Grant Maloy Smith

Thursday, February 15, 2024 · 0 min read

How They Did it: Testing the Boeing 747 Jumbo Jet

In 1963 the US military opened a competition to build a new strategic transport. They wanted an aircraft that could carry enormous loads to farther distances. Boeing entered the competition against major rivals like Lockheed, Douglas, General Dynamics, and Martin Marietta. They designed a huge transport that could load military tanks and other cargo from the front via a hinged nose, elevated cockpit, and hump behind it. Ultimately they lost the competition to Lockheed (now Lockheed Martin), but they had done a lot of design work about how to build such an enormous aircraft.

Not long after, Pan American Airlines president Juan Trippe proposed to Boeing president William Allen that they build the world’s largest commercial passenger jet. Pan Am was one of the biggest and most successful airlines in the world at the time. They wanted a wide-body jet that could carry more passengers at a lower cost per mile. Larger aircraft could carry more fuel and thus travel longer distances, opening up new markets.

Trippe agreed to order such an airplane if Boeing would build it. Allen reportedly said, “If you buy it, I’ll build it.” Six months later, Pan Am sent Boeing a $500 million order to deliver 25 of the world’s first wide-body commercial jet airplanes for $20 million each (5B USD equivalent of 4.6B € today).

The development was daunting. Numerous challenges stood in their way. But armed with the Pan Am purchase order, Boeing undertook the most ambitious development project in its history, to design, test, and then build the Boeing 747.

Boeing’s company newspaper after the Pan Am order was placed, April 14, 1966

The first big challenge was the lack of a facility big enough to assemble the 747. Boeing cleared a forest just north of Seattle to build it. This factory remains the largest building in the world at 13,385,378 cubic meters (472,370,319 cubic feet). It encompasses almost 100 acres (39.9 ha). Despite its enormous original size, it has been expanded several times over the decades to handle the 767, 777, and 787 programs.

Another challenge was that many airports simply didn’t have runways long enough for huge airplanes like the 747. And airport terminal jetways were not equipped to handle such an airplane. Airport infrastructure would have to grow to accommodate the new era of widebody aircraft.

The program manager

In the Summer of 1965, Boeing reassigned engineer Joe Sutter from the 737 development program to the 747 program. Joe reported to Malcolm Stamper, the top man in charge of the program. Sutter was later described as the "father of the 747" by the Smithsonian’s Air & Space magazine. 

Joe Sutter, photo by Kathy Stauber, University of Washington, Smithsonian magazine

Joe wrote an autobiography called 747: Creating the World's First Jumbo Jet and Other Adventures from a Life in Aviation, in which he described the development and testing process of the 747. Joe was of Slovenian descent. his father, Franc Suhadolc came to America when he was 17. His surname was converted to “Sutter” when he arrived. 

Finding the engines

Boeing engineers were experts at building airframes, but they needed another company to build the engines that would power them. The problem was that the enormous jet engines that the 747 required only existed for military aircraft. As it happened, West Hartford-based Pratt and Whitney had developed a very powerful JTF14 engine for the US Air Force’s C-5 Galaxy program, but they had lost the contract to General Electric. 

Hearing about the upcoming 747 aircraft, Pratt and Whitney proposed using their JTF14 as the basis for a powerful new engine to push the Boeing 747 into the sky. Boeing contracted with Hartford-based Pratt & Whitney and the race was on.

Looking inside the JT9D Turbofan jet engine. Olivier Cleynen, CC BY-SA 3.0 via Wikimedia Commons

Boeing experienced numerous troubles with the JT9D engines provided by Pratt & Whitney (now called United Technologies). According to Joe Sutter’s autobiography, he and others at Boeing believed that Pratt & Whitney wasn’t sufficiently serious about addressing the troubles.

In 1968 they performed a test flight of the JT9D by mounting it onto a Boeing B-52 aircraft. The engines delivered to Boeing, however, proved to be somewhat fickle, and no two of them performed the same way. 

In his book, Joe Sutter describes watching chief test pilot Jack Waddell bring four engines up to full power when suddenly one of them stopped cold. The torque was so high that the entire nacelle moved upwards dangerously on the wing. This could have been tragic if they had been flying. Investigation revealed that a shaft had snapped and collided with the turbine blades. 

Pratt & Whitney provided replacement shafts, but the cause of the problem was a mystery. Boeing and Pratt & Whitney engineers investigated and learned that the low-speed and high-speed compressors and turbines were not winding up or down at the same rate. They worked well when running at a constant speed, but abrupt throttle changes could lead to flame-outs or worse.

Pratt & Whitney president Barney Schmickrath and his team flew from Hartford to Seattle to meet with Boeing president T. Wilson, program manager Joe Sutter, and chief test pilot Jack Waddell to confront the problem. There were some tense moments when Wadell sensed that Schmickrath wasn’t taking the issue seriously enough. So Wadell invited him and his senior staff on a test flight, during which he pulsed the engines to demonstrate the problem. 

When the turbine contained excessive fuel when one of these events occurred, a loud bang could be felt everywhere in the 747, and a long flame would come out of the engine. Schmickrath saw the problem and committed his company to fixing the problem.

Testing revealed that stresses were causing the engine casings to be flattened, or “ovalized” enough that the turbine blades made contact with the inner walls. Boeing engineers did the structural analysis and handed the results to Pratt & Whitney engineers so that they could stiffen the casings and fix this problem permanently.

Being the first large, high bypass ratio turbofan jet engine to power a wide-body airliner, the JT9D turbofan jet engine provided 235,700 N (53,000 lb) of thrust. Each engine weighed 4,153 kg (9,155 lbs) by itself. The engines were used for years on many versions of the Boeing 747, and 767 as well as the Airbus A300 and A310, among other aircraft. 

Boeing ramps up production

By 1968 roughly 20,000 Boeing employees were working on the 747. In time more than 40,000 people were working on the program. The first 747 to roll off the assembly line in Everett, Washington was used for the flight testing regimen that Boeing engineers had planned. She was christened as the”City of Everett,” honouring her place of birth.

The first Boeing 747 was shown to the public, in September 1968. Public domain photo from Wikimedia Commons.

The first flight test

RA100 takes off for the first time. Photo courtesy of Boeing

The first test flight took place on February 9, 1969. Journalists, engineers, and spectators alike were stunned watching the world’s largest passenger plane roll down the runway and take off effortlessly from Paine Field. The 747’s official FAA registration number was N7470, but the flight crew and engineers referred to it by its Boeing serial number, RA001.

The flight crew was made up of pilot Jack Waddell, copilot Brien Wygle, and flight engineer Jess Wallick. Joe Sutter called this crew “the three Ws.” 

Left to right: pilot Jack Waddell, co-pilot Brien Wygl, and navigator Jess Wallick. Photo courtesy of Boeing

The gray Washington skies turned blue as the pilots took RA001 into the sky. They performed a variety of tests that day, including a simulated loss of hydraulic power and subsequent recovery as well as a variety of cross-control maneuvers such as the Dutch Roll (a combination of roll and yaw). 

Stall testing

Boeing Test Pilot Jack jack Waddell had expressed concern about the possibility of the four jet engines stalling at the same time in the event of a dramatic pitch-up of the 747.

A stall is when an aircraft’s angle of attack is too steep, i.e., when the nose of the aircraft is pitched up more than about 15 degrees. This results in a vortex forming above the wings, which interferes with their lifting capabilities. Unless the stall is corrected, the aircraft’s speed will drop and the nose will pitch down, sending it into a downward spiral. Stalls have caused numerous accidents because the pilots had faulty information about their speed and angle of attack, because they didn’t pull the aircraft’s nose down soon enough, or were unable to pull the aircraft out of the downward spiral.

RA001 was subjected to stall testing to certify that normal flight operation could be recovered and controlled after a stall condition. On this first day of testing, the world’s first 747 performed well during her stall tests, and Wadell was pleased with the results.

Video from Boeing, showing stall, ground effects, flutter, and other tests (stall test starts at 1:50)

Control surface testing

Unfortunately, RA001 had some trouble when the crew began control surface testing. When her flaps were extended from 25 to 30 degrees, the aircraft began to shudder badly. Wadell quickly returned the flaps to 25 degrees and the shuddering stopped. Flight engineer Wallick went aft and looked out the windows at the wings. He discovered that part of a starboard wing flap had become loose. When he brought this unhappy news back to Wadell and Wygle, they decided to end the day’s testing schedule early, and let Boeing engineers investigate the problem.

Although the testing was over, RA001 was cleared to fly again that day to allow a second aircraft to fly along with her and take documentary photos, one of which is shown below. When that was done the aircraft was brought back to Boeing, so that the engineers could work on the flap issue.

RA001 on her first day of flight. Photo by the Museum of Flight

Despite the abbreviated first flight, Pilot Wadell gave a glowing review of the 747, saying

(it’s)... is a pilot's dream; it's a two-finger airplane. I only needed to move my forefinger and thumb around the control wheel to fly it, and it gave responsive movements.

Then more months of testing

In his autobiography, Joe Sutter described the euphoria that the Boeing engineers and test pilots felt after the first flight. In the weeks and months that followed they tried to do test flights every day. 

Landing gear testing

At first, they focused on cycling the 18-wheel landing gear, and on landing the huge airplane in general. Plenty of testing of the immense landing gear system had been performed on the ground, but never in actual operation. They found no significant issues with the landing gear during taxiing, take-offs, or landings.

Prototype Boeing 747 landing gear. Image by Clemens Vasters from Viersen, Germany, CC BY 2.0, via Wikimedia Commons

Performance testing

They increased RA001’s speed over time, working their way up to her top-rated speed of Mach 0.85, or about 652 MPH (1049 km/h). They would eventually exceed that speed and push RA001 beyond its limits. They also began loading the 747 with more and more weight, to measure the effects on handling and damping.

Fatigue testing

Following the explosions of two de Havilland Comet jetliners in the 1950s, the FAA and other agencies worldwide began testing aircraft for fuselage fatigue. This condition can result from repeated cabin pressurization and depressuration during normal use. 

Jetliners operate at altitudes up to 35,000 feet, and the cabins are pressured to the equivalent of 8,000 feet, for passenger comfort. The Boeing 747 was tested against fuselage fatigue by pressurizing and depressurizing the cabin thousands of times on the ground, in addition to a host of individual component tests.

A 52-page fatigue analysis report written by Max Spencer of Boeing in 1972, reveals the extent of the company’s obsession with making the 747 a “fail-safe” aircraft that could withstand the most extreme operating conditions and yet remain safe and intact. According to Boeing, the 747 is designed to withstand 150% of the highest loads it could experience during normal operation. 

One figure from Boeing’s 747 Fatigue Integrity Program shows the doublers and stiffeners used to address cracks in the fuselage skin

Fatigue tests subject the aircraft to several normal operating lifetimes, to verify that they will last and perform safely for decades, assuming proper maintenance. 

Spreading the testing workload

Before long, Boeing built four more aircraft to use for testing. So Sutter and his teams divided the remaining tasks among them. The first ship, RA001, was used primarily for flutter and stall testing. Ship 2 was used as an engine testbed platform as well as for system tests. Ship 3 was used for structural testing, and the last two ships were used for destructive static tests on the ground. The goal was to operate all five ships every single day to complete FAA certification in an unprecedented 10 months.

Damping and flutter testing

As the name implies, “flutter” occurs when an aircraft’s inertial force, elastic force, and aerodynamic pressure combine to “self-excite” various modes, i.e., vibrations in the fuselage or control surfaces. The various structures of an airplane are designed so that their natural frequencies cancel each other out. Such an aircraft is considered to be “well-damped.” If an aircraft is poorly damped, dangerous resonances can develop, and a structure can tear itself apart.

During flutter testing, the natural frequencies and damping coefficients of the normal modes of flight are measured as airspeed is increased. Measuring and then mitigating it by mechanical damping and other means is extremely important. The testing itself can be dangerous because pilots must intentionally fly the aircraft at potentially troublesome speeds that are outside of the aircraft’s approved operating envelope.

B747 scale model in a wind tunnel undergoing flutter testing at extreme speeds

During the flight test phase of the 747, Boeing discovered that the 747’s damping was too low. Test flights came to a sudden stop as Boeing engineers investigated. They installed accelerometers and motion sensors in critical areas of the test plane. 

The test pilots “pulsed” the aircraft at high speeds by making sharp movements of the controls. They also used shakers to induce vibrations at specific locations on the airframe. After doing many tests at different fuel loadings and centers of gravity they had enough data to analyze. After looking at the data, the engineers discovered that dangerous vibrations were developing between the wings and engine nacelles. The damping they had engineered didn’t work properly when the wings were heavy with fuel.

It was clear that the nacelles would have to be redesigned, but in the meantime, Boeing Joe Sutter's top technical manager Everett Webb, a structural engineer, devised a way to add weight at precise locations on the nacelles to achieve the required damping. After a month's delay, testing resumed.

As with any mechanical structure, large or small, modal analysis is used to measure the effects of stresses on the natural frequencies of structures, so that they can be mitigated.

Dewesoft Modal Testing and Analysis solution

Learn more about the modal analysis here:

Rejected take-off testing

If a take-off attempt must be aborted, how long would it take for the aircraft to stop safely before reaching the end of the runway? How will the brakes handle the extreme stresses and heat, which can reach 1400°C (2552 °F)? What if reverse thrusters cannot be used, and the aircraft is at its maximum rated weight? Will the fuse plugs in the tires automatically deflate them after the aircraft stops, to prevent them from exploding? The Rejected Take-Off test (RTO) is an extreme condition test.

In 1969, Boeing flew one of their test planes to Edwards Air Force Base in the California Desert north of Los Angeles. Test pilot Jack Waddelltaxied down the runway, increasing speed to V1 (where the nose wheel comes off the ground), and then slammed on the brakes to see how quickly the jumbo jet could come to a stop. Unlike today’s composite brakes, these brakes were made of steel, which was not as good at handling the incredibly high heat that built up in mere seconds. The 747 passed these tests easily, bringing a 700,000 lb. (317,500 kg) aircraft to a screeching stop in less than the required distance.

Boeing 747-8 performs an ultimate rejected takeoff test

Velocity minimum unstick testing

During a Velocity Minimum Unstick test, the tail of an aircraft is deliberately along the runway during takeoff. This is achieved by pitching the nose up until the tail makes contact with the ground. This test determines the minimum speed required for takeoff. A tail bumper is added to the rear fuselage to prevent permanent damage to the aircraft. Sparks fly as it is dragged along the runway at high speeds.

VMU test on an Airbus 350-1000 shows the special tail bumper making contact with the runway

In 1969, Boeing flew one of their test planes to Moses Lake Airport 200 miles (320 km) east of Everett to perform minimum velocity unstick testing. According to regulations, this test must be performed at minimum takeoff weight, with one engine idling to simulate a failure. While today there are specially engineered bumpers that can be installed on the aft section to prevent damage to the fuselage, Boeing used a long strip of oak wood. Boeing president T. Wilson invited himself to sit in the cockpit during these somewhat dangerous tests, as program director Joe Sutter observed multiple unstick tests from the ground. The oak strip emitted long tails of fire from the friction with the runway.

Taxi testing

A “dress rehearsal” before the first flight testing, in taxi testing the aircraft is run up and down the runway at various speeds. The performance of the brakes is tested as well, ensuring smooth and consistent operation across a wide range of taxi speeds and aircraft weights.

Taxi testing of the 747-8

Other taxiing tests were performed, such as those involving ice, snow, and water on the runway.

Climatic and wind tunnel testing

Countless other tests were performed, including extreme environmental testing. Scale models are subjected to wind tunnel testing, but full-scale facilities were also used to test whole aircraft or large sections of them. Climatic chambers were used to simulate wind, rain, snow, and ice. Static testing of virtually every component was performed on the ground, subjecting them to loads and stresses far higher than normal conditions. 

Over 10 months, more than 1,000 tests were performed on five different 747 test planes. These aircraft were flown for more than 1,400 hours. In addition, countless tests of components and whole aircraft were performed on the ground.

The first commercial flight of the 747

On 30 December 1969, the US Federal Aviation Administration (FAA) certified the 747 for passenger service. A little over three weeks later she was flown commercially for the first time.

Pan Am Boeing 747-121 N732PA. Image by Bidini Aldo GFDL 1.2 via Wikimedia Commons

On 22 January 1970, a 747-121 christened by Pan Am as the “Clipper Young America” made its first commercial flight. 335 passengers and 20 crew flew from New York to London. But by this time, several other airlines had ordered their 747s as well, including TWA, American Airlines, United Airlines, National Airlines, Branniff International, Air Canada, Air India, Japan Airlines, and more.

Upgrades and other 747 versions

The original Boeing 747 could carry 362 passengers in three classes. Several upgraded variants of the 747 were introduced over the years. The 747-300 could carry 400 passengers in three classes, but the most successful was the 747-400 which carried 416 passengers in three classes. Boeing delivered about 700 of this version. Boeing stretched the 747-400 again in 2012, dubbing it 747-8, which carried 467 passengers in three classes. Additional passenger and freighter models like the 747-8F and 747-8i are still at work in the skies.

Hauling the US space shuttle

NASA bought two standard versions of the 747 and modified them to haul the orbiter from the Space Shuttle long distances. The orbiter normally landed at the Kennedy Space Center, but from time to time was required to land at Edwards Air Force Base. NASA used these upgraded Shuttle Carrier Aircraft (SCA) to return the orbiter to the Kennedy Space Center in Florida for its next mission. 

NASA added struts to handle the additional weight and stresses caused by the orbiter’s load. Two vertical stabilizers were added to the tail section for increased stability. They also removed most of the furnishings and added load and weight monitoring instrumentation that is monitored in flight. NASA orbiters weighed ​​78,000 kg (about 172,000 lbs), so the load was significant.

The Orbiter Endeavor gets a ride from a Boeing 747 SCA. Photo by NASA, Public domain, via Wikimedia Commons

Carrying the US president

The original aircraft used by the President of the United States was a Douglas Dolphin amphibian airplane used by President Roosevelt during World War II. In the 1950s President Eisenhower flew on a Lockheed Constellation. Toward the end of the 1950s, the jet era came, and three Boeing 707-120s were used for the president. 

But in 1990 a special military version of the Boeing 747-200B designated VC-25A took over the job of carrying the president and his staff around the USA and the world. These VC-25As can fly 12,600 km (about ⅓ of the way around the earth), and are capable of midair refueling.

Air Force One in 2021. Image by Kevin McCarthy, Public domain, via Wikimedia Commons

The VC-25A has many classified and unclassified military upgrades to protect it in case of a calamity or even attack. The front of the aircraft is known as the “White House” since this area is configured as the President’s living quarters, office, and meeting rooms. It has advanced communication equipment that allows it to connect the president with secure systems on the ground, and even to broadcast live addresses to television and radio networks.

The planes are referred to as “Air Force One” only when the president is aboard, otherwise, their model name is VC-25A. The USAF has ordered two upgraded Boeing 747-8s (VC-25B) to serve as the next presidential aircraft.

The 747 as a jet engine testbed

The 747 has been a favourite testbed aircraft of several major jet engine manufacturers for decades. Why? Because of its four-engine design, several test engines can be added, leaving plenty of backup propulsion power. In addition, cable-based mechanical systems are sometimes preferred over more modern systems in this application. 

General Electric (GE), a major jet engine manufacturer, has several 747 testbeds in its fleet. In 2018 GE recently retired their oldest testbed, the 747-100, which was built in 1969. Purchased by GE in 1994, the 747-100 testbed was used to test and certify eleven different engines and almost 40 variants. These include the powerful GE90 and the GEnx engines for the Dreamliner. GE has acquired a much newer 747-400 as a testbed aircraft. Other companies that use the 747 as a testbed include Rolls Royce and Pratt & Whitney Canada.

GE’s 747-400 testbed. Image courtesy of General Electric Aviation.

The iconic hump

Perhaps the most iconic feature of the 747 is the hump above and behind the nose. There are several reasons why the hump was added. First, Boeing intended that the 747 should be made for both passenger and freight carrying. 

The 747 freighter’s hinged nose tilts for cargo loading Aleksandr Markin Русский CC BY-SA 2.0, via Wikimedia Commons

When the nose tilts up it’s faster and easier to load freight straight back into the fuselage rather than through side doors. Unlike any commercial aircraft before, the 747 could load in two 8 x 8 ft. (2.44 x 2m44 m) standard cargo containers side by side simultaneously. Originally the area in the hump behind the flight deck was going to be used for the crew to rest during very long flights, but Pan Am president Trippe saw it as a first-class cabin for upscale passengers who would pay for more luxury and privacy. As a result of the hump, a pilot’s eye level is 29 feet (~9 meters) above the ground, about the height of a three-story building.

The end of an era

The Boeing 747 held the record for the largest passenger capacity for nearly 40 years. It wasn’t until 2005 when the Airbus A380 surpassed it. The first 747 rolled off the assembly line in Everett, Washington on 30 September 1968. This event revolutionized air travel by lowering prices and creating the wide-body jet era. In less than six months the 747 had carried more than a million passengers. More than half a century later, the last 747 ever made was delivered to Atlas Air, a freight company that owns 51 of the mighty aircraft. 

The 747 project was a large financial risk, even for a company the size of Boeing. If the 747 had been a failure, the company would likely have folded. But they persisted despite the huge technical challenges and setbacks to follow a 28-month development plan that most insiders considered impossible to achieve. The first 747, RA001, was donated to the Seattle's Museum of Flight. 

1,574 Boeing 747s of many types were built, and about one-quarter of them are still flying. As of December 2023, an impressive 441 Boeing 747s were in active service, primarily freighters. Boeing designed the 777 to replace the 747, and it has already overtaken it with more than 2,200 aircraft ordered, and more than 1,700 delivered so far.

Although the 747 lost its crown to the Airbus A380 as the world’s largest passenger airplane, it had more staying power. 251 Airbus A380s were produced across 18 years, while 1,574 Boeing 747s were produced across 55 years, between 1968 and 2023.

The term “jumbo jet” was coined for the 747. The world’s most famous airplane in her day was also dubbed “Queen of the Skies.” Signage at Seattle’s Museum of Flight calls the 747

...the most significant engineering achievement ever undertaken by private industry without significant governmental support.

For his accomplishments, in 1985 Joe Sutter was awarded the National Medal of Technology by US President Ronald Reagan.