Can a Boeing 737 Max 8 glide

Why aircraft stall stalls are so dangerous

In principle, commercial aircraft do not simply crash, even at low speeds. They are built in such a way that they have buoyancy even at speeds of just 280 kilometers per hour. The lift is created by the special wing shape. Put simply: the wing deflects the air downwards and thereby generates its own lift upwards.

This works as long as the air on the surface flows cleanly backwards. In the rear wing area there is a larger volume of air and thus a negative pressure that pulls the wing upwards.

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High angle of attack - vortices on the upper side of the wing

But that only works if the wing is at the right angle to the air flowing around it. If the angle becomes too steep (from about 15 degrees), the streamlines at the rear end of the wing first loosen. Vortices are created. That is already a first warning signal.

It gets even worse if there is no countermeasure: The pilot has to press down the nose of the aircraft to reduce the angle of attack. In this way he avoids the eddies and can ensure buoyancy. If he does not do this and the aircraft is always steeper in the air, a dangerous stall occurs from an angle of attack of around 18-20 degrees. This means that the air above the entire wing swirls.

The wing loses its lift and with it all of its function. The plane tips forward and falls into the fall. When turning, the stall can only occur on one wing. The plane then spins and falls like a stone. Only at very high altitudes can experienced pilots regain control of a falling aircraft.

Speed ​​also plays a role

Such situations almost always end in a crash, especially when climbing. It is during this phase of the flight that airliners have the most frequent accidents. Because in addition to the angle of attack, it is also about the speed of the aircraft: the slower an aircraft flies, the higher the angle of attack must be so that the aircraft has enough lift. If it does not reach the necessary "tear-off speed", the flow will stall.

Shortly after take-off, an aircraft needs considerable thrust to increase its speed and gain altitude at the same time. If the thrust decreases when climbing, this inevitably leads to a considerable loss of speed.

Which sensor should the pilot rely on?

In any case, it is important that the pilots know their speed and the angle of attack of the wings. If a sensor that supplies such data is defective, the pilots have to switch to a second - redundant - sensor. However, they must also be able to recognize which of the two sensors is wrong. If you now rely on the defective sensor, it can quickly lead to disaster.

The investigators lifted the flight recorder of Air France Flight 447 from the depths of the Atlantic Ocean

In three flight accidents in the past few decades, an incorrect speed measurement with a so-called pitot tube was the cause of the crash: Birgenair flight 301 crashed in 1996 while climbing in the Dominican Republic. There was probably dust in the speedometer there. The crash of Aeroperu flight 603 that same year had a very similar cause. Only the speedometer was not dirty there, but taped off as a precaution. The problem: Nobody had removed the adhesive strips before the start.

In both cases the speed measuring tube signaled to the pilots that the speed was far too high. In the case of the Birgenair flight, the pilot tried to counteract this by pulling up the aircraft's nose - a devastating mistake. The pilot ignored the correct data from a second speedometer and a warning signal about the impending stall, because he was probably confused and overwhelmed by the incorrect speed information.

During the Aeroperu flight, the team was able to initiate a landing maneuver. It was only during the attempt to land that the flow stalled and the result was a crash.

The pitot tube probably iced up on Air France Flight 447 in 2009. However, here the plane was already at cruising altitude. When the autopilot then switched itself off, the pilots were probably distracted by an abrupt tilting of the aircraft and tried to get the jet back under control by stalling it - too much. So they also triggered a stall that led to the crash over the Atlantic.

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The Indonesian chief investigator Nurcahyo Utomo reports on findings about the Lion Air crash in 2018

Are robots really the solution?

Aircraft manufacturers try to deal with the known danger in two ways: On the one hand, pilots are specifically trained to deal with incorrect measurement data from sensors and to interpret them correctly despite confusion and possible panic.

On the other hand, the technology should improve and also intervene when stressed pilots make wrong decisions. Boeing has introduced a "Maneuvering Characteristics Augmentation System" (MCAS), which can recognize critical flight situations in the 737 Max models and intervene in the event of an impending stall, but only when the autopilot is switched off. This can be the case, for example, shortly after take-off in a climb, but also when sensors deliver unreliable measurement data - as was the case with the Air France flight.

However, the MCAS system is also associated with the crash of Lion Air Flight 610 in October 2018. Apparently it was not the speedometers that were faulty here, but the sensors that determine the angle of attack of the wings. The two sensors deviated from each other by up to 20 degrees. This accident also occurred shortly after the start in the critical climb phase.

While the investigation is still ongoing, there is some evidence that the MCAS tried repeatedly to descend before the crash, while the pilot tried 26 times to pull the aircraft's nose up again.

When Ethiopian Airlines flight 302 crashed on March 10, 2019, there was at least one indication of a connection to the MCAS system. The flight monitor service "Flightradar 24" recorded an "unstable vertical speed". That could mean that pilots and robots may have been working against each other. However, it will be clear at the earliest when the flight recorder has been found and evaluated.

  • 50 years of the jumbo jet: an icon celebrates its birthday

    The nickname makes sense

    A British Airways jumbo jet approaching Heathrow Airport. The picture shows why the Boeing aircraft with the type designation 747 quickly got its nickname "Jumbo" shortly after its market launch 50 years ago: The four-engine jet is simply gigantic.

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Old friends

    Boeing President Bill Allen and the head of the US airline Pan Am Juan Trippe (right) on February 9, 1969 after the maiden flight of the first 747. The two had a long friendship. Trippe is said to have approached everyone while fishing about the plans for the new wide-body aircraft: "If you build it, I'll buy it". The answer: "If you buy it, I'll build it".

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Travel with glitz and glamor

    The new 747 was not only famous for its technical innovations, it also spread glamor. With a lounge where cocktails were served, it promised an elegant and relaxed travel experience. With a length of more than 70 meters and a wingspan of almost 60 meters, it offered - depending on the seating - between 366 and 550 passengers.

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Disasters

    The jumbo is also associated with great misfortunes. The crash of a Lufthansa plane in 1974 shortly after take-off in Nairobi cost 59 people their lives. In 1977 two jumbos collided at Tenerife airport - 583 dead. 270 people were killed in 1988 when a jumbo crashed after a terror bomb exploded on board over the Scottish Lockerbie (see photo).

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Big mouth

    The "hump", the upper deck, which also houses the cockpit, is characteristic of the Jumbo. This construction enables a front door for the cargo version of the machine, which allows generous loading. Boeing now sells the four-engine and therefore fuel-guzzling aircraft practically only in the cargo version.

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Shuttle carrier

    The US space shuttle Discovery rides piggyback on our giant - a photo from 2012. The Shuttle Carrier Aircraft (SCA) were two modified aircraft of the type 747-100 used to transport the space shuttles of the US space agency NASA. They were used to transport the shuttles back to the Kennedy Space Center, provided the shuttle landed at an alternative landing site.

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Winner flyer

    In July 2014, Lufthansa insisted on flying the German national soccer team with the World Cup in their luggage from the final in Rio de Janeiro to Berlin. The 747-8 was quickly repainted as the "Fanhansa's winning aircraft".

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Iron Maiden's Ed Force One

    The British heavy metal band Iron Maiden landed at Düsseldorf Airport in their chartered Boeing 747 in May 2016. Metalheads and plane spotters secure the best seats to watch the plane, the "Ed Force One" - named after the band's mascot, the monster Eddie. Incidentally, the machine was flown by lead singer Bruce Dickinson himself.

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Dinosaur of the skies

    Like the even larger Airbus A380 jet (in the foreground), the 747 no longer meets the economic requirements of the airlines, who prefer to use twin-jet aircraft like the A350 or the Boeing models 777 and 787 on long-haul routes. Last year there were 18 new orders for the Jumbo, only 24 unprocessed orders are in the books.

  • 50 years of the jumbo jet: an icon celebrates its birthday

    Air Force One

    In addition to the 747 aircraft delivered in 1548, only a few will be added, even though US President Donald Trump has also ordered the next presidential aircraft, "Air Force One" based on the 747. The Japanese Emperor and the Sultan of Brunei also appreciate the "Queen of the Skies" as government machines.

    Author: Klaus Ulrich