1972 Puerto Rico DC-7 crash

Clemente and a relief committee had leased the aircraft for $4,000 from a local airline, American Air Express Leasing Company,[9][10] which was owned by a 27-year-old Puerto Rican named Arthur J. Rivera.

Unknown to Clemente or to the pilot, the four-engine Douglas DC-7 had suffered a non-fatal taxiway accident just 29 days before the fateful flight took place. This accident damaged the No. 2 and No. 3 propeller blades and the No. 3 engine cooler scoop.[11] Advised to replace one of the engines, Rivera pressed his mechanics to do what they could to inspect the engine and keep it in service, but after inspecting the engines, the mechanics could not find a reason to justify replacing one. The standard procedure after the sudden stop of piston engine is to disassemble the engine to magnaflux its parts for cracks, but this was not done. An FAA maintenance inspector inspected the propeller shaft limits after the sudden stoppage repairs and found them within tolerances, though a later report said that he merely witnessed the inspection.[10]

The post-war era in which cargo carriers operated surplus piston-driven prop planes was at its end, as high maintenance costs restricted the ability to keep up with newer aircraft technology. Rivera had just regained his FAA clearance to operate a cargo plane, claiming that it was his only livelihood. Struggling to keep American Air Express Leasing afloat against a tide of change in the airline industry, he began to cut corners.

Clemente's father, his wife Vera and son Roberto, Jr. warned him that they had bad feelings and premonitions about the flight, and just a few days before the flight, Clemente had a dream about overlooking his own funeral. However, Clemente insisted on conducting the planned relief mission and taking off on December 31 as planned, despite bad weather having hit the area near Isla Verde International Airport.[12]

After volunteers spent most of the afternoon loading the aircraft, pilot Jerry Hill boarded the plane as the sole member of the flight crew. Owner Rivera sat in the co-pilot's seat, though he was only certified to fly the twin-engine Douglas DC-3, which had Pratt Whitney Twin Wasp engines. Rivera may not have understood the added complexity of the DC-7’s Wright R-3350 Duplex-Cyclone engine, which was nearly twice the size and power. Fransisco Matias, a fill-in mechanic employed by another airline who was moonlighting with several other mechanics for cargo carriers at the same airport, sat in the flight engineer's seat because Rivera and Hill had made several unsuccessful attempts to secure a flight engineer.[10][11]

Hill, a well-qualified, seasoned pilot, was in command. He had been found by chance several days earlier while watching the plane being loaded. After another pilot had failed to show from a waitlist of itinerant pilots, Hill flew back from Miami on short notice. He sat in the plane for the first time the previous morning of the flight, and slept all day in a crew bunk to rest for the flight.

This was the aircraft's first flight since Rivera had purchased it several months earlier, and it was the first time that the pilot had flown with either Rivera or Francisco. Clemente boarded with associate Angel Lozano around the same time as the aircraft's crew.

National Transportation Safety Board (NTSB) investigators later estimated that the aircraft's gross weight at takeoff was 148,943 pounds, based on a fuel receipt and customs declaration. The plane had been loaded by a ground crew led by a qualified loadmaster. The calculation was based on the following estimates and measurements:

• Plane: 72,763 pounds
• Fuel: 32,830 pounds (round-trip)
• Cargo: 38,288 pounds (from flight plan)
• Crew: Five members, 1,000 pounds (200 pounds per person)
• Fuel reserve (one hour): 4,063 pounds

At its takeoff weight, the aircraft was three percent (4,193 pounds) over the 144,750-pound maximum takeoff weight for a DC-7C. The crew submitted a flight plan with a cargo weight of 38,288 pounds (including crew and reserve fuel); without fuel, that resulted in a takeoff weight of 116,110 pounds, which was under the 144,750-pound limit. At a minimum, the 1,420-mile one-way flight would need 14,240 pounds of fuel, which would mean a takeoff weight of 130,350 pounds, again under the limit. With a one-way fuel load, the pilot would have to divert to somewhere such as San José, Costa Rica or Panama City to refuel for the return leg. It is possible that the flight's fuel was donated, and there would be a challenge finding fuel in the devastated city of Managua. This left the investigators to guess how much fuel was actually on board, stating that "The actual weight and balance computation made by the crew was not found." Secondary documents indicated that enough fuel for a round trip was purchased and presumably loaded onto the plane,[11]

The commercial airline industry of the 1970s used general factors in calculating takeoff weight and maximum weight limits. However, applying more current scientific knowledge may lead to a more accurate estimate of the aircraft's takeoff weight.

The aircraft's fuel weight was a focus of conjecture. The investigation found evidence that the aircraft was fueled for a round trip. The investigators calculated the weight of that fuel and pushed the aircraft weight to 148,943 pounds. The 1,420-mile distance to Nicaragua was less than half the 3,605-mile range of a fully loaded DC-7C,[13] which holds 7,825 gallons of fuel.[14] At takeoff, the plane was filled to 60% of fuel capacity. However, the 1972 investigation was limited, because at the time, the effect of temperature on fuel density and weight was not well understood by the airline industry.[15] Depending on temperature, gasoline ranges from under six pounds per gallon to 6.75 pounds per gallon at 60 °F. The fuel needed for the four-hour flight to Nicaragua and four-hour return to Puerto Rico was somewhere between 28,480 and 32,400 pounds, a 3,900-pound difference. Puerto Rico has a hot climate, with the December temperature usually above 80 °F, so the fuel would have actually weighed on the low side. The estimated 4,193 pounds by which the flight was overweight is equivalent to 707 gallons of fuel, 9% of a DC-7's fuel capacity, sufficient for an hour of flight.

Another concept introduced after the 1970s is zero-fuel weight, the total weight of the airplane and all its contents minus the total weight of the usable fuel on board. The weight of fuel in the wings has less of a structural effect than that of the fuselage—modern planes have a zero-fuel weight that allows for increasing the maximum takeoff weight when that weight is in fuel.

Air density affects the maximum takeoff weight. Colder air provides more buoyancy, more engine performance and a wider safe engine operating band. The later takeoff time meant the air temperature was 76 °F, 10 °F cooler than daytime evening air. The cooler, denser air provided both better buoyancy and increased engine performance, which at sea level provides a considerable increase to the aircraft's weight capacity (a DC-7C could see a 7,000-pound change with a 10° drop in air temperature).[16][17][18]

The flight might not have been overloaded after all. Regardless, Hill would not have known the concepts of fuel density, zero-fuel weight or air buoyancy adjustment, though by experience he should have become familiar with the engine performance change. The NTSB investigators found that while weight was a factor in the accident, it was not the cause.

A more interesting calculation is that the plane could not have landed with both a full cargo load and enough fuel for a return trip; if at takeoff the aircraft was 148,943 pounds, then in Nicaragua it would have landed after burning 14,240 pounds of fuel and would weigh 134,700 pounds, which is 25,700 pounds over the landing-weight limit. To land at a specified weight and have return fuel would limit the cargo to 12,600 pounds.

The takeoff and landing limits are commercial peacetime limits, but war emergency load limits may be up to 20% higher. The war emergency takeoff limit would be 178,000 pounds and the landing limit would be 160,000; the DC-7C was within those limits, which are set for new military aircraft receiving military maintenance. Hill would have been aware of these emergency cargo limits from his time as a U.S. Air Force major flying the Douglas C-124 Globemaster II on a transpacific route (possibly the Douglas C-74 Globemaster). Whether the Nicaraguan earthquake relief was an event allowing for emergency cargo limits is an open question.

Clemente's wife said she was concerned that the plane seemed old and overloaded.[4]

As air traffic control is merely responsible for directing traffic and cannot be expected to determine if a flight should take off, a lawsuit was brought charging that the FAA should have prohibited the takeoff. The FAA argued that the aircraft was overloaded rather than mechanically unsound.[11] The court ruled that because the FAA had not inspected planes at that airport previously, it was not liable, despite its knowledge of the aircraft's condition and its failure to act. The court stated that the takeoff decision is ultimately the pilot's responsibility, though it did not find Hill at fault.[19]

On the dark, moonless night of December 31, 1972, at 9:11 p.m. local time, after the previously aborted takeoff and additional mechanical work, the plane taxied around the airport's runway 7. By then the weather had cleared and visibility was at 10 miles, with only a few clouds visible.

After engine run-up by the crew, the flight was cleared for takeoff at 9:20:30 p.m. for the four-hour flight to Nicaragua. The aircraft took an exceptionally long takeoff roll and gained very little altitude.[20] A left turn was commenced towards the north, and at 9:23:15 p.m., the San Juan tower received the following transmission: "N500AE coming back around." To land safely, the aircraft would have first needed to dump 32,000 pounds of fuel; as a typical dump rate is one to two tons of fuel per minute, this process would have taken between 16 and 32 minutes.

At or soon after the time of the last radio transmission, the plane experienced a catastrophic failure of engine No. 2. It is possible that engine No. 3 was also lost. Engines 2 and 3 are closest to the fuselage and contain the hydraulic pumps. If both were lost, the pilot would be forced to rely on a controls reversion system. With reduced control and possibly loss of electrical power, the pilot was then faced with the challenge of ditching the aircraft into the sea while maintaining a relation to the horizon over water on a moonless night. In this scenario, the aircraft was essentially unflyable.

With an engine lost, the airplane slowly descended and about 10 to 30 seconds later crashed into the Atlantic Ocean at a point approximately 1​¹⁄₂ miles offshore, and 2​¹⁄₂ miles on the 040-degree radial from the western end of Runway 25.[11] In that time, 500 to 2,000 pounds of fuel would have dumped. In the last seconds of flight, ground effect would have kept the plane aloft, skimming the wave tops.

Delgado Cintrón, a mechanic who witnessed the takeoff from the ground at the airport, testified that the engines sounded even and normal. However, the plane was too low at 25 feet off the ground. Other witnesses estimated that the plane gained altitude to 100 feet. After the aircraft was out of sight behind trees, the engines sounded fine and then, a few seconds later, Cintrón heard three backfires and a large explosion, which he thought was the impact with the ocean, followed by silence.[11][10]

That a DC-7 had lost an engine on takeoff was not unexpected. During World War II, twice as many aircraft were lost because of problems with the same type of engine (in aircraft such as the Boeing B-29 Superfortress) as were lost to enemy fire.[21]

The Wright R-3350 engine that powered the DC-7 had started as a problematic multi-row radial design that was rushed into wartime production. In post-war civilian use, those problems continued, and aircraft with this engine were less favored for commercial flights and often were converted to cargo planes.

Clemente would have had no reason to have known the history of the R-3350, but Captain Hill would have had a good understanding from over 12,000 flight hours piloting multi-row, piston-powered, radial-engine aircraft over his nearly 30-year career, including the DC-4, DC-6, DC-7, C-46 and the USAF Globemaster.

An issue related to the cooling of additional rows of radial cylinders in multi-row radial engines was understood and well known. A key concern was a lean air/fuel condition causing detonation because of the high supercharger boost on such engines, which by itself and with cooling issues was dangerous. Problems with cooling had plagued the engine since its use in the hot Pacific climate during World War II, prompting the military to add an air scoop to the top of the engine cowling that directed air to cool the back cylinders. Those problems only added to the engine's sensitivity to detonation, particularly on takeoff.