YOUR SPEED. YOUR DEVICE.
Ground Speed vs Airspeed: What Your Phone Measures
A phone on a plane measures ground speed: how fast the aircraft moves over the surface of the earth. Airspeed is the aircraft's speed relative to the surrounding air, and it is the speed that produces lift. Wind separates the two, so with a strong tailwind your phone can show a ground speed well above the aircraft's airspeed.
XpeedHow the FAA defines each speed
The FAA's Pilot/Controller Glossary gives short definitions:
- Ground speed is the speed of an aircraft relative to the surface of the earth.
- Airspeed is the speed of an aircraft relative to its surrounding air mass.
- Indicated airspeed is the speed shown on the aircraft's airspeed indicator, and it is what pilots and controllers mean by the plain word "airspeed".
- True airspeed is the airspeed relative to undisturbed air, used mainly in flight planning and the en route part of a flight.
A passenger phone can only report the first of these.
Wind connects the two numbers
NASA's Glenn Research Center explains that airspeed is the vector difference between ground speed and wind speed. On a perfectly still day, airspeed equals ground speed. NASA also notes that airspeed is the one that generates lift and drag, which is why it matters to the crew.
A simple example with round numbers shows the size of the effect:
- An aircraft with a true airspeed of 800 km/h and a direct 100 km/h tailwind moves over the ground at 900 km/h.
- The same aircraft into a direct 100 km/h headwind moves over the ground at 700 km/h.
A crosswind adds a sideways component. It changes the aircraft's path over the ground, and the ground speed differs from the airspeed by less than the full wind speed.
Why a phone cannot show airspeed
GPS receivers calculate position and time from one-way satellite signals. A browser speedometer turns those positions into speed over the ground, either from a speed value the device supplies or from the change in position over time. Nothing in that chain measures the air around the aircraft, so it cannot know the wind or the airspeed.
The same applies at any altitude. Two flights with identical airspeed can show different ground speeds on your phone because the wind is different along each route.
Using a phone as a passenger
The FAA says cell phones and portable electronic devices must be used in airplane mode or with the cellular connection disabled. You may use the aircraft's Wi-Fi if it has one and the airline allows it. Crew instructions come first.
GPS reception inside a cabin is not guaranteed. GPS.gov lists signal blockage among the main causes of poor accuracy, and Apple notes that GPS accuracy depends on how many satellites are visible. A window seat may give the phone a better view of the sky. Whether GPS stays available in airplane mode depends on your device and its settings.
Xpeed's airplane mode is experimental and has not been field-tested on an aircraft. When no valid recent reading is available, it shows a dash.
Reading the number on your screen
Xpeed's airplane mode starts in knots, one nautical mile per hour. You can switch units on the tool, and the speed converter turns knots into km/h or mph.
A ground speed that looks too high or too low for the aircraft often reflects the wind along the route. The same route can show different ground speeds in each direction because the aircraft flies with the wind one way and against it the other.
The phone reading is for curiosity. It must not be used for piloting, flight planning or navigation.
Questions and answers
Why is my phone showing a speed above the plane's cruising speed?
A tailwind adds to ground speed, which is what the phone measures. The aircraft's speed through the air can be much lower than the number on your screen.
Can my phone get GPS in airplane mode?
GPS reception does not transmit anything, but whether it remains available in airplane mode depends on the device and its settings. Follow the airline's and crew's rules on device use.
Does Xpeed work offline on a flight?
Only if the page was opened and saved on your device while you were online, and only if the phone receives a location. The offline guide explains what Xpeed saves.