The First Steps to a Turboprop Transport, Part One

The YC-131C in flight. Note the 3-bladed propellers.

By the time of the Korean Armistice in 1953, the US Air Force was busy absorbing the lessons of airlift accrued over a less-than-ten-year span from the logistics flights to support World War II to the Berlin Airlift to the strategic airlift partnership forged with the commercial airlines in the Korean War. In pace with advances in aerodynamics and propulsion, the USAF began a four-step process in exploring the possibilities of turboprop propulsion given that pure jet engines of the day were still incredibly fuel-thirsty. The first steps were taken in 1945 with the test program of the Convair XP-81 turboprop fighter that also had an Allison J33 jet engine for additional power. The next steps were the testing of turboprop engines on existing high-speed jet designs that would result in the XF-84H "Thunderscreech and test versions of the Boeing B-47 Stratojet and the McDonnell XF-88 that flew with turboprop engines. The third step was the installation of turboprops on existing transport designs to evaluate their performance on large transports. And the final step was the introduction of production-standard turboprop transports which would result in the Lockheed C-130 Hercules and the Douglas C-133 Cargomaster. 

On 15 June 1954, the headquarters of the Military Air Transport Service (MATS) activated the 1700th Test Squadron (Turboprop) at Kelly AFB, Texas, with the task of developing maintenance procedures and techniques for the employment of turboprop transport aircraft pending the arrival of the C-130 and C-133 into the USAF service. The squadron had three flights with each flight dedicated to a single type for the testing of standard transport aircraft that had been converted to turboprop power. The first of the three flights to be activated would operate the Convair YC-131C. Two aircraft were converted from standard C-131 Samaritan transports (the USAF version of the CV-340 airliner) to use early test versions of the venerable Allison T56 turboprop. The YT56 turboprops replaced the piston engines of the C-131 and drove three-bladed Aeroproducts propellers. As this was the combination planned for the Lockheed C-130A, Allison was, needless to say, keen on being involved in getting flight time for the new engine. Tail numbers 53-7886 and 53-7887 were pulled from USAF service and modified by Convair at their Fort Worth facility at Carswell AFB. After initial flight testing at Edwards AFB, the first YC-131C was flown to Kelly AFB on 20 January 1955 with the second aircraft arriving three days later. 

Ground run of the YT56 engines at Convair Fort Worth.

As the goal of the test program set up by the 1700th TS was to fly the turboprop aircraft assigned to it as much as possible, the YC-131Cs were assigned to a scheduled military passenger service that operated between Kelly AFB in San Antonio and Andrews AFB outside of Washington, DC. The first services began on 14 March 1955 as the first scheduled turboprop passenger services in the United States. Covering a distance of just over 1,200 miles, the YC-131Cs took 4 hours 20 minutes on the first flight, approximately 20 minutes faster than a piston C-131 on the same route. By May of that year a regular flying schedule was established that would have the 1700th TS flying the two YC-131Cs 3,000 flight hours in nine months. As maintenance at destination stations was not expected to be adequate, each scheduled route flown by the YC-131C always returned back to Kelly AFB where the squadron had proper maintenance facilities. By July the Civil Aeronautics Administration (the CAA, the predecessor agency to the FAA) assigned four pilots to the 1700th TS to gain knowledge and experience in scheduled turboprop transport operations. That particular month, the second YC-131C became the first American-built turboprop aircraft to exceed 1,000 flight hours. 

The second YC-131C being handed over the USAF.

In the first six months the YC-131Cs were flown intensively, sometimes over 30 hours per day between the two aircraft. The initial time between overhaul (TBO) on the Allison YT56 engines was set at 100 flight hours at the start of the program but the engine proved to be highly reliable and as the program progressed, the TBO was increased progressively up to 200 flight hours. Though the engines could have safely flown with a longer TBO than 200 hours, Allison engineers were anxious to teardown and study the engines to improve the planned production T56 that would be used on not just the Lockheed C-130 but also on the same company's L-188 Electra airliner. During the nine month test program, 55 engines were changed out and sent back to Allison for analysis. The three-bladed Aeroproducts propellers also had TBO limits, starting at 300 hours and then extended out to 1,000 hours by the end of the test program. With a reliability well in excess of what was possible with piston engines, the two YC-131Cs also became the first USAF turboprops to exceed 1,000 flight hours with one day a record being set with an astounding 46 hours and 20 minutes flown in a 24-hour period, evenly split between the two aircraft. 

On 15 December 1955 the test program with the YC-131C ended, 45 days early thanks to the reliability of the YT56 engine. The USAF gained important data on fuel planning for turboprops, ATC procedures, holding patterns and ground operations that was also shared with the airline industry. In addition, the first squadrons that would be receiving the first C-130A Hercules aircraft at Sewart AFB, Tennessee, Ardmore AFB in Oklahoma, and Eglin AFB in Florida, sent their initial cadre of maintenance personnel to the 1700th TS in San Antonio for familiarization with the T56 engine. Both aircraft were eventually declared surplus and passed on to civilian owners before being scrapped. 

The next blog post will look at the second of the three turboprop conversions operated by the 1700th TS. Stay tuned!

Source: Remembering an Unsung Giant: The Douglas C-133 Cargomaster and Its People by Cal Taylor. Firstfleet Publishers, 2005, p29-43. Photos: Smithsonian Institution, SDASM.