Archive for category ATP
We all know that as we climb higher into the atmosphere the indicated airspeed will not change while the true airspeed will increase. This is a known feature of climbing higher. What may be unknown is that at a certain range of altitudes the true airspeed will no longer increase but slowly decrease. Why is that?
Notice how the true airspeed slows down as the altitude is increased. The other piece of information not listed on this picture is the aircraft is cruising at a constant Mach number. The Mach number is a ratio of airspeeds where the current true airspeed of the aircraft is divided by the speed of sound. The interesting thing about the speed of sound is that the speed is directly related to the temperature of the surrounding air.
One of the characteristics of our atmosphere is different temperature rates in the parts of the atmosphere. For example, the troposphere we currently live in has a temperature lapse rate of around 2 degrees per thousand feet. The troposphere extends from the surface of the earth to around 25,000 to 35,000 feet depending on the season of the year and latitude. The tropopause is right above the troposphere and is characterized by a constant temperature. This is where most corporate jets fly. In the example above, the tropopause starts at FL360.
So what is happening with the chart above? The answer lies in how the true airspeed interacts with the Mach number. As the airplane climbs, at some point the crew will transition from using indicated airspeed to Mach speed. Depending on the aircraft, that altitude may be different but almost always above FL250. Our original premise that true airspeed increases with altitude is absolutely correct but what is now changing is our indicated airspeed.
For our aircraft to fly at a constant Mach number, the airplanes indicated airspeed will decrease with altitude. The chart to the left shows an aircraft who is climbing at 300 knots indicated airspeed until the aircraft reaches .81 Mach at FL320. Above this altitude the airplane continues to climb at .81 Mach till reaching FL500 (which is higher than most aircraft fly). We can plainly see that our indicated airspeed decreases and that is the reason why our true airspeed decreases just a little bit before stabilizing at 465 K.
As you get farther along into your flying career, you will most likely find gauges for different types of air temperatures. You are probably thinking, what all I need to know is one air temperature but that really isn’t true. For some reason, aircraft manufacturers have used separate air temperatures as the basis for their calculations in the performance charts.
For example, the early Learjet aircraft, 20 series, used a ram air temperature (RAT) gauge while later model Learjet aircraft use static air temperature gauges (SAT). Bombardier Challenger 604 aircraft use true air temperature (TAT) in their calculations as well as SAT readings.
What do all of these things mean?
- Outside Air Temperature (OAT): The free air static temperature obtained from either ground meteorological sources or from inflight temperature indications adjusted for instrument error and compressibility effects
- Static Air Temperature (SAT): The total air temperature obtained from onboard temperature measurement adjusted from compressibility effects. (Inflight SAT is equal to OAT.)
- Total Air Temperature (TAT): Static air temperature plus adiabatic compression (ram) rise.
- Ram Air Temperature (RAT): The static air temperature corrected for full adiabatic compression rise corresponding to the true Mach number, and multiplied by a recovery factor.
Confused yet? Each of these definitions rely on a previous definition creating a circular definition that doesn’t really answer anything. Though a search of the web doesn’t really give us a better definition. Also unfortunately for us, these definitions come from multiple airplanes.
This is what I know. OAT is always reported by the ground station. As the airplane takes off, the temperature of the air (where it is measured) rises due to adiabatic compression. The onboard computers will either directly report that value (as in the case of older airplanes) or will convert the RAT or TAT to SAT and display that value to the pilot. This provides one consistent temperature for the pilot. The chart below will convert a RAT scale to a SAT/OAT scale.
There are times where knowing the TAT or RAT is preferred and that mostly comes when the aircraft enters icing conditions. If you take a look at the Mach number and RAT gauge you will notice an increase in RAT value with an increase in Mach number. Meaning the faster an airplane goes, the hotter the air temperature. Remember watching Apollo 13 when the capsule was coming back into the Earth’s atmosphere. The temperature was so hot they were worried that the heat shield would fail.
How does this help with icing conditions? Well… a wise old friend of mine told me once that if you ever get into icing conditions the best thing to do after turning on any anti-ice / de-ice equipment is to keep the speed up. The faster you can go the easier it is to get rid of the ice on the airplane. He then lamented, that the 250K speed limit under 10,000 FT MSL doesn’t really help matters in icing conditions. Here is another chart that graphically shows what the table above displays.