No, Your Flight Didn't Go Supersonic. That Was Just Really Fast Wind.

In August 1961, a newly-built Douglas DC-8-43 passenger airliner climbed to over 52,000 feet over Edwards Air Force Base in California, during a routine certification test flight. The crew nosed the plane over, and in a controlled dive through 41,000 feet, the four-engine jet broke the sound barrier, reaching a speed of Mach 1.012.

Until the Mach 2 Concorde entered service in the mid-1970s, that was the only documented example of a plane intended for passenger service exceeding Mach 1, or the speed of sound, which is about 760 mph at sea level.

But lately, social media has been lighting up with excited reports that Boeing 787s have been pushing close to 800 mph. It's happened before, but this week a Virgin Atlantic flight was clocked over 800: precisely at 801 mph, over Pennsylvania. That's supersonic, right? Well, no.

When we see reports of a plane reaching over 800 mph or other similarly humongous speeds, well above the usual speeds around 550 mph, that’s the aircraft’s groundspeed. But planes are surrounded by moving air, and they fly inside of it. So the speed that really matters to pilots is known as airspeed, or how fast the plane is moving relative to the air around it. That's the number that counts when going supersonic, for example.

So when the otherwise tame jetliner taking you to London is doing a blistering 800 mph, it means that, just like a boat in a river’s current, it has hitched a ride in a fast-moving stratospheric jetstream. That's the wind that blows west to east at high altitudes, and can make eastbound flights much faster than in the opposite direction.

So, you may be flying over the ground at 800 mph, but within the jetstream, the plane’s airspeed is still Mach 0.85. You'll get to London faster, sure, because you're moving faster over the ground. But in airspeed, you're still quite subsonic.

A tale of two cargo flights seen over the Atlantic on Wednesday helps understand the concept. At the top in the image below is an MD-11 tri-jet flying from Philadelphia to Cologne, Germany. The label tells us that it's at Flight Level 350, or 35,000 feet, and going at 550 knots over the ground (well, over the sea, but you get the point.) At the bottom, an Etihad flight — a Boeing 777 freighter bound to Miami from Zaragoza, Spain at 34,000 feet — is doing only 457 knots despite having similar speed capabilities and being in pretty much the same patch of sky as the other plane. The 777 is facing a jetstream going at about 100 knots, and the MD-11 is being pushed by it. They have the same airspeed, but different groundspeeds.

Modern airliner flight decks show groundspeed and airspeed next to each other, as well as the wind's speed and direction. Check out the image below, showing the navigation display of an airliner photographed recently above the US and headed almost due west, on a magnetic heading for 254 degrees on the compass (you can see HDG 254 MAG at the top of the left-hand screen.) Now focus on the red arrow, added in Photoshop, in the upper left corner. That “260° / 54” means that the wind is coming from almost dead ahead, 260 degrees on the compass, at a speed of 54 knots, or 62 mph. Above the wind indication, you’ll see the airplane’s speeds. Relative to the surrounding air (true airspeed, or TAS) the plane is moving at 480 knots; but relative to the ground below (GS, or ground speed) it’s doing just 426 knots, or 490 mph. The difference between those two speeds is exactly equal to the speed of the wind, 54 knots. If you point the plane the other way, the headwind becomes a tailwind, and groundspeed will be faster — by those 54 knots.

What about that DC-8 from 60 years ago, though? That one really went supersonic, in airspeed, not just fake-supersonic in groundspeed. It managed to do it in a dive, and thanks to peculiarities of design and aerodynamics. In level flight, the plane normally cruised at Mach 0.82, or about 550 mph. The cruise speed of commercial planes hasn’t changed much since then: the newest Boeing 787s and Airbus A350s fly between Mach 0.85 and 0.89. Most jets can push it a bit faster, but at huge increases in fuel consumption.

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That’s because all modern jetliners have been designed to meet a specific airline mission that’s a balance between speed, range and payload. With today’s aerodynamic and technological advances, we’re going further than we used to on less fuel, but not a whole lot faster than 50 years ago at the dawn of the jet age.

There are some real speedsters among business jets, such as Cessna, Gulfstream and Bombardier planes that can routinely cruise above Mach 0.90. And looking like a cross between Concorde and a jet fighter, the proposed Aerion eight-seat supersonic bizjet is targeting Mach 1.4. That would be the first civilian supersonic since the late, lamented Concorde.

Could a 787 break Mach 1, though, in exceptional circumstances?

In a controlled test flight, if a 787 crew followed a similar flight profile as that DC-8 and used a dive to try to push through the sound barrier, perhaps. But in normal cruise flight, with passengers onboard? Nope. Because the Dreamliner is not designed to fly supersonically, the pilots will not exceed the performance guidelines set by the manufacturer. In other words, the Dreamliner is designed to fly at Mach 0.85, and that's exactly what it will do. To break the speed of sound, the aircraft would have to fly at about 574 knots of true airspeed, at an altitude of 35,000 feet.

What you can do, while you wait for supersonic commercial flight to become a thing again, is look at the flight data on the entertainment display in front of you. It'll always show you the groundspeed, and when flying eastbound in winter, when the jetstream's stronger, those numbers can be pretty crazy. For example, on a recent flight over the Pacific on Philippine Airlines, I witnessed what might well be my personal (ground)speed record: 731 mph, thanks to the December jetstream at 39,000 feet.

There's even a whole site dedicated to crazy groundspeeds, called, as you guessed, Groundspeed Records. If you're asking yourself what's the fastest jet on there, that is a bit of an unfair question, since it includes jet fighters too — like a US Air Force F-16 doing 1,120 knots, or 1,288 mph. And you just can't do that in a 787, jetstream or not.

Howard Slutsken and Mike Arnot contributed reporting for this story.