Understanding Speed Metrics in Aviation

On a recent Monday, a Virgin Atlantic Boeing 787 achieved a remarkable ground speed of 801 miles per hour, as reported by the Washington Post. This impressive speed resulted in an early landing, much to the delight of passengers. However, it's important to note that the aircraft was likely cruising at its usual speed for commercial jets and certainly wasn’t maintaining 801 mph through the air at cruising altitude. The distinction between a plane's airspeed and its ground speed is crucial. In this instance, the aircraft was assisted by strong tailwinds due to the jet stream while flying from Los Angeles to London. Therefore, while it was moving swiftly over the ground, its speed relative to the air was standard.

“This is primarily a weather phenomenon,” explains Richard P. Anderson, a pilot and director at the Eagle Flight Research Center at Embry-Riddle Aeronautical University. “The airplane itself is unaware of its ground speed.”

Anderson emphasizes that a plane cannot determine its ground speed simply by observing the surrounding air. The aircraft's performance is unaffected by its ground speed.

The weather event in question saw the jet stream traveling at an astonishing 231 mph. (This measurement was taken over New York, while the plane was above Pennsylvania when it reached 801 mph.) The plane effectively became a passenger on this powerful wind. Typically, from November to March, the jet stream is stronger and situated over the continental U.S., making it an ideal time for eastbound flights to benefit from these swift currents.

To fully grasp the situation, it's also essential to understand how speed is quantified in aviation.

In a vehicle, the speedometer provides a straightforward readout of your traveling speed. However, airplanes utilize multiple metrics to express their velocities. Anderson notes that there are three primary measurements: ground speed, indicated airspeed, and true airspeed. Pilots have access to all three measurements on larger aircraft. Ground speed is calculated using GPS, while indicated airspeed is gauged via sensors known as pitot tubes. True airspeed is derived from calculations based on indicated airspeed and is inherently different. As altitude increases, the gap between indicated and true airspeed widens.

True airspeed is the most intuitive concept: it represents the plane's speed relative to the surrounding air.

While pilots monitor indicated ground speed and true airspeed, passengers are primarily concerned with ground speed, as it determines arrival times. Traveling west to east across the U.S. to Europe, one hopes the jet stream will facilitate a quicker arrival. Tailwinds expedite journeys, while headwinds can cause delays; both serve as apt metaphors for various life experiences.

A fitting analogy for a plane that is cruising normally yet boasts a high ground speed due to tailwinds is to imagine standing in a moving train car and throwing a paper airplane in the same direction the train is traveling. The paper airplane maintains a normal airspeed; however, when you factor in the train's speed, its ground speed becomes remarkably high.

Regarding the 801 mph ground speed recorded by the Virgin Atlantic flight, a Boeing representative declined to comment, stating that they do not monitor such statistics. Dreamliners, the aircraft type involved, usually cruise at approximately 85 percent of the speed of sound.

Anderson notes that such speeds are rare and largely influenced by weather conditions.

“As a pilot, tailwinds are what everyone desires,” he adds. “We are always aware of our true speed and feel exhilarated when nature provides the opportunity for significantly higher speeds than our aircraft can typically achieve.”

This video explains how the jet stream can push planes to incredible speeds, showcasing the phenomenon observed during the Boeing 787 flight.