F vs. C Temp Fight Is Over for Pilots

Like many of you I grew up flying in a bifurcated world of air temperature. On the ground we used Fahrenheit but as soon as we lifted off air temperature considerations were in Centigrade. Or Celsius some called it.

Actually, I’m not so sure it was that clean cut of a situation when I learned to fly 45 years ago. As I remember the winds and temperature aloft forecasts were reported in degrees C. But I also remember a lot of conversion from C to F going on in ground school and when making flight plans.

The rest of the world had been on the Centigrade scale for hundreds of years. There was a concerted effort in the U.S. to move entirely to the decimal system when I was in school during the 50s and 60s. We were told the decimal system made more sense, was easier to use, and the U.S. couldn’t hold out against the world.

Highway signs began to appear in both statue miles and kilometers. The same for speed limits. Even domestic cars grew a second scale on the speedometer to show kilometers per hour. The transition from the English system of weights and measurements to the international decimal system was a certainty many people believed.

But the change didn’t happen. Even the English have largely converted to the international decimal system, but in the U.S. we haven’t. That says something about the U.S. psyche but I’m not sure what. In any case, that’s beyond our concern as pilots.

Aviation in the U.S. did finally make the transition to the international system for air temperature. We haven’t gone to meters for distance and altitude, or hectopascals for altimeter seeting, but we did change to C for air temperature. I think the reason is that using degrees C for air temp is just so simple and easy in flying that why would anyone continue to resist.

In aviation we care most about two air temperatures—freezing and ISA, the international standard atmosphere. We care about freezing because that’s the temperature were water in the air can become ice. We care about ISA because all airplane performance is measured against that standard.

On the C scale 0 is the temperature where water freezes. Ok, all of you with access to Google can save your emails. I know that the C scale was modified years ago to match with Kelvin and water freezes and ice melts at about .01 degrees C above zero. Since I don’t have a thermometer that good I’m sticking with zero as the temperature for ice.

ISA air temperature at sea level is 15 degrees C. An easy number to remember. Any air temperature above ISA robs engines and airfoils of performance, while temperature below ISA adds to potential power output and lift. We need to know if we are above or below standard when planning any takeoff to know how the airplane will perform.

Another great benefit of using degree C for flying is that the standard lapse rate—the rate at which air cools with altitude–is 2 degrees per thousand feet. On the F scale the lapse rate is 3 point something degrees which is really hard to work in your head compared to working with 2.

The standard lapse rate is essential to know because if the air temperature is above standard at your altitude your airplane loses performance while it gains if the temp is colder. You can always look up ISA for your altitude in the POH, but it’s so easy to multiply your altitude by 2, adjust for the sea level standard of 15, and know ISA for your altitude. For example, at 5,000 feet the standard air has cooled 10 degrees. Subtract 10 from the sea level of 15 and you know ISA is 5 degrees C for 5,000 feet. Look at the air temp gauge and you know if your conditions are above or below ISA.

That calculation works up to 36,000 feet where ISA stops cooling with altitude and remains at -56.5 degrees C. If you are ever able to fly above 75,000 feet the air—what little is left of it—starts to warm up with altitude.

U.S. airplane manufacturers were the first to make the switch to all-C temperatures in performance charts. When we listened to ATIS the airport temperature was still being reported in degrees F until about 15 years ago, but the charts in the book used C for larger airplanes first, and then most GA airplanes by the 1970s and 80s. It was just an easier and more logical system to use.

Finally, in the mid 90s the FAA converted to the METAR international standard for airport weather observation reporting. In the old sequence report observations of ceiling and visibility came first, and surface temperature was listed in degrees F. The METAR moved the wind into first place and changed surface temperature and dew point to degrees C. Gone was the need to convert F to C to use the charts to calculate takeoff performance.

It took some time but I made the total transformation from F to C when flying years ago. The only temperature conversion I now work is after I listen to the ATIS for arrival. I still haven’t made the complete switch to C for my personal comfort so when I hear the ATIS report of airport surface temp in C I glance at the dual scale on the air temp gauge to know if I will be hot, cold or just right when I open the airplane door after landing.

 

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30 Responses to F vs. C Temp Fight Is Over for Pilots

  1. John Sayle says:

    Mac, A fun read. I’m an “old dog pilot,” and I’ve almost caught up to where you are regarding F and C temps. Of course, I think the US does most of the flying in the world, so the rest of the world should use our systems. Going to the alphabet airspace classification I think was a big step back from our TCAs, etc. At least our letters stood for something. I always look forward to your articles. I respect your perspective, experience and comments on all things airborne. Can’t wait to see what you have to say next. Best regards.

  2. Mac says:

    Thanks, John. And I still have to translate Class B into TCA for it to make sense to me. And I’m not sure I’ll every fully understand Class E. But an airport traffic area still makes sense, and I could even figure out TRSA when those were invented. The best news is that new electronic altimeters don’t care about inches or hectopascal and will do the conversion at the push of the knob. But when it comes to air temp, C sure is easier to use than F.
    Bests,
    Mac Mc

  3. Jeff Welch says:

    Freezing and ISA may be your only two concerns as a private pilot. For most of us that fly for a living, fog is a concern. In as much as it generally gets real foggy when the temp and dew point match, I prefer to read the degrees in F for a higher degree of accuracy. Frankly the same goes for icing.

    1.0 C on the temp gauge might mean 1.5 C to 0.5 C as the scale only reads in whole increments. Assume it is really 0.5C when reading 1.0C …..and the dew point gauge reading 0.0 C is actually 0.5 C……..oops……the 1.0 (2F) spread that you thought you had as insurance….is actually 0.0!

    Better have a good alternate!

    I’ll take the F scale any ole day……because I R ole…………..

    Speaking of old and fog…..do you remember the old weather symbols? Most of us think WOXOF meant Indefinite Ceiling -Sky Obscurred -Zero Vis Fog….right? It actually meant Wow-Clear-Xtra Clear-Fly!

    Have a good day.

  4. Thomas Boyle says:

    It’s a funny thing – I’m fine with gallons, liters, feet, meters, inches, cm, mm, km/h, mph and kt.

    But, like Mac, my “personal comfort scale” is stuck on the one I learned first!

    (And, AFAIK, almost the whole world uses ft for altitudes, because thousands of feet are a particularly useful unit, and 300m isn’t. 100m is too small (for now), and 500m is wasteful.)

  5. Pingback: F vs. C Temp Fight Is Over for Pilots | Left Seat | EAA Chapter 1523

  6. John says:

    “Freefall” by William and Marilyn Mona Hoffer is about an Air Canada 767 that ran out of gas at FL410 because of confusion in converting units of jet fuel. Why were they allowed to depart without working fuel gauges?

    Multiply degrees C by 2, subtract 10% and add the result to 32. If C is negative, subtract the result from 32.

    • Mac says:

      Hi John,
      I can answer your question about the non-operating fuel gauges in the Air Canada Boeing. All airlines, and most jet operators of all types, have approved master minimum equipment lists that describe how or if you can fly with failed equipment. When a cockpit fuel gauge is not working reliably the MMEL typically allows a flight to be made if the mechanical fuel gauges–often called drip sticks because they are under the wing–show a much increased fuel reserve for the flight.
      In the Air Canada case the fuel order was mixed up between liters and gallons. And of course jet fuel gauges read in pounds because engines burn fuel by mass, not volume.
      Without MMELs it would be very difficult for a large operator to function. And they work pretty well. That Air Canada crash must have been about 30 years ago and I can’t name a similar accident since.
      Mac Mc

      • Robert says:

        The 767 landed in Gimly, and became infamous as the “Gimly Glider”

        • Robert says:

          … so it didn’t really “crash”

          • Mike F says:

            Technically they landed off-airport, since the runway was converted to a race track, and they did damage the nose gear even though they did eventually fly the plane back out. I think it qualifies as a “crash”.

          • Robert says:

            any landing you can walk away from is a good landing. any landing where you can reuse the plane is an excellent landing.

            the nose landing gear damage was more a result of the design — it wouldn’t descend and lock w/o power, than a rough landing.

  7. Jeff says:

    “In the Air Canada case the fuel order was mixed up between liters and gallons. And of course jet fuel gauges read in pounds because engines burn fuel by mass, not volume.”

    So, if gas turbines burn fuel by mass, not volume, then why don’t the gauges read in units of mass, such as grams or slugs? A pound is not a unit of mass. It is not a unit of volume.

    And let’s not be confused by units of mass in the metric system (gram) being used as units of weights here on earth. Grams, Newtons, pounds, slugs get all mixed up.

    • deadstick says:

      “then why don’t the gauges read in units of mass”

      Because that’s a pretty expensive indicator to make. The only DIRECT way to measure mass is to measure response to an unbalanced force, and that doesn’t work very well in a fuel tank. The practical way is to measure the volume (with a simple fuel gauge or a dipstick) and the temperature, and then correct for thermal expansion.

      Actually, there are two kinds of pounds. A pound-mass (lbm) is indeed a unit of mass: in fact, it’s the BASIC unit of mass in the Imperial system. A pound-force (lbf) is the force exerted by standard Earth gravity on a mass of one lbm.

      The metric system isn’t “metric” any more: since 1960 it’s been the International System of Units, abbreviated SI from the French name (which is quite fair, since they invented it). And its basic mass unit is not the gram: it’s the kilogram.

      • Robert says:

        I was taught pounds are force, slugs are mass. “Pounds-mass” are akin to the “kilogram of Force”, in that they are units of convenience.

        the SI system has 2 “standards”, mks and cgs, meters-kilograms-seconds, and centimeters-grams-seconds, so one could say either kg or g are basic mass units.

  8. John says:

    Mac,
    Without getting into whether the MMEL should allow flight without cockpit fuel gauges, I wonder if in the Air Canada case, did the crew check the “drip sticks” when they learned of the problem.
    Seems to me a case of relying too much on technology, instead of basics like checking a secondary source of information (drip stick) when the primary (cockpit gauge) is out of service.

    • Mac says:

      Hi John,
      As I recall the report the direct read fuel gauges were checked. An extra large fuel load was calculated to build in the greater reserve required. The fuel order was placed that would bring the onboard level to the required amount but the fuel was delivered in liters instead of gallons. Should the mechanical gauges been checked again after fueling. You bet. Should you always put the gear down. You bet. Every pilot who slid down the runway on the belly got that answer right.
      Mac Mc

  9. Wes Felty says:

    Nice article as always. I always read your column first. But, we shouldn’t call our antiquated system of measures the “British System” since it no longer is their “legal” system. The USA and Borneo are about the only nations still using it.

    By the way, the “other” temperature system is Celsius not Centigrade. The Fahrenheit temperature scale is also a Centigrade scale. Centigrade just means “Centi”, 100, and “grade”, graduations, a scale with 100 graduations. The scientists Fahrenheit and Celsius just used different test points for their zero and 100 degree points. Celsius used the normal freezing and boiling points of pure water at sea-level and Fahrenheit used the lowest temperature that water can freeze at for zero and the “normal” body temperature of warm blooded animals for his 100. He was a little off, so we came out around 98.6. For me, that is a slight fever. As you add salt to water, its freezing point lowers and you can only add so much salt before the water solution is saturated and it won’t dissolve any more. That is what Fahrenheit used for his zero point. And, the boiling point of water lowers as pressure drops so the boiling point of 100 C is at Standard Atmospheric Pressure. I used to demonstrate to my Science classes this pressure aspect by “boiling water with an ice cube”.

  10. Wes Felty says:

    Nice article as always. I always read your column first. But, we shouldn’t call our antiquated system of measures the “British System” since it no longer is their “legal” system. The USA and Borneo are about the only nations still using it. Of course the Brit’s are ignoring the official system as much as they can get away with.

    By the way, the “other” temperature system is Celsius not Centigrade. The Fahrenheit temperature scale is also a Centigrade scale. Centigrade just means “Centi”, 100, and “grade”, graduations, a scale with 100 graduations. The scientists Fahrenheit and Celsius just used different test points for their zero and 100 degree points. Celsius used the normal freezing and boiling points of pure water at sea-level and Fahrenheit used the lowest temperature that water can freeze at for zero and the “normal” body temperature of warm blooded animals for his 100. He was a little off, so we came out around 98.6. For me, that is a slight fever. As you add salt to water, its freezing point lowers and you can only add so much salt before the water solution is saturated and it won’t dissolve any more. That is what Fahrenheit used for his zero point. And, the boiling point of water lowers as pressure drops so the boiling point of 100 C is at Standard Atmospheric Pressure. I used to demonstrate to my Science classes this pressure aspect by “boiling water with an ice cube”.

  11. Horay for C…never knew the nicer than F features. Maybe pilots like F since when pronounced, it has the sound of HEIGHT in it. OK lousy humor but its morning in Bluemont VA and the temp is -17C.

  12. McMetric says:

    An easy way to calculate the cloudbase:
    Difference between temperature and dewpoint X 200=cloudbase in feet.
    Temperatures are in Celcius of course.

  13. Mike Brown says:

    > Even the English have largely converted to the international decimal system…

    Not completely, though. Just to keep things interesting, while they sell gas (petrol, sorry) by the litre, the British use miles for road distances and miles per hour for speed limits and speedometers. Fuel economy is reported in miles per Imperial gallon (5 quarts US) or in litres per 100 km, for yet another point of confusion – neither matches both the fuel and the distance measures in use in the UK at the same time.

    A few years ago I drove into Northern Ireland (part of the UK) from Ireland (Eire, an independent country). With today’s EU open borders, there’s really nothing to mark the transition. I carefully slowed down as the speed limit signs changed from 100 to 60 – and annoyed a whole lot of people because the speed limit didn’t change, there are just two different measurement systems on the same island.

  14. Howard Riley says:

    Mac, where in heaven’s name did you come up with the designation “international decimal system”? In stead of confusing your audience with contrived terms, call it what is-the Metric System!

    • deadstick says:

      Well, no, neither name is right. It’s the International System of Units, abbreviated SI for “Systeme International d’Unites”, and it has been for 53 years now.

      • Howard Riley says:

        Thanks for the clarification! I finished my last Engineering physics course more than 53 years ago! Interesting, though, when we started to design cars (Omni’s & Horizon’s) using SI in 1975 we called it the Metric system for simplification. Of course when we get the barometric pressure from ATIS it’s called “altimeter” for simplification!

        • deadstick says:

          Interesting to note that the kilogram is the last fundamental unit defined by a tangible object: a hunk of platinum-iridium alloy in a vault outside Paris. And even that will fall by the wayside in a few years: the kilogram is scheduled to be redefined in terms of Planck’s Constant. With that done, the entire set of standards will be reproducible anywhere in the world or off it, starting with nothing but information.

          • John Patson says:

            The Economist had a story on it a while back.
            The problem is that even though it is kept in a temperature and moisture controlled glass case, except for annual calibrations, it still rusts — although your need a scale measuring atoms to calculate this.

  15. Howard Riley says:

    Responding to Jay’s question of MPH vs. knots-
    Using knots makes navigation easier, since sectionals have the degrees of arc on the longitudes. One degree is 60 nautical miles! Or a minute of arc is one nautical mile. Same on marine charts! Also, if you are off course by 30 sec of arc, you are about 3000 ft. off, since a nautical mile is 6,076.12 ft., or approximately 6,000 ft..

    • Robert says:

      the knot sounds eligant, but the 19th century method of measuring it involved a wooden panel with a long line attached to it.

      the line had knots tied every 47 feet 3 inches. while one sailor counts the knots as the line is drawn out, another sailor times intervals of 28 seconds using a sand-glass.

      it works, but you have to just love the constants.

  16. Andrew Grant says:

    Mac: In addition to moving to an all-Celsius reference – with new dial stickers for some of our OAT gauges -could we reduce the confusion for new pilots from the use of both statute miles and nautical miles in the FAA rules? This is an exceptionally unnecessary bit of bumbledom – of course we all ignore it once we’ve passed the stupid tests! But, just like the totally inexplicable Class E definitions ( how about doing a survey of active pilots to see how many of us actually understand the Class E definition – rules MUST be intelligible to have any value ), it is an unnecessary barrier – get rid of it, FAA!!!

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