NASHUA, N.H. — Look at the flight map on the seatback screen and there's a good chance the altitude will settle somewhere around 35,000 feet. Another flight might cruise at 33,000 feet or climb above 40,000, but commercial jets spend much of their time in a generally narrow band of the atmosphere.

Flying high makes sense. The air gets thinner with altitude, which lessens aerodynamic drag and can help an aircraft cover more distance for the fuel it burns. If thinner air is better, though, why stop around 35,000 feet?

The answer comes down to a balance. An airplane still needs enough air to generate lift, its engines still need to produce thrust and the aircraft has to maintain enough performance to operate safely. Eventually, climbing higher starts taking away some of the advantages that made altitude attractive in the first place. NASA describes cruise efficiency as a function of the aircraft, its engines and operating conditions rather than a single ideal altitude that works for every flight.

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Thin air helps an airplane until it doesn't

An aircraft moving forward has to push its way through the atmosphere, but climbing changes the equation. As air density decreases, so does some of the drag acting on the aircraft. That makes high-altitude cruise particularly attractive for jet-powered airplanes, which can travel efficiently through the thinner air.

The wings, however, generate lift by moving through air. Thinner air means fewer air molecules are available to produce the aerodynamic forces that keep the aircraft aloft. The airplane can compensate in part through speed and angle of attack, but it can't keep climbing indefinitely while expecting the same performance.

Engines face their own limits as atmospheric pressure and density decrease. Together, those aerodynamic and propulsion constraints create an operating range where the aircraft can take advantage of thinner air without giving up too much performance.

That's why 35,000 feet isn't a magic number. Many commercial jets operate across a broader range of cruise altitudes depending on the aircraft and conditions.

The best altitude changes with the weight of the airplane

Even for the same aircraft, the ideal cruise altitude doesn't stay fixed throughout a flight. A vehicle begins a long trip carrying thousands of pounds of fuel that it will gradually burn along the way. Early in the flight, that weight can make a higher altitude less efficient or leave the aircraft without enough performance margin to climb there comfortably.

Several hours later, the situation has changed. The plane weighs less because much of that fuel is gone, and an altitude that wasn't attractive after takeoff may now offer better cruise efficiency. NASA describes optimum cruise altitude as “closely tied” to aircraft weight. As fuel burns and the airplane becomes lighter, the altitude that offers the best performance generally moves higher. Rather than continuously climbing as the airplane gets lighter, crews can make the change in stages.

Step climbs let the aircraft move higher along the way

On a long flight, passengers may notice the aircraft rise even though it reached cruising altitude hours earlier. That can be a deliberate move known as a step climb.

The vehicle begins at an altitude appropriate for its initial weight. After it burns enough fuel, it can climb to another flight level that better matches its lighter weight. The process can happen again later in the trip if another altitude becomes more favorable.

The most efficient altitude on paper isn't necessarily the altitude the airplane will actually get, though. Air traffic control has to maintain separation among aircraft, and crews may have to remain at a different flight level because other traffic occupies the one they want. Weather and winds can also make another altitude more attractive.

That means cruise altitude isn't simply a number calculated once before departure. Dispatchers and flight crews can consider aircraft weight, winds, weather and traffic as the flight progresses.

Going higher leaves less room for trouble

Aircraft also need performance margin. At high altitude, the airplane has less excess performance available than it would closer to the ground. The engines have less dense air to work with, while the wings still have to generate enough lift to support the plane.

Every aircraft has an upper operating limit, but crews don't normally treat that number as a cruise target. The useful question is not how high the airplane can fly. It's how high it can fly efficiently while maintaining the performance required for the conditions at that point in the trip. That altitude can move as weight, temperature and winds change.

Sometimes the fastest route isn't the highest one

Altitude isn't determined by aircraft performance alone. Winds vary with altitude, and a higher flight level may place an aircraft in a stronger headwind that wipes out some of the efficiency gained by climbing.

Weather can push a flight away from its preferred altitude as well. Crews may request a different level to avoid turbulence or other conditions, while air traffic can prevent a climb until enough space becomes available.

The flight management system and airline dispatchers can account for many of those variables when planning and adjusting the route. The result is an altitude chosen for the flight taking place at that moment, not an altitude that is universally best for every airplane. NASA research has found that the optimum depends partly on aircraft weight and the prevailing wind profile.

So when the seatback map shows 35,000 feet, the aircraft isn't sitting at a predetermined sweet spot simply because commercial airplanes belong there. It has reached a compromise between aerodynamic efficiency, engine performance, aircraft weight and the atmosphere around it.

Later in the flight, that compromise may move a few thousand feet higher.