While a plane may be traveling at a constant speed in terms of its Mach number (which represents the ratio of the aircraft's speed to the speed of sound), the actual airspeed of the plane can vary due to several factors. Here are a few reasons for different speeds in a plane, even when it is traveling at a constant Mach number:
Altitude and Air Density: As an aircraft climbs to higher altitudes, the air density decreases. To maintain the same Mach number, the plane needs to increase its true airspeed to compensate for the thinner air.
Wind: The presence of wind can affect the groundspeed of an aircraft. Groundspeed is the actual speed of the plane relative to the ground. If the plane is flying with a headwind, it will have a higher groundspeed compared to its airspeed. Conversely, a tailwind will result in a lower groundspeed.
Temperature: Temperature can influence the speed of sound. As the temperature decreases, the speed of sound also decreases. Therefore, to maintain a constant Mach number, the plane would need to reduce its true airspeed in colder temperatures.
Aircraft Configuration: The speed of an aircraft can be influenced by its configuration, such as the deployment of flaps or landing gear. When the plane's configuration changes, its lift and drag characteristics alter, potentially requiring adjustments to the throttle or airspeed.
Air Traffic Control Instructions: Air traffic control may provide instructions to the pilots, which can result in variations in speed. For example, the pilots might be asked to increase or decrease their speed for spacing between aircraft or for traffic management purposes.
In summary, while a plane may maintain a constant Mach number, variations in altitude, air density, wind, temperature, aircraft configuration, and air traffic control instructions can cause differences in the plane's actual airspeed and groundspeed.