Lockheed L-188 Electra

Development

By the mid-20th century, Lockheed had established a strong position in commercial airliner production with its piston-engined Constellation series. Further development brought turboprop engines to the Constellation airframe with the Lockheed L-1249 Super Constellation.

In 1951, Lockheed was approached by Capital Airlines to develop a new turboprop airliner, which was designated the YC-130, but no other carriers had any interest, so the design was dropped. Subsequently, Capital Airlines went on to order 60 British Vickers Viscounts. In 1954, as a result of American Airlines' interest in developing a twin-engined aircraft, the idea resurfaced and the company offered a twin-engined design now designated the CL-303. This newer design was a high-wing type and would allow for 60 to 70 passengers. This design was also shelved for lack of interest from other carriers.

The following year, American Airlines revised its requirement to a four-engine design for 75 passengers with 2,000 miles (3,200 km) range. Lockheed proposed a new design, the CL-310 with a low wing and four Rolls-Royce Darts or Napier Elands. The CL-310 design met the American Airlines requirements, but failed to meet those of another interested carrier, Eastern Air Lines. Its requirements were for a longer range, a minimum cruising speed of 350 miles per hour (560 km/h), and increased seating capacity to the 85-to-90-passenger level. Lockheed redesigned the CL-310 to use the Allison 501-D13 turboprop engine, a civilian version of the T56 developed for the Lockheed C-130 Hercules military transport. The airframe was stretched to allow for more seats and handle the increased performance. This design, led by chief design engineer Lou Storey, was launched as the Model 188 with an order for 35 by American Airlines on June 8, 1955. This was followed by Eastern Air Lines with an order for 40 on September 27, 1955. The first aircraft took 26 months to complete, and by that time Lockheed had orders for 129. The prototype, a Model 188A, first flew on December 6, 1957, two months ahead of schedule. Lockheed was awarded a type certificate by the Civil Aeronautics Administration (CAA) on 22 August 1958. The first delivery – to Eastern Air Lines – was on October 8, 1958, but it did not enter service until January 12, 1959.

In 1957, the United States Navy issued a requirement for an advanced maritime patrol aircraft. Lockheed proposed a development of the Electra that was later placed into production as the P-3 Orion, which had much greater success – the Orion has been in continual front-line service for more than 50 years.

Design

The Model 188 Electra is a low-wing cantilever monoplane powered by four wing-mounted Allison 501-D13 turboprops. It has a retractable tricycle landing gear and a conventional tail. It has a cockpit crew of three and can carry 66 to 80 passengers in a mixed-class arrangement, although 98 could be carried in a high-density layout. The first variant was the Model 188A, followed by the longer-range 188C with room for 1,000 US gallons (3,800 L) more fuel and maximum take-off weight 15000 kg / 33069 lbs.

Operational history

Civilian operations

American Airlines was the launch customer. Eastern Air Lines, Braniff Airways, and Northwest Airlines followed. The Electra suffered a troubled start. Passengers of early aircraft complained of noise in the cabin forward of the wings, caused by propeller resonance. Lockheed redesigned the engine nacelles, tilting the engines upwards 3°. The changes were incorporated on the production line by mid-1959 or as modification kits for the aircraft already built, and resulted in improved performance and a better ride for passengers.

Three aircraft were lost in fatal accidents between February 1959 and March 1960. After the third crash, the FAA limited the Electra's speed until the cause could be determined. After an extensive investigation, two of the crashes (in September 1959 and March 1960) were found to be caused by an engine-mount problem. The mounting of the gearbox cracked, and the reduced rigidity enabled a phenomenon called "whirl mode flutter" (analogous to the precession of a child's spinning top as it slows down, an interaction of propellers with airflow) that affected the outboard engine nacelles. When the oscillation was transmitted to the wings and the flutter frequency decreased to a point where it was resonant with the outer wing panels (at the same frequency, or harmonically related ones), violent up-and-down oscillation increased until the wings would tear off.

The company implemented an expensive modification program (the Lockheed Electra Achievement Program, LEAP) in which the engine mounts and the wing structures supporting the mounts were strengthened, and some of the wing skins were replaced with thicker material. All Electras were modified by the factory at Lockheed's expense, with the modifications taking 20 days for each aircraft. The changes were incorporated in later aircraft as they were built. However, the damage had been done, and the public lost confidence in the type.