The Cessna T-37/A-37

v1.2.0 / 01 nov 02 / greg goebel / public domain

* A jet trainer / light attack aircraft may not be the same sort of status symbol as first-line fighter jet, but the need to perform adequate flight training and the low cost, ease of maintenance, and combat effectiveness of light attack aircraft has ensured that the trainer-attack type is in common use over the world.

One of the most prominent of the trainer-attack types is the Cessna "T-37/A-37", known in various forms as the "Tweety Bird", "Tweet", "Dragonfly", or "Super Tweet". This small, neat aircraft flew for decades as a primary trainer for the US Air Force, and served with distinction in the attack role during the Vietnam War. The T-37/A-37 has also provided excellent service in the air forces of other nations. This document outlines the history of the T-37/A-37.

[1] ORIGINS: MODEL 318 (XT-37)
[2] T-37A / T-37B / T-37C
[3] A-37A
[4] A-37B

[1] ORIGINS: MODEL 318 (XT-37)

* The Cessna Aircraft Company of Wichita, Kansas, earned a good reputation with the US Army during World War II with the company's excellent utility, light transport, and observation aircraft, particularly the "O-1 Bird Dog" series.

In the spring of 1952, the US Air Force (USAF) issued a request for proposals for a "Trainer Experimental (TX)" program, specifying a lightweight two-seat basic trainer for introducing USAF cadets to jet aircraft.

Cessna responded to the TX request with a twin-jet design that featured side-by-side seating. The USAF liked the Cessna design, which was given the company designation of "Model 318", and particularly liked the side-by-side seating since it allowed the student and instructor to interact more closely than would be possible with tandem seating. In the spring of 1954, the USAF awarded Cessna a contract for three prototypes of the Model 318, along with a separate contract for a single static test aircraft. The Air Force assigned the designation of "XT-37" to the type.

The first XT-37 performed its initial flight in October 1954. The aircraft featured a straight low wing, with the engines buried in the wing roots; a clamshell-type canopy, hinged to open vertically to the rear; a control layout similar to that of contemporary operational USAF aircraft; ejection seats; and tricycle landing gear with a wide track of 4.3 meters (14 feet).

The wide track and a steerable nosewheel made the aircraft easy to handle on the ground, and the short landing gear eliminated the need for access ladders and service stands. The aircraft was designed to be simple to maintain, featuring more than a hundred access panels and doors. An experienced ground crew could change an engine in about a half hour.

The XT-37 was very aerodynamically clean, and so an air brake was fitted behind the nosewheel door to reduce landing speed. Given that the landing gear was short and the engine intakes were consequently close to the ground, the XT-37 was equipped with screens that pivoted up over the intakes from underneath when the landing gear was extended to prevent foreign object damage.

The XT-37 was fitted with two Continental-Teledyne J-69-T-9 turbojet engines with 4.1 kN (420 kg / 920 lb) thrust each. These were actually French Turbomeca Marbore engines, built under license. The engines incorporated thrust deflectors to allow the engines to remain spooled up during landing approach, permitting shorter landings while still allowing the aircraft to easily make another "go-round" in case something went wrong. Total weight of the XT-37 was 2.27 tonnes (5,000 pounds).

Tests showed the XT-37 had a maximum speed of 630 KPH (390 MPH) at altitude, with a range of 1,500 kilometers (935 miles). The aircraft was unpressurized, and so limited to a ceiling of 7.6 kilometers (25,000 feet) by USAF regulations.

* The initial prototype crashed during spin tests. The later prototypes incorporated new features to improve handling, including long strakes along the nose, and an extensively redesigned and enlarged tail. After these modifications, the USAF found the aircraft acceptable to their needs, and ordered it into production as the "T-37A".


[2] T-37A / T-37B / T-37C

* The production T-37A was similar to the XT-37 prototypes, except for minor changes to fix problems revealed by the flight test program. The first T-37A was completed in September 1955, and was delivered to the Air Force in June 1956. The USAF began cadet training in the T-37A during 1957.

Instructors and students found the T-37A extremely pleasant to fly. It handled well and was agile and responsive, though it was definitely not overpowered. It was capable of all traditional aerobatic maneuvers.

The T-37A had one very noticeable and unpleasant characteristic: it was very noisy, even by the standards of a jet aircraft. Its little turbojets emitted a high-pitched shriek that led some to describe the trainer as a "Screaming Mimi", and it was referred to as the "6,000 pound dog whistle". The piercing whistle quickly gave the T-37 its name: "Tweety Bird", or just "Tweet".

The Air Force ordered 444 T-37As, with the last produced in 1959. During 1957, the US Army evaluated three T-37As for battlefield observation and other combat support roles, but eventually procured the Grumman OV-1 Mohawk for the mission instead.

* The Air Force liked the T-37A, but felt it was underpowered. As a result, the service ordered an improved version, the "T-37B", with uprated J-69-T-25 engines. The new engines provided about ten percent more thrust and better reliability. Improved avionics were also specified for the new variant.

The first T-37B was delivered in 1959. A total of 552 new-build T-37Bs were built through 1973. All surviving T-37As were eventually upgraded to the T-37B standard as well.

   CESSNA T-37B:
   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                10.32 meters        33 feet 10 inches
   length                  8.92 meters         29 feet 3 inches
   height                  2.79 meters         9 feet 2 inches

   empty weight            1,840 kilograms     4,056 pounds
   max loaded weight       2,980 kilograms     6,570 pounds

   maximum speed           685 KPH             425 MPH / 370 KT
   service ceiling         7,620 meters        25,000 feet
   range                   1,500 kilometers    930 MI / 810 NMI
   _____________________   _________________   _______________________

Due to a series of accidents caused by bird strikes between 1965 and 1970, all T-37s were later retrofitted with a new windscreen made of Lexan polycarbonate plastic 12.5 millimeters (a half inch) thick. The new windscreen was capable of tolerating the impact of a 1.8 kilogram (4 pound) bird at a relative speed of 460 KPH (288 MPH).

In 1962, Cessna suggested the T-37B as a replacement for the North American F-100 Super Sabre as the mount for the USAF aerobatic demonstration team, the Thunderbirds. The T-37B was an excellent aerobatic aircraft, was economical to operate and support, and could be flown from small airports. However, the USAF was happy with the F-100 and were not interested in trading it in for the Tweet.

* The T-37A and T-37B had no built-in armament and no stores pylons for external armament. In 1961, Cessna began development of a modest enhancement of the T-37 for use as a weapons trainer. The new variant, the "T-37C", was intended for export and could be used for light attack duties in a pinch.

The prototype T-37C was actually a modified T-37B. The primary changes included stronger wings, with a stores pylon under each wing, outboard of the main landing gear well. The T-37C could also be fitted with wingtip fuel tanks, each with a capacity of 245 liters (65 US gallons), that could be dropped in an emergency.

A computing gunsight and gun camera were added. The T-37C could also be fitted with a reconnaissance camera mounted inside the fuselage.

The primary armament of the T-37C was the General Electric "multi-purpose pod", which carried a 12.7 millimeter (0.50 caliber) machine gun with 200 rounds; two 70 millimeter (2.75 inch) folding-fin rockets; and four practice bombs. Other stores, such as folding-fin rocket pods or even Sidewinder air-to-air missiles, could be carried as well.

The changes increased the weight of the T-37C by 650 kilograms (1,430 pounds). As the engines were not upgraded, this reduced top speed to 595 KPH (370 MPH), though the wingtip tanks increased maximum range to 1,770 kilometers (1,100 miles).

* T-37s, including both new-build and ex-USAF aircraft, were supplied to a number of foreign operators, including:

The Portugese Air Force used the T-37C as the mount for their aerobatic demonstration team, the "Asas de Portugal (Wings of Portugal)" starting in 1977, and put on hundreds of shows all over Europe with these brightly-painted birds.

* T-37 production ended in 1975. The list of exports above amounts to 273 T-37Cs. Adding this to the 444 T-37As and 552 T-37Bs gives a total of 1,269 aircraft built.

The type remained in service with the USAF into the 21st century, having survived various attempts to find a replacement. However, the Tweet is now being phased out in favor of the turboprop-powered Raytheon T-6A, going into retirement after over four decades of reliable service.


[3] A-37A

* The growing American military involvement in Vietnam in the early 1960s led to strong interest in "counter-insurgency (COIN)" aircraft, and in late 1962 the US Air Force's Special Air Warfare Center at Eglin AFB in Florida evaluated two T-37Cs for the role.

The Air Force found the T-37 promising, but wanted an improved version of the aircraft that could carry a much larger warload, and had much greater endurance, as well as better short-field performance. This meant a heavier aircraft with more powerful engines, and during 1963 the Air Force awarded a contract to Cessna for two T-37s with the necessary modifications.

The two prototypes were given the designation "YAT-37D. Changes included:

These changes meant a drastic increase in aircraft weight, and the aircraft now had to carry a serious warload as well, so Cessna replaced the two Continental J-69 engines with General Electric J85-J2/5 turbojet engines with 10.7 kN (1,090 kg / 2,400 lb) thrust each, twice as powerful as the old Continental J85s.

The first YAT-37D flew in October 1964, followed a year later by the second prototype. The second prototype had four stores pylons under each wing, rather than three, and the first prototype was upgraded to this configuration as well.

Test results were good, but USAF interest in COIN aircraft had faded for the moment. The program went into limbo for a time, with the second prototype "put out to pasture" at the Air Force Museum at Wright-Patterson Air Force Base in Ohio.

However, the war in Southeast Asia continued to escalate. Losses of Douglas A-1 Skyraider close-support aircraft in US and South Vietnamese hands proved greater than anticipated, and so USAF interest in COIN aircraft revived. The YAT-37D seemed like a promising candidate for the job, but the Air Force felt the only way to be sure was to evaluate the aircraft in combat.

As a result, the USAF issued a contract to Cessna for a preproduction batch of 39 YAT-37Ds, with a few minor changes relative to the prototypes, to be rebuilt from existing T-37Bs. These aircraft were initially designated "AT-37D", but the designation was quickly changed to "A-37A". The second prototype YAT-37D was yanked out of the Air Force Museum and upgraded to A-37A standards as part of the test program.

The A-37A had a gross takeoff weight of 5,440 kilograms (12,000 pounds), of which 2,130 kilograms (2,700 pounds) was warload. The A-37A retained the dual controls of its T-37B ancestor, allowing it to be used as an operational trainer.

In combat "forward air control (FAC)" operations, the second seat was occupied by an observer. Only one crewmember normally flew in the aircraft for close support missions, permitting a slight increase in warload.

In August 1967, 25 of the A-37As were sent to Vietnam under the "Combat Dragon" evaluation program, and flew from Bieh Hoa Air Force Base on USAF "air commando" missions, including close air support, helicopter escort, FAC, and night interdiction. Warloads included high-explosive bombs, cluster munition dispensers, unguided rocket packs, napalm tanks, and the SUU-11/A Minigun pod. For most missions, the aircraft also carried two additional external tanks on the inner stores pylons.

The A-37As flew thousands of sorties. None were lost to enemy fire, though two were wrecked in landing accidents. The A-37A was formally named the "Dragonfly", but most pilots called it the "Super Tweet". It is an interesting question if that meant the A-37A was even noisier than the T-37.


[4] A-37B

* The Combat Dragon program was very successful, but unsurprisingly the combat evaluation revealed some of the deficiencies of the A-37A. The worst problem was that the aircraft lacked range and endurance. Other concerns were heavy control response during attack runs and the vulnerability of the aircraft's non-redundant flight control system.

The USAF signed a contract with Cessna in early 1967 for an improved Super Tweet, designated the "A-37B". The initial order was for 57 aircraft, but this was quickly increased to 127. The A-37Bs were primarily intended to be supplied to the South Vietnamese Air Force as replacements for their Skyraiders. The A-37B prototype was rolled out in September 1967, with deliveries to the South Vietnamese beginning in 1968.

   CESSNA A-37B:
   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                10.94 meters        35 feet 10 inches
   length (no probe)       8.92 meters         29 feet 3 inches
   height                  2.82 meters         9 feet 3 inches

   empty weight            2,815 kilograms     6,210 pounds
   max loaded weight       6,800 kilograms     15,000 pounds

   maximum speed           815 KPH             505 MPH / 440 KT
   service ceiling         12,700 meters       41,800 feet
   range                   1,480 kilometers    920 MI / 800 NMI
   _____________________   _________________   _______________________

The A-37Bs were all new-build aircraft. They were stronger than the A-37A, stressed to 6 gees instead of 5, and had a longer fatigue life of 4,000 hours. Field experience would show that 7,000 hours was more in line with reality.

The A-37B weighed almost twice as much as the T-37C. A remarkable fraction of the loaded weight, 2.67 tonnes (5,880 pounds), could be external stores. In practice, the A-37B usually operated with at least two and sometimes four underwing fuel tanks to improve combat endurance.

To get this greater weight off the ground, the A-37B was fitted with General Electric J85-GE-17A engines, providing 12.7 kN (1,300 kg / 2,850 lb) thrust each. These engines were canted slightly outward and downward to improve single-engine handling. Air commando pilots in Vietnam operating the A-37A had found single-engine cruise an effective means of improving their flight endurance.

Modifications were made to control surfaces to improve handling. To improve aircraft and crew survivability, the A-37B was fitted with redundant elevator control runs that were placed as far apart as possible. The ejection seats were armored, the cockpit was lined with nylon flak curtains, and foam-filled self-sealing fuel tanks were installed.

The A-37B added a refueling probe to the nose, leading to pipes wrapped around the lower lip of the canopy, for probe-and-drogue midair refueling. This was an unusual fit for USAF aircraft, which traditionally are configured for boom refueling. Other improvements included updated avionics, a redesigned instrument panel to make the aircraft easier to fly from either seat, an automatic engine inlet de-icing system, and revised landing gear. Like its predecessors, the A-37B was not pressurized.

20 millimeter and 30 millimeter cannon pods were developed for the A-37B to give it more punch, but apparently such pods were rarely or never used in operation.

* A total of 577 A-37Bs were built, with 254 delivered to the South Vietnamese Air Force. They reportedly performed well in actions against Communist forces. When South Vietnam fell in early 1975, a South Vietnamese Air Force officer, Captain Nguyen Thanh Trung, defected to the Communists and led strikes against South Vietnamese government positions in his A-37B.

There were about 187 A-37Bs in South Vietnamese hands when the country fell. 92 were recovered by the Americans, while the other 95 were later used by the Communist Vietnamese in missions over Cambodia. These "renegade" aircraft were phased out of service in the late 1970s or early 1980s, no doubt due to lack of spares.

After the war, the USAF passed their A-37Bs from the USAF Tactical Air Command to Air National Guard and Air Force Reserve units. In the early 1980s, these aircraft were assigned the FAC role and given the designation "OA-37B". The OA-37Bs were eventually phased out and replaced in the FAC mission by the much more formidable Fairchild OA-10A Warthog.

The A-37B was also exported to Latin American air arms, mostly during the 1970s. It was well suited to their needs because of its simplicity, low cost, and effectiveness for insurgent warfare. Most of the A-37Bs exported south had the refueling probe shortened to act as a single-point ground refueling probe, or deleted completely.

Latin American countries that received the A-37B included Peru (36 aircraft), Chile (34), Columbia (14 originally, plus 12 reportedly supplied later), Ecuador (12), Uruguay (8), Honduras (15), Guatemala (13), and El Salvador (18). Super Tweets remain in service in some of these countries in slowly declining numbers.



* Cessna proposed a number of interesting variants of the Tweet that never went into production.

In 1959, Cessna built a prototype of a light jet transport version of the T-37, designated the Cessna "Model 407", which was stretched 60 centimeters (two feet) to accommodate a four-place pressurized cockpit with an automobile-type configuration. The prototype performed well in tests, but Cessna couldn't find any buyers. The company also proposed a similar four-place military light transport, the "Model 405", with a big clamshell canopy, but it was never built.

In response to a US Navy "Tandem Navy Trainer" (TNT) requirement, Cessna proposed a T-37 with a modified fuselage featuring a tandem cockpit. The Navy chose the North American T-2 Buckeye instead, and the Cessna concept never got off the drawing board.

Cessna proposed various other trainer derivatives for the US Navy and Air Force, including a vertical take-off version based on the TNT configuration and incorporating lift-jet pods in the wings, but none of them got to the prototype stage.

Cessna also proposed enhanced strike variants of the T-37. The AT-37E/STOL (short takeoff & landing) concept incorporated more powerful engines, thrust reversers, and big flaps. Side-by-side and tandem seating versions were offered.

The A-37D also had more powerful engines, a centerline gun pod, and a single-seat configuration based on the TNT fuselage. As with the improved trainer concepts, the improved strike versions never flew.

* I have a certain partiality to jet trainers. This is not just because I tend to get a bit bored with fighters and bombers. Trainers just look they would be tremendous fun to fly. The Tweet is doubly interesting, because as a trainer it does give that sporty feel, but then, given a steroid upgrade to a strike aircraft, looks like it definitely is a little tough guy who means business.

It has also been unambiguously successful, having been built in good numbers, served the USAF for decades in training, and also given a fair account of itself in combat, though admittedly it would have a hard time surviving in a battlefield environment where an enemy had serious air defenses.

I've never actually seen an A-37, and only seen a T-37 once. One conveniently flew at low altitude the top of my house in Loveland, Colorado, heading due south, I suspect on its way to USAF facilities in Colorado Springs. "Good Bob, it really is a Screaming Mimi!"

* Although I found scraps of data on the Tweet in a few books, this document is essentially an outline of a single source:

* Revision history:

   v1.0   / 01 dec 98 / gvg
   v1.1   / 01 mar 01 / gvg / Minor cosmetic update.
   v1.2.0 / 01 nov 02 / gvg / Cosmetic update.