The Rise and Fall of the Concorde: An Engineering Marvel's Tragic End

The Concorde, a joint venture between Britain's BAC and France's Aerospatiale, was an anomaly in commercial travel, funded entirely by the British and French governments, totaling 2.8 billion dollars. Despite its commercial failure, it was lauded as a triumph of ingenuity, transforming military-only technology into a luxury flying experience.

Developing a supersonic plane that could also satisfy the demands of wealthy commercial customers presented unique challenges compared to military designs. Unlike military aircraft where fuel efficiency and passenger comfort are secondary, the Concorde's design had to account for these needs, resulting in its sleek exterior and powerful engines, which were developed from the iconic British warplane, the Avro Vulcan.

The Rolls-Royce Olympus 593 engines, when flying at Mach 2, were engineered to be among the most efficient ever created. Initially, these engines, used in the Avro Vulcan, produced 49 kilonewtons of thrust. However, for the Concorde, their thrust was more than tripled to 169 kilonewtons.

A significant portion of this thrust increase, 20%, came from newly incorporated afterburners. This technology, typically found in military planes, works by injecting fuel directly into the superheated, high-pressure exhaust of the turbine section, essentially creating a 'rocket stage' effect that further expands and accelerates the exhaust through the nozzle.

Unlike typical airliner engines with fixed nozzles, which are optimized for a narrow range of subsonic speeds, the Concorde required two controllable nozzles to manage its massive speed range. These nozzles allowed the engine's power to be adequately controlled across various flight conditions.

The primary nozzle, located at the jet pipe's exit, utilized petal-like structures to vary its diameter. When the afterburner was engaged, it caused a significant pressure increase in the exhaust. The primary nozzle would open wider to reduce this pressure and prevent choked flow, ensuring a consistent mass flow rate through the engine. This actuation also influenced the low-pressure spool's rotational speed, optimizing engine performance across its wide speed range.

The secondary nozzle, with its 'eyelid doors,' functioned as a diverging duct to accelerate exhaust to supersonic speeds, forming a controllable converging-diverging nozzle similar to rocket designs. This advanced system enabled the Concorde to carefully optimize its nozzle profile for different flight conditions, representing a remarkable feat of engineering.

No jet engine can operate efficiently with supersonic airflow directly entering it, as shock waves would cause internal chaos. Therefore, the Olympus 593 engine required a system to decelerate supersonic air entering its unique square inlets. These inlets incorporated a series of adjustable ramps and doors designed to create a smooth, laminar subsonic flow for the engine's compressors.

During takeoff, the engines needed maximum air intake to help the delta wings generate sufficient lift. In this phase, the two variable ramps were retracted, a spill door in front of the intake was held open for additional airflow, and two bypass doors (controlling air around the engine's cooling passage) were closed. As the Concorde accelerated to supersonic cruise, the inlet ramps actuated, creating a converging inlet that slowed and compressed the incoming supersonic air, significantly improving the engine's thermal efficiency despite the massive fuel consumption primarily driven by aerodynamic drag.

The Olympus 593 engine achieved an exceptionally high cruising pressure ratio of 80 to 1, significantly surpassing the Boeing 787's GE NX engines, which manage 58 to 1. This elevated pressure ratio, resulting from supersonic air ramming into the unique square inlets, generated extreme internal temperatures.

These temperatures necessitated using high-temperature alloys typically reserved for turbine blades, such as titanium for early compressor stages and nickel alloys for later, hotter stages. Engineers also had to remove a center bearing immediately after the high-pressure compressor due to the intense heat, thickening the drive shaft into a bottle-like shape to maintain stiffness in the new gap between bearings.

The Concorde's ogival delta wings were critical for generating sufficient lift at low speeds, which is typically a challenge for delta-wing aircraft. This compound delta wing design, with its unique curve, created vortex lift at high angles of attack. When flying at these angles, separated airflow would form stable cone-shaped vortices over the wings, where high air speeds and low pressure effectively generated lift.

However, the high angles of attack needed for low-speed operations, such as takeoff and landing, created severe visibility issues for pilots. The unique solution was the 'droop snoot,' a mechanized nose that could drop by 12.5 degrees to provide pilots a clear view of the runway during these maneuvers. The mechanism had four distinct positions, controlled by the pilot, including a fully retracted position for supersonic flights and a 5-degree down position for takeoffs and taxis, along with a specialized heat shield visor for cabin window protection.

To further mitigate risks, an extendable tail landing gear was installed. While not always necessary, it served to protect the plane's engines from potential tail strikes, which would typically be the first point of contact during difficult landings for other airliners.

The Concorde's fuselage experienced significant thermal expansion due to extreme temperature variations between takeoff and supersonic flight, causing the plane to grow by 20 cm on every flight. This constant expansion and contraction introduced thermal stress that, if not properly managed, could lead to catastrophic fatigue failure.

To address this, engineers opted for Hiduminium RR58, an aluminum alloy initially developed during WWII for gas turbine engines, which offered the necessary heat resistance and weight savings over titanium. Interior panels were designed to slide, and wiring and other components were corrugated to accommodate the movement. A visible gap, for instance, would form between the flight engineer's console and the bulkhead at cruising speed, demonstrating the extent of this expansion.

The Concorde's continual expansion and contraction cycles created serious issues for its fuel tank sealants. The aircraft utilized Viton, a high-temperature sealant, but even this material would harden and crack over time due to the extreme thermal stress, leading to fuel leaks.

This degradation necessitated frequent and extensive maintenance. Approximately every 1,100 flying hours, or roughly once a year, the Concorde underwent a three-week maintenance period. Technicians had to manually inspect and replace the sealant in all 13 fuel tanks, a massive job often requiring them to work by touch alone due to limited access, particularly in tanks 5A and 7A on the outermost part of the wing. These tanks were crucial, as they held 119,000 liters of fuel, enabling transatlantic flights.

As the Concorde accelerated, the center of pressure of its wings would shift backward by as much as 1.8 meters, which could cause significant control issues, potentially forcing the plane into a nose dive. Unlike slower airliners that use elevators for pitch stabilization, the Concorde lacked a horizontal tail, and large elevons would have created excessive trim drag at supersonic speeds.

To counteract this, Concorde engineers devised an ingenious system: they used the aircraft's extensive fuel tanks to manage balance. As the plane approached Mach 2, up to 20 tons of fuel would be pumped from the forward trim tanks to the rear trim tanks and collector tanks in the wings. This strategic movement shifted the center of gravity backward, precisely matching the center of pressure shift.

This method maintained the aircraft's balance without introducing additional drag from control surfaces, proving to be an elegant solution that utilized the existing resources onboard the plane.

One of the last remaining Concordes, nicknamed Alpha Bravo, is a static fixture at Heathrow Airport, visible to thousands of passengers. It made its final flight just one month after the tragic Air France 4590 crash.

At the time of the fleet's retirement announcement, Alpha Bravo was undergoing significant interior upgrades. However, these planned modifications, intended to prevent future disasters like Air France 4590, were never completed, leaving the plane in an indefinite state of limbo.

At 2:40 p.m., Air France Flight 4590 began its takeoff sequence from runway 26R. Just 20 seconds later, after the first officer called out 'V1'—the point of no return—a loud crash was heard from the left wing. Flames billowed from the plane, which then crashed moments later into a nearby hotel.

Investigators faced an immense challenge at the scorched debris field southwest of Charles de Gaulle Airport. Initial findings, including clipped trees to the north of the crash site, indicated the plane veered left while heading south. Irregular marks on the concrete and rubber chunks on the runway strongly suggested a left tire failure had occurred.

Flight data revealed an immediate loss of speed and thrust in engine number two, quickly followed by engine number one. The plane struggled to maintain flight before its final, tragic impact.

Investigators discovered a 42 cm long titanium strip with irregular drilled holes on the charred runway, along with damaged rubber tire pieces. This mysterious strip did not match any components of the Concorde, leading them to conclude it was foreign debris present on the runway before Flight 4590's takeoff.

By scrutinizing maintenance and engineering documents for all flights departing from the same runway after 1:00 p.m., investigators traced the debris to a McDonnell Douglas DC-10, servicing a Continental flight, which had departed just five minutes before the Concorde. Maintenance documents for the DC-10 referenced a metallic wear strip and indicated that repairs a month prior were not up to manufacturing standards. Further investigation confirmed the DC-10 was missing a wear strip with matching irregular drilled holes, strongly suggesting it was the source of the runway debris.

A critical piece of evidence was a 32 cm long cut found in the rubber tire debris on the runway. To definitively link the 42 cm metal strip to this damage, investigators conducted a simulation. They built a test wagon, modified to fit a Concorde tire loaded with 25 tons to replicate the aircraft's weight, and tested the impact of the metal strip.

The simulation proved that the strip cut the tire precisely as observed in the crash debris. While this confirmed the cause of the tire burst, it did not immediately explain the subsequent fiery crash. The Concorde had experienced 57 tire malfunctions during its 30-year program, with 22 occurring during takeoff and 19 caused by foreign objects. Although 12 of these resulted in structural damage and six in fuel tank penetrations, none had caused a fire like that of Flight 4590.

Previous incidents, like a 1979 Concorde tire burst from Dulles Airport that punctured three fuel tanks, resulted in a fuel leak rate of 4 kg per second without fire, leading to a report that deemed the risk of fire low due to slow leakage and dispersion. However, Flight 4590 presented a stark contrast.

Investigators quantified the fuel leak rate of Flight 4590 using visual evidence from videos and images of the crash. They estimated the flames trailing behind the wing to be 50 meters long, 3 meters in diameter, and 10 cm thick. This allowed them to calculate a fuel leak rate of 60 kg per second, a figure confirmed by a surviving fuel dial stuck in its final position. This was an astonishing 15 times greater than the previous incident's leak rate.

This massive difference in fuel leak rates indicated a fundamentally different failure mechanism was at play in the Air France 4590 crash.

A crucial piece of evidence found on the runway was a 30 cm by 30 cm piece of tank number five, located directly above the landing gear, which was unburned, indicating it detached before the fire started. A material science expert analyzing the fracture face of this piece noted it was matte and angled at 45 degrees, which is characteristic of static ruptures in ductile materials.

Further analysis indicated that the piece of rubber from the tire burst struck the underside of the wing with sufficient force to deform the fuel tank. Because fuel is incompressible, this impact caused a massive pressure wave inside the tank. This pressure wave then ejected a section of the tank, creating a large hole that resulted in the catastrophic fuel leak observed.

The sequence of events concluded: a poorly positioned metal strip caused a tire to burst, which then slammed against the underside of the wing. This impact generated a massive pressure wave inside the fuel tank, rupturing it and leading to a catastrophic loss of power, ultimately causing the plane's uncontrollable descent.

The Air France 4590 crash ignited a decade-long legal battle between Air France and Continental, culminating in a 2010 court ruling that fined Continental 1 million euros. In response to the disaster, safety modifications were proposed and implemented, including adding Kevlar liners to the fuel tanks to prevent future static ruptures, relocating and reinforcing potential electric ignition sources, and introducing new damage-resilient tires.

However, these significant modifications came too late for the financially struggling Concorde. The crash, coupled with already high ticket prices and plummeting demand, made the supersonic jet financially unviable. Ultimately, this proved to be the final nail in the coffin, leading both British Airways and Air France to cancel their supersonic flying programs and marking the end of the Concorde era.