If you've ever been on a commercial airline flight, and you hear the whistling of the wind get louder as the aircraft climbs up higher into the sky, you might be wondering just how fast you are actually traveling. One might think that as you climb higher, the plane can travel faster than it was during takeoff and low altitudes. While the theory of this is technically correct, some factors actually make the answer be a combination of traveling slower and faster at the same time.
Sounds confusing, right? Well in this article I'll explain the two main factors that determine exactly how fast commercial airliners fly at cruising altitude, and why it matters.
Airspeed
Believe it or not, the airspeed a commercial jet is moving at while on cruise is actually slower than when it is lower to the ground. Why does this happen? Well, it's all about air density. At higher altitudes, air is less dense than it is closer to the ground, so the amount of airflow over the wing needed to generate the same amount of lift at 5,000 feet vs 35,000 feet is much higher.
The average airspeed of a jet at cruising altitude is about 270-280 knots indicated (which is the direct measurement of airflow over the aircraft), while at lower altitudes during climbs and descents, the airspeed is usually around 300-320 knots indicated. Below 10,000 feet aircraft are speed restricted to 250 knots indicated or less, so below that altitude is being ignored for the sake of this topic.
Ground Speed/Mach Number
In addition to the air being thinner at altitude, the margin of safety for maximum airspeed decreases as altitude increases. So flying at 300 knots indicated at 35,000 feet could cause structural damage to the aircraft compared to flying at the same speed at 15,000 feet. This is because the ground speed of the aircraft is a constantly increasing factor with altitude, thanks to the concept of needing more airflow over the wing at altitude due to decreased density. On average, the ground speed of an aircraft flying at 280 knots indicated at 35,000 feet is between 450 knots and 500 knots over the ground. This is where the Mach number comes into play.