No. Most flights over a few hundred miles attain their optimum cruising altitude. It may be 34,000 to 38,000 ft, depending on plane type, weight, and weather. (Tailwinds are great for gas mileage.)

In 2026, the cruising altitude of a flight is determined more by the aircraft type, weight, and distance than by whether the flight crosses an international border. However, international flights often utilize larger, wide-body aircraft (like the Boeing 787 or Airbus A350) designed for long-haul travel. These planes are engineered to fly at higher altitudes—typically between 35,000 and 43,000 feet—to take advantage of thinner air, which reduces drag and improves fuel efficiency. Shorter domestic flights often use smaller narrow-body planes that may cruise slightly lower, around 30,000 to 35,000 feet, because they don't always need to climb to maximum altitude for a brief journey. Additionally, "step climbing" is common on long international hauls: as the plane burns massive amounts of fuel and becomes lighter, it can safely climb to higher, more efficient altitudes. So while there is no "law" that international flights must go higher, the physics of long-distance travel usually results in them reaching the upper limits of commercial airspace.