What about those air temperatures?

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.

Understanding Weather Reports

I was standing inside the airport terminal and rain was pouring down outside on the tarmac.  I figured this would be a good time for a teachable moment with my student who just got inside from pre-flighting the aircraft when the down pour started.  I pulled out my cell phone and called the local AWOS phone number to get an updated weather report.

I could almost repeat what the weather is going to be like, very strong winds with gusts a lowered visibility and very heavy rain.  What I got, my to my dismay on the teaching moment, was strong winds with no gusts and only light rain.  Both my student and I looked at each other puzzled and he asked me why the AWOS weather was not correct.  I said, honestly, I don’t know. 

A few minutes later, the storm moved quickly through and the wind calmed down and only light rain was falling.  I thought to myself and wondered what the AWOS would say.  Winds very strong and gusting with heavy rain.  TO say the least, I lost my teaching moment and promised my student I would figure out what this all means.  I was hoping the AWOS system was out of tolerance and needed repair but that was not the case.  A review of AC 00-45 Aviation Weather Services shed clues on what I was missing and it may be interesting to you as well.  I was also able to get a copy of the OFCM Surface Weather Observations and Reports Handbook.

Did you know that most of the weather listed above is averaged over a given period of time?  If the weather is changing quickly enough, the newly received weather from the AWOS system may be obsolete as it was in my case above.  It begins to make you think about the accuracy of the weather for certain situations, doesn’t it? 

For example let us examine the wind component.  First of, we know that printed winds are always in respect to TRUE NORTH and spoken winds are always with respect to MAGNETIC NORTH (see note in AIM 7-1-10).  Did you know the wind shown above is the average wind over the preceding two minutes?  It may not even be accurate anymore but we can be assured the new wind is less than 60° from what it says.  The VRB wind is used when the wind over the previous two minutes vary more than 60° and the speed is less than six knots..  A variable wind 060V150 is for speed greater than six knots.

A gust is a little more complicated is the AWOS / ASOS system examines all the wind over the preceding 10 minutes and will report a gust if a rapid fluctuation between low speed and high speed has a difference of 10 knot or more.  It is quite possible that no gust exists any longer even though the METAR report is only one minute old.

A wind shift happens when the wind changes by 45° in less than 15 minutes with wind speeds greater than 10 knots.

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The visibility group is pretty interesting.  The weather report can be automated and / or manual by the tower.  If it is automated, there is 10 minutes of sensor data used in the prevailing surface visibility.

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The present weather group (in this case -RA) has a time frame associated with it as well. For example, over the previous six minutes, the accumulation rate is observed and reported in the METAR. It makes sense now why I first received a light rain when it was a heavy downpour and a heavy rain when it just stopped a couple minutes before.

There are also distance requirements for the prevailing weather group.  If the prevailing weather is at the airport which is defined as with a 5 mile radius there is will nothing with prevailing weather as in our case above (-RA).  If the prevailing weather is between 5 miles and 10 miles from the airfield, the code VC is used for “in the vicinity” like this: VCSH for there are showers in the vicinity of the airport.  Weather greater than 10 miles from the airport uses DSNT for distant weather and is usually associated with lightening.

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What does this all mean to me?  I know that the currently reported weather may or may not have changed and I should be ready for anything that happens to show up.  I much prefer to have a human observer to an automated station and will trust weather from a military station more than a civilian airport.    As always, frequent updates from tower personnel is highly desirable.

Calculating the Headwind and Crosswind Component

Is there an easy way to calculate the headwind and crosswind component while in the airplane?  What about confirming the planned winds (you did do a flight plan right?)  Is it possible to come close to get this information correct in the airplane?

Absolutely, there is a way to quickly get these numbers using simple math.  For this method, it is important to memorize a couple numbers associated with the 0°, 30°, 45°, 60° and 90° angles.  The other angles can be converted to the closest values.  This will typically yield a very small error for wind speeds below 20 knots or so.  I also remember the power of 60 rule.  This rule states that for every 60 knots of airspeed, the wind speed decreases by 1/2, then 1/3, 1/4, 1/5 etc.  For example, if your true airspeed is 60 knots, there is a direct one to one relationship between the crosswind angle and the crosswind speed.  As the airplane accelerates to 120 knots, there is a one to 1/2 relationship and for every 2 knots of crosswind there is a one degree of crosswind correction.  At 180 knots, a one to 1/3 relationship and so on.  This also works for speeds less than 60 knots but the crosswind correction is opposite.  For a 30 knots true airspeed, there is a one-to-two relationship and at 15 knots (can we fly this slow?) a one-to-three relationship.

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A little example, let’s say you are traveling on a 090° true heading with a true airspeed of 120 knots.  There is a wind coming from 120° at 20 knots.  What is our headwind and crosswind components and crosswind correction?  The headwind component from the chart up there is .9 * 20 or ~18 knot headwind making the groundspeed around 102 knots.  The crosswind component is .5 * 20  or 10 knots.  According to power of 60 rule will will divide the speed by two to get a 5 degree correction to the right.  Thus the true course is 095° cruising at 102 knots.

Notice, it isn’t quite exact but it is definitely close enough for calculations in the cockpit.  One more example, you are cruising at 14,000 feet at 180 knots on a 210°  true heading.  There is a wind outside at 150° at 24 knots.  The same kind of analysis of what the headwind, crosswind and crosswind correction is.  The wind is at 60°  off the left side so we can see that the headwind is 12 knots making the groundspeed 168 knots.  The crosswind component is .9 * 24 ~ 21 knots.  If we take power of 60 rule we will divide that 21 by three and get a 9°  crosswind correction.  Thus our groundspeed is 168 knots and our true course is 201°.  Again, it is not perfect but it will definitely get in into the ballpark.

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What if want to work the numbers a little bit differently and get the wind speed and direction from the other four values.  All we need to do is to work the numbers  backwards.  Let’s say your planned course is 310° with a planned TAS of 90 knots.  We are currently flying a heading of 300° with an actual groundspeed of 80 knots.  What is the wind speed and direction?

Start off by determining the crab angle.  In this case it is 10°.   Applying the power of sixty rule we can see that 90 knots is half way between 60 and 120 so the ratio is 1 to 1.5.  Multiply 10° by 1.5 to get 15 knots of wind.  At this point, find a ratio between 10 knots and 15 knots for the wind angle.  In this case it is around 50°.  Since the heading is lower than the course the wind must be coming from the left so we will subtract 50° from our course of 310° to get a wind of 260° at 15 knots.

Again, the formula isn’t perfect but it is much fairly accurate to confirm whether or not a wind shift has occurred.  We all know that a wind shift indicates that we passed a front so knowledge of wind shifts provide other really important information.

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OK, so why don’t we try some examples:    My answers I got using the formulas are below as well as the mathematically correct answer.

1.

2.

3.

4.

5.

6.

My Answer	Mathematically Correct Answer
1. TH = 50° and GS = 109 knots	TH = 50° and GS = 109 knots
2. TH = 124° and GS = 202 knots  (Tailwind)	TH = 127° and GS = 191 knots
3. TH = 130° and GS = 100 knots (direct Tailwind)	TH = 130° and GS = 100 knots
4. TH = 148° and GS = 134 knots	TH = 148° and GS = 135 knots
5. WD = 060° and WS = 10 knots	WD = 050° and WS = 18 knots
6. WD = 310° and WS = 5 knots	WD = 302° and WS = 7 knots

Like I said, it’s not perfect but it gets us in the ball park and that is what is important.  For a more accurate method pull out the E-6B and do the calculation.  Remember, aircraft control is #1 though.