Why plane cockpits have a special setting for flying over China

Everything about the profession of aviation is made to measure. But when it comes to height, distance and speed, the array and inconsistency of how things are measured is a far more dizzying curiosity.

Australia converted to the metric system for measurement in 1971, going from miles per hour to kilometres per hour, pounds to kilograms, inches to centimetres, and so on.

Pilots flying internationally need to understand a dizzying range of different measurements.Credit: iStock

However, the International Civil Aviation Organisation is the custodian that determines the global standardisation for measurement in aviation. In 2010, ICAO provided recommendations for the International System of Units measuring things such as aircraft speed, height, wind speed and distance. In 1979, however, it permitted the list of “temporary” units of measurement still in use today. Effectively, individual countries can thumb their nose at these global standards and do their own thing.

To kick things off, airspeed is measured in knots, and that system is consistent around the globe. Travelling at one knot is the equivalent of 1.85km/h. A Boeing 787 will normally cruise at 500 knots, which in still air is approximately 925km/h. Commonly, jet aircraft are required to fly at 250 knots or 462km/h below 10,000 feet. Which brings in the issue of measuring height.

While Australia uses the metric system, height is measured in feet in aviation circles. And that unit of measurement is consistent in most regions around the globe. Only China, Mongolia and several of the “stans” – Kazakhstan and Uzbekistan are examples – use metres for measuring height. This meant aircraft manufacturers like Boeing and Airbus had to install a button on the flight deck that allowed the altimeter to display the aircraft’s height in metres and feet to fly over those countries.

Altitude is measured in feet, unless your flying over China or several Central Asia countries, which use metres.Credit: iStock

When talking distance, pilots think in nautical miles. A nautical mile (NM) aligns with the knot terminology from before. One knot means travelling one nautical mile per hour. The distance between Sydney and Melbourne is around 385NM, while Sydney to Los Angeles is about 6800NM.

At a smaller level, the length of a runway is measured in metres. The longest runway in Australia is at Sydney airport and reaches 3962 metres, while pilots taking holidaymakers to Hamilton Island have a runway of only 1766 metres to land on.

In North America, however, they still measure runways in feet. The longest runway at Los Angeles (LAX) airport reaches 12,923 feet, but if you convert that to metres it is 3939 metres, similar to the long runway in Sydney.

Measuring visibility is another curious oddity. Most countries use metres. Normal instrument approaches in Australia require a forward visibility of 800 metres for pilots to land safely. Zipping around in a light aircraft needs a forward visibility of 5000 metres. Again, heading back to North America, they measure visibility in statute miles or sometimes even feet.

This requires some mental gymnastics by pilots to remember that one statute mile is 1600 metres, so for a visibility of 800 metres, you need half a statute mile in the US.

Is this starting to make your head hurt?

Wind speed is normally measured in knots, although some countries use metres per second, while the rate of climb of an aircraft is measured in feet per minute and is globally consistent except for those countries listed earlier, including the giant land mass of China.

When it comes to weights, the fun continues. Think about Jules and Vincent in the car in the movie Pulp Fiction discussing a quarter pounder in France. Aircraft manufacturers tailor their weights to the country the aircraft will operate in. So, while the American company Boeing builds planes in pounds, they are converted to kilograms for Australian carriers. Hence the maximum take-off weight (MTOW) of a B787 is converted from 561,500 pounds to 254,692 kilograms.

Whereas French Airbus operates under the metric system – a Royale with cheese instead of a quarter pounder – so the MTOW of an Airbus A321 is 97000 kilograms, a nice even number in the metric world. However, US operators would use 213,848 pounds as their reference weight.

Moving on to fuel, where from a Boeing perspective all the numbers are long and complex because they have been converted from pounds to kilograms. However, when the refueller fills the tank, that figure is provided to the pilots in litres. This figure then needs to be converted into kilograms depending on the specific gravity of the fuel – something that can vary with temperature – so a cross-check can be done to make sure the right amount of fuel has been loaded for the flight.

In 1983, an Air Canada flight famously ran out of fuel at 41,000 feet after the incorrect fuel order was loaded. Several factors contributed to this, but one was the mistaken conversion of pounds to kilograms in the aircraft’s computer system. Fortunately, that flight became known as the “Gimli Glider” because it still successfully landed with all 69 passengers and crew alive.

Time is the one true constant for all pilots who take to the sky. Universal Time Coordinated is behind every on-time departure or disappointing delay. It is also known as Greenwich Mean Time (GMT) or Zulu (Z) time. The zero-meridian line of longitude is located at Greenwich in the United Kingdom and is the baseline for all aviation timing.

The east coast of Australia (in non-daylight saving time) is 10 hours ahead of Greenwich, so for pilots reviewing a flight plan, a 10am departure from Sydney will be listed as 0000Z (effectively midnight in London), while an 11.30am arrival into Melbourne is listed as 0130Z. This system comes into its own when flying across vast time zones to provide a consistent measurement of telling the time.

So, whether it is knowing a knot from a nautical mile, metres from litres, or pounds from feet, the ability to be fluent and understanding of all manner of measurements and terminology is just another skill of being a pilot.