Select text to annotate, Click play in YouTube to begin
- What does the world of aviation have in common with your car tires? Well, the short answer is that aviation and tires use units of measurements that look inexcusably complicated but why is that? Today, we're going to have a look at how we got into this mess and also how it could change in the future or maybe not. Stay tuned. (playful chime)
On a nice summer day back in 1983, Air Canada Flight 143 took off from Ottawa, heading towards Montreal in Canada but when this flight was about halfway to its destination, cruising happily at 41,000 feet, first one then both engines suddenly flamed out. Incredibly, the Boeing 767 had run out of fuel. This story became known as the Gimli Glider accident,
named after the disused airport that the pilots finally managed to land on and I have actually covered it in detail on a video over on the Mentour Pilot Channel but how on Earth could it happen? Well, like nearly all incidents and accidents, several different factors came into play here in what we know as Dr Reason's Swiss Cheese Model and one of the holes in the cheese slices in this accident was Canada's
and Air Canada's then recent switch over to the metric system. When this happened in 1983, the Boeing 767 was a very new aircraft and it was the first aircraft in Air Canada's fleet to come with some new metric indications in the flight deck. There were, of course, plenty of exceptions to that where the old units were still being used but the newly changed indications included the fuel quantity in the plane's tanks.
Instead of pounds, the Air Canada 767's fuel systems measured their fuel in kilos and this required a different kind of conversion when going from volume to weight. Now when you buy fuel for your car, you generally measure it by volume in either liters or gallons, right? Well, the same goes for airlines as well except that we pilots have to then convert that volume of fuel into weight for our weight and balance calculations
and we do so using a figure appropriate for the specific density of the jet fuel but obviously that calculation is going to be very different depending on whether we are starting with gallons or liters and end up with kilos or pounds. Well, you get the idea. Again, you have to watch my video on this accident to get the full detailed picture of what happened here but there's no question that the switch to a new system of measurement
played a major role in it. And this is why the Gimli Glider is often used as an example of how this process can potentially go wrong. But why did Canada and many others switch to the metric system in the first place? - Canada is in the throes of a full scale metric muddle. - Well, metrification is a gigantic topic that I'm not really even qualified to analyze but the adoption of some form
of decimal system for measurements in currencies has been ongoing for literally centuries as it serves as a simpler way to work with different scales of units and to minimize conversion errors. Back in the day, once the ball got rolling and one country started using one particular system, other countries generally followed suit and that was partially because they also thought that the new measurements were really, really good
but also to harmonize their systems with their neighbors to forward international trade. Today, many online sources say that the only countries in the world that still haven't fully adopted the metric system officially are Liberia, Myanmar and the United States. - Kylo-grams. - But that's a bit debatable for a couple of reasons. Firstly, to be perfectly honest, quite a few supposedly metric countries
still use other units for, well, kind of a lot of stuff and that's because some old habits just die really, really hard. As an example, I live here in all-metric Spain and I can tell you the diagonal of my TV or my computer screens in inches but I have no idea how much they are in millimeters. Also, convincing, for example, plumbers or makers of plumbing equipment
to use millimeters instead of inches for all pipes and fittings hasn't happened in many countries at all and the same thing is true for a lot of other industries. Secondly, I had to choose my words very carefully just now when describing what Liberia, Myanmar and the United States are doing. And that's because, technically, at least, the United States has switched to metric a long, long time ago. They just did it in a bit of a different way.
- [Announcer] America is going metric. It's a darn good thing. - Now I don't want to tease my American viewers too much so believe me when I say that some things I will say a bit later here in the video won't go down very well with some die-hard metric system fans either so, at least, you can look forward to that. Now I really don't want to get into a discussion about which system is better here but let's have a look at some facts. The former names for the Freedom Units
which are used in the States are the Traditional Systems of Weights and Measures or the United States Customary Units and the use of the words traditional or customary is a big clue to where we're going. In 1975, the United States passed a law called the Metric Conversion Act which defined the metric system as the preferred system for the country and in many fields, in the military, for example, the United States had actually been using
certain metric units even decades before that. And even more importantly, the United States officially started defining its own units by using the metric system as a reference, way back in 1893 with something called the Mendenhall Order. And that means that, for example, today, the inch is not defined as the length of three barley corns which genuinely was its official definition for centuries,
instead the inch is now officially defined as 25.4 millimeters and all other US customary units have similar metric definitions which they've had for well over a century by now. Now I am oversimplifying things a little bit here because units based in surveying in America still have their own different definitions and also the metric system has evolved into International System or SI
which is a long story in itself which I'm not going to be touching in this video but the point is that, generally speaking, if you want to claim that a system is superior to another system, well, it's hard to make that argument stick if you then use that other system as the basis for all of the units in your own system, just saying. Now I know that people on both side of this argument have some really strong feelings about it
because you always let me know when I forget to use both meters and feet or kilos and pounds in my videos but thanks to global long-standing industries that refuse to update their practices and because of simply old habits, no one's metrification is really complete yet. And nowhere is this more obvious and more messy than in aviation.
Although there are actually some justifications for it which could make this mess stick around for a while, technically, it should be on the way out. And I'll explain all of that after this: Just like aviation relies on sometimes overly complex units of measurements, your online security relies on expert tools to help you stave off cyber threats and prying eyes, which is actually a problem
that's getting bigger and bigger every single day. When I'm out traveling myself, I'm always acutely aware of the threat of my data getting stolen whenever I'm using Wi-Fi in airports, restaurants and bars and so on and that's why I am so happy to have today's sponsor, NordVPN, on my team. Nord's extra security features warns me whenever a network is not safe and it also allows me to watch my favorite shows wherever I am
as well as giving my team access to geoblocked content for our research and since Nord has hundreds of servers all over the world, the connection is always super fast and reliable. So if you, like me, are traveling a lot or relying on global access to websites for research, I highly recommend you to try them out, using the link below, which is nordvpn.com/mentournow. That will both support my channel and give you
four months completely cost-free if you sign up for a two-year deal with Nord and remember, with Nord, you always have a 30-day money-back guarantee. Thank you, Nord. Now back to the video. As with many other things in aviation, in order to understand how we ended up with the units that we use, we have to look at the maritime world. Believe it or not, one of the most basic units that we use in our super sleek airliners today, started off with some old sailors, keeping one eye on an hourglass
while keeping the other eye for tracking a rope and a small wooden plank but for that to make sense, we first have to talk about the nautical mile. If you go into flight training, you will learn a couple of definitions for the nautical mile, one being that a nautical mile is the meridian arc length, corresponding to one minute of a degree of latitude. In other words, the full circumference of the earth is 360° and a nautical mile
is one 60th of one degree at the equator but that's a historical definition. Today, a nautical mile is defined using the metric system like everything else and it is exactly 1,852 meters. But if we want to get really nerdy here which we obviously do, so I can use my nerd bell. (nerd bell chiming) (Petter chuckles) - Why are you the way that you are? - There is also another historical definition
of the nautical mile which also uses the one 60th of a degree expressing the result in meters. That's assuming that the distance from the equator to the North Pole is 10 million meters. So 10 million divided by 90° from the equator to the pole and divided again by 60, to give us minutes, makes 1,851.85 meters which we can round out to 1,852.
Now, however you choose to define it, the nautical mile is not a recognized SI unit and it's far from the only unit of length that we use in aviation. For example, we use nautical miles when we want to give our distance from a waypoint or an airport, for example. But when you read a METAR and you want to get weather at your destination, in most parts of the world, you will see the visibility expressed in meters
which 9999 being the maximum, indicating a visibility of 10,000 meters or more. However, if you are in the United States, Canada or Mexico, METAR will likely show visibility in miles, statute miles, not nautical miles. And for the length of a runway, in most of the world, we now use meters but again in North America, they typically use not nautical miles
or statute miles but feet. - But we have no reason to be ashamed for using feet. - And if you think that that seems like an excessive number of units for measuring length or distance, well, I haven't even started mentioning any vertical units yet. So for altitude, in most part of the world, we use feet and feet per minute for climb rate. But in China and a few other countries, they measure altitude in meters and their rate of climb is meters per second.
But perhaps the unit with the strangest history that we use in aviation is the speed unit. Now I'm not going to go into the differences between our different air speeds indicated, true and ground speed and so on because that's not really relevant to what we're discussing here but they're all expressed in the same unit, the knot, well, unless we use Mach numbers, I guess. But did you know where the word knot actually comes from?
Obviously, saying knots is much faster and more convenient than saying nautical miles per hour which is what a knot is. So using that name clearly makes sense since we use nautical miles for our distances. But the origin of the word, once again, is a funny remnant of how aviation drew heavily from the maritime world early in its history. And this is where that wooden plank, the rope and the hourglass that I mentioned before
comes into play. Believe it or not, the name knot is a direct reference to actual knots tied on the rope. One end of the rope was then attached to a big spool which the rope was wound up on and the other end of the rope was attached to a type of triangular wooden board in a very specific way. The idea was actually pretty simple. The wooden board which was called the chip log was then thrown overboard from the back of the ship and one of the sides of the wooden ship
had a lead weight to keep it vertical on the surface of the sea. As the ship then sped forward, the wood was supposed to stay mostly still on the surface of the water, with the rope quickly unwinding from the spool. On the rope, there were knots spaced out on regular intervals either every 47 feet and 3 inches or every 48 ft, depending on which source you use. The idea was that the sailors would use a small hourglass
which measured either 28 seconds or 30 seconds, again, depending on the source and they would count how many knots went by in that time and the answer that they then derived from this exercise was the ship's speed, measured in literal knots and, curiously, mathematically, if the wood chip log would really sit still on the water, the result would be almost exactly the same speed as the modern definition of a knot
but, obviously, there was quite a bit that could go wrong with this type of a measurement, in pitching and rolling seas and with friction between the spool, the rope, the wooden chip log and well, basically everything else. Also, the measured speed was with relation to the water. So if the boat was traveling in a current, well, then the crew would need some more references just like we do if we're dealing with headwinds or tailwinds when we fly. Now, getting your speed in this fashion
was as much of an art form as it was science and I'm sure that you'll agree that this is a pretty strange unit to pick to measure the speed of a flying aircraft. Now I should add here that, officially, the American Federal Aviation Regulations or FAR, only specified that distances should be expressed in nautical miles and speed in knots after 1969. Before then, the FARs actually called for statute miles
and miles per hours to be used. So the direct link between maritime and aviation usage of knot is probably a little bit debatable. Other than our speed, we also use knots to measure wind speed although, again, in some parts of the world, wind is measured in meters per second instead. Now don't worry, I'm not going to spend the rest of this video listing different units of measurements that we use in the cockpit and how they differ around the world because actually,
it would be much quicker to list the units that don't vary between countries. These are time, hours, minutes and seconds, degrees for heading, turn, slip and attitude, basically everything that use degrees of an angle, and temperatures which is measured in Celsius. Even the United States uses Celsius for flight-planning purposes although some temperatures for certain aircraft systems might still be measured in Fahrenheit.
Now the important point here is that with very, very few exceptions like maybe China, wherever you are in the world as a pilot or an air traffic controller, you will be using both metric and SI units and also some form of more traditional Imperial or US Customary Units and it's all very, very messy. So are we going to do something about it? Well, officially, at least, ICAO, the United Nation body
that is responsible for aviation, says that we should. ICAO has been calling for the metrification of the aviation industry for quite some time now. And already, in 1979, ICAO was listing the use of knots nautical miles and feet as appropriate for use only on a temporary basis and more recently, in 2010, ICAO, again, recommended that we switch to kilometers per hour for speed,
meters second for wind speed, kilometers for distance and meters for elevation but like before, these ICAO recommendations did not come with a deadline. It only stated that the switch to metric must be made eventually. Now there are actually some arguments against such a metrification and they're not just coming from the United States. One simple concern is practical
and it's has to do with the existing cockpit instruments in thousands of aircraft all around the world. Of course some instruments and settings, for example, our QNH for altimeter setting already comes with dual-setting options that allow us to switch between hectopascals or millibars and inches of mercury, so basically between metric and non-metric but that's obviously necessary to allow us to fly between countries
that use different systems but we can't really do the same with all of the other analog air speed, altitude and fuel fuel gauges and all of the analog instruments that are out there. This would be particularly true for smaller aircraft who, for the most part, still use analog instruments. Now most modern airliners generally have electronic flight instrument systems or EFIS displays which could, in theory, be reprogrammed to indicate whatever units that we like
but, in practice, things aren't quite that simple. For example, in China, Mongolia, North Korea and several former Soviet countries, they use meters for altitude. But as far as I know, at least, the Boeing and Airbus aircraft in these countries still use feet in the altitude tape of their primary flight displays. Now they can have altitude in meters displayed as a secondary indication but they still would have feet
as primary units for altitude which means that this is what they will have to use to program the autopilot and other aircraft systems, at least as far as I know. Let me know, in the comments, if you know more about that. Now if we really wanted to recalibrate everything to metric units, I'm sure that that could be done. It would just take a lot of time and effort but here's a question for you: would it actually be a good idea?
Is the situation we have now in aviation, just the result of old habits dying hard like non-metric house plumbing or is there actually some logic behind the madness? You see, one thing that we normally really like about the metric system is how easily we can use, for example, millimeters, meters or kilometers without even thinking about it. We don't need to make any mental calculations at all to convert between them, we just multiply or divide it by 1,000.
But in aviation, we often need to express different values at the same time and often over a busy radio frequency. For example, ATC could tell you that you need to be at a certain altitude at a specific distance from the runway and for these, units now we use feet and nautical miles respectively. But if we want to use metric, we would use meters for altitude and kilometers for distance and it's not difficult to imagine
someone either mishearing kilometers as meters or maybe ATC getting stepped on right as they say kilo in kilometers, potentially creating some real confusion. - Okay, here (indistinct). - Okay, what?! - See, I told you there will be a downside to the metric system. Now even in relatively new fields of aviation like drones, some authorities are actually using meters to express horizontal distance and feet to express vertical distance
just to avoid confusion and we sort of do the same thing when we're out flying on a daily basis for visibility horizontally and cloud height over the ground, for example. And actually if you think that metric systems will win out eventually, consider that some countries have actually switched away from metric, at least in part. Russia, for example, who had been one of the few countries more widely using the metric system in aviation
started moving more toward feet for altitude as recently as in 2017, although it appears that they're still using meters for lower altitudes near airports. So to summarize, there might actually be some real practical reasons why we want to stick to different units for different things, basically to avoid confusion even though that really sounds like a contradiction, and again, saying knot is very nice and short
although I think, in theory, we could probably rename a metric unit to make it sound a bit different, kind of like in some military applications where they say a click instead of a kilometer. I guess we could use that. Finally, it's probably worth bearing in mind that even even if we totally metrify or metrificate aviation, we still would need to make conversions like between liters and kilos of fuel, for example
and that's not as simple as times 10 or times 1,000. You could probably even argue that people who grew up with Imperial or American Customary Units have a little bit of an advantage here because they're more used to having to make these unusual conversions all the time. The point is that the use of the units that we have in aviation now isn't as unjustified as many people think even though there are definitely too many of them right now.
As always, I hope that you have subscribed to the channel and let me know what your thoughts on this theme is in the comments here below. If you want to support the work that I do, then you can leave a Super Thanks, get yourself a awesome t-shirt or a hoodie or even better, join my awesome Patreon crew. Have an absolutely fantastic day and I'll see you next time, bye-bye.
End of transcript