters or BARO settings can also be off adding to the error.
Bottom line, reporting speed with an airspeed indicator for the purpose of flight test and comparing planes is not real accurate. The best way is using a GPS and accurately accounting for the wind effect. Even then the pilot needs to report the conditions: power and density altitude.
George
