4 min read · Sep 10, 2023
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If you’ve ever flown from east to west and back, you’ll find that the flight flying east is considerably shorter than flying the same route west. For example, Los Angeles to New York takes around 5 hours, whereas New York to Los Angeles takes around 6 hours. These two flights are flying the same route — so why is there a difference in their flight times? Shouldn’t they be the same?
Many people would assume it’s because of the Earth’s rotation. The Earth rotates from west to east. Under this method of thinking, a flight from east to west should be faster than the same flight from west to east, which is not the case. In addition, Earth’s rotation is irrelevant in this case because as the Earth rotates, the air in the atmosphere rotates with the Earth due to gravity. Due to air resistance, the aircraft’s location would be stationary relative to the atmosphere, hence stationary relative to the Earth. Because of this, the aircraft wouldn’t have an advantage flying one direction over the other.
What are Jet Streams?
In simple terms, jet streams are streams of high-speed air currents at high altitudes. These jet streams are formed by complex interactions between multiple variables, such as locations of high and low-pressure systems, warm and cold air, uneven heating of the Earth by the Sun, and seasonal changes. And, you guessed it — they move from the west to the east, in both hemispheres.
Jet streams can reach a top speed of a whopping 250kts. As a result, many airlines started to plan routes that take shorter flight times with the help of these jet streams. Flights may intentionally take a longer route to utilize a jet stream that propels the aircraft forward, which significantly increases the ground speed of the aircraft.
The Great Circle Route is the flight path that minimizes ground distance covered by an aircraft. Notice that the Jet Stream Route is only used for flights from west to east, as jet streams blow to the east. When flying west, planes will take the Great Circle Route to minimize covered distance (hence avoiding head-on jet streams and minimizing flight time).
Speed Records!
- Virgin Atlantic 787–9 flying LAX-LHR at a peak speed of 696 knots (regular cruising speed: 494 kts)
- Delta 717–200 flying LAX-JFK at a peak speed of 589 knots (regular cruising speed: 438 knots)
- American 737–800 flying ORD-JFK at a peak speed of 609 knots (regular cruising speed: 460 knots)
A British Airways 747–400 operating as flight BA112 once flew a top speed of 717 kts (or 825 mph, or 1328 km/h, or mach 1.08) from JFK to LHR. This smashed the transatlantic subsonic speed record with a whopping time of 4 hours and 56 minutes, and you guessed it — this was with the help of a violent jet stream.
Note that the airspeed of the aircraft was subsonic; the groundspeed of the aircraft was supersonic.
Despite all the pros of flying in a jet stream, there are a few downsides as well. These include flying perpendicularly into a jet stream or flying head-on into a jet stream. Flying perpendicularly into a jet stream means severe crosswinds and increased difficulty of direction judgment while flying head-on into a jet stream slows the ground speed of your aircraft significantly.
Clear Air Turbulence (CAT)
Another issue is clear air turbulence. CAT can cause sudden severe turbulence in clear skies, which results in sudden and unpredicted violent shaking of the aircraft. CAT is usually caused by small-scale wind gradients right next to the jet stream, and the interaction of slow winds with fast winds causes disturbance in air movement. There have been a few fatalities in the past where the seatbelt sign was turned off due to clear skies, but clear air turbulence hit the aircraft.
