Should aviation switch to the metric system ?
Here are three weather reports (a.k.a. METARs) I got from www.easymetar.com. The first one if from New-York JFK, the second Geneva, and the third from Moscow Domodedovo. They all contain the same information: wind, visibility, clouds, temperature, and pressure. Unless you’re a pilot used to fly on various continents, something should surprise you.
KJFK 291751Z 15013KT 10SM FEW050 SCT100 BKN140 BKN250 28/18 A2982
LSGG 291820Z VRB06KT 9999 FEW050 FEW060TCU FEW070CB 29/15 Q1017 NOSIG
UUDD 291800Z 24002MPS 6000 BKN023CB BKN100 14/14 Q1004 TEMPO TSRA
Winds for New-York and Geneva are given in knots (KT), but Moscow ones are in meters per second (MPS). Good pilots know that 1 meter per second is 1.94 knots… pragmatic pilots call that 2. The other look in Google.
Then come the visibility. 10 Statute Miles (SM) in New-York, 9999 meters (10 km or more) in Geneva, and 6000 meters in Moscow. Funny to see that wind New-York is given in Nautical Miles per hour (knots), and visibility in Status Miles. And what about Geneva ? Wind in knots, visibility in meters. One must give that to the Russians, they’re more coherent: both wind speed and visibility are given in meters.
But when it comes to clouds cover… all three airports report them in feet ! Brilliant harmony, but not for long. The New-York air-pressure (QNH) is given in inches of mercury, whereas both Geneva and Moscow report it in millibars.
This simple example is just scratch on the surface of the units chaos that exist in aviation. Airspeed indicators can be graduated in knots, MPH, or kilometers per hour. Depending which manufacturer publish them, weight and balance sheets can be in kilograms or pounds. Fuel can be delivered in liters or US gallons. For larger aircrafts, it comes in tons or pounds. Oil comes in liters, or US gallon quarters.
Take-off and landing performance calculations can be critical. The tools (tables or graphs) used to determine the required distance for taking-off or landing produce results in meters… or in feet.
When it comes to flying, the things could also get complex. In Russia, altitudes in clearances are given in meters. Jeppesen publishes charts with values in feet, with conversion tables. A typical Moscow departure initial climb clereance is 3550 feet, corresponding to 900 meters.
How safe is such a unit mixture ? For a crew operating under normal conditions, this requires some additional attention. Put some extra-pressure (bad weather, technical problem, may be an emergency), and this is one more possible trap. Units problems were a contributing factor in several accidents, including the infamous Gimli Glider – a 767 that ran out of fuel at 41′000 feet, partly because of a metric units problem.
The problem is clear, so what about the solution ? Glass cockpit systems can switch between various units, reducing the number of calculations to be made by the crew. This is a good step, but not yet a full solution. So here comes the quesiton again: should aviation switch to the metric system ?
This switch would impact an incredible number of areas. On aircraft side, it means avionics upgrade, fuel system upgrade, new weight and balance documents, updates to the aircraft manuals, and probably pilot training. On ground, ATC systems shall be adapted to display speeds, altitudes, winds, pressures in metric units. Controllers shall be trained to new procedures with metric values. Maintenance engineers, fuel and dispatch staff shall also be familiarized with the new units systems.
The cost of such a switch would be simply huge. The next problem is to manage the transition. The whole aviation system could not be adapted overnight. Handling that mixed situation safely would not be an easy thing. How many Gimli-glider-like accidents could this change create before all crews get used to the new system ?
To me, it all boils down to two questions:
- Will airlines pay for this change ?
- Is the solution more hazardous than the problem ?
My crystal ball is undergoing maintenance right now, so I can’t answer those questions. If yours is working, I’d be glad to know your predictions.
PS: Many thanks to Paul from www.askacfi.com, who asked this question in a comment on his own blog.
9 Comments, Comment or Ping
Paul
Did you know that there are only 3 countries in the ENTIRE WORLD that have NOT adopted the metric system? Liberia, Myanmar and the United States. That is messed up. The word “inch” means the same as the word “thumb” in some languages. The “foot” dates to 2575 B.C!!! It is seriously embarrassing that the leader of the free world bases it measurements on human body parts.
Jun 30th, 2008
Yellowbird
“It is seriously embarrassing that the leader of the free world bases it measurements on human body parts.”
No more so than the idea that we should base measurements on the distance light travels in 1⁄299,792,458 of a second in a complete vacuum.
I personally use the FFF (Furlong/Firkin/Fortnight) system and base all my flight planning on a cruising speed of 371197.2178780147 furlongs per fortnight
Jul 1st, 2008
Paul
I understand your point but If we took your rationale we would still be telling time by the sundial. Human body parts obviously vary in length (how’s the joke go? Why do women make bad carpenters…”) and the speed of light and time is something pretty quantifiable and measurable. There is a known constant scientific standard. Besides, I like units of 10 a lot better than dividing things by 12, mulitplying by 3, multiplying by 1760, 5280 or whatever…don’t you? We vastly improved our methods of measuring, or should I say MOST of us have. We made the whole world speak english, maybe we should compromise and give them the metric system.
Jul 1st, 2008
Yellowbird
Paul,
Perhaps you didn’t see the tongue in my cheek? I guess the distance it protruded wasn’t noticeable enough to justify it’s use as the basis of a unit of measurement.
Jul 1st, 2008
Paul
No, I saw it, I was just having some fun. I try and talk to my wife about this stuff and she just looks at me and says “how much time do you spend thinking about this? why don’t you do something productive like clean your desk” It’s nice to discuss something of meaning…
Jul 2nd, 2008
Physics
With glass cockpits becoming popular a change won’t be as hard. It will be as easy as loading new firmware on the hardware side. Most pilots on earth already know how to use metric so maybe just a check out flight and a short exam over new procedures would do it along with regular recurrent training and flight reviews.
Apr 19th, 2009
Vincent
@Physics: from an equipment point of view this would indeed be an easy change… but you have to think of what is behind. The ATC software engineer in me is talking here. All systems should be adapted to metric system, all procedures and charts should be re-published, and air traffic controllers should be re-trained to the new values, even if the procedures remain the same. Can you imagine the change for a controller who knew for years that the ILS interception for a given airport is at 7′000 feet and that the minimum radar vectoring altitude in the area is 5′000 feet having to change all of that to meters ?
Not to mention the transition phase where ATC should be able to offer both metric and non-metric clearances ? What a mess…
Apr 19th, 2009
Physics
Well to minimize the disruption it would be best to do it when fewest pilots you the system. Which would be a winter during an economic downturn. Controllers change the areas they serve from time to time, possibly not often but does happen, so they need to relearn values for a specific location. It should be easy to implement in class A airspace before the rest since it is altimeter setting separated anyway. I think relearning values would at most be a few hours of ground school and possibly a written test and stressed on the regular flight review. On the controllers side is there any regular proficiency review or test or training anyway? I assume there is so it could be part of that. Charts are published on a few week cycle anyway so one day a new set would be metric only. For the dual use situation would be dangerous. The switch should be in the middle of the night on a normally quiet ATC day. Possibly have extra controllers for that night. I understand it would be tough for a few weeks but everyone would get used to it. We change the time twice a year and a few days are wacky, especial since Zulu is always constant. It might take some time to for the revised procedures to be created/converted but it would be an opportunity to possibly hire more people for a few months.
And no Vincent I am not a pilot but I would to be. I have at least six more years of University before I can even think of being a pilot. I live in USA so I might be making assumptions about the charts being republished and training. Thanks for the reply.
Apr 25th, 2009
Vincent
@Physics: I see that you already thought a lot about that. Being in the business of ATC systems, let me show you some of the problems typically relating to such a transition.
1) The whole system must be adapted at once, on both ATC and aircraft side. What to do with light aircraft without glass cockpits ? Two altimeters ? Altimeters with two scales ? Glass-cockpits shall not be an issue as they can be updated with a bi-units software, but there are still lot of aircraft flying with mechanical instruments… not to mention the backup instruments of glass-cockpits.
2) Controllers have to be trained and checked on new procedures, whatever the change. They go through theory and simulator sessions. ATCOs shortage is a common problem in Europe and getting them out of the normal roster for long training times can be complex.
3) If no serious safety advantage is demonstrated, no airline or air traffic authority will take the step. Being a nice thing from an engineering point of view does not means it brings any benefit.
4) All centers (and in Europe this means all centers from several ATC organisations) shall adapt at the same time, to make possible to exchange flights at pre-defined altitudes.
Finally, you can think of RVSM. This consisted in the addition of new flight levels, thanks to the increased accuracy of baro-altimeters. It took several years until sufficient aircraft were equipped and all ATC systems adapted… The rate of innovation in ATC is rather slow, but may be this will come. Not soon.
Apr 26th, 2009
Reply to “Should aviation switch to the metric system ?”