Design Origins
Although the short-range, low-capacity segment of the market was saturated with twin-jet designs, inclusive of the Sud-Aviation SE.210 Caravelle, the British Aircraft Corporation BAC-111, the McDonnell-Douglas DC-9, the Boeing 737, and the Fokker F.28 Fellowship, Dassault-Breguet believed that a larger twin, accommodating at least 150 passengers and operating over sectors no longer than 932 miles, was needed to fill the gap between these other designs and the 336-passenger Airbus A300.
Market studies conducted in 1968 indicated the need for as many as 1,500 aircraft in this category during the next nine years. Of the existing types, only the Caravelle 12 and the DC-9-50 accommodated 130 single-class passengers in high-density configurations, while the others carried considerably fewer. The BAC 1-11-200, for instance, had a capacity of only 89, and the F.28, even in its ultimate stretch as the F.28-4000, accommodated five fewer. Dassault-Breguet’s intended airliner was seen as an early Airbus, economically operating short-range, high-density routes, such as those between France’s major population centers.
Unlike four of these five other twin-jets, its own configuration, resulting from several sizes and design solutions, was a conventional low-wing monoplane with a fuselage wide enough, like that of the 737, for six-abreast coach seating, two pylon-mounted engines, and an equally conventional tailplane. Its overall appearance, in fact, was very much a mirror image of that 737, except that its turbofans were hung lower and ahead of the wing, as opposed to Boeing’s underside attachment arrangement, and its fuselage was longer to permit six additional six-abreast seat rows to be installed.
Project go-ahead was contingent upon receipt of at least 50 firm orders for the eventually named “Mercure,” or “Mercury,” but only Air Inter, the French domestic carrier, committed itself to ten on 29 January 1972. So convinced was Dassault-Breguet of the integrity of its design and the world market’s need for it that it launched the program on the basis of a single airline order.
The French government covered 56 percent of the launch costs, with 14 percent borne by Dassault-Breguet itself and the remainder by its five risk-sharing partners, which included Aeritalia of Italy, Canadair of Canada, CASA of Spain, the Eidgenössisches Flugzeugwerk of Switzerland, and SABCA of Belgium. Each was responsible for parts and systems manufacture: Aeritalia, the tail unit and the tailcone; Canadair, the wing panels, the leading edge, the flap tracks, and the engine pylons; CASA, the first and second fuselage sections; Eidgenossisches Flugzeugwerk, the engine air intakes and the cowling panels; and SABCA, the flaps, the spoilers, the ailerons, and the airbrakes. Final assembly would take place at the Bordeaux-Merignac production plant in Southern France.
Mercure Design Features
The Mercure 100 in its initial—and, in the event, only—version featured an all-metal, five-section, semi-monocoque fuselage of circular cross-section wide enough for six-abreast coach seating and measuring 112.11 feet in length, giving the aircraft an overall length of 114 feet 3.5 inches. Of the five other low-capacity, short-range twin-jets, only the Boeing 737 offered sufficient width for this configuration.
The airframe was designed for 30,000 flight cycles and 40,000 airborne hours.
The low-set wings, whose two-spar torsion boxes were comprised of a continuous spar, eight skin panels, stiffeners, and machined ribs, were equipped with three-section out- and downward-extending leading edge slats (as opposed to the five fitted to the prototype), two-section triple-slotted trailing edge flaps, an aileron, three spoiler panels, and two airbrakes. All were hydraulically operated by dual actuators, which were themselves fed by three independent circuits.
The spoilers, interrupting lift upon main-wheel spin-up, operated in conjunction with the aileron in flight to provide lateral control.
The 100.3-foot wingspan, resulting in a 1,250-square-foot area, featured a 25-degree sweepback, producing a 96.25 pounds-per-square-foot loading. Other wing surface areas included the trailing edge flaps (261.6 square feet), the ailerons (45.2 square feet), the spoilers (49.5 square feet), and the speed brakes (36.6 square feet).
Wing leading edges were deiced by means of engine bleed air.
The conventional tailplane, again similar to the 737’s, consisted of two-spar horizontal and three-spar vertical surfaces, resulting in a 37-foot, 3.25-inch overall aircraft height. The former, mounted with three degrees of dihedral and of the variable-incidence type, had a 41.11-foot span and a 257.3-square-foot area with 86.1 square feet of hinged, hydraulically actuated elevators. The 166.3-square-foot fin was provisioned with a 64.1-square-foot, hydraulically-operated, dual-section rudder.
Power was provided by two nacelle-encased, sound absorption-treated, thrust reverser-equipped, 15,500 thrust-pound Pratt and Whitney JT8D-15 low (1:1) bypass ratio turbofans pylon-mounted to the wing leading edge underside. Unlike the direct-wing underside arrangement of the 737, the configuration placed the engine below and ahead of the airfoil, necessitating a longer undercarriage strut. Several versions of the engine powered the Boeing 727, higher-capacity Caravelle variants, the DC-9, and the 737 itself.
Fuel, whose capacity was 4,860 US gallons, was introduced at the starboard wing’s outer leading edge, with additional points on the actual surface. Oil capacity was 11.9 US gallons.
The Mercure 100 rested on a hydraulically-operated, twin-wheeled undercarriage consisting of Kleber-Colombes tires, Messier-Hispano wheels, oleo-pneumatic shock absorbers, Messier-Hispano brakes, and antiskid units. The nose gear, with a tire size of 30 x 8.8 and a pressure of 123 psi, was steerable and retracted forward. The main gear, whose tire size was 46 x 16 and whose pressure was 141 psi, retracted laterally into the wing and fuselage wheel well fairings. The undercarriage could be manually extended in the event of a hydraulic failure.
Nose wheel steering and track enabled the aircraft to be turned 180 degrees on the ground on a 70-foot-wide surface.
Entry was provided by 71-by-33-inch forward and aft port-side doors, the first of which was standardly equipped with an extendible, lighted, handrail-provisioned airstair, and the second of which could be optionally equipped with one. The forward and aft, starboard-side galley-servicing counterparts measured 71-by-33- and 65-by-33-inches, respectively. Four 39.4-by 20-inch overwing emergency exits, two on each side, facilitated egress.
Although the Mercure 100 was standardly operated by a two-person crew, two additional observer seats were provided. Vision was through two forward and four side windows, as well as two eyebrow panes. Incorporating technology designed for Dassault-Breguet’s famous fighters, it was provisioned with a head-up display, reducing the time between sky scanning and instrument monitoring for more immediate control and corrective action, if required.
Three 3,000-psi independent hydraulic systems powered the primary and secondary flight controls. The first two, driven by Abex engine-driven pumps, powered the leading-edge slats, trailing-edge flaps, spoilers, tailplane, nose-wheel steering, undercarriage, and brakes. The third, powered by a Vickers electrically driven pump, provided backup power to the ailerons, elevators, and rudder.
The 83.7-foot-long, 11.11-foot-wide, and 7-foot-2.75-inch-high cabin, with an 882-square-foot floor area and 5,717-cubic-foot volume, was both wider and longer than that of any twin-jet up to that time and thus facilitated numerous class divisions, seat pitches, and seat densities. This flexibility was further enhanced by its rectangular, closely spaced, polarized, light-filtering passenger windows and the four forward and aft access doors.
Cabin Layout and Passenger Configuration
Originally outfitted with a wide cabin look, it featured a flat ceiling with recessed, diffused lighting; individual, upward-opening overhead storage compartments for carry-on luggage; and paneled sidewalls. Galleys, lavatories, and garment closets could be installed in 11 locations. The galleys themselves could be simply or extensively provisioned with ovens, refrigeration units, coffee makers, storage compartments, counters, waste receptacles, and meal and beverage carts, and could serve anything from an aperitif to a hot meal appropriate to the time of day.
Installation and the number of units included the forward, right side (two units), the forward, left side behind the passenger door (two units), the aft, right side ahead of the servicing door (two units), the aft, left side ahead of the passenger door (one unit), and the aft, right side behind the servicing door (one unit).
Lavatories, which standardly consisted of a lockable door, a chrome sink and counter, dispensers for tissues, cups, and toilet paper, a lighted mirror, and a recirculating toilet, could equally be installed in a number of locations, including on the forward, right side behind the servicing door; on the forward, left side behind the passenger door; on the aft, left side before the passenger door; on the aft, left side behind the passenger door; and on the aft, right side behind the servicing door.
Garment storage closets could also be installed in seven forward and aft locations.
Seating densities, configurations, upholstery, color, and design, like galley, lavatory, and garment closet locations, varied according to customer specification, but each seat itself was provisioned with a seatbelt, an ashtray, a pull-down tray table, and a literature pocket. Each also reclined.
A dual-class, 120-passenger arrangement entailed 12 four-abreast first-class seats at a 38-inch pitch, their pairs spanning 54 inches in width and thus permitting a 31-inch aisle, and 108 six-abreast coach ones at a 34-inch pitch, their triples spanning 61 inches and the resultant aisle measuring 17 inches in width.
A dual-class, 132-passenger configuration consisted of 12 first-class seats at a 38-inch pitch and 120 at a 34-inch pitch, but the additional capacity was achieved by relocating or removing galley, lavatory, and/or garment storage closet facilities.
An international, single-class arrangement entailed 135 to 140 six-abreast seats at a 34-inch pitch, while its single-class domestic counterpart increased the capacity to 150 at a 32-inch pitch. The type’s maximum single-class capacity was 162 at a 30-inch pitch, which was 32 more than the comparable 737-200’s and 23 more than either the Caravelle 12’s or the DC-9-50’s.
Garett air conditioning and Hamilton Standard pressurization systems, using engine bleed air, produced an 8.3-psi cabin differential.
Baggage, cargo, mail, and live animals were carried in three lower-deck holds.
The first two compartments could accommodate five and four 727-type cargo containers, respectively, while the third accepted only bulk or loose loads. All were heated, lighted, and pressurized, and housed oxygen and potable water supply tanks, as well as lavatory waste receptacles. A powered loading system could be optionally installed.
The Mercure 100 offered the following weights: an empty operating weight of 70,039 pounds, a payload of 32,850 pounds, a maximum ramp weight of 121,250 pounds, and a maximum takeoff weight of 120,150 pounds.
Weights for export aircraft—that is, those ordered by carriers other than Air Inter—differed, as follows: a maximum zero-fuel weight of 105,820 pounds, a maximum ramp weight of 125,660 pounds, and a maximum takeoff weight, after taxi fuel consumption, of 124,560 pounds.
Performance and Payload Limitations
Fulfilling its design goals, the Mercure 100 offered a higher payload, a greater passenger capacity, and more cargo hold volume than any previous twin.
It was also able to transport more passengers for its weight than any other twin-jet, achieving superior operating efficiency.
Only the 727-200, which was also powered by the JT8D engine, could exceed the Mercure 100’s capacity with 169 passengers, but at a considerably higher average weight of 145,000 pounds and with an additional powerplant.
The Mercure 100 also offered the lowest fuel burn per passenger on a typical 630-nautical-mile European sector when operating at its long-range cruise speed at 33,000 feet with a 137-mile diversion allowance.
These comparisons indicate four basic design superiorities.
- The highest fuel mileage per seat of all jetliners carrying fewer than 250 passengers.
- The highest average speed of all short- to medium-range commercial aircraft.
- The lowest empty weight per seat.
- The highest payload of all twin-jets weighing less than 200,000 pounds.
Optimized for short flight times, the Mercure 100 covered 500 nautical mile segments in five to 15 minutes less than comparable twins because of its high climb, cruise, and descent rates. Combined with its quick turnaround capability, these performance figures facilitated high daily utilization rates.
The aircraft required a 4,630-foot runway for landing based upon a 136-mph (118-knot) approach speed and a 109,790-pound maximum landing weight.
Built for Fast Turnarounds
Short turnaround times were made possible by a tailcone-installed Garrett-AiResearch GTCP-85-163C auxiliary power unit (APU) that provided ground power for cabin conditioning, engine starting, and an emergency electrical supply; the forward, left (and optional aft, left) Aerozur airstair; and the starboard side galley servicing and lower deck baggage and cargo hold access doors. Because sectors were short, galley servicing and cabin cleaning were limited. And while the aircraft could be refueled in ten minutes, its high takeoff-to-landing weight ratio enabled it to operate several sectors before refueling was required.
Turnaround times thus varied from 10 to 15 minutes at intermediate stops and from 20 to 25 minutes at terminating points, where more extensive provisioning was required.
Aircraft at their maximum gross weight required a 6,300-foot runway for takeoff and could climb at 3,300 fpm.
Despite these seemingly superior design capabilities, the type’s primary deficiency was its payload-range inflexibility. On a typical Air Inter flight with 150 passengers cruising at Mach 0.78 and at 32,000 feet with a 200 nautical mile diversion allowance, for instance, the Mercure 100’s range was only 800 nautical miles. Only payload restrictions would have increased this—to 1,200 miles with a 25,000-pound payload and 1,600 miles with a 21,000-pound payload—thereby rendering it unable to exploit its passenger-cabin-and-cargo-hold advantage on routes that exceeded short-range ones.
First Flights
The first Mercure 100 prototype, designated 01 and registered F-WTCC, first flew from Bordeaux on 28 May 1971, powered by two 15,000-thrust-pound Pratt & Whitney JT8D-11 turbofans, but incorporated a shorter fuselage than that intended for production versions.
Because performance proved below design targets, it was retrofitted with 15,500-thrust-pound JT8D-15s and returned to the sky four months later, on 7 September. But the additional power did little to improve its poor low-speed handling characteristics, which were, in part, remedied by adding horizontal stabilizer dihedral, leaving it to fly again, now in this configuration, still two months later, on 18 November.
The second prototype, 02, and registered F-WTMD, joined the flight test program the following year on 7 September. The higher-thrust engines and the tailplane dihedral, along with full-span leading-edge slats, delivered the intended performance.
The first production version of the Mercure 100, intended for launch customer Air Inter and incorporating all of these modifications, along with the full-length fuselage, first flew from the new Istres production plant on 17 July 1973, and was awarded its French type certificate the following year on 12 February.
Air Inter Service
Air Inter took delivery of the first of its ten aircraft on 16 May, integrating the type into its shuttle-type French domestic route network. It received its second example seven days later, on 11 June.
Of the ten ordered, the first six were delivered in 1974, and the remaining four the following year, alphabetically registered F-BTTA through BTTJ.
Although its first four aircraft were only provisioned with Category II landing capability, its remaining six featured the upgraded Category III standard, enabling them to conduct approaches with a 500-foot runway visual range (RVR) and a 50-foot decision height (DH). The initial four were subsequently brought up to this capability.
Accommodating 150 passengers in a single-class, six-abreast, 3-3 configuration with 32-inch seat pitches, its ten Mercure 100s were well received by passengers and operated multiple daily round trips between a dozen French domestic cities and towns, with sectors ranging from 50 to 70 minutes, demonstrating its rapid turnaround capability.
Its capacity, however, proved excessive relative to demand. Lower-than-anticipated load factors, along with escalating fuel prices, ironically prompted Air Inter to ultimately acquire a fleet of Sud-Aviation Caravelle 12s, operating them on many of the same routes, but with lower costs. Although it briefly flirted with the idea of selling its Mercures, the French government mandated their continued operation, but provided a subsidy to do so.
Structurally, however, the Dassault-Breguet airliner was sound and offered technological comparability to other twin-jets.
During the first five years of operation, the ten Mercure 100s logged more than 90,000 hours, comprising some 100,000 53-minute segments and achieving an average dispatch reliability of 98.5 percent. Each aircraft operated eight to ten daily flights.
Equipped with a fully monitored automatic pilot and head-up instrument displays, they conducted Category IIIA landings in poor weather conditions and still achieved a 98- to 99-percent reliability rate.
On 25 March 1985, Air Inter took delivery of the second prototype, F-WTCC. Having already been provisioned with the longer fuselage, the higher-rated engines, the leading-edge slats, and the tailplane dihedral, it was brought up to full production standard, giving Air Inter an 11-strong fleet.
Program Conclusion
Because no other carrier or operator ordered the Mercure 100, which, to a degree, was designed for Air Inter’s frequent domestic route system, only a dozen, including the two prototypes, were ever built. Although the design was an aerodynamic success and it was extremely reliable in daily operations, it was a financial failure for several reasons.
- Like the Vickers VC10 and the Hawker Siddeley HS.121 Trident, which had respectively been designed for British Overseas Airways Corporation (BOAC) and British European Airways (BEA), it was geared toward Air Inter and its unique high-density, multiple-frequency, short-range domestic routes, a system which no other carrier maintained.
- Dassault-Breguet, perhaps without choice, employed then-current technology, incorporating the existing Pratt and Whitney JT8D engines that powered other twin-jets, such as the DC-9, the 737, and the later versions of the Caravelle, and therefore elected to use its thrust for payload and passenger capacity rather than range.
- Because it launched the Mercure program late, these and other twin-jets had already captured most of the market and offered the necessary payload and range variations to do so. While the ultimate stretches of the Caravelle, the DC-9, and the 737 resulted in maximum passenger capacities of 139, 139, and 130, respectively, their ranges were significantly greater, offering operators more flexibility.
- Although market studies for an Airbus-type aircraft were accurate, their timing was not. Only after the 1980s was the 150-seat twin prevalent.
- Finally, the Mercure 100’s operating economics could not be fully exploited if its additional capacity was not filled.
While all of these factors led to the type’s demise and resulted in its paltry production run, Dassault-Breguet officially attributed the reason for its discontinuation as, “Fabrication stopped after completion of the tenth (production) aircraft due to the extremely penalizing evolution of the US dollar exchange rate.”
Despite what was certainly a failure by many measures, the Mercure 100, whose last scheduled flight occurred on 29 April 1996, achieved an enviable operational record. Air Inter’s 11 aircraft carried some 44 million passengers, logging 360,000 airborne hours during 440,000 flights with a 98-percent dispatch reliability during their 22 years of service, never having caused a single fatality.
