AirSpaceMag.com

Back in the 1990s, I spied a piece of burned paper in a dusty Illinois junk shop. For 20 bucks, I purchased a singed envelope that had been placed in a mailbox in 1919, postmarked in New York, and sent by airmail to Sutiff & Case Co., in Peoria. The letter was never delivered. Also inside the cheap wooden frame — with a broken string attached, indicating that it once hung on a wall — was an apologetic letter dated May 28, 1919, from W.B Carlile, the Chicago postmaster:

The enclosed piece of mail matter was damaged in the aero-plane accident at Cleveland, Ohio, May 25th.
The incident is very much regretted.

When those papers were first displayed, American airmail service was little more than a year old — the U.S. Army had begun flying regularly scheduled flights on May 15, 1918, with deliveries between Washington, Philadelphia, and New York. In August, the Army turned the operation over to the Postal Service. The accompanying newspaper clipping described the crash of Frank McCusker, who was flying the newly inaugurated New York-to-Cleveland route, and was the second Postal Service air pilot to perish when his de Havilland DH-4 caught fire after taking off from Cleveland. The doomed and parachute-less pilot jumped to his death.

Those seminal years were extremely dangerous for airmail pilots — flying surplus aircraft, with little more than a sight of a river or road for navigation. Some were veterans of the Great War; others were barely out of flight school. Between 1919 and 1926, 35 pilots died hauling Christmas missives, overdue bills, and humdrum human communication now delivered with a mouse click.

Indeed, by 1975, the term “air mail” would be dropped from Postal Service lexicon; today nearly every letter travels most of its journey as unremarkable baggage beneath the feet of thousands of bored flyers dozing toward their hurried connection in Atlanta or Dallas. And yet, it’s hard to look at that charred letter and not think of the bravery of Frank McCusker as he climbed into an open cockpit 94 years ago, daring the clouds that lay ahead.

Ever since my first trip with my Dad, who worked for National Airlines and Pan American, I’ve always loved when the engines throttle up and the runway lights start to track by. I don’t understand dropping the shade for a lousy movie or snoozing in the aisle seat. A window, always.

See that faint, upside-down "Y" running vertically down the middle of the photo?

A few times I’ve seen what I believe to be a shock wave forming on the wing of the jet I was traveling on. Ernst Mach told us that at sea level, the speed of sound — Mach one, or the ratio of the speed of sound at a given altitude and the speed of the vehicle — is around 750 miles per hour. The outside air temperature, or OAT, is important here (which I also remember being a factor during test engine runs as a crew chief) because the density of air at a given altitude affects the speed of sound.

Recently, during an American flight from Atlanta, I saw what I thought was a shock wave dancing along the upper surface of the wing; it rode back and forth, divided, re-combined, and stayed visible until we began to descend. I even got a picture.

Based on a quick, and quite possibly faulty, Internet search, my best guess at the speed of sound at our announced altitude — 39,000 feet — is about 660 mph, or 574 knots. The cruising speed of the latest 737s (Next Generation) is 514 mph, with a max speed of 544 mph. Theoretically, the difference between the speed of sound at that altitude and the speed of our jet was as low as 116 mph — a few feet outside the window of a near-sighted airplane geek with his tray table down in seat 15A.

I checked with Gordon Leishman, a professor of aerospace engineering at the University of Maryland in College Park, who emailed back:

Yes, what you have is a photo of a shock wave (well, it is the shadow of the shockwave cast onto the wing). So, we call such images shadowgraphs or shadowgrams. Seen it many times, have many photos, generations of my students have taken photos, etc. Science from your airplane window! Of course, we also use the technique in the laboratory.

Commercial jet aircraft cruise at transonic speeds (Mach 0.8 to 0.86), so there is generally always a shock or a series of bifurcated (i.e., the upside down “y”) and interconnected shocks) over the upper surface of the wing. On early jets, the shocks were nearer to the leading edge (727 is a good example), but on newer generations of aircraft with their better transonic airfoils, you will see the shocks further aft on the wing chord (777). You can also sometimes see shocks on the engine nacelle or between the nacelle and the fuselage. If the lighting is right, you may even see the optical distortion of the shock extending well off the wing surface.

Getting a good photo of the shocks is all about lighting (in this case where the sun is relative to the aircraft) so that the light rays refract and cast a shadow of the shocks on the wing. Often it is luck. In this case you can clearly see the bifurcated shock pattern as the flow over the wing interferes with the fuselage flow.

Whew. And, I thought what I saw was just due to the free drink coupons!