The Far Side of the Storm: The crash of Northwest Orient Airlines flight 705
On the 12th of February 1963, a Northwest Airlines Boeing 720 was climbing out of Miami, Florida, when it encountered severe turbulence over the Everglades. Minutes later, amid heavy rain and lightning, witnesses heard an explosion and saw a fireball descending from the clouds, plummeting at great speed into the watery wilderness. When search crews arrived at the scene, they found the wreckage of the four-engine jet scattered across a vast expanse of marsh, along with the bodies of 43 passengers and crew. None of those aboard had survived.
When Civil Aeronautics Board investigators arrived at the scene, they faced several burning questions. The state-of-the-art jet was only a year and a half old, and this was the type’s first fatal crash in passenger service. What could have brought it down so suddenly? Using little more than a primitive, four-parameter flight data recorder, investigators began to piece together a disturbing story of a severe weather encounter, a massive in-flight upset, a vertical dive, and finally a violent breakup as the pilots tried desperately to recover. Even worse, it wasn’t the first time this had happened, nor would it be the last. All over the world, pilots were losing control of large, swept-wing jet airliners under conditions where safe flight should have been possible. The crash of flight 705 would prove to be one of the keys which unlocked the mystery: it wasn’t the planes, and it wasn’t the weather — it was the way pilots were reacting. This discovery would lead not only to major changes in the way pilots were trained, but to the virtual elimination of turbulence-related crashes as a category, no doubt saving the lives of countless future air travelers.
Between 1958 and 1959, the world of commercial aviation changed forever with the introduction of the first successful large passenger jets, the Boeing 707 and the Douglas DC-8. This new generation of airliners represented a massive technological leap, allowing commercial flights to operate higher and faster than ever before. Travel times were cut in half, seemingly overnight, and the airlines’ demand for jets soon became insatiable.
Capitalizing on this growing market, in the summer of 1960 Boeing introduced a now largely forgotten third, or maybe half past second, member of the new airliner family: the 720. Known today mostly for being the only Boeing jet airliner that didn’t follow the company’s 7x7 naming scheme, the 720 was really a heavily modified 707 designed to carry fewer passengers on shorter routes to smaller airports. Differences from the 707 included a fuselage that was 2.5 meters shorter, a redesigned wing, a lighter base weight, and after the first few months of production, a new set of Pratt & Whitney JT3D turbofan engines which produced significantly more thrust. The version with the new engines was referred to as the 720B.
Of the 154 Boeing 720s which were built between 1958 and 1967, several were delivered to Minneapolis-based Northwest Airlines, then one of America’s largest air carriers. At that time, the company was doing business as Northwest Orient Airlines as part of an ongoing brand effort centered around its routes to East Asia, and although it was still legally known as Northwest Airlines, everything passenger-facing, from the check-in gates to the tickets to the livery on its aircraft, would have said Northwest Orient.
Early in the afternoon on February 12th, 1963, a Northwest Orient Boeing 720B, registered as N724US, arrived in Miami, Florida after a flight from Chicago. Having reached the end of their duty day, the crew disembarked; but on his way out, the outgoing captain stopped to talk with Roy Almquist, the pilot who was to take over N724US for its journey back to Chicago. He had little to report, except for the fact that a heavy squall line was positioned a short distance to the northwest of Miami International Airport, and that on the way in he had flown over the storms at cruising altitude, overshot the airport, and then approached from the east in order to avoid the worst of the weather. His suggestion was that Almquist should reverse this pattern on his way out. Almquist thanked him, and the two men parted ways.
The 47-year-old Captain Roy Almquist was something of a renaissance man — he was president of a bank, co-owner of a Ford dealership, president of a school bus company, director of a machine tooling R&D firm, and president of his local Lions Club back home in Minnesota. He also somehow found time to fly for Northwest, where he had worked since 1942, accumulating over 17,000 flight hours and a wide variety of type ratings. Just a few months earlier he had upgraded to a jet for the first time, receiving training to become a Captain on the Boeing 720, where he had since accumulated a mere 150 hours. Most likely, if one were to put him under pressure, they would have found that he was still a turboprop pilot by nature.
Joining him were two more flight crewmembers, consisting of 38-year-old First Officer Robert Feller, whose 11,800 hours included about 1,100 on the 720, and a 29-year-old Flight Engineer, Allen Friesen. Also on board were five flight attendants and a rather light load of just 35 passengers. There would have been 36, but one passenger took one look at the weather, decided she would rather not fly that day, and turned around and went home.
As the pilots prepared to operate Northwest Orient flight 705 to Chicago, Spokane, Seattle, and Portland, they began by reviewing the latest weather information at the company operations office. The data corroborated the outgoing captain’s report, indicating the presence of heavy thunderstorms on a band running southwest to northeast and centered a few miles northwest of Miami International Airport. A SIGMET, short for “significant meteorological information,” had been issued by the US Weather Bureau warning of moderate to severe turbulence within the squall line, but it had expired a few minutes earlier. It was up to Almquist to decide whether conditions were suitable for departure, and in the end he concluded that they were. Black clouds and lightning were visible in the near distance, but there were gaps to the south, and while at least one captain did decide to hold off, other planes seemed to be departing just fine.
At 13:23, as flight 705 was preparing to leave the gate, Captain Almquist contacted ground control and asked, “How are they vectoring out? We’re going IFR [Instrument Flight Rules] to Chicago… any chance for a radar vector around some of this?”
“Yes sir, they’re doing the best they can,” the controller replied. “It’s a pretty thick line northwest of us. Most of the… uh… through a southwest climb or a southeast climb, and then back over the top of it [is] what most people are doing.”
Minutes later, as they were taxiing out, the pilots received their route clearance from Clearance Delivery. “Northwest seven zero five’s cleared to the Chicago via J forty-one radials, Saint Petersburg, flight plan route, maintain three thousand. Expect further clearance at flight level two five zero, ten minutes after Cypress intersection. After takeoff, turn right heading three six zero for vector to J forty-one radials.”
In order to take off into the wind, planes were departing to the west on runway 27L before turning left, to the southwest or southeast, to stay away from the bad weather. But for some reason the controller wanted flight 705 to turn right and head due north, directly into the storm. After switching frequencies to contact the tower, which was responsible for radar vectors, Almquist voiced his displeasure: “They gave us a right turn to three six zero,” he said. “We would kinda like to take that southeast vector, if they’ll give it to us.”
The tower controller told flight 705 to standby while he coordinated with other nearby sectors, ensuring that there would be no traffic conflicts. After a moment, he returned and offered a left turn after takeoff to a heading of 180, or due south. “That’s real fine,” Almquist replied.
At 13:35, flight 705 took off from runway 27L, turned to the south, and began to climb toward 3,000, and then 5,000 feet. All the while, the controllers provided the flight with new headings to keep it away from the storms on their radar: first right to 240, then left back to 180, right to 240, right again to 270, then right to 300. Throughout these turns the flight meandered in a generally southwesterly direction, paralleling the squall line. Controllers appeared unable to route it out to the east over the ocean because of conflicting traffic.
Now level at 5,000, the pilots could see on their radar scope that they were flying directly toward a thunderstorm. “Ah, Departure,” said First Officer Feller, who was now operating the radio, “it looks like we’re going to run right back into this at this altitude. Ah, is there a chance to go back to the southwest or southeast, or climb?”
“Northwest seven zero five, roger,” said the tower. “You’ll enter a precipitation area in about four miles, and you should be out in the clear for about three miles, then back into it again. However, north of the localizer and northwest of Jersey intersection you should break out in the clear and it should be okay from there on.”
“Ah, we’re in the clear now,” said Feller. “We can see it out ahead… looks pretty bad.” The pilots clearly did not want to fly through the storm — they preferred to climb over it.
“Okay, Northwest seven zero five, we’re working on a higher altitude now,” the controller replied. Moments later, the tower granted permission to climb to 25,000 feet.
As flight 705 ascended through the storm, powerful turbulence battered the plane, rattling the cabin and shaking the pilots’ instruments. Their speed fluctuated wildly, and the nose oscillated nauseatingly up and down. To the tower, First Officer Feller reported, “Ah, moderate to s — to heavy turbulence, right through where you vectored us.”
“Can you turn right now?” the controller asked.
But the center of the squall line was still to their right, and they weren’t high enough to pass over it. “Ah, negative,” said Feller. “You vectored us right into that moderate to heavy turbulence… we’re out of ten [thousand] now, we’ll turn right as soon as we can.”
“Northwest seven zero five, the vector I gave you was to the least turbulent area that I have indicated on my scope… stand by,” said the controller.
“Okay, you better run the rest of them off the other way then,” said Feller, sounding cross.
The controller didn’t acknowledge the criticism. Instead, he simply told the crew to contact the Miami area control center on 118.5. But for whatever reason, the pilots could not contact Miami center, either because of interference from the storm or because the cockpit was shaking too heavily to enter the frequency. “We’re unable, one eighteen point nine,” Feller said to the tower.
“Northwest seven zero five, roger,” said the controller. “Stand by on this frequency. Turn right heading three six zero to intercept the J forty-one radials.”
This time, the pilots were able to comply with the request to turn right, heading north across the squall line. Climbing through 15,000 feet, they seemed to have now escaped the worst of the turbulence, and things started to settle down. Now able to contact Miami Center, the pilots called the new controller and reported that they were at 17,500 feet, followed by a few garbled words. The controller called back to ask for clarification, but there was no reply. In fact, no one would ever hear from flight 705 again.
At that moment, in a remote area of Everglades National Park, a group of couples out fishing in the marsh heard a loud boom some distance to the north, where heavy storm clouds had gathered. Turning her eyes to the sky, one member of the party spotted a flash of light, like an explosion, near the base of the clouds, which then plunged rapidly to earth. Several seconds later, another dull boom rolled in from the northern horizon. Timing the delay, they estimated that the explosion had taken place some 18 kilometers northwest of their position. Abandoning their fishing trip, they made their way to the nearest outpost of civilization, some two hours by boat, and reported what they had seen.
Meanwhile, with flight 705 having vanished from both radar and radio contact, a search and rescue mission was launched near its last known position, deep in the inhospitable Everglades. It was not until near nightfall that a Coast Guard helicopter at last spotted the wreckage, strewn across the trackless marsh, some 69 kilometers southwest of Miami International Airport. “This is Coast Guard three-oh-four,” the helicopter pilot reported. “We have the wreckage in sight — it’s all busted up, consumed by fire.” It was evident that there were no survivors.
For investigators with the Civil Aeronautics Board, the predecessor to today’s NTSB, just getting to the crash site would prove a challenge, let alone finding the cause. The wreckage was located in an area of mixed marsh and cypress groves some 24 kilometers from the nearest road; the only way to reach it was by helicopter or all-terrain vehicle. And to say that there was a single crash site would be incorrect — the remains of the Boeing were in fact strewn over an area 24 kilometers long and two kilometers wide, indicating that the plane had disintegrated catastrophically in flight. The westernmost piece of debris was the upper part of the rudder; 150 meters further to the east was the first of the four engines, which were deposited sequentially over distance of 800 meters; and 150 meters northeast of the last engine lay the severed cockpit, along with the bodies of the flight crew. Meanwhile, the tips of both wings were found together some 450 meters due east of the rudder fragment. 365 meters beyond that was center fuselage section, with the majority of the wings still attached, lying inverted in the grass. Much of the fuselage had been destroyed by fire. Finally, the tail section lay still 300 meters further on, and beyond that, light debris, such as panels, papers, and other small objects had been blown by the wind along an eastward trajectory stretching more than 20 kilometers. Scattered throughout were the remains of the 43 passengers and crew, many of them still strapped into their seats amid grass soaked in jet fuel. Although none had survived, some lesser creatures did — a bag of tropical fish, carried in the cargo hold, was found intact in the wreckage with the fish still swimming around inside, apparently unharmed.
In the face of this challenging scene, the CAB set to work, gathering the wreckage and laying it out in the field before transferring it to a hangar at Opa Locka, where they began to reassemble the shattered jet from its constituent parts. The CAB’s dragnet would ultimately recover 97% of the aircraft by weight, allowing a detailed reconstruction. Examination of the reassembled wreckage ruled out almost all the leading theories rather quickly. There was no evidence of metal fatigue in any primary structure. None of the engines had failed before the breakup. In-flight fire and explosion damage was limited, and suggestive of a brief flash fire after the plane had already begun to disintegrate. There were no signs of a bomb, a lightning strike, or a catastrophic failure of any control surface. As far as investigators could tell, the plane had been in perfect mechanical order before it suddenly plunged from the sky and broke apart.
That the “plunging” came before the “breaking apart” was one of the main takeaways from the wreckage examination. Analysis showed that the horizontal stabilizer at the tail had failed first, due to aerodynamic overload in a downward direction; the wings failed next in the same manner. After that, the cockpit separated upward, and the rest of the plane spiraled, on fire, to its doom.
When a plane dives at high speed and the pilot attempts to pull the nose up to effect a recovery, the resulting G-forces can, if the speed is high enough, overstress the airframe sufficiently to break the wings and tail. The damage to flight 705 was consistent with such an explanation, and an examination of the control systems provided a potential clue as to why: the jackscrew which controls the horizontal stabilizer was found set to the maximum possible nose down position.
The horizontal stabilizer, or tailplane, looks like a smaller pair of wings, and in fact it is, albeit upside-down. The purpose of the horizontal stabilizer is to generate downforce, or lift in a negative direction. An aircraft’s center of lift is deliberately positioned just behind its center of gravity, which would cause the nose to drop; the downforce from the horizontal stabilizer counters this tendency, pushing the tail down and bringing the nose up. The balance between the center of lift, center of gravity, and the horizontal stabilizer is what makes stable flight possible (hence the term “stabilizer”).
On most smaller, propeller-driven aircraft, the horizontal stabilizer is fixed in position. On these aircraft types, should the pilot need to adjust the amount of downforce, they may do so using elevator trim, which holds the elevators in position to apply a constant nose up or nose down command. But as jet airliners were being developed, it became apparent that these aircraft differed in a number of key ways, which came together to necessitate the invention of an adjustable, or trimmable, horizontal stabilizer.
Because jets are designed to fly at high speed and high altitude, they needed to be capable of operating closer to the speed of sound than previous aircraft types. This required contending with an aerodynamic quirk: because the airflow accelerates over the top of the wing (which is part of why a wing generates lift in the first place), it may exceed the speed of sound even while the plane is in subsonic flight, generating shockwaves which reduce lift. During the 1940s and 1950s, manufacturers found that the onset of these shockwaves could be delayed by introducing wing sweep — the now-ubiquitous practice of building wings which trail backward toward the tips. Without these swept wings, flight close to the speed of sound would be impossible.
The speed of sound is not constant — it varies with temperature, and consequently with altitude as well, so that in standard temperature conditions its value will drop from about 661 knots at sea level to about 574 knots at 34,000 feet. For this reason, the exact velocity of an aircraft at high speed and high altitude is not so important as its Mach number — its velocity as a percentage of the speed of sound. Most jets cruise at a Mach number of 0.8 to 0.85, much higher than turboprops with straight wings. But the swept wings on these high-speed jets, which helped them achieve these speeds in the first place, came with another side effect. As Mach number increases, an aircraft’s center of lift moves aft, creating a larger nose-down movement due to the greater distance between the center of lift and the center of gravity. On swept-wing aircraft, this effect is amplified, because wings which trail aft allow the center of lift to move farther aft than would otherwise be possible. At cruising speeds the resulting nose-down moment would be so great that excessive force on the elevators would be needed to counteract it. The solution, therefore, was to allow the entire horizontal stabilizer to move up and down, using its much greater surface area to assist the pilot in stabilizing the plane’s pitch.
The concept of such a trimmable horizontal stabilizer first entered widespread use on the Boeing 707 and Douglas DC-8. The basic design hasn’t changed much since: a pair of electric motors, or a manual backup, drive a jackscrew through a nut, torquing the stabilizer up to pitch the nose down, or down to pitch the nose up. Whenever a change in an aircraft’s configuration affects its center of gravity or center of lift — for example, if the flaps are extended, or several passengers move to the rear — the pilot or autopilot can adjust, or trim, the stabilizer to compensate, even at high speeds where the force required to perform this same task using the elevators would be too great. Therefore, the intended use case of the two forms of pitch control was as follows: elevators for small, one-time adjustments; and stabilizer trim for keeping the plane balanced over longer timeframes.
Returning now to the wreckage of flight 705, the fact that the extremely powerful stabilizer was found in the fully nose down position was highly unusual, and pointed to a large stabilizer input as the cause of the plane’s fatal dive. However, no malfunctions of the stabilizer control system were found, suggesting that it was instead moved to full nose down by a pilot.
This interpretation was further supported by the contents of the plane’s primitive flight data recorder. Although cockpit voice recorders were not yet widespread in 1963, the 720 did have an FDR which etched traces corresponding to altitude, airspeed, heading, and vertical acceleration into a rotating spool of foil. These data revealed that the flight came to a shocking and dramatic end.
As flight 705 climbed toward 15,000 feet after takeoff, most parameters were normal, except for vertical acceleration, which indicated that the plane was in heavy turbulence for about three minutes. This turbulence then stopped until the flight reached 17,250 feet, whereupon it leveled off for 12 seconds, then began to climb steeply. Within seconds it was ascending at an astonishing rate of 9,000 feet per minute, far faster than could be sustained under normal conditions. Its speed began to bleed off, falling from 270 to 215 knots as it rocketed upward. The climb continued until the plane reached an altitude of 19,285 feet, at which point it violently pitched down into a near-vertical nosedive, which continued until the plane broke apart and the FDR ceased recording. The question was, were these massive pitch excursions the result of turbulence, or pilot inputs?
To learn more about the weather at the time of the crash, the CAB turned to the Weather Bureau, which conducted a study of the conditions in the Miami area. The study found that the thunderstorms were quite typical for that part of the country, and although they were strong, they were not unusually so. These storms would have generated heavy or even severe air turbulence, with a slim chance of localized extreme turbulence, but the chances that there was any turbulence strong enough to tear the plane apart in flight were vanishingly small.
In order to better understand the 720’s bizarre flight path, investigators also simulated the flight using an IBM supercomputer based on the known weather conditions and the data from the FDR. What the computer determined was that the 9,000-feet-per-minute climb could feasibly have been caused by an updraft associated with a developing thunderstorm, perhaps augmented by pilot inputs. It was noted that while an updraft causes a plane’s altitude to increase, it also reduces the amount of downforce on the tail, causing the plane to pitch down as it is pushed upward; this could cause the pilot to react instinctively by pulling up, steepening the climb. Whether this happened was uncertain, but what the simulation did prove was that the subsequent reversal from climb to descent was so violent that it likely had little to do with the weather. A downdraft strong enough to trigger such a dive would have to be inconceivably stronger than the strongest turbulence predicted by the Weather Bureau’s analysis. Instead, the CAB and Boeing believed that the only way such a rapid descent could have developed was if the pilot simultaneously applied full nose down elevator and full nose down stabilizer trim.
The question, then, was why a pilot would ever do this. In the CAB’s view, Captain Almquist would not have made such an input unless he believed his aircraft to be in extraordinary danger. The only conclusion, then, was that when the updraft caused the plane to ascend steeply, Almquist saw their high rate of climb and plummeting airspeed and concluded that the plane was in danger of stalling. The only way to prevent this, in his view, would have been to push forward on his controls as hard as he could. As he did so, he used the electric trim switches to move the stabilizer in the nose down direction, altering the plane’s stable pitch angle in order to reduce the force required to move the elevators.
The problem here is that the stabilizer is not supposed to be used in this manner. As mentioned earlier, its purpose was to compensate for longer-term changes in the plane’s stability characteristics over the course of the flight. It should not be used to recover from an updraft, which may only last a few seconds — that’s what the elevators are for. Pushing the elevators to full nose down requires considerable force, but then when the updraft goes away, which it inevitably will, the pilot can simply let go of the controls and the plane’s inherent stability will cause it to resume whatever flight profile it was maintaining before. On the other hand, using the stabilizer trim to push the nose down will make it easier to move the elevators, but it will also alter the plane’s inherent stability, so that when the updraft goes away, its natural desire will be to dive. On flight 705, that was exactly what happened: trimming the stabilizer to full nose down did counteract the effect of the updraft, but then the updraft went away, and now the plane was configured incorrectly for the conditions. The technical term for this is “out of trim.”
The CAB believed that the updraft in fact disappeared almost concurrently with Captain Almquist’s elevator and stabilizer inputs. With both pitch control surfaces at full nose down, and no more updraft to counteract them, the plane pitched down so suddenly that the occupants experienced a mind-bending 2.8 negative G’s — the equivalent of being pulled toward the ceiling at nearly three times the force of gravity. The plane was embarking on a downward trajectory, while everything inside it, due to conservation of momentum, attempted to keep going up. This would have been incredibly disorienting for the pilots and passengers alike. The pilots would have been lifted out of their seats, restrained only by their seat belts, and were most likely unable to properly reach the controls. Loose objects — baggage and books, pencils and cups, trash, dirt, and dust — would have been flung violently upward. Any passengers who had ignored the fasten seat belt signs would have been seriously injured. The pilots’ vision would have blurred, and the cockpit would have been filled with a dazzling cacophony of sound as the G-forces momentarily activated every aural alarm at the same time.
Throughout this maneuver, which lasted for eight seconds, the simulation indicated that the pilots’ control columns must have remained at full nose down, continuing to drive the plane into its dive. Studies carried out by the CAB in fact found that at high negative G-loads, pitch control forces on the 720 tended to lighten considerably or even reverse. If the feedback force on the controls lightened to zero, then the pilots’ control columns would have remained at full nose down, exactly where they left them, even if they let go. If a reversal occurred, then it would actually have taken more force to pull the controls back to nose level than to push them further nose down. Either of these scenarios would have been quite contrary to normal operation, where aerodynamic forces should push the elevators (and consequently the control columns) back to the neutral position when the pilot lets go. This phenomenon therefore explained why the elevators did not return to neutral, even though the pilots should not have been able to reach the controls under a vertical acceleration of -2.8 G.
By the time the pilots managed to grab the controls and attempt a recovery, the plane was in a near-vertical dive, descending through 16,000 feet with rapidly increasing airspeed. According to the simulation, the pilot most likely pulled the control column back to the neutral position, held it there for a few seconds, then wrenched it back to full nose up. However, not only was he trying to recover from the extreme aircraft attitude, he was also fighting against the stabilizer, which was still set to full nose down, creating an out of trim condition. Captain Almquist most likely would have attempted to use the electric switches to drive the trim back to nose up, but if he did, he would have found this to be impossible.
As mentioned earlier, the stabilizer moves up to pitch the nose down, and down to move the nose up. However, the elevators, which are attached to the back of the stabilizer, work the opposite way, moving up to raise the nose and down to lower it. Therefore, moving the elevators up creates a hinge moment which attempts to force the stabilizer up as well. Because “stabilizer up” means “nose down,” pulling up using the elevators will actually increase the nose down aerodynamic loads on the stabilizer. At very high speeds during a dive, such as that experienced by flight 705, this aerodynamic force can be so great that it exceeds the capability of the electric trim motor to drive the stabilizer in the opposite direction, causing the motor to slip and making it impossible to bring the airplane “in trim.” The only way to recover is to relieve the force on the stabilizer by pitching down, trimming nose up, and only then pulling up with the elevators.
However, this maneuver prolongs the dive, and the crew of flight 705 didn’t have enough time or altitude to carry it out. Instead, the pilots attempted to overpower the stabilizer by brute force. At first it seemed to work — the load factor reversed from -2.8 G to +1.5 G, indicating that the plane was beginning to pitch up. But it was too late. Seconds later, the load factor reversed again, trending back into the negative. The plane pitched over to 90 degrees nose down, and then kept going, beyond the vertical. The plane was rocked by heavy buffeting, like that which precedes an aerodynamic stall — only this wasn’t a regular stall, it was a negative stall. Most regular readers will by now be familiar with the basic concept of a stall — if the angle of the wings into the airstream, or angle of attack, becomes too great, then smooth airflow over the wings ceases, heavy buffeting occurs, and then the plane stalls and falls from the sky. On the other hand, even many pilots may not have considered that it is possible to stall an airfoil at high negative angles of attack as well. In this case, with the plane pitched more than 90 degrees nose down but still traveling on its original heading due to conservation of momentum, the airstream was impacting the wings from above rather than below, resulting in a negative angle of attack, and eventually negative stall buffet. As for the significance of this fact to the accident sequence, there is none — by this time the situation was completely unrecoverable. Nevertheless, the fact that such a thing occurred serves to illustrate the extreme forces at work during flight 705’s final moments.
As for why the pilots’ efforts were overcome, causing the dive to steepen, investigators identified two main reasons. One was their speed — in fact, they were traveling so fast that the airspeed stylus in the flight data recorder had reached its mechanical stop at 470 knots, causing it to record a flat line. The actual speed achieved is not known for sure, but it may have reached as high Mach 0.95. At such high Mach numbers, the airflow over the horizontal stabilizer can become supersonic, resulting in shockwaves which reduce elevator effectiveness, stymieing the pilots’ ability to pull out. The second possible factor was Mach tuck — the tendency of an aircraft to pitch down due to the aft movement of its center of lift at high Mach numbers. If the Mach number is sufficiently great, this “Mach tuck” can overpower the elevators, making it impossible to pull up. One or both of these factors likely explained why the dive steepened despite the pilots’ efforts.
However, a study conducted by Boeing showed that in the end, it didn’t matter one way or the other. Per Boeing’s analysis, recovery would only have been possible if full nose up elevator was applied before the airspeed reached 320 knots, and even then, over 300 pounds of control force would have been required. If this were to be accomplished, the plane would pull out at 5,000 feet. Any later, however, and the aforementioned high-speed effects would make it impossible to pull out no matter how much control force the pilots applied.
In the event, the pilots did not manage to achieve full nose up elevator until the speed was already in excess of 470 knots, by which point it was too late. The plane pitched back down again, the dive angle reached 95 degrees, the speed approached Mach 1.0, and the vertical acceleration reached -3.5 G’s, exceeding the ultimate limits of the airplane’s structure. The horizontal stabilizer ripped off in a downward direction, followed almost immediately by the outboard portions of both wings. The fuel tanks exploded, triggering the blast seen and heard by witnesses, and then the fuselage broke into two sections, which plummeted to earth a few short seconds later, killing everyone on board. The entire maneuver, from the start of the climb to the moment of breakup, lasted just 45 seconds.
Although this was a satisfactory explanation for what happened, the root causes had yet to be elucidated. After all, why would Captain Almquist think it was reasonable to use stabilizer trim to deal with a transient updraft? Had someone taught him this technique? The answer would change how the industry saw the crash of flight 705.
As the CAB investigation proceeded, the investigators could not help but notice that similar events seemed to be occurring regularly around the world, at different airlines and in different countries, involving not only the Boeing 720, but also the Boeing 707 and the Douglas DC-8 — all of the large, swept-wing jets then in service. A few of these events resulted in a crash; most did not, but they all involved steep dives while flying in turbulence. For instance, six months after the crash of flight 705, a United Boeing 720 was attempting to climb through turbulence at 37,500 feet over Nebraska when it went into a dive and plunged almost five miles before the pilots managed to regain control. A similar incident occurred on a Pan Am 707 in 1959; that plane dived from 35,000 feet to 6,000 feet before recovering. All told, more than two dozen accidents and incidents were identified which may have been related. Some of them, including the United Airlines incident, were found to have flight profiles almost identical to that of Northwest 705, as shown above.
Investigators found that while each of these incidents had many unique aspects, they were related in that each was caused not by the turbulence itself, but rather by pilot responses which were out of phase with the turbulence. That is, the turbulence would cause the plane to move one way, the pilot would attempt to counter, and by the time the inputs took effect, the turbulence was already moving the other way. These out-of-phase inputs could become divergent, feeding each other and growing in amplitude until a loss of control occurred. Researchers dubbed these events “jet upsets,” because they only seemed to happen in jets. Unlike older planes, the streamlined design and high cruising speeds of the new jet airliners allowed loss of control events to escalate more quickly, taking pilots by surprise. Furthermore, the position of the cockpit so far forward of the center of gravity tended to create discomforting sensations that misled pilots about the seriousness of turbulence encounters, and made it harder to read the instruments. And once an upset occurred, recovery was often hampered by the design of early attitude indicators, which featured only a white horizon line on a solid black background. If the plane pitched far enough up or down, all reference markings would roll back into the frame of the instrument, and it would be impossible to tell what control inputs were required to return to level flight.
Compounding these issues were the new jets’ more powerful controls, especially the horizontal stabilizer, which was capable of placing the aircraft into a dangerous upset condition much faster than any control surface on a turboprop or piston propeller plane. As it turned out, jet pilots generally did not appreciate this reality, and developed habits which were suboptimal or even dangerous. One of these was the practice of moving the stabilizer every time one moved the elevators. The technique was not taught by Boeing or by the airlines, but pilots taught it to themselves and to each other as a way to reduce the sometimes tiring control forces required to move the manual elevators on the 707, 720, and DC-8. As a result, many pilots simply reached for the trim switches to trim out their inputs every time they wanted to adjust the plane’s pitch, even if it was unnecessary. When flying in heavy turbulence, where large but transient changes in speed and pitch are possible, pilots would sometimes react with a large stabilizer trim input, then the gust would subside while the trim setting remained, and they’d find themselves in yet another “jet upset.”
Part of the problem was the way pilots were taught to fly in turbulence. The emphasis traditionally had been on maintaining a narrow speed band which provided an adequate margin above the stall speed and below the maximum speed, so that a powerful gust would not propel the plane outside of its operating envelope. The main way in which pilots fine-tune their airspeed is by increasing pitch to slow down or decreasing pitch to speed up. However, studies conducted after the crash of flight 705 found that when a jet pilot flying in heavy turbulence is told to maintain a strict speed, they will make large pitch inputs which almost invariably end up being out of phase with the true motion of the aircraft, at best worsening the turbulence and at worst causing a “jet upset.” Trying to maintain a perfect nose-level attitude led to similar results. On the contrary, to the surprise of many pilots, the most effective technique seemed to be to do nothing! Countless simulations in fact showed that if the pilots barely touched the controls while flying in turbulence, a jet’s inherent stability would nearly always be sufficient to keep it within the operating envelope regardless of how hard the turbulence blew it around. All the pilots needed to do was make small pitch inputs to tamp down excessive deviations, and everything would be fine. Under no circumstances were they to touch the stabilizer trim, nor should they let the autopilot do so — in fact, the conclusion was that if the autopilot tries to move the stabilizer in turbulence, it should be turned off.
In a series of widely read technical papers written in 1963 and 1964, Northwest Airlines Manager of Flight Operations R&D Paul Soderlind laid out these findings and much more besides. His main thesis was that when flying manually in turbulence, the instrument of reference should be the attitude indicator. Control of altitude and airspeed are secondary. Had Captain Almquist of Northwest Orient flight 705 been taught this technique, he probably would have responded to the updraft with only a moderate pitch down, the airspeed would have fallen but stayed within limits, and the flight would have continued merrily on its way as soon as the updraft dissipated.
As a result of the massive effort undertaken to research the “jet upset” problem, a number of significant changes were made both to the design of passenger jets and to the way pilots were taught to fly them. Several reforms were made to certification requirements for jets, including demonstrations of in-flight upset recovery capability; airspeed-based stick force gradients, to prevent lightening or reversal; demonstrations of safe aircraft behavior in out-of-trim situations; changes to the concept of maximum operating speed; and, for the first time, the mandatory installation of an overspeed warning. Attitude indicators were redesigned with pitch markings throughout their full range of motion and with a light-colored sky to contrast with the dark-colored ground. The CAB also pointed out that on early jets it was possible to apply much more nose down stabilizer trim than was conceivably necessary, so the FAA introduced measures limiting the nose down range of motion of the stabilizers on both existing and future models.
In the field of pilot training, jet pilots were taught new turbulence penetration techniques, based around the following principles:
1. The attitude indicator is the primary instrument of reference.
2. Use the recommended turbulence penetration speed, but don’t chase it — if your stabilizer is set to the correct pitch angle to maintain the penetration speed, the plane will always return to this speed by itself.
3. Don’t penetrate turbulence at very high or very low altitudes where there is no room to maneuver.
4. Don’t use the autopilot in altitude hold mode, because it will try to make too many pitch changes.
5. Use moderate control movements and avoid large pitch inputs, even if large attitude changes occur.
6. Don’t try to chase your assigned altitude — the plane will return to it with minimal help — and definitely don’t use the stabilizer trim.
The recommended turbulence penetration speed was itself increased by 25 to 30 knots in order to provide a better margin above the stall speed, and better training about when and how to use the stabilizer trim was introduced in order to crack down on dangerous habits. And lastly, efforts to provide ATC with more accurate weather information were accelerated, eventually giving controllers the tools they needed to keep planes further away from thunderstorms when assigning radar vectors.
As a result of all these changes, both human and mechanical, the phenomenon of jet pilots losing control in heavy turbulence essentially vanished by the second half of the 1960s. In fact, no passenger jet has crashed due to an encounter with turbulence since 1966. The problem of turbulence-related upsets proved not to be about turbulence at all, but rather the inexperience of the aviation industry as a whole with jet operations, as pilots developed new flying techniques and encountered new, unforeseen conditions in the course of day-to-day operations. In 1963, most pilots had very limited experience with the concept of jets, let alone much time spent flying them — for instance, Captain Roy Almquist, a veteran pilot by all accounts, had spent only a tiny fraction of his long career in the cockpit of a Boeing 720. At that time, even those pilots responsible for developing flying techniques and procedures often had less jet experience than the average captain at a major airline today.
With our improved understanding of the dos and don’ts of flying in turbulence, the risk of a serious crash is now virtually zero. Although turbulence strong enough to tear a plane apart does exist, it is very easy to avoid, especially with modern weather forecasting technology. And the revised turbulence penetration techniques have since helped countless pilots to guide their airplanes safely through turbulence strong enough to make veteran air travelers clutch their armrests and pray to their deities of choice. For many people, turbulence is the scariest part of the flying experience, but while it can be unnerving, fear of turbulence from a safety standpoint is misplaced. The best advice is simply to put your tray table up, fasten your seat belt, and enjoy the ride — the airplane will come through just fine.
I would like to acknowledge the work of Theresa Trebon, who maintains a dedicated website which serves as both a memorial to the victims and an archive of materials related to flight 705. Her work has greatly increased the amount of information about this crash which is available to the public, and has helped flight 705 families reconnect across decades.
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