British de Havilland Comet, the First Commercial Jet Aircraft in Aviation History
and Why Airplanes Don’t Have Square Windows
On May 2, 1952, the first commercial jet flight in history departed London for Johannesburg, South Africa, a British Overseas Airways Corporation aircraft.
On May 2, 1953, one year to the day after the maiden flight of the British-made de Havilland Comet, aircraft G-ALYV departed Calcutta Airport for Delhi as BOAC Flight 783. A few miles out of the airport, the flight encountered a severe thunderstorm. While the pilot and air traffic control were both aware of it, the storm did not appear severe enough to restrict flight through it. Furthermore, the captain was well-qualified, had considerable experience on this route, and had experience in similar weather conditions. Just six minutes after take off, while climbing to 7,500 feet, radio communication was lost. About this same time, witnesses at various ground locations saw “an aircraft coming down in a blaze of fire through severe thunderstorm and rain” and then crash into the ground. All 37 passengers and six crew members were killed.
The most notable lesson learned from the Comet disaster is that viewing windows are no longer designed square but with rounded edges to reduce any stress concentrations. Another immediate lessons is that crack-stoppers are now placed between frame-cutouts that take the shape of circumferential stiffeners that break-up the fuselage into multiple sections and thus prevent the crack from propagating from one window to the next. Most importantly however, before and during the Comet era the aircraft design philosophy was predominantly SAFE-LIFE, which means that the structure was designed to sustain the required fatigue life with no initial damage and no accumulation of damage during service e.g. cracking (1). The Comet accidents showed that around stress concentration cracks would initiate and propagate much earlier than expected, such that safety could not be universally guaranteed in the SAFE-LIFE approach without uneconomically short aircraft service lives.
For this reason the FAIL-SAFE design philosophy was developed in the late 1950’s. All materials are assumed to contain a finite initial defect size before entering service that may grow due to fatigue loading in-service. The aircraft structure is thus designed to sustain structural damage without compromising safety up to a critical damage size that can be easily detected by visual inspection between flights. All inspections are coupled with crack propagation calculations that guarantee that an observed crack is not susceptible to grow to the critical size between two inspection cycles, in which case adequate repair is performed. Furthermore, the structure is designed to be damage tolerant with multiple load paths and built-in redundancies that impart residual strength to the aircraft in case the primary structure is compromised in-service.
A couple of weeks ago, I turned 65 and it is still astonishing to me the entire history of commercial jet aviation has taken place in my relatively short lifetime. The first flight took off just 85 days before I was born. Nine years later in 1961, Shannon and I took our first flight on a jet to Dallas to visit our uncle and attend the grand opening of Six Flags Over Texas.
From that one single jet flight leaving London in 1952 to over 100,000 flights each day around the world.