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.
5 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
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