Transcription
In the cold December sky over Scandinavia, a passenger airliner is climbing its way through an overcast layer of low-lying clouds. The aircraft has just taken off, but a series of loud bangs and the violent shaking are leaving the passengers terrified and the flight crew puzzled. As the pilots focus on troubleshooting the problem and discuss returning to the airport, both engines begin to spool down. A frightening realization hits the flight crew. They might not make it back to the airport before running out of altitude. Yet, still flying through a dense cloud, they can't see ahead and don't know where to land. With 129 lives at stake, the captain must make a quick decision, testing the limits of his skills and airmanship. Can he turn his bad luck into a miracle? Or will this event become one of Sweden's darkest days?
This is the story of SAS Flight 751. On the late night of December 26th, 1991, an SAS Macdonald Douglas MD81 arrived in Stockholm, Sweden after a 2-hour flight from Zurich, Switzerland. The crew concluded their day with this flight, leaving the MD81 at the international gate overnight. Returning from Zurich, the airliner had used only a portion of the fuel it carried, arriving in Sweden with its wing fuel tanks more than half full. After 2 hours at high altitudes, the temperature of the fuel inside the wing tanks had dropped to below -50° C and the fuel was gradually cooling the surface of the wings.
It was snowing in Stockholm that night with the outside air temperatures lingering around 0° C, rising to just above or dropping to barely below zero. As the night progressed, the snow began to accumulate on the surface of the wing, turning into a wet slush, collecting on the area of the wings surrounding the fuel tanks, commonly referred to as the cold corner. The snow froze, creating a layer or thick clear ice. A particular characteristic of clear ice is a lack of bubbles or other deformationations within it. As the name implies, such a layer can become completely transparent with a smooth, glossy finish. These characteristics of clear ice make it especially hard to identify unless the wing is inspected closely. In most cases, a visual check is not sufficient, and the most effective way to determine the presence of clear ice is by touching the wing.
[Music]
On December 27th at 2:00 a.m., a maintenance technician conducted a routine inspection of the aircraft and noticed the slush on the wings. He did not use a ladder that was long enough to reach the cold corner during his procedure and did not see the clear ice that had formed on the wings. Although another maintenance technician completed a pre-eparture inspection in the morning, he too did not notice the ice on the wings. The quick 350 mi hop to Copenhagen was going to take approximately 1 hour from takeoff to landing. Operated by Captain Stefan G. Rasmusen, a Danish national with 8,000 flight hours of experience and close to 600 hours on this aircraft type. Olf Sedmar from Sweden, who was accompanying Captain Raasmuson on this flight, had accumulated over 3,000 hours of total time, having only completed his training on the MD81 in November of 1991, he had about 80 hours of flight time on the aircraft. Both flight crew members were former Air Force pilots with experience in flying fighter jets. At 8:30 a.m., flight 751 left the gate carrying 123 passengers and six crew members.
Before we continue, I want to take a moment to thank the paid partner of today's video, Better Help. Life doesn't always go according to plan. Stress, uncertainty, or even just the weight of everyday responsibilities can sometimes feel overwhelming. I still remember when I launched one of my first projects. I had poured months of work into it, but it completely flopped. At the time, it felt like a dead end. What helped me push through was talking it out, getting perspective, and slowly rebuilding the confidence to try again. That's what BetterHelp is built for. It's the world's largest online therapy platform, connecting you with a large network of credentialed and experienced therapists. And the best part, you can talk to them however it works best for you through video calls, phone, or even chat. No waiting rooms, no long commutes, just professional support when you need it. And if you feel your first match isn't the right fit, you can switch therapists at any time. With a 4.5 star trust pilot rating, millions of people have already used BetterHelp to take that important first step toward feeling better. If you'd like to give it a try, visit betterhelp.com/aircrashinvestigations or select aircrashinvestigations at sign up for 10% off your first month because sometimes the hardest part is just starting and BetterHelp makes that easier. Now, let's get back to the video.
After starting the engines and completing the necessary departure checks, the pilots taxied the aircraft over to the deicing bay. Per the instructions from the crew, a qualified ground crew began to spray a hot glycol solution over the wings to remove the snow from the surface. Concerned about the frost, the captain asked the ground crew to pay special attention to the underside of both wings. The de ice workers confirmed to him that all frost was removed, but neither employee at SAS realized that the cold corners remained covered in clear ice. Through a series of slips and cracks in the process, SAS Flight 751 left the ramp with significant contamination on its wings, putting 129 lives in grave danger without their knowledge.
Given the go-ahead from the ground crew, Captain Raasmuson took the aircraft onto the taxi way and headed out toward runway 08. Within a few minutes, the pilots completed the required pre-takeoff checks. And at 8:45, the MD81 was lined up on the runway and ready to begin its journey. As the captain applied takeoff power, the engines spooled up and the aircraft began to accelerate. The MD81 lifted off the ground shortly thereafter, starting a gentle climb from wintry Stockholm. These were the last few peaceful moments on board Flight 751.
Only 25 seconds later, as the airplane was crossing an altitude of 1,100 ft above the ground and entering a cloud layer, a series of loud bangs coming from the tail section shook the entire cabin, alarming the pilots. "I believe it's the compressor stall," the first officer noted, correctly identifying the problem with the right engine during the liftoff. Subjected to the weight of the aircraft and the aerodynamic forces involved. Both wings flexed upwards, breaking the clear ice that had not been removed. Lifted up by the flow of the oncoming air. The ice flew straight into the tail-mounted engines of the MD81 entering the compressor section. As its name suggests, this section is responsible for compressing the incoming air before it is mixed with fuel and ignited producing thrust. The compression is completed using a series of blades that rotate at high speeds. Therefore, a constant and uniform flow of air into the engine is critically important for optimal operation. At a rotational speed of 8,000 revolutions per minute on takeoff, it does not take much more than a brief contact with a small piece of ice to cause severe damage to the compressor blades, creating small cracks within the blades or even bending them. The resulting disruptions in the air flow known as a compressor stall significantly affect the pressure supplied to the combustion chamber. If not resolved in time, the difference in air pressure between the compressor and the combustion chamber can cause the air flow to momentarily reverse and push in the opposite direction. This phenomenon is known as an engine surge. The surges can worsen the disturbance in the entire combustion process and produce the loud bangs that were heard by everyone on board flight 751. Compressor surges are somewhat similar to a backfiring car engine with one major difference. If a compressor surge is allowed to develop further, irreversible damage and a complete failure of the engine may occur.
Assessing the situation and aiming to limit the damage from the compressor surge, Captain Rasmusen pulled the thrust lever of the right engine slightly back to reduce the pressure differential within the engine and stabilize the air flow. Yet, what followed next was going to turn into a far greater crisis. Unbeknownsted to either flight crew member or anyone at SAS, the MD-81 came equipped with an automatic thrust restoration or ATR system, continuously monitoring multiple engine parameters. The system was designed to automatically apply maximum available power if an engine failure is suspected. When the surging began, the ATR recognized it as an engine failure and moved both thrust levers to a setting beyond takeoff power. Hence, when Captain Rasmuson pulled the right thrust lever back, he set the engines to nearly the same power setting at which the surging occurred. As he released his hand from the thrust lever, the ATR once again pushed it forward toward the maximum allowable setting. A few seconds later, the banging noises intensified, shaking the aircraft even harder. The left engine had also begun surging. Busy with trying to take control of the situation, the pilots did not notice the issue. A flight attendant tried to call the flight deck and let the flight crew know what was happening, but she did not use the emergency communication mode, and her calls were not noticed. Climbing through 25,500 ft above the ground, the captain turned the autopilot on, hoping to reduce his workload and delegate the control of the aircraft to the automation. However, the state of the aircraft and its behavior fell beyond the limits of the autopilot and it would not engage. Instead, an audible message filled the noise of the flight deck, informing the pilots that the autopilot had disconnected.
Autopilot >> only a few thousand ft in the air. In a violently shaking aircraft flying through a thick layer of clouds with oral alerts repeatedly adding to the chaos, the situation was getting more and more stressful. A brief moment later, to the relief of all passengers, the vibrations and the loud banging stopped. But with it came a deafening silence and a terrifying realization at the front of the aircraft. The surging, which was not remedied by the flight crew in time, had disrupted the combustion process enough to completely extinguish it, causing both engines to flame out. A little over 1 minute after flight 751 lifted off from runway 08 in Stockholm, the aircraft lost both of its engines, turning a routine flight to Captain Rasmuson's home country into the greatest challenge of his career.
>> "We are trying to make a restart of the engines and make a slow turn to the left."
At only 3,000 ft above the ground and without engine power, the crew of SAS Flight 751 could not stay up in the air for too long. The aircraft was 2 mi east of the airport, moving further from it, and a decision had to be made quickly. Suddenly, another warning rang through the flight deck. Running at high power, the engines had internally warmed up to a temperature of over 800° C, triggering a fire warning. Although no actual fire broke out, first officer Cedar activated the fire extinguishing system as a precaution. Moments later, he called Stockholm ATC to inform them of the emergency.
>> "Have problems with our engines. Please uh uh um go back to number."
Captain Rasmmanson began a left turn toward the airport. Halfway through the turn, he realized that the aircraft was losing too much air speed and he did not have enough altitude to trade for more air speed. Despite the controller's instruction to continue the left turn for runway 01 in Stockholm, the aircraft leveled its wings and pointed north. Seconds later, the captain's electronic flight instrument system shut down due to a power interruption. He had no time to troubleshoot the issue, leaving him with only his standby flight instruments and a severely crippled aircraft.
Watching the nightmare unfold before his eyes through the open flight deck door, one of the passengers seated in the front section of the cabin sprung out of his seat and rushed to help the pilots. In a rare stroke of luck during an otherwise dark moment for the crew of flight 751, this passenger turned out to be a fellow SAS pilot. Jumping into action without hesitation, he started the auxiliary power unit at the captain's request. At last, things were starting to look up.
As the aircraft continued to descend toward the ground, Captain Rasmuson had to choose a landing spot, but they were still in the clouds and could not see anything ahead of them. He knew that once the aircraft descended below the clouds, he would only have seconds to choose a good landing spot and land. It was going to be a hard landing. So, Captain Rasmmanson instructed the cabin crew to get ready. Descending through about 1,300 ft, still completely in the blind, the flight crew began to gradually extend flaps in preparation for landing. A few seconds later, the aircraft finally broke out of the clouds, giving the captain a glimpse of what was lying ahead. He obtained a clear view of the farm fields ahead as the aircraft crossed the last 1,000 ft of altitude. Losing approximately 30 ft with every second, Flight 751 had less than a minute before it would reach the ground. In another stroke of luck, Captain Rasmusen saw a large enough open field straight ahead of him and within reach. It was his best chance at saving the lives of everyone on board, and it was the only chance he had. Turning slightly to the right to avoid multiple houses along the field, the crew extended the landing gear and braced for impact. A chilling transmission reached the control tower at Arlander Airport.
75.
[Music]
7 seconds after the last transmission, the right wing of the MD81 made contact with the trees in its way, sending forceful thuds through the aircraft before breaking off. The sound of creaking metal mixed with the screams of panicked passengers filled the entire cabin as the tail section hit the ground and a sudden wave of cold air rushed inside. Hitting the ground at over 100 knots, the fuselage broke into three pieces, sliding through the snow and the mud and leaving a trail of flammable fuel behind. No longer controllable by the crew, the pieces of the fuselage continued to slide for 300 ft before finally coming to rest in the snow. Despite the destructive damage to the aircraft itself and a major fuel leak, everyone on board the airliner survived the harsh landing, marking the last and the most important stroke of luck in the story of SAS Flight 751.
How could a crash of this severity result in zero fatalities? It was a result of a combination of good fortune and skill. Captain Rasmuson's quick decision-making helped him identify a suitable landing spot within seconds of making visual contact with the ground. Despite inflicting severe damage to the wings, the trees had significantly slowed down the aircraft before it struck a slight downslope with a thick layer of snow cushioning the impact. Lastly, because the fuselage broke into several pieces, the passengers were able to evacuate much faster.
The story of flight 751 underlines the importance of proper training and understanding of aircraft systems as well as the environment in which it is operated. It is not unlikely that had the flight crew been aware of the ATR and the authority it had over engine management, they would have reduced the power to a setting that would have allowed them to keep both engines operational and safely return to Stockholm. However, Captain Raasmuson was presented with a situation that neither he nor anyone at his company could expect, and thus he had no training on engine surges and how to handle them. Despite the fact that the crew did not follow any emergency checklists, Captain Rasmuson's excellent aircraft handling skills have undoubtedly played a significant role in the survival of everyone on board. More importantly, it was his ability to delegate tasks and communicate with the other crew members that allowed him to focus on flying the aircraft all the way to the point of impact, saving the lives of everyone on board against all odds stacked against him on this fateful day.
Although the ultimate responsibility for establishing that the aircraft is safe to fly always remains with the pilot in command and in this case it was Captain Rasmuson. The board of investigation of Sweden turned the spotlight on the company SAS and determined that the accident was caused by the deficiencies within the organization as a whole. Namely, it was the airlines routines and instructions which were found to be not sufficient to ensure that clear ice was removed from the wing surface. Contributing to the accident were the lack of training on engine surging and the operation of the ATR.