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Larsen JL-6
The JL-6 was a lightly modified Junkers F.13 airplane brought to market in the United States by John Miller Larsen in 1920. In all, Larsen made no more than 23 sales of JL-6 aircraft: eight to the U.S. Air Mail Service; four to the U.S. Navy; three to the U.S. Army Air Service; two to Mercury Aviation; two to Imperial Oil in Canada; two to the Mexican government; two to Roald Amundsen (actually, these were not sold but donated, and in any case were used aircraft); and one was converted to the armored JL-12 attack plane, which did not sell.

Larsen imported F.13 aircraft and then assembled them at his airfield on Long Island. Small modifications were added, such as replacing with window glass with safety glass, upgrading the cooling system, and putting on better shock absorbers and wheels. Because the modifications were minor, the JL-6 shared the nearly all of the design and performance characteristics of the Junkers F.13.

Records
As of August 1920, the JL-6 was said to have claimed the following records:


 * speed: flying six passengers from Atlantic City to Philadelphia and back, a distance of 130 miles, in 59 minutes.
 * altitude: climbing to 20,600 feet in 97 minutes carrying six passengers, then flew a distance of 140 miles.
 * non-stop flying: starting with 970 pounds of fuel (about 158 U.S. gallons) and 45 pounds of oil, traveling from Omaha to Philadelphia, a distance of 1200 miles, in 10 hours 58 minutes. Flying weight was 4184 pounds.
 * economy: flying eight passengers 130 miles in 88 minutes, using 12 ½ gallons of gas, costing $3.00.

In December 1921, Eddie Stinson and Lloyd Bertaud set a record for sustained flight, staying aloft for 26 hours 19 minutes.

Use by the Post Office
There were several reasons why the Post Office choose the JL-6 to fly the transcontinental airmail. Sharing every feature with the Junkers F.13 on which they were based, they had large capacity for freight, they were sturdy, and they were economical to fly. The all-metal design gave them an advantages in rough conditions. They could land in a short distance. They were powerful enough to climb over the mountains encountered along the route. The deciding factor, though, was that John Larsen offered to provide at his expense the use of two JL-6 aircraft along with crew.

Safety issues
The JL-6 quickly attracted negative attention because it was found the aircraft had a tendency to catch fire in mid-air. There were numerous crashes and at least six died. In mid-September 1920, the Post Office grounded all of the JL-6s until the cause of the engine fires could be determined.

An investigation by the U.S. Army Air Service found two problems. First, the German engines used, particularly the BMW-IIIa, needed to use benzole (or benzol) as fuel on take off. If unmixed gasoline was used the engine would overheat, leading to a failure on take-off. Once in the air, the engine could cruise well on gasoline. This was a problem caused by simple pilot error. Second, the fuel line between the fuel pump and carburetor was hard-soldered and brittle. Eddie Rickenbacker, who promoted sales of the JL-6 in 1920, correctly blamed the rigid fuel line, which would “break under pressure and vibration.” Fuel then spilled into the engine compartment, and the engine, “starved of fuel, would backfire, which would in turn ignite the spilled gasoline.” The Army Air Service recommended that "Breakage of gas lines was prevented by the installation of flexible hose connections.”

Why did the Post Office drop the JL-6?
The Post Office had been using the deHavilland DH-4 on airmail routes. The deHavilland had many limitations. It was not well suited to long distance flying and was not capable of flying at altitudes over 10,000 feet. They were also very bad in crash landings. When opening transcontinental airmail service, the plan had been to phase out the deHavillands and replace them with JL-6 aircraft. However, during the time the JL-6s were grounded, arrangements for Larsen to import the F.13s from Junkers collapsed. Junkers had become fed up with dealing with Larsen and would no longer sell to him. On 11 October 1920, Hugo Junkers was determined to end the relationship with Larsen and he announced his intention at a meeting of his board of directors in Dessau. Without Larsen, for several years there was no dealer to bring the airplanes here. The Post Office needed airplanes right away so made the decision to stick with the DH-4s. Their fleet of remaining JL-6s were eventually sold to private buyers as surplus.

Canada
In 1920 Imperial Oil made one of the great oil field discoveries of the 20th century. It was in the Northwest Territory, near Fort Norman (now know as Tulita), and so remote they needed airplanes to deliver geologists and engineers to explore and establish leases. In 1921, they purchased two JL-6 aircraft for that purpose.

The aircraft delivered to Canada used either a Mercedes engine or a BMW-IIIa engine modified so that it could run on ordinary gasoline. They also had some additional modifications installed by Larsen: insulated oil lines, more efficient radiator shutters, and carburetor intake heaters.

Roald Amundsen
When Roald Amundsen was planning his 1922 attempt to fly over the North Pole in an airplane, he selected the JL-6 as the airplane to do it. He could see that the plane had proved itself to be a capable workhorse in sub-arctic conditions in Canada, and the endurance record set by Stinson and Bertaud showed that it was capable of sustained flight sufficient to carry him from Pt. Barrow, Alaska to Spitzbergen, Norway, over the Pole. The expedition failed because the JL-6 was forced down in a violent hail storm, crashing in a field in Pennsylvania.