Beechcraft AT-7 Navigator

The Beechcraft Model 18 (or "Twin Beech", as it is also known) is a 6- to 11-seat, twin-engined, low-wing, tailwheel light aircraft manufactured by the Beech Aircraft Corporation of Wichita, Kansas. Continuously produced from 1937 to November 1969 (over 32 years, a world record at the time), over 9,000 were built, making it one of the world's most widely used light aircraft. Sold worldwide as a civilian executive, utility, cargo aircraft, and passenger airliner on tailwheels, nosewheels, skis, or floats, it was also used as a military aircraft."Beechcraft D18S Twin Beech."
'' National Air and Space Museum'' of the Smithsonian Institution, Washington, D.C. Retrieved: December 17, 2014."Fact Sheet: Beech C-45H Expeditor."
''National Museum of the U.S. Air Force'', Wright-Patterson Air Force Base, Dayton, Ohio. Retrieved: August 5, 2017."Twin Beech: The 1930s airplane that set Beech Aircraft Corporation on a course towards 50 years of success"
'' Flying Magazine'', February 1982, pp. 26-30, Retrieved: August 5, 2017

During and after World War II, over 4,500 Beech 18s were used in military service—as light transport, light bomber (for China), aircrew trainer (for bombing, navigation, and gunnery), photo-reconnaissance, and "mother ship" for target drones—including United States Army Air Forces (USAAF) C-45 Expeditor, AT-7 Navigator, and AT-11 Kansan; and United States Navy (USN) UC-45J Navigator, SNB-1 Kansan, and others. In World War II, over 90% of USAAF bombardiers and navigators trained in these aircraft."Fact Sheet: Beech AT-11 Kansan."
''National Museum of the U.S. Air Force'', Wright-Patterson Air Force Base, Dayton, Ohio. Retrieved: August 5, 2017.

In the early postwar era, the Beech 18 was the pre-eminent "business aircraft" and "feeder airliner". Besides carrying passengers, its civilian uses have included aerial spraying, sterile insect release, fish stocking, dry-ice cloud seeding, aerial firefighting, air-mail delivery, ambulance service, numerous movie productions, skydiving, freight, weapon- and drug- smuggling, engine testbed, skywriting, banner towing, and stunt aircraft. Many are privately owned, around the world, with 240 in the U.S. still on the FAA Aircraft Registry in August 2017.Bauschspies, James S. and William E. Simpson
"Research and Technology Program Perspectives for General Aviation and Commuter Aircraft"
NASA Contract NASW-3554 for NASA, Sept. 1982, N83-17454#. Retrieved: Dec. 18, 2014. (In particular, see: Table 2.4 "COMMUTER CARGO FLEET IN 1981 - TOP TEN AIRCRAFT MODELS - NUMBER IN FLEET," which notes Beech 18 units are more than the next two aircraft ''combined'' (Convair 500/680 and Douglas DC-3), and more than the next three general aviation aircraft combined."Beech 18" FAA Aircraft Registry.
'' Federal Aviation Administration''. Retrieved: August 5, 2017.

Design and development

By the late 1930s, Beechcraft management speculated that a demand would exist for a new design dubbed the Model 18, which would have a military application, and increased the main production facilities. The design was mainly conventional for the time, including twin radial engines, all-metal semimonocoque construction with fabric-covered control surfaces, and tailwheel undercarriage. Less conventional was the twin-tailfin configuration. The Model 18 can be mistaken for the larger Lockheed Electra series of airliners, which closely resemble it. Early production aircraft were powered either by two 330-hp (250-kW) Jacobs L-6s or 350-hp (260-kW) Wright R-760Es. The 450-hp (336-kW) Pratt & Whitney R-985 became the definitive engine from the prewar C18S onwards. The Beech 18 prototype first flew on January 15, 1937.

The aircraft has used a variety of engines and has had a number of airframe modifications to increase gross weight and speed. At least one aircraft was modified to a 600-hp (447-kW) Pratt & Whitney R-1340 powerplant configuration. With the added weight of about 200 lb (91 kg) per engine, the concept of a Model 18 fitted with R-1340 engines was deemed unsatisfactory due to the weakest structural area of the aircraft being the engine mounts. Nearly every airframe component has been modified.

In 1955, deliveries of the Model E18S commenced; the E18S featured a fuselage that was extended higher for more headroom in the passenger cabin. All later Beech 18s (sometimes called Super 18s) featured this taller fuselage, and some earlier models (including one AT-11) have been modified to this larger fuselage. The Model H18, introduced in 1963, featured optional tricycle undercarriage. Unusually, the undercarriage was developed for earlier-model aircraft under an STC by Volpar, and installed in H18s at the factory during manufacture. A total of 109 H18s was built with tricycle undercarriage, and another 240 earlier-model aircraft were modified with this."Model 18 Specifications."
''Beechcraft Heritage Museum''. Retrieved: August 24, 2008.

Construction of the Beechcraft Model 18 ended in 1970 with a final Model H18 going to Japan Airlines. Through the years, 32 variations of the basic design had flown, over 200 improvement modification kits were developed, and almost 8,000 aircraft were built. In one case, the aircraft was modified to a triple tail, trigear, humpbacked configuration and appeared similar to a miniature Lockheed Constellation. Another distinctive conversion was carried out by Pacific Airmotive as the PacAero Tradewind. This featured a lengthened nose to accommodate the tricycle nosewheel, and the Model 18's twin tailfins were replaced by a single fin."Beechcraft 3NMT Expeditor."
''Canadian Museum of Flight.'' Retrieved: August 13, 2012.

Operational history

Production got an early boost when Nationalist China paid the company US$750,000 for six M18R light bombers,"Beechcraft page."
''Aerofiles.'' Retrieved: August 12, 2008. but by the time of the U.S. entry into World War II, only 39 Model 18s had been sold, of which 29 were for civilian customers. Work began in earnest on a variant specifically for training United States Army Air Forces (USAAF) military pilots, bombardiers, and navigators. The effort resulted in the Army AT-7. Further development led to the AT-11 navigation trainer, C-45 military transport, and F-2 (the "F" standing for "Fotorecon", short for "photographic reconnaissance"). The United States Navy first adopted the Beech 18 as the JRB-1, equivalent to the F-2, followed by the JRB-2 transport; the JRB was initially named the Voyager, but this name did not enter common use, and JRBs were generally called Expeditors like their USAAF counterparts. The first JRB-1 obtained by the Navy, bureau number (BuNo) ''09771'', was converted from the last civil Model 18 built before production was earmarked solely for the military for the duration of the war. The Navy subsequently obtained more Model 18s as the JRB-3 (C-45B), JRB-4 (UC-45F), SNB-1 Kansan (AT-11), SNB-2 (AT-7), and SNB-2C (AT-7C). Existing naval Twin Beeches were subsequently modified into the SNB-2H air ambulance, SNB-2P reconnaissance trainer, and SNB-3Q electronic countermeasures trainer. The United States Coast Guard acquired seven JRB-4 and JRB-5 aircraft from the Navy between 1943 and 1947; they were primarily used as utility transports, with one aircraft later converted for aerial mapping, and another used for proficiency flying.

After the war, the USAAF became the United States Air Force (USAF), and the USAF Strategic Air Command had Model 18 variants (AT-11 Kansans, C-45 Expeditors, F-2 Expeditors, and UC-45 Expeditors) from 1946 until 1951. In 1950, the Navy still had around 1,200 JRB and SNB aircraft in inventory. From 1951 to 1955, the USAF had many of its aircraft remanufactured with new fuselages, wing center sections, and undercarriages to take advantage of the improvements to the civil models since the end of World War II. Eventually, 900 aircraft were remanufactured to be similar to the then-current Model D18S and given new designations, constructor's numbers, and Air Force serial numbers."C-45H."
''Beechcraft Heritage Museum''. Retrieved: August 24, 2008. The USN had many of its surviving aircraft remanufactured as well, resulting in the JRB-6, the SNB-5, and SNB-5P. The Coast Guard retired its JRBs in 1956 and sold most of them as surplus in 1959, but one was retained by the United States Coast Guard Reserve until at least 1972. With the adoption of the 1962 United States Tri-Service aircraft designation system, the Navy's SNB-5 and SNB-5P became the TC-45J and RC-45J respectively, later becoming the UC-45J as their primary mission shifted from aircrew training to utility transport work. The C-45 flew in USAF service until 1963, the USN retired its last UC-45J in 1972, while the U.S. Army flew its C-45s until 1976. In later years, the military called these aircraft "bug smashers" in reference to their extensive use supplying mandatory flight hours for desk-bound aviators in the Pentagon.O'Rourke, G.G, CAPT USN. "Of Hosenoses, Stoofs, and Lefthanded Spads." ''United States Naval Institute Proceedings'', July 1968.

Beech 18s were used extensively by Air America during the Vietnam War; initially more-or-less standard ex-military C-45 examples were used, but then the airline had 12 aircraft modified by Conrad Conversions in 1963 and 1964 to increase performance and load-carrying capacity. The modified aircraft were known as Conrad Ten-Twos, as the maximum takeoff weight (MTOW) was increased to . The increase was achieved by several airframe modifications, including increased horizontal stabilizer angle-of-incidence, redesigned undercarriage doors, and aerodynamically improved wingtips. Air America then had Volpar convert 14 aircraft to turboprop power, fitted with Garrett AiResearch TPE-331 engines; modified aircraft were called Volpar Turbo Beeches, and also had a further increase in MTOW to ."Air America: Beech/Volpar Turbo Beech 18".
''University of Texas at Dallas'', 2006. Retrieved: August 5, 2017.

Spar problems

The wing spar of the Model 18 was fabricated by welding an assembly of tubular steel. The configuration of the tubes in combination with drilled holes from aftermarket STC modifications on some of these aircraft have allowed the spar to become susceptible to corrosion and cracking while in service. This prompted the FAA to issue an Airworthiness Directive in 1975, mandating the fitting of a spar strap to some Model 18s. This led, in turn, to the retirement of a large number of STC-modified Model 18s when owners determined the aircraft were worth less than the cost of the modifications. The corrosion on unmodified spars was not a problem; it occurred due to the additional exposed surface area created through the STC hole-drilling process. Further requirements have been mandated by the FAA and other national airworthiness authorities, including regular removal of the spar strap to allow the strap to be checked for cracks and corrosion and the spar to be X-rayed. In Australia, the airworthiness authority has placed a life limit on the airframe, beyond which aircraft are not allowed to fly.

Variants

Manufacturer models

Unless otherwise noted, the engines fitted are Pratt & Whitney R-985 radials.

;Model 18A
:First production model with seating for two pilots and seven or eight passengers, fitted with Wright R-760E-2 engines of , MTOW: Four built.Pelletier 1995, p. 68

*Model S18A
:Version of Model 18A capable of being fitted with skis or Edo 55-7170 floats; MTOW:

;Model A18A
:Version fitted with Wright R-760E-2 engines, MTOW:

*Model SA18A
:Seaplane version of Model A18A, MTOW:

;Model 18B
:Version powered with Jacobs L-5 engines. Four built.McKillop, Jack
"Beech JRB Expedition (sic), Beech SNB Kansan and Navigator".
''microworks.ne.'' Retrieved: August 28, 2008.

*Model S18B
:Version of Model 18B capable of being fitted with skis or floats.

;Model 18D
:Variant with seating for two pilots and nine passengers, fitted with Jacobs L-6 engines of , MTOW: . Twelve aircraft built.

*Model S18D
:Version of Model 18D capable of being fitted with skis or , MTOW: "S18D."
''Beechcraft Heritage Museum.'' Retrieved: August 12, 2008.

;Model A18D
:Variant of 18D with MTOW increased by to .

*Model SA18D
:Seaplane version of Model A18D, but same MTOW as S18D.

;Model 18R
:Model with Pratt and Whitney R-985, , seven built, one to Sweden as an air ambulance, six to Nationalist China as M18R light bombers

;Model 18S
:Nine-passenger pre-World War II civil variant, powered by served as basis for USAAF C-45C

;Model B18S
:Nine-passenger pre-World War II civil variant, served as basis for USAAF F-2

;Model C18S
:Variant of B18S with seating for eight passengers, and equipment and minor structural changes"Beech C18S Type Certificate."
''Federal Aviation Administration''. Retrieved: August 12, 2008.

;Model D18S
:First post-World War II variant introduced in 1945, with seating for eight passengers and MTOW of , 1,035 built