How are the cruising altitudes of aircraft determined? Is it quicker to fly east than west?

Cruise is that phase of aircraft flight that starts when the aircraft levels off after a climb until it begins to descend for landing. Commercial or passenger aircraft are usually designed for optimum performance around their cruise speed and a number of factors including payload, centre of gravity, air temperature and humidity influence cruise speed and altitude.

It is a common observation among flyers traveling eastbound that the journey takes a significantly lesser amount of time than for the same journey westbound.

Representative | Source

Does it have anything to do with the earth’s spinning? Not really.

Jet streams are narrow bands of strong wind in the upper levels of the atmosphere. The winds blow from west to east in jet streams but the flow often shifts to the north and south.

How do they affect flights?

These currents are formed as a result of atmospheric heating from the sun’s radiation and the earth’s Coriolis force. Together, these factors produce streams of fast flowing air that are responsible for flight times looking significantly different east to west. Airplanes flying eastward in a jet stream get a powerful boost while those flying westward must fight an equally powerful headwind.

The polar stream and the subtropical stream are the two most prominent jet-streams. The polar stream is the stronger of the two, and it causes much faster winds compared to the subtropical. This feature is exploited by most airlines that have their paths over the Atlantic or the Pacific.

National Weather Stream

Now that being said, how are the cruising altitudes determined?

According to Mark Vanhoenacker, a British Airways pilot and author of Skyfaring, an airplane’s altitude is a “vertical wayfinding in the ocean of air.”

Apparently, there happens to be a mnemonic, which is quite popular among pilots, that helps remember what altitudes to fly at under visual and instrument flight rules and it goes something like this:

The East Is Odd, West is Even Odder


What does that mean?

To get a perspective, here’s something else you should know:

  • East and West are defined by the magnetic headings of zero to 179 degrees (East) and 180-359 degrees (West)
  • Odd and Even refer to the altitudes
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For instance, a commercial flight flying under IFR at 33,000 feet will cruise eastbound at “odd” flight levels while a westbound flight will cruise at even numbered flight levels-for instance 32,000 feet (FL320) or 34,000 feet (FL340). This automatically results in the aircraft being separated vertically.

This is also where RVSM ( reduced vertical separation minima) comes into play. Modern regulations allow aircraft to be separated by as little as 1,000 feet above or below each other- thereby allowing for a more efficient use of airspace (between FL290 and FL410).


Furthermore, flying at higher altitudes also means planes can avoid birds, drones, and light aircraft and helicopters, which often fly at lower altitudes.

Representative | Boldmethod

In 2020, a British Airways Boeing 747-436 reached speeds of 825 mph (1,327 km/h) as it rode a jet stream accelerated by Storm Ciara. The aircraft flew from JFK to LHR in a record 4 hours and 55 minutes, thus beating the previous record of 5 hours and 13 minutes by the Norwegian.

Representative | InxeloTechnologies

Following factors also contribute in determining the cruising altitudes of aircraft:

  • Turbulence: Although a common event, it occurs less frequently at higher altitudes
  • Weather: Troposphere is the part of the atmosphere where most of the weather event takes place and hence planes generally fly above this level
  • Emergencies: In the unlikely event of an emergency, flying at higher altitudes gives the flight crew ample time to fix the situation or find a safe place to land as opposed to flying low in a lighter aircraft
  • A plane’s altitude is majorly determined by its current weight and the atmospheric conditions at the time of flight- a no-brainer

Ultimately, its up-to the dedicated team of flight dispatchers and the ATC who are in charge of planning safe and efficient routes and altitudes for flying.

Representative | BAA Training

There are also laws in place that prohibit flying below 1,000 feet over a built-up area or 500 feet over any person, vehicle or structure.



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