1. Design Origins:
Transition periods sometimes prompt transition solutions. During the 1950s, the piston airliner, in the form of the Douglas DC-6 and DC-7 and the Lockheed L-649/749 and -1040 Constellations, were moving toward their technological peaks, yet the pure-jet engine, other than that powering the ill-fated de Havilland DH.106 Comet I and emerging military aircraft, had yet to reach commercial aviation maturity. The compromise, at least in terms of speed, seemed to be the turboprop engine, which combined elements of both and had already been introduced by the Vickers Viscount in the UK.
It was during this period-specifically 1954-that American Airlines, supported by interest from Eastern, submitted design specifications for what it considered a new class of airliner. Those included a greater than 400-mph cruise speed, profitable operations on sectors ranging from 100 to 2,700 miles, a passenger capacity of at least 65, and the type of short-field performance that would enable it to serve all of the country’s 100 major airports.
In short, it sought greater speed, comfort, and economy than that offered by the current generation of quad-engine piston transports, but that could operate multi-sector flights without requiring enroute refueling and attain profitability with load factors as low as 50 percent.
“American and Eastern had demanded a plane equally adept at short- and long-haul operations,” according to Robert J. Serling in “The Electra Story: Aviation’s Greatest Mystery” (Bantam Books, 1963, p. 15). “This was mostly achieved by the thirteen-and-a-half-foot props, which swept their mighty air stream over all but nine feet of the wing area.”
Toward that end, Lockheed elected to employ the same C-130 Hercules design team and Allison T-56 engines that powered the type, creating the US’s first turboprop-powered airliner, the L-188.
“Lockheed opened America’s commercial jet era by hanging a propeller on the jet engine,” according to Jim Upton in “Lockheed L-188 Electra” (Specialty Press Publishers and Wholesalers, 1999, p. 7). “Research left Lockheed convinced that, while jets without propellers (would be) excellent on long-range fights, airlines would be better served by having an effective vehicle for segments which historically showed little or no profit-(that is), short to medium routes.”
The aircraft was almost the product of an equation which read: “Jet power + propeller efficiency = proper performance and economy.”
Aside from its design team and powerplant, it also shared another aspect of the manufacturer’s lineage: its name. Ensuring that its products would bear the designation of a star, as had occurred during the 1920s and 1930s with names such as “Orion,” “Vega,” “Sirius,” and “Altair,” it would borrow the nomenclature of its twin piston engine L-10 Electra, L-12 Electra Junior, and L-14 Super Electra.
Eastern and American respectively placed orders for 40 and 35 L-188 second generation Electras in 1955.
2. Design Features:
“(The Lockheed L-188 Electra) has a purposeful and powerful profile,” according to veteran American Airlines Captain Arthur Weidman, who had flown DC-3s, Convairliners, DC-6s, and DC-7s. “The nose slopes downward sharply to provide good forward visibility on the ground and in the air. Then, her lines go straight back along a perfectly cylindrical fuselage to give her a wider cross section than the DC-7… There is a graceful upsweep to its dorsal fin and rudder, effecting a sleek, trim, streamlined look. Slender nacelles jut forward like giant probes, offering a minimum of frontal resistance.”
With a 104.6-foot-long and 11.4-foot-diameter fuselage, the Electra featured large, square passenger windows.
One of the keys to its design was its wing. Appearing proportionately too short in span for the aircraft it supported, mounted with considerable dihedral, and sporting square tips, it was only 5.5 feet shorter than the fuselage itself, at 99 feet in length, and offered both a low-drag and -aspect ratio. Its trailing edge flaps ran from the fuselage root to the ailerons, or just beyond the outer engines’ exhaust nozzles, and almost 80 percent of its span was subjected to lift-generating prop wash, facilitating low-speed handling.
Power was provided by four 3,750-eshp (equivalent shaft horse power) Allison 501-D13 turboprops, which turned 13.6-foot-diameter, single-rotation, hydraulically-controlled, constant-speed, reversible pitch, four-bladed propellers. Compared to the pure-jet engine, the prop jet featured reduction gear that drove both the propeller and additional gas turbine section stages, resulting in a 90:10 thrust production ratio, or 90 percent created by the propeller and ten percent by the exhaust gases.
The ailerons, elevator, and rudder were operated by push-pull, tube-linked hydraulic booster units, while engine compressor bleed air provided anti-icing of all control surfaces.
The aircraft’s 5,520-US gallon fuel capacity was stored in four wing integral tanks, divided into the two, 1,100-gallon inboard and two 1,660-gallon outboard ones. In-flight fuel cross-feeding was only necessary on long-range sectors exceeding 1,800 miles.
The L-188 rested on a twin-wheeled, hydraulically actuated, forward-retracting tricycle undercarriage, which had the provision for gravity free-fall extension in the event of either hydraulic or electrical system failures.
Integral, fuselage extendable air stairs, along with other self-contained features and its low-to-ground, support equipment-independent position, facilitated turn-arounds at transit stations where fueling was not required in as little as 12 minutes.
The Electra was standardly flown by a three-person cockpit crew, with a duplicate throttle quadrant on the captain’s and first officer’s sides and the flight engineer’s station behind both in the center on domestic routes, while a fourth crew member, the navigator, was employed on international ones and positioned on the aft, left side, occupying the location of the otherwise observer’s eat.
Although passenger cabin configurations and densities varied according to the operator, Lockheed initially offered several options, facilitated by the installation of seat tracks. Either 66 four-abreast, 20-inch-wide seats at a 38-inch pitch with a 26-inch aisle or 85 five-abreast, 18-inch-wide ones with a 17-inch aisle could be installed, both of which also featured a six-place, tail-located lounge arranged in a semi-circular configuration. Installation of aft, as well as the standard mid, lavatory reduced the capacity to 83, while the maximum was 99 five-abreast in 20 rows. Alternatively, 127 passengers in a six-abreast, 32-inch pitch configuration was available, but required structural modifications and additional exits to meet evacuation criteria.
A maximum, 6.55-psi differential, achieved by two engine-driven superchargers, provided cabin pressurization and temperature was maintained by radiant heating.
Baggage, cargo, and mail were carried in two underfloor, starboard door-accessed holds.
Featuring a 113,000-pound maximum takeoff weight, the initial, domestic L-188A version had a 2,200-mile range and attained a 373-mph cruise and 448-mph maximum speed.
“There were… two basic versions, the L-188A for US domestic operation, with a fuel capacity of 5,520 US gallons, and the L-188C with 900 US gallons more fuel and a higher gross weight of 116,000 pounds… ,” according to Michael Hardy in World Civil Aircraft since 1945 (Charles Scribner’s Sons 1979, p. 93).
Its range was 3,500 miles.
3. Test Flights:
Piloted by Captain Herman “Fish” Salmon, First Officer Roy Wimmer, Flight Engineer Laurie Hallard, and Flight Test Engineer Bill Spreurer, the L-188 Electra made its inaugural flight from the Lockheed Air Terminal in Burbank, California, on December 6, 1957, after which Spreurer commented, “The smoothness and quietness of the aircraft (were immediately apparent). The vibration level was very low and the engines were so quiet that you could hear the chase aircraft.”
After a four-airplane, 655-hour flight test program, the type was certified on August 12, 1958, five weeks ahead of schedule, permitting first delivery of aircraft 1007 to launch customer Eastern Airlines two months later, on October 8.
Accolades of the Electra’s design and performance capabilities accrued, as pronounced by the Air Line Pilots Association evaluation committee.
“Members were very much impressed with the rapid power application possible and with the immediate airplane response in climb performance,” they proclaimed. “It definitely exceeded their balked landing and pull-out.
“High-speed stability is good… good control response at touchdown speeds… responded well to the flare-out on landing… crosswind take off and landing characteristics to be most normal…
“The stall characteristics of this airplane in all configurations was exceptionally good. There was no fall-off on one wing or any other adverse tendencies.
“This committee is more than reasonably confident that the manufacturers, the operators, the pilots, and the public will be satisfied with the record of safety, efficiency, and economy which will be achieved.”
American Airlines Captain Arthur Weidman expanded upon this after his first flight in the type.
“Electra is every inch a pilot’s airplane,” he wrote in Douglas J. Ingells’ “L-1011 TriStar and the Lockheed Story” (Aero Publishers, Inc., 1973, p. 124.) His initial impression was that the aircraft exuded “functional beauty.”
Despite its powerful prop-jet turbines, he found it quieter during taxi and acceleration in the cockpit than in comparable pistonliners.
“… It got off in a hurry and climbed rapidly,” he stated. “Obviously, there was a lot of power packed into her streamlined nacelles (and) thrust to spare in the noticeably wide, flat blades of the propellers (p. 127).
A throttle advance to the “flight regime” stage initiated the aircraft’s acceleration roll at a 13,820-rpm speed of its engines, causing the L-188 to achieve its rotation “before it would occur in a Piper Cub. Power is there and speaks through performance.
“The low sound and vibration level make the take off seem effortless and the airplane lifts off… ” he continued (p. 129). “The thumping vibration of piston engines and the long, slow climb out are things of the past.”
Contrary to the throttles on other aircraft, those on the Electra controlled the blade angle, not the engine’s rotations, which remained constant throughout all flight phases. Thrust changes therefore only depended upon changes to their pitch, but needed to be coupled with elevator inputs.
Climbs equaled 2,200 to 2,500-fpm and speeds exceeded 400 mph.
Lift and wing efficiency were considerably enhanced by the prop wash over the upper surface.
“She responds to control actions more like a fighter than a sixty-ton airliner,” he commented (p. 129).
The aircraft’s power reserve was almost astronomical: it could climb on any two engines and maintain altitude on any one.
Landing only required ground contact and a short deceleration roll, aided by brake applications and the reversible pitch of the propellers.
4. Airline Operations:
First and largest of the 14 original operators with 34 L-188As and six L-188Cs, Eastern Airlines inaugurated the type into scheduled service on January 12, 1959, configured for 66 single-class passengers, along with the aft, six-place lounge, on several dual-sector routes, including New York (Idlewild)-Atlanta-Tampa, Miami-New York-Montreal, and Detroit-Cleveland-Miami. It was both the air shuttle’s first- and second-section (to the DC-9) aircraft between 1965 and 1977, linking La Guardia with Boston and Washington.
American, the second operator with 35 L-188As, inaugurated its Electra service the same day as Eastern on the New York-La Guardia to Chicago-Midway route.
National Airlines, which had ordered 15 L-188As, offered a 72-passenger and six-seat lounge interior and connected New York (Idlewild) with Miami as of April 23, 1959.
Braniff, which offered a similar 75/6 arrangement, served the Texas cities of Dallas, Houston, and San Antonio from New York-Idlewild and Chicago-Midway airports.
“Advertised as ‘a totally new dimension in jet-age travel,’ Western Airlines began Electra-jet service on August 1, 1959 between the West Coast cities of Los Angeles, San Francisco, Portland, and Seattle,” according to John Proctor, Mike Machat, and Craig Kodera in “From Props to Jets: Commercial Aviation’s Transition to the Jet Age” (Specialty Press, 2010, p. 91). “Two months later turboprop flights were added to Salt Lake City, Denver, and Minneapolis, as the fleet expanded to five 66-seat, first class-configured airplanes. Seven more Electras followed with the last delivered in 96-seat, all-coach layouts, lacking a lounge.”
Inaugurating service on September 18, 1959 with the first of 18 72-seat L-188Cs, Northwest served its Minneapolis fight base with it, along with operating a transcontinental segment from New York-Idlewild to Seattle.
KLM Royal Dutch Airlines, with 12 67-passenger international L-188Cs, became the only European operator of the Electra, inaugurating it into service on December 9, 1959 on routes such as Amsterdam-Dusseldorf-Vienna, Amsterdam-Frankfurt-Budapest, and those to the Middle East. Its aircraft featured the rounded-tip Hamilton Standard propellers and cockpit navigator’s stations.
The type was also operated as far afield as Hong Kong, Indonesia, and Australia with, respectively, the likes of Cathay Pacific, Garuda, and Qantas.
5. Braniff Flight 542:
While the conclusion of the Air Line Pilots Association concerning the fact that the Electra’s “record of safety, efficiency, and economy will be achieved” was optimistically predictive, the first of its three tenets was, in the event, not realized.
Scheduled to operate the multi-sector route from Houston to New York with intermediate stops in Dallas and Washington as Braniff Flight 542, aircraft N-9705C, the carrier’s fifth L-188A-which itself had only been delivered two days earlier-accepted its 28 passengers on the warm, humid night of September 29, 1959. There was no hint as to the airplane’s fate. Or was there?
Of the six crew members aboard, First Officer Dan Hallowell commented to an Allison representative before departure, “This aircraft trims up funny.” Hallowell could not elaborate, nor did the representative understand his implications. The aircraft’s logbook noted no maintenance, trim-related anomalies. Perhaps it was nothing more than an uneasy, unexplainable feeling.
Divorcing itself from the runway at 2244, the Electra reached its assigned, 15,000-foot altitude 13 minutes later, at 2300, maintaining a 275-knot speed on its relatively short sector to Dallas.
After reporting its position over Leona, Texas, five minutes later, it was instructed, “Request you now monitor Fort Worth on a frequency of 120.8,” which was recorded in the logbook as “Transmission completed, 2307.”
It was its last.
The subsequent event was heard before it was seen by the tiny town of Buffalo, Texas, as most of its inhabitants had already retired for the night. It was an assault of the senses. Shrills and deafening whistles, of varying pitches, preceded a faint roar that culminated in a thunderous cacophony. Like an exploding bomb, it next visually manifested itself as tornadoes of heavy metal shards, fractured fragments of some considerably sized craft. Finally, it entered the nostrils as rain reeking of jet-propelling kerosene, all remnants of Braniff Flight 542 and all at a time when almost 100 L-188s routinely carried 20,000 daily passengers.
Although witnesses on the ground from the predominantly farmland area described various, pre-impact sounds, perhaps the most accurate of them came not from humans, but from canines, when a farmer observed, “When the sound came, every coon dog for miles around started howling.” Why did it affect them so severely?
Reflected on the ground, to a degree, was the aftermath image of what must have occurred in the air, of what had sparked the airliner’s plummet and disintegration. A crater apparently bored by its nose contained the forward fuselage section and a few seats, dismembered from the rest of its body, and behind it, first at periodic feet, and then mile, locations, were its remnants: the center cabin at 225 feet; the vertical tail, rudder, inboard stabilizers, and tail cone at 230 feet; a large section of the right wing at 1,760 feet; the starboard stabilizer at 2,020 feet; the port stabilizer at 4,080 feet; the number four engine nacelle covering at 5,300 feet; the left wing, number two engine nacelle covering and propeller, and number four engine at 8,640 feet; the number one propeller and gear box at 9,600 feet; and a nine-inch section of the number two fuel tank’s hydraulic line at 2.3 miles. Indeed, a 17-mile linear pattern of wreckage stretched from the crater to the LEONA VOR.
Painstaking reconstruction revealed that the Electra had shed its left wing, at which point fire erupted and the limbless airplane dove earthward, shattering from the gravity-induced forces.
Part of the investigation focused on ground witness accounts and claims about the high-pitched sound in the sky before they were even aware of its origin, indicating, perhaps, that the turning propellers had for some reason reached supersonic speeds. The physiological responses of the collective coon dogs was also not to be discounted, since they reacted as if the sound had pierced their ears. But how and why? And what, if anything, did all of this have to do with the first officer’s pre-departure comment about the airplane’s “funny trim?” Could this have been the result of an autopilot or stabilizer malfunction or even a fuel imbalance?
And what was the significance of the damage marks that revealed that the number one propeller had whirled at an angle of up to 35 degrees from its normal plane of rotation? Would it not have been the natural result of the stresses and strains of the left wing as it tore off? Or was it the cause?
Yet exhaustive investigation and analysis revealed no definitive answer-no probable cause-and hence no design modifications could be recommended to correct the undetermined error or flaw.
By March 17, 1960, it was concluded that only the unlikely repeat of the Braniff Electra accident could pinpoint the reason for its demise and the loss of all on board. And on that day, that is exactly what occurred.
6. Northwest Flight 710:
Aircraft N-122US was ironically the first L-188C delivered to Northwest Airlines and had logged fewer than 1,800 hours, but it would not be in service for long. Operating as Flight 710 on March 17, 1960, it had covered the first of its two segments, from Minneapolis to Chicago-Midway, in one hour, fur minutes; however, it was quickly airborne again, now destined for Miami, at 1438 local time, at a 105,000-pound gross weight, reaching 18,000 feet and advising Indianapolis Center seven minutes later that it was over Millford, Illinois.
Proceeding to its next radio checkpoint of Scotland, Indiana, at 1513, it advised, “Maintaining 18,000 and estimating Bowling Green (Kentucky) at 1525.”
Fifteen minutes later, Flight 710 was instructed to contact Memphis Center on frequency 124.6, to which it replied, “Acknowledged.” It was the last transmission received.
The weather was clear, but, based upon the subsequent events, apparently not very cooperative. Penetrating the powerful, unpredictable phenomenon designated “clear air turbulence” (CAT), the Electra was allegedly reduced to a helpless victim, releasing two puffs of white smoke and then a huge black one as its fate was audibly registered as two, ground witness evidenced explosions.
Reduced to an airborne amputee, the airplane shed its right wing and retained little more than the stub of its left one. Initially oblivious, the limbless body continued in a straight-and-level path, but, unable to generate lift and helpless to create or correct a bank without ailerons, it was no longer able to tame one of the three axes of flight the Wright Brothers had so scientifically identified 57 years earlier and succumbed to the instability of air above and the pull of gravity below.
Nosing over, trailing smoke, and shedding structure, it dove like an air-to-ground missile, plunging into a soybean field near Tell City, Indiana, at 618 mph. Gauging snow, dirt, mud, and vegetation, it more than adequately demonstrated Newton’s Third Law of Motion-“for every action there is an equal and opposite reaction”-when the earth ricocheted and spat chunks of itself 250 feet into the air.
What remained was a 30-by-40-foot wide, 12-foot deep crater of smoldering smoke, molecular disintegration, and the obliteration of the 63 passengers and crew on board, since not a single recognizable body was ever found.
Could clear air turbulence have been the culprit?
The only significant piece of wreckage was later discovered in the crater itself.
“The huge fuselage had telescoped and compressed into a mass of molten metal only one-third its overall length,” wrote Serling in “The Electra Story: Aviation’s Greatest Mystery” (op. cit., p. 49). “Of the 63 occupants, there was not enough left to identify-eventually-more than seven bodies. The aluminum fuselage that was their coffin was so hot that five days later a steam shovel picked up pieces that still were burning.”
11,291 feet from the impact point was the severed right wing. The clues were strangely reminiscent of the Braniff accident near Buffalo, Texas. What was the commonality between the two?
One aspect differed. Clear air turbulence and a more than 100-mph jet stream at 18,000 feet, the Northwest flight’s altitude, had intercepted its flight path at a 90-degree angle and had affected other aircraft in the vicinity at the time. But it begged the question: why, if it had been so severe, had they not succumbed to a similar fate?
Clear air turbulence for all its properties, had suddenly become visible to the Federal Aviation Agency. Although the L-188 had more than exceeded its structural expectations, it differed from other propeller airliners, since it represented, to a degree, transition technology: it combined traditional props with still-untraditional turbines, enabling it to eclipse speed boundaries between those of, say, the DC-6 and the emerging military jets.
Like the adolescent who tries to grow up too fast, perhaps it had entered a realm for which it was not sufficiently ready, as the Comet had at high-altitude regions with insufficiently thick fuselage skin gauges. Combined with CAT, perhaps it had proved catastrophic.
Fighting to ground the aircraft, yet unable to identify the definitive cause, the FAA elected to keep the Electra in the sky, albeit at an initially imposed 275-knot speed restriction, coupled with the deactivation of its autopilots and the installation of impact-sustainable flight recorders. When it was realized that this had been the speed of the Braniff aircraft, it was further reduced to 225 knots.
What exactly was happening? The aircraft had, after all, been subjected to rigorous, pre-certification tests.
“… (But) nowhere in the Electra blueprints-which, laid end to end, would stretch forty miles-nowhere in the reports of thousands of hours of ground and test flights-nowhere in 20,000 separate design studies or 7,000 pages of mathematical calculations-was there any mention of a scientific phenomenon known as ‘whirl mode,'” Serling pointed out (Ibid, p. 19).
7. Mystery Solved:
Both laboratory (theoretical) and airborne (practical) exploration and analysis, parts of the Lockheed Electra Achievement Program or LEAP, probed the mystery behind the Braniff and Northwest accidents, and entailed two daily, ten-hour flights, in which various loads, parameters, and speeds were explored, even red-line eclipsing ones. Initially, they only proved the L-188’s design integrity, until a clue, which was not even interpretable, finally surfaced.
Energy propagates and exerts its effects at its final destination. In the Electra’s case, it was ascertained that heavy motion loads had produced a far greater effect on its outboard engine nacelles during severe turbulence penetration than structural tests had revealed, producing a wing bending force from there to the tips, as proven during flight tests over the California mountains that produced tornado-strength updrafts called the “Sierra waves.” The turbulence they created wreaked havoc with the aircraft’s flight controls and structure.
Progressive damage from the number one and number four engines of, respectively, the Braniff and Northwest aircraft had been the result of uncontrolled flutter. Diagonal, saw-tooth fractures indicated the presence of pre-structural failure–cyclic, repetitive, and powerful oscillations—but what could not be answered was why the lack of turbulence over Buffalo, Texas, had caused the same phenomenon as that over Tell City, Indiana. What exactly had sparked the same destructive flutter in the atmospheric-dissimilar mishaps?
Focus next shifted from the weather to the engine nacelles themselves, which opposed each other in installation on the respective Electras involved.
Analyses of what remained of the eight propellers indicated that that turned by engine number one on the Braniff aircraft had, for some reason, wobbled. An over-speed catalyst or condition had caused the tips to reach sonic velocities and with that realization the light of truth had been lit. Both accidents had been caused by propeller whirl mode.
Because a propeller has gyroscopic tendencies, it remains in its plane of rotation until and unless it is displaced by an external source, causing it to adhere to Newton’s “equal and opposite reaction” law. In this case, the propeller continued to rotate in one direction, while the induced whirl mode removed it from its uniform place of rotation and caused it to vibrate in a different one.
If not dampened, removed, or reversed, it develops a wildly wobbling gyroscope, transmitting its energy to that which it is mounted-like an illness that spreads and infects everything in its path-in this case, the wing-or, more precisely, the outer wing. In the Braniff accident, it was the left one. In the Northwest accident, it was the right one.
A strut fairing failure, occurring in the number four engine of the latter Electra, eliminated the restraint that had restricted the engine from moving upward and to the left, resulting in abnormal, omni-directional loads, which caused the engine to experience large cycle motions. These ultimately cracked the propeller’s reduction gear box.
The result, as demonstrated by a one-eighth scale L-188 model in a NASA Langley wind tunnel, was expressed as follows.
“With simulated damage in the nacelle area, propeller auto-precession, a self-sustained, wobbling motion of the spinning propeller involving coupling of gyroscopic and aerodynamic forces, occurred.”
The aircraft’s design flaw did not necessarily entail the inadequate strength of the nacelle structure, but its lack of sufficient stiffening. Affected by previous damage, it developed into a chain reaction of destruction. After its engine had wobbled, so, too, had its propeller and, as its motion was transmitted to the outer wing, it flexed, fluttered, and snapped, leaving the limbless fuselage to the grip of gravity.
Although clear air turbulence had obviously been the spark that lit the chain reaction in the Northwest accident, it could only be surmised that a hard landing, not noted in the logbook, had served as the similar ignition in the Braniff one. Undetected, could this early, not yet catastrophic wobble have not been the reason behind the first officer’s comment that the airplane had “trimmed funny?”
And dogs do not lie, coon or otherwise. As the supersonic speed of the propeller tips emitted painful pitches that virtually pierced their hypersensitive ears, they reacted with a collective howl.
A $25 million, Lockheed financed modification program, applied to both in-service and assembly line aircraft, entailed structural improvements, which resulted in a seven-percent increase in stiffness, and the installation of top and bottom struts, designated “vibration isolators,” were installed in the engine’s reduction gearbox. Its air inlet was relocated and new, stronger engine mounts prevented lateral movements, all resulting in the addition of 1,400 pounds of structural weight.
The aircraft was FAA recertified on December 30, 1960 and, in order to increase public confidence, which had understandably been marred as a result of the accidents, airlines redesignated their modified aircraft “Electra IIs” and “Super Electras.”
8. Program Sunset:
The last three of the 170 L-188As and -Cs produced, registered PK-GLA, -GLB, and -GLC, were acquired by Garuda Indonesian Airways, while the type was given a second lease on life as Central and South American airliners, cargo liners, fire bombers, and as the platform of the foreshortened P-3C Orion antisubmarine patrol aircraft. Alaska-based Reeve Aleutian Airways operated three pure-passenger and combi examples on scheduled services as late as the turn-of-the-century, demonstrating the type’s ruggedness and reliability.
But, as a main line bridge between the piston and pure-jet eras, its crossing was brief and it was quickly replaced by the likes of the Sud-Aviation SE.210 Caravelle, the Boeing 727-100, and the Douglas DC-9-10 and -30 by the mid-1960s.
Hardy, Michael. “World Civil Aircraft since 1945.” New York: Charles Scribner’s Sons, 1979.
Ingells, Douglas J. “L-1011 TriStar and the Lockheed Story.” Fallbrook, California: Aero Publishers, Inc., 1973.
Proctor, John; Machat, Mike; and Kodera, Craig. “From Props to Jets: Commercial Aviation’s Transition to the Jet Age, 1952-1962.” North Branch, Minnesota: Specialty Press, 2010.
Serling, Robert J. “The Electra Story: Aviation’s Greatest Mystery.” New York: Bantam Books, 1963.
Upton, Jim. “Lockheed L-188 Electra.” North Branch, Minnesota: Specialty Press Publishers and Wholesalers, 1999.