The Laté 631, with its ambitious design and tragic operational history, stands as a reminder of the boldness and perils of aviation

On March 12, 1936, the French Air Ministry’s civil aeronautics department sought proposals for a commercial seaplane.

It needed to weigh no more than 88,185 lb and carry at least 20 passengers, sleeping berths, and 1,100 lb of cargo for 3,730 miles.

Additionally, it needed to counter a 37 mph headwind and maintain a cruising speed of 155 mph. This large passenger aircraft aimed to service transatlantic routes to both Americas.

Marcel Moine, a leading engineer at Latécoère, had been developing a similar aircraft since late 1935.

He had designed a plane for North Atlantic service, weighing up to 142,200 lb. Deemed too ambitious, Moine modified the design to align with the 1936 request. Consequently, he proposed the adjusted aircraft to the Air Ministry, naming it the Latécoère 630.

Contents

• Gigantic Swingspan
• High-Mounted Wing
• Wing Flutter at 143 mph
• No Survivors

Laté 631 Gigantic Swingspan

The Laté 630 was a six-engine, all-metal flying boat featuring retractable floats. It utilized a 930 hp, liquid-cooled Hispano Suiza 12 Ydrs to power its 98,860 lb frame. With a wingspan of 187 ft and a length of 117 ft 9 in, it could travel 4,909 miles.

On November 15, 1936, order 575/6 requested detailed design work and a wind-tunnel test model for the Laté 630. Then, order number 637/7, issued on April 15, 1937, requested a single prototype.

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However, the Air Ministry cancelled the Laté 630 on July 22, 1937, advocating for a larger, more capable aircraft due to aeronautical advancements.

Meanwhile, construction of the Potez-CAMS 161, designed to the same specifications, continued.

Considering new aeronautical advancements, the Air Ministry requested an aircraft with a maximum weight of 154,323 lb.

This aircraft would need to transport 40 passengers and 11,000 lb of cargo at a normal cruising speed exceeding 186 mph.

To comply, Moine and Latécoère adapted and powered up the Laté 630 design, resulting in the Laté 631. In October 1937, they ordered detailed design work and a wind tunnel model for the Laté 631.

The following year, on July 1, they issued order number 597/8 for a single prototype. Simultaneously, a prototype of Lioré et Olivier H-49, which transitioned into the SNCASE SE.200, was ordered under the same specification as the Laté 631.

Laté 631 High-Mounted Wing

The Latécoère 631 was an all-metal flying boat featuring a two-step hull. Its monocoque fuselage had an aluminum frame and aluminum sheeting.

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Inside, the hull housed several passenger compartments, including a lounge/bar beneath the radio/navigation room, possibly in the nose in some designs, and a rear kitchen.

The cockpit and radio/navigation room sat above the main passenger compartment, just in front of the wings.

Interestingly, the cockpit was situated quite far back from the aircraft’s nose. Additionally, the design included numerous access doors in the nose, cockpit side, and fuselage sides.

The high-mounted wing, blending into the fuselage top, housed six engines in individual nacelles. It contained two main spars and a false spar, dividing into an inner section with engine nacelles and an outer section.

Remarkably, each wing’s outer engine nacelle featured a retractable float extending behind the trailing edge. Due to interference, deploying the float partially was necessary before lowering the flaps.

A leading-edge passageway in the wing, accessible from the radio/navigation room, provided access to the engine nacelles.

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Each nacelle featured two downward-opening doors behind the engine, acting as maintenance platforms. A removable firewall section allowed engine rear access from within the nacelle.

Additionally, a compartment between the inboard engine and the fuselage, situated in the wing’s leading edge, was intended for mail cargo.

Everything was on a Big Scale

Initially, the Laté 631 was set to utilize 1,500 hp Gnôme Rhône 18P radial engines. However, due to potential availability issues, a switch to 1,600 hp Wright R-2600 radial engines occurred.

The team also pondered using Gnôme Rhône 14R and Pratt & Whitney R-2800 engines. Nonetheless, the 14R was not available, and exporting R-2800 engines posed restrictions.

Each engine propelled a three-blade, variable-pitch propeller, spanning 14 ft 1 in, crafted by Ratier. Subsequently, larger propellers came into use, though sources dispute their size, citing either 14 ft 5 in or 15 ft 1 in. Both larger sizes might have been tested at different times.

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Moving to the aircraft’s rear, twin tails anchored to a horizontal stabilizer boasted a 16.7-degree dihedral. All control surfaces featured an aluminum frame and a leading edge shrouded by aluminum, while the remaining surface was fabric-covered.

The control surfaces moved, enhanced by a servo-controlled electrohydraulic system, with a disengagement option for pilots.

The slotted aileron on each wing divided into two sections, inner and outer. Additionally, Flettner servo tabs adorned the ailerons, serving to trim the aircraft and optionally boost roll control when engaged.

Six wing tanks, holding 7,582 gallons of fuel, each supplied one engine. In Flight, six hull tanks, containing an additional 5,785 gallons, replenished the wing tanks.

Consequently, the Laté 631 boasted a total fuel capacity of 13,367 gallons. Furthermore, each engine benefitted from its individual 111-gallon oil tank.

Speed of 245 mph

Regarding dimensions and performance, the Latécoère 631 spanned 188 ft 5 in, measured 142 ft 7 in in length, and stood 33 ft 11 in tall.

It achieved a maximum speed of 245 mph at 5,906 ft and 224 mph at sea level, while maintaining a cruising speed of 183 mph at 1,640 ft.

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The aircraft weighed in at 89,265 lb when empty and maxed out at 163,347 lb. Finally, it could traverse 3,766 miles with an airspeed of 180 mph, countering a 37 mph headwind.

Construction of the Laté 631 commenced quickly after contract issuance. Yet, on 12 September 1939, work stopped to prioritize vital military aircraft production following the declaration of war on Germany.

Subsequently, after the French surrender, work resumed in July 1940 but faced another halt on 10 November by German order.

Negotiations between the French and Germans, centered on the civil transportation project, unfolded. Consequently, the Germans permitted construction to persist, and a second prototype was ordered on 19 March 1941, under contract 597/8.

The 35 Wright R-2600 engines, initially stranded in Casablanca, Morocco due to the 1939 war outbreak, were astonishingly released and delivered at 1941’s end.

The first prototype, Laté 631-01, registered as F-BAHG, was completed in Toulouse, France, during the 1942 summer.

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Then, it was disassembled and transported 310 miles to Marignane in southern France, not without challenges. Subsequently, it was reassembled for ensuing tests on Étang de Berre.

Laté 631 Wing Flutter at 143 mph

Meanwhile, its competitor, the SNCASE SE.200, was approaching completion at Marignane. Reassembly of Laté 631-01 concluded in October 1942, making its maiden flight on 4 November, piloted by Pierre Crespy.

Accompanying him, seven others, including Moine, joined as crew and observers. A follow-up flight on 5 November encountered aileron and wing flutter at 143 mph.

Troubleshooting traced the issues to a faulty aileron control circuit component that had failed.

Laté 631-01 underwent repair, but the German occupation in November 1942 paused further flight tests. However, on 23 November, order 280/42 commissioned two additional Laté 631s, totaling four aircraft.

Flight restrictions lifted in December 1942, enabling Laté 631-01 to soar again. Nonetheless, German orders sporadically halted test flights.

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In April 1943, tests resumed under the condition of donning German colors, markings, and hosting a Lufthansa pilot on board.

Consequently, Germany, having essentially seized Laté 631-01 and SE.200, envisioned its post-“quick” war commercial transport utility.

The Germans also explored arming the flying boat for maritime patrols. Consequently, Laté 631-01 was repainted with the German code 63+11.

Flight testing of Laté 631-01 picked up again in June 1943. On 20 January 1944, its 46th flight pushed the gross weight past 154,323 lb for the first time.

Another flight occurred at 157,630 lb. Testing affirmed that, at 88,185 lb, the Laté 631 managed with three same-side engines offline. At 154,323 lb, it coped with both outer engines off. Flutter signs appeared but weren’t fully deciphered.

World War Two

Laté 631-01 underwent repair, but the German occupation in November 1942 paused further flight tests. However, on 23 November, order 280/42 commissioned two additional Laté 631s, totaling four aircraft.

Flight restrictions lifted in December 1942, enabling Laté 631-01 to soar again. Nonetheless, German orders sporadically halted test flights.

In April 1943, tests resumed under the condition of donning German colors, markings, and hosting a Lufthansa pilot on board.

Consequently, Germany, having essentially seized Laté 631-01 and SE.200, envisioned its post-“quick” war commercial transport utility.

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The Germans also explored arming the flying boat for maritime patrols. Consequently, Laté 631-01 was repainted with the German code 63+11.

Flight testing of Laté 631-01 picked up again in June 1943. On 20 January 1944, its 46th flight pushed the gross weight past 154,323 lb for the first time.

Another flight occurred at 157,630 lb. Testing affirmed that, at 88,185 lb, the Laté 631 managed with three same-side engines offline. At 154,323 lb, it coped with both outer engines off. Flutter signs appeared but weren’t fully deciphered.

Propaganda Flight of Laté 631

On 28 September 1945, Laté 631-02 experienced a severe right roll due to an autopilot issue, damaging its wing. Consequently, over 8,000 rivets needed replacement to repair it.

Nonetheless, rapid repairs enabled a propaganda flight to Rio de Janeiro, Brazil, on 10 October 1945.

However, that day, a collision with a submerged concrete block while taxiing ripped a 6 ft 7 in gash in the hull. Following this incident, French authorities nationalized the Latécoère factories.

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Production of the final six Laté 631 aircraft was distributed among AECAT, Breguet, SNCASO, and SNCAN. SNCASO at Saint-Nazaire mainly handled production of aircraft numbers 6, 8, and 10, while SNCAN at Le Havre managed numbers 7, 9, and 11.

Laté 631-02 eventually accomplished its flight to Rio de Janeiro, landing on 25 October 1945 with 45 passengers.

Tragedy struck the Laté 631 program on 31 October 1945. Laté 631-02, while flying from Rio de Janeiro to Montevideo, Uruguay, encountered a propeller failure on its No. 3 engine with 64 people aboard.

The ensuing imbalance caused the No. 3 engine to completely detach from the aircraft. A flying blade not only damaged the No. 2 engine’s propeller but also nearly severed the engine from its mounts.

Additionally, a separate blade pierced the fuselage, resulting in one immediate and one subsequent passenger death.

An emergency landing was executed on Laguna de Rocha in Uruguay. Investigation traced the Ratier propeller failure to its aluminum hub, which was later swapped for a steel unit.

Recovery efforts involved substituting the absent engine with one from the right wing. With a reduced crew, the four-engine aircraft flew to Montevideo on 13 November for comprehensive repairs, which spanned three months.

Saint-Nazaire, France

In February 1946, Argentina bought three Laté 631 aircraft but never paid, causing the deal to collapse. Conversely, in May 1946, Air France agreed to acquire three Laté 631 aircraft.

Jean Prévost piloted the first flight of Laté 631-03, registered as F-BANU, at Biscarrosse on 15 June 1946. Shortly afterward, it transferred to Air France, christened Henri Guillaumet.

Meanwhile, Laté 631-04, holding the registration F-BDRA, made its initial flight at Biscarrosse on 22 May 1947. This aircraft became the second Laté 631 to join Air France.

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Additionally, Laté 631-05 took its maiden flight on 22 May 1947, under the registration F-BDRB. Later, on 9 November 1947, Laté 631-06, labeled F-BDRC, flew for the first time near Saint-Nazaire, France, and became Air France’s third aircraft.

Moreover, Laté 631-07, registered as F-BDRD, had its inaugural flight on 27 January 1948 but tragically crashed on 21 February during a test flight.

Taking off in unsuitable weather and lacking proper instruments for such conditions, it plummeted into the English Channel off Utah Beach. The tragic incident took the lives of all 19 crew and Latécoère engineers onboard.

The exact cause remains undetermined, with speculation around spatial disorientation or low flying due to the poor weather and possible collision with leftover D-Day wreckage.

No Survivors from Laté 631

Tragedy struck again on 1 August 1948 when Air France’s Laté 631-06 F-BDRC was lost over the Atlantic.

This catastrophe between Fort-de-France, Martinique, and Port-Etienne, Mauritania, left no survivors among the 52 onboard.

Recovered wreckage hinted at an in-flight breakup, potentially involving fire or explosion, yet without a confirmed cause.

At the accident time, F-BDRC had logged 185 flight hours. Consequently, Air France withdrew its remaining two Laté 631s from service, using Laté 631-04 F-BDRA in a 75-hour search for survivors, which included a single 26-hour flight.

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The era of the flying boat concluded in the decade between the ordering of the Latécoère 631 in 1938 and its service initiation with Air France in 1947.

Moreover, World War II’s aviation advancements underscored the future belonging to landplanes in commercial aviation.

After the accidents, any prospect of utilizing the Laté 631 as a commercial airliner vanished. Therefore, with four aircraft complete and another four in production, the decision emerged to transform the Laté 631 into a cargo plane.

Following the Crash

On 28 November 1948, 631-08 F-BDRE took its inaugural flight from Saint-Nazaire. Originally designed for Air France, it became orphaned following the crash of Laté 631-06.

Consequently, Laté 631-08, along with Laté 631-03, were transferred to SEMAF, a company established in March 1949. SEMAF aimed to repurpose the Laté 631 as an air freighter.

After converting F-BDRE to a cargo plane by reinforcing its airframe and adding a large cargo door, it made its modified debut flight on 8 June 1949.

Quickly, it began transporting fabric and products between France and various African locations, completing 12 trips by March 1950.

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Contrastingly, 631-09 F-BDRF had already soared through its first flight on 20 November 1948 at Le Harve before F-BDRE.

Also, Laté 631-10 F-BDRG lifted off for the first time on 7 October 1949 from Saint-Nazaire. Both planes were eventually stored in Biscarrosse with the incomplete Laté 631-11 F-BDRH, and later received new registrations as F-WDRF and F-WDRG.

Tragedy Strikes Again

In a different vein, 631-03 F-BANU, re-registered as F-WANU, underwent tests for airframe and engine vibrations, partially to investigate the loss of F-BDRC.

A tragic incident occurred on 28 March 1950 during its second flight of the day from Biscarrosse. After ramping up engine power, the left wing began to flutter, causing the left aileron’s outer section to detach.

The aircraft spiraled, inverted, and crashed into the water, instantly killing all 12 people on board. Subsequent investigation into the wreckage indicated that a resonance issue between the engine, propeller, and wing components led to rapid part fatigue.

This fatigue resulted in aileron system failure, initiating flutter, complete control system breakdown, and ultimately, a catastrophic loss of control over the aircraft.

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At the time of the accident, Laté 631-03 utilized R-2600 engines with a .5625 gear reduction. Interestingly, these engines, while installed on later models and retrofitted on earlier ones, contributed to only a fraction of Laté 631-03’s 1,001 hours.

Nonetheless, this was sufficient to fatigue the aileron control to its failure point. Tragically, the loss of Laté 631-03 precipitated the downfall of SEMAF.

Implementing Modifications

Understanding the crash cause, a new company formed to enhance the Laté 631 fleet and convert them for cargo service. La Société France Hydro took control of Laté 631-02 and Laté 631-08, re-registering the latter as F-WDRE.

Implementing modifications to avert a repeat of the 631-03 disaster, Laté 631-08 reentered cargo service in late 1951. Then, starting in March 1952, it navigated a 7,460-mile route from Biscarrosse to Saigon.

Impressively, it departed Bizerte, Tunisia with a takeoff weight of 167,000 lb, a record for a Laté 631. By 1953, it was transporting cotton from Douala, Cameroon to Biscarrosse.

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Finding some profitability, cargo-conversion of Laté 631-02 commenced. Also, Laté 631-05 transferred to France Hydro, but with little subsequent activity.

However, disaster struck on 10 September 1955, when Laté 631-08 disintegrated during a fierce thunderstorm over Sambolabo, Cameroon, killing all 16 onboard.

This tragedy marked the end of the Latécoère 631’s operational life, with no further usage attempts thereafter.

In a further blow in February 1956, Laté 631-05, -10, and -11 incurred irreparable damage when the Biscarrosse hangar’s roof collapsed due to heavy snowfall.

Consequently, the remaining Latécoère 631s faced scrapping, predominantly in late 1956. Lastly, in 1961, Swiss recovery teams lifted and subsequently scrapped the remains of Laté 631-01 and the SE.200 prototype from Lake Constance.

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