Posts Tagged temperature
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.
I’ve wanted to blog about this topic for some time and as I started writing the blog a couple months ago I realized that there are fundamental principles that I needed to blog about first. The reason for this blog is to answer the “why” behind how things work in aviation.
To recap the previous posts, I first talked about how the curvature of the earth affects the altitude of the glideslope. It was that blog that was originally designed to talk about the temperature effects on the glideslope but as I researched the topic I found some really interesting things as I detailed in that blog. One of the things that I discussed was the fact that the glideslope beam does not bend and is not affected by the weather of the day.
I later realized that I needed to blog about how non-standard pressure and non-standard temperature affects the altimeter before I could blog about how the altitude on the glideslope changes with non-standard pressure and temperature. In this post, we learned the current pressure of the air is dependent on the weight of the air above that column of air. We know, through other sciences that the weight of the air is dependent on its pressure and temperature. As explained in the blog, both pressure and temperature differences from standard affect the accuracy of the altimeter. Thus the saying, “FROM HIGH TO LOW AND HOT TO COLD… LOOK OUT BELOW”
Warmer temperatures and higher pressures will make the altimeter read lower than the actual aircraft. You might want to read that statement again. At first glance, this statement appears to contradict the trusty ol’ statement above but it doesn’t. It all depends on your reference point. The first statement is in reference to true altitude and the second statement is in reference to true altitude. It is this second statement that will shed light on how temperature and pressure affect the location where the glideslope meets the intermediate altitude on the approach.
In the picture above, each aircraft is at the same true altitude and has intercepted the glide slope. Each aircraft is over the outer marker, where typically the final approach fix is for the localizer approach. A couple things become quickly apparent. The first thing is that warmer temperatures and higher pressures will make the altimeter read lower than the actual aircraft. Take a look a the 30°c aircraft on the far left of the picture. Notice that at a warmer temperature, the altimeter will read below the glide slope intercept altitude at the outer marker. As the temperature cools down, the altimeter will slowly rise till it is higher than the glide slope intercept altitude.
In a previous post, I explained how the altimeter correction formula in really cold temperature works. Transport Canada developed the formula and if it is applied to our current discussion it would agree with our second statement. “Warmer temperatures and higher pressures will make the altimeter read lower than the actual aircraft.” In this case of a pilot shooting an ILS approach, they will stay at the intermediate MEA till glide slope intercept and come down at the proper descent rate.
Let’s take a look at an ILS example into Los Angeles, CA. I like this approach because it has quite a few different step down fixes. The TDZE for Runway 25L is 98’ and for our discussion we can say the standard temperature at KLAX is 15°c. Any temperature above standard will cause the airplane to intercept the glideslope outside of LIMMA and a colder than standard temperature will cause the airplane to intercept the glideslope inside LIMMA.
I have always taught my students to check the altitude as we pass over the localizer final approach fix to verify the altimeter is working correctly. I know teach them, that the altimeter may read outside the ±100 limits typically set. If that is the case, we know that either non-standard pressure or temperature is affecting the altimeter. Period.
There is one other consideration that must be discussed to finish this topic. Most pilots will hear a phrase from ATC similar to this. “N12345 turn left heading 270° maintain 9000’ till on the localizer, cleared for the approach”. In this example, the pilot will most likely elect to stay at 9000’ and capture the glideslope and come down. There is nothing wrong with this procedure except that in warmer temperature, it can cause the pilot to descend below the MEA’s on the intermediate segments. Remember, since the glideslope does not change with pressure and temperature, the altimeter will read lower than your true altitude with warmer temperature.
The FAA has long since known this and has issued an Information for Pilots document stating the pilots are required to honor the intermediate fix altitudes and not to fly below them in warmer temperatures.
Why is it that the FAA has deemed a minimum 2000’ MEA on IFR low enroute charts in mountainous terrain instead of the standard 1000’ minimum for non-mountainous terrain. Is it possible that our altimeter is less accurate in the higher terrain of the mountains? Maybe, but the altimeter is calibrated at least once every 2 years to be able to be used under IFR flight. So what is actually happening? The answer lies within how the altimeter determines the current pressure altitude and the assumptions that are made with respect to how the atmosphere works.
A quick review of the atmosphere. We know the standard atmosphere at sea level is 15° C and 29.92” Hg and weighs 14.7 pounds per square inch. We are taught that the standard pressure decreases at a rate of 1” Hg per 1000’ and at 18,000 feet the atmosphere is half of the atmosphere at sea level. We are also taught the standard temperature decreases at a rate of 2° C per 1000’. The pressure of the air at sea level is determined by the weight of the air above it. Weight is affected by the temperature of the air as well as the pressure.
A standard altimeter uses an aneroid wafer that senses the pressure of the air and translates that pressure into an altitude. Inside the aneroid wafer is a sealed chamber with a certain pressure (not important to know but it is speculated to be at standard pressure 29.92” Hg). As the pressure of the air outside the aneroid wafer changes either due to an increase in pressure or weather conditions (i.e. low or high pressure system), the aneroid wafer will expand or contract and through gears and linkages will adjust the altitude on the altimeter.
A couple things to notice about this picture. The only thing that moves the gears and linkages is the pressure changes on the aneroid wafer. There is a way to set the altimeter setting which adjusts the gears as well but doesn’t have an effect on this discussion just yet. For the altimeter to work correctly, and to pass the required altimeter checks of §91.411 it must be calibrated to the standard pressure and rates of change. This means the gears and linkages are calibrated to read 1000’ MSL when the pressure is 28.92” Hg and the temperature reads 13° C. The altimeter is constructed to assume standard conditions all the way up through the atmosphere.
How often is the atmosphere standard? A recent friend of mine made a poignant statement and said the atmosphere has never been standard in the 40+ years of his aviation career. Interesting.
The altimeter setting for a field is the barometric pressure that field would be at if it was at sea level. Put another way, the altimeter setting is a correction for non standard atmospheric conditions to ensure the altimeter is accurate at the field.
I always assumed that the early pilots came up with the slogan “From High to Low look out below” dealt with the altimeter of the day, a pressure altimeter. An altimeter without a Kollsman window. In fact, a sensitive altimeter (one with a Kollsman window) is subject to the same errors though they are corrected somewhat. In the graphic below, we can see that a higher temperature (or a higher pressure) will cause our true altitude (above MSL) to increase above our altimeter referenced altitude. If the air pressure and/or temperature are above standard the true altitude will be higher than the altimeter referenced altitude and if the pressure and/or temperature are lower, the altimeter referenced altitude will be lower. Non-standard pressure has a greater effect on altimeter errors than non-standard temperature.
Unfortunately, it is quite difficult to determine with any reliance non-standard pressure trends and relay them to the altimeter so the instrument reads correctly. With that said, we might start to understand why there is a higher MEA for mountainous terrain than non-mountainous terrain. Let me explain, the altimeter setting we use is from a station and the altimeter is calibrated to read true altitude (MSL) at that station. The farther away from the station the less accurate the altimeter becomes. Since, we cannot quantify non standard pressure, in reference to the altimeter, a large error can happen. Unfortunately, the mountain tops don’t adjust themselves for non-standard pressure so a larger safety margin is necessary due to the distance the plane is from the altimeter setting source. To prove this point, I have pulled up some METAR reports from the Rocky mountains with weather stations on top of the passes versus METAR reports from the airports below. If you take a look at this graphic to the right, you can see that Monarch Pass and Cottonwood Pass both have altimeter setting higher than the lower airports. Monarch Pass is currently at 30.75” Hg and Cottonwood Pass is at 30.48” Hg. The lower airports are at 30.37” Hg and 30.36” Hg respectively. The surface analysis chart shows a high pressure system over the area during these METAR readings.
There is a way to calculate the effects non standard temperature has on true altitude. Our Canadian friends to the north of us are required to compensate for cold temperature when they shoot an instrument approach. It will be a discussion for another blog on how the chart is calculated but a decent formula to us is for every degree below standard and altitude above altimeter station (in 1000s feet) subtract four feet. We can see that a non standard temperature has a greater effect the higher above the station than at a lower station. In essence, the temperature error decreases to zero as the pilot completes the instrument approach. It is those intermediate segments that can cause issues for the non acquainted pilot to cold weather.
In the end, we know the altimeter is affected by both non-standard pressure and non-standard temperature. The FAA has designed safety into the airway system to minimize the effects pressure and temperature have on the altimeter and although the FAA is not yet there with requiring cold weather corrections to instrument approaches I will imagine they will soon follow suit like our Canadian friends to the north.