Oh God, you’re saying.  What an incredibly boring subject.  And you’d be right, except that as a pilot you had better know that lack of proficiency with units could result in you having a really bad day.


The pilots of Air Canada flight 143 had a really bad day on 23 July 1983 when they ran their Boeing 767 out of fuel.  Double flameout.  They unwillingly entered the history books as the “Gimli Glider” due to where they landed.  The reason why?  Incorrect unit conversion.  The 767 had the “new and improved” metric system, and they took off with less than half the fuel they needed – they had 22,300 pounds and they needed 22,300 kilograms.  Oops.


A very common error that I see new pilots make is units of miles.  There are statute miles and nautical miles.  Older aircraft often have POH’s and airspeed indicators with statute MPH.  Newer aircraft (after 1976) are generally marked with nautical miles (knots).  GPS’s can be configured to display nautical, statute or even kilometers per hour.  As a pilot you may be required to work with both MPH and knots, and you must know that there is a 15 percent conversion between them.  A statute mile is 5,280 feet.  A nautical mile is 6080 feet, so 6080/5280 = 1.15 – that’s where the 15 percent comes in.  Make sure you apply the 15 percent the right way!  A nautical mile is bigger than a statute mile, so any speed you are travelling is always less in knots than in MPH.


Once you figure that out, next you need to learn the difference (and convert) between indicated, calibrated, equivalent and true airspeed.  And then groundspeed.


Another common error is true vs magnetic heading.  Unless you never leave Thunder Bay, you must be easily able to quickly convert from true to magnetic heading.  If you go up north, both of these are equally useless.  At the North Pole, all headings are south.  For that matter, you have to know the difference between a heading and a track, and how to convert from one to another.


And altitudes.  You must learn the difference between indicated, pressure, density and true altitude – and once again, how to convert between them.  When do you use pressure altitude?  And density altitude affects your aircraft performance and determines whether you will be able to take off or not without crashing.  Google “high density altitude takeoff crash”.  The Stinson 108-3 video is terrifying.  True altitude determines whether or not you’re going to hit anything.  If you’re going to get an instrument rating, you had best learn to perform the cold temperature corrections required in the CAP GEN for instrument approaches.


Back to fuel units.  I regularly see US gallons, Imperial gallons, liters, pounds and kilograms used as units of measurement of fuel.  And as noted above, you had better be able to quickly and accurately convert between them, or you’re going to have a career-ending accident.  Note that temperature affects the conversion factors between volumes and weights because both avgas and jet fuel expand and contract in hot and cold temperatures.  In Canada we regularly see +40C and -40C so this cannot be ignored.


And of course, time.  If you don’t travel anywhere, this doesn’t sound like a big deal, but as a pilot you are always crossing time zones and taking into account when daylight savings time change occurs.  If you fly in North America, you simply must know how many hours off Zulu (sorry, UTC) that Newfoundland, Atlantic, Eastern, Central, Mountain and Pacific time zones are, so that you can convert from local to Zulu and back again.


I know this stuff isn’t a whole lot of fun, but like doing your taxes, it’s not something you can ignore.




Dec 2014