Every 50 Seats

You will never see a plane with 51 seats. Why? The FAA requires one flight attendant per 50 seats. That 51st seat can generate 2% more revenue (on a sold out flight) but requires 100% more flight attendant cost (on every flight). 52 seats offers 4% more revenue, 53 6% more. How many seats does it take to make enough money to justify a second flight attendant?

You’re looking at a histogram of number of planes with a number of seats. The horizontal axis is number of seats on a plane; the vertical axis the number of commercial planes in the US with that number of seats. I collected this data from Wikipedia (e.g., American Airlines Fleet) into a Google Spreadsheet. For instance: the “40” column goes up to 116. There are 116 planes with 31-40 seats. What does this graph (detailed, interactive version) tell us?

There are ten times as many planes with 41-50 seats (1112) as there are with 31-40 seats (116). Once you’re paying a flight attendant, you want to get your money’s worth.

There are no planes with 51-60 seats. In fact, the next plane has 64 seats. Without talking to accountants, we know it takes 14 seats of revenue to offset the cost of one flight attendant.

The same cliff happens at 100 seats and 150 seats. The effect weakens at 200 seats and beyond because of first and business class. Premium service requires more flight attendants, so the FAA requirements are less demanding than first class customers.

Popular planes make their own peaks. The 140 bump is due to Southwest’s 587 (!) identical 737-300s. The bump at 180 and 190 is the venerable 757 (it’s split over two values because different airlines put in different size first class cabins).

Ahead of its Time: the de Havilland Comet

The de Havilland Comet was the Neanderthal Icarus of jetliners. “Neanderthal” in that it came first, but rooted no family trees. “Icarus” because it flew too high and failed.

The Comet was the first jetliner. It first flew in 1949 and carried paying passengers in 1952. It looks modern enough (all-metal body, swept wings, pressurized cabin) but for the engines and the windows. It has 4 (instead of 2), small (instead of large) jets mounted inside the wings (instead of podded) and rectangular (instead of rounded) windows.

Those windows were the Achilles Heel of the wax of its Icaru-- Wait. Too many metaphors. More simply: you’ve never head of the Comet because it windows were critically flawed. Pressurization pushed each window out with a ton of pressure (16.6 inches wide * 14 inches high * 8.25 psi). The window’s glass held, but passed the weight to the metal frame unevenly: the square shape pushed much more at the corners. Like bending a paper clip back and forth, it fatigued the frame until even a small force could break the metal.

Two Comets blew apart in mid-air in 1954. Authorities blamed sabotage, until forensic evidence proved the planes failed in the same way. Very smart people figured out what I described above and De Havilland spent 4 years reengineering the Comet. On October 4th, 1958, a Comet operated the first London-New York passenger jet flight. The Comet’s monopoly didn’t last long: later that month, Pan Am flew the same route with the larger, faster, more efficient, better looking, more popular 707. The Comet’s biggest contribution to aviation is a stark reminder that, at 30,000 feet, little details matter.