On Friday a flight-enthusiast passenger noticed that a Spirit Airlines (SAVE) plane that had taken off a few spots ahead of his United plane (UAL) from the Houston airport was now well behind him, according to a flight tracker.
The passenger, who is also a licensed flight dispatcher, guessed that the Spirit flight was likely flying slower to save money, like a frugal driver on the highway. (Fuel economy usually decreases at 50mph according to the DOE.)
This interesting observation exposed an open secret in the world of airlines and flights: They don't all fly at the same speed. Two identical itineraries with identical weather conditions can have two different ETAs.
Despite the occasional pilot informing passengers (proudly) of an intention to “make up time in the air,” people usually assume planes are going as fast as they safely can, like a speed limit, similar to a bus or a train. Even Scott Keyes, the deal-finding guru behind Scott’s Cheap Flights, tweeted his surprise at this fact.
Bernstein airline analyst Daniel Roeska told Yahoo Finance that these speeds all depend on the specific airline and its goals. Some airlines like Ryanair and Wizz “typically fly at the optimum speed which minimizes en route fuel burn,” he said.
“Network airlines [non-low cost carriers] sometimes fly faster to enable a higher productivity (given that their planes are on the ground a long time),” he said. “But it’s all policy: if you fly faster, you can cram more flights (revenues) into your day, if you fly slower — fewer flights and lower fuel cost.”
In other words, speed is dictated by the airline’s goals, be they on time performance, cramming flights in, or saving money on fuel — things that are all in flux depending on the airline, the day, and the situation.
“The differences are not huge,” Roeska said, usually something like 466 mph to 547 mph, which is a difference of covering 81 more miles in an hour.
A lot of factors in play
Airplane physics complicate the speed equation further, longtime airline analyst Robert W. Mann told Yahoo Finance.
Evaluating the Spirit and United situation, Mann said that usually airplanes looking to make up speed will fly lower, “but in this case, it could be due to the United aircraft operating at a lower weight (fewer passengers/less cargo and fuel onboard) than the Spirit aircraft.”
This would make the Spirit plane slower to climb with a slower cruising speed, he said.
“Airlines still try to save fuel when they can,” he said “The physics of flight, generally, is that flying faster and heavier at any given altitude requires greater fuel flow.”
Altitude is tricky since it’s often a product of weight, and Mann explained that long-haul flights often have to “step climb,” which is when a plane will slowly climb as it burns off fuel, because the fuel itself represents a significant weight cost.
“They can't reach the final cruising altitude at their initial departure weight,” he said, noting that in the future, electric planes will re-write the long-haul playbook, since a battery doesn’t burn off fuel and become lighter, creating a challenge for electric propulsion.