How Fast Do Passenger Planes Travel and What Affects Their Speed?

When considering the speed of passenger planes, one might wonder if these aircraft fly as fast as they do in movies or TV shows. In reality, the typical range of a passenger plane's speed is 450 to 550 knots. However, certain aircraft types can achieve slightly higher speeds. For instance, the iconic Boeing 747 is known to fly a bit faster within this range. But when it comes to understanding what really determines the speed of a plane, a completely different context needs to be considered.

Understanding the Concept of Air Speed Indicators (ASI)

Let's delve into a little piece of information that might surprise some enthusiasts. Commercial jets, while impressive in size and capability, are not significantly faster than single-engine private airplanes in terms of their ASI (Air Speed Indicator) readings. Generally, small private piston engine airplanes can cruise at speeds exceeding 200 mph, which is comparable to the ASI readings of a commercial jet, typically around 260 mph.

The Impact of High Altitude

The real difference lies in the altitude at which these planes operate. Commercial jets fly at very high altitudes, often around 37,000 feet. At this elevation, the air is much thinner, and to achieve the same pressure speed as a slower plane, the commercial jet must move much faster. This results in a much higher true air speed (TAS) of around 591 mph, compared to the ASI reading of 260 mph. In contrast, piston-engine aircraft typically don't fly as high because they do not have pressurized cabins. Without pressurization, these planes require supplemental oxygen above 12,500 feet, an added and inconvenient expense for most private pilots.

Power and Altitude

Another significant factor that limits the altitude of piston-engine aircraft is the power output of these engines. Without turbocharging or similar enhancements, piston-powered planes can only maintain a certain altitude before the thinner air reduces engine performance. As a result, they typically cruise around the mid-teens in altitude, far below the cruising altitude of commercial jets.

The Common Cruise Speed of Commercial Jets

Commercial airliners today cruise at around Mach 0.79 to save fuel, which equates to approximately 595 mph or 520 knots at 35,000 feet. This speed is well within the limits set by VMO (Maximum Operating Velocity) and MMO (Maximum Operating Mach Number). The VMO for commercial airliners is usually around 593 mph at 35,000 feet, while the MMO is around 515 knots. The true air speed at these altitudes is approximately 954 km/h or 1,030 km/h.

Private Jets and Military Cargo Jets

Private jets generally travel faster than commercial airliners, often reaching speeds around Mach 0.90 or 593 mph at 35,000 feet. On the other hand, military cargo jets tend to be slower, typically cruising around Mach 0.85 or 567 mph. These differing speeds reflect the varying purposes and requirements of these aircraft.

In conclusion, while passenger planes like the Boeing 747 can achieve impressive speeds, the true capabilities of these aircraft are often influenced by their cruising altitude. The air speed indicators provide a clear measure of the plane's speed, but true air speed is a critical factor that separates high-altitude commercial airliners from piston-engine private aircraft.