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Panel to review approval of Boeing 737 Max flight controls


CHICAGO – A global team of experts next week will begin reviewing how the Boeing 737 Max’s flight control system was approved by the U.S. Federal Aviation Administration.

The FAA says experts from nine international civil aviation authorities have confirmed participation in a technical review promised by the agency.

Former National Transportation Safety Board Chairman Chris Hart will lead the group, which also will have experts from the FAA and NASA. They will look at the plane’s automated system including the way it interacts with pilots. The group will meet Tuesday and is expected to finish in 90 days.

The Boeing jetliner has been grounded around the world since mid-March after two crashes killed 346 people. Investigators are focusing on anti-stall software that pushed the planes’ noses down based on erroneous sensor readings.

In a statement Friday, the FAA said aviation authorities from Australia, Brazil, Canada, China, the European Union, Japan, Indonesia, Singapore and the United Arab Emirates have agreed to help with the work, called a Joint Authorities Technical Review.

The group will evaluate the automated flight control design and determine whether it complies with regulations. It also will decide if changes need to be made in the FAA’s approval process.

Chicago-based Boeing is working on a software fix to the planes’ anti-stall system, known by its acronym, MCAS. In both an October crash off the coast of Indonesia and a March crash in Ethiopia, a faulty sensor reading triggered MCAS and pushed the plane’s nose down, and pilots were unable to recover.

Pilots at U.S. airlines complained that they didn’t even know about MCAS until after the October crash. They then received computer training that described the system and how to respond when something goes wrong with it.

On Wednesday, Boeing CEO Dennis Muilenburg said the company completed its last test flight of updated flight-control software. Muilenburg said test pilots flew 120 flights totalling 203 hours with the new software. The company is expected to conduct a crucial certification flight with an FAA test pilot on board soon, possibly next week.

“We are making steady progress toward certification” and returning the Max to service, Muilenburg said as he stood in front of a Max jet at Boeing Field in Seattle.

Muilenburg said he went on a test flight that day and saw the updated software “operating as designed across a range of flight conditions.”

In the U.S., United Airlines has removed its 14 Max jets from the schedule until early July, while American, with 24, and Southwest, with 34, are not counting on the planes until August.

It could take longer before foreign airlines can use their Max jets. Regulators outside the U.S. once relied on the FAA’s judgment in such matters but have indicated plans to conduct their own reviews this time.

Foreign countries may impose additional requirements, delaying the use of the Max by their carriers.

For example, FAA experts concluded in a draft report that while pilots need training on the anti-stall system, they do not need additional time in flight simulators. Canada’s transportation minister said this week, however, that he wants simulator training for Max pilots. Air Canada has 24 Max jets.

Copyright (c) 2019 The Canadian Press


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1 Comment » for Panel to review approval of Boeing 737 Max flight controls
  1. Mohammad Syed Husain says:

    The design for the Boeing 737-800, the Max, was altered to accommodate more fuel. The result was an altered wing, which in my opinion is the cause of the two crashes in Indonesia and Ethiopia. The software problem may be there, but that may not be the actual cause, but a smokescreen.

    Why did the two airliners crash elsewhere but not in Europe and America? The answer may lie in the weight carried or the position of the centre of gravity, the interplay between the centre of pressure and centre of gravity, or the loading of the aircraft

    S.M.Husain

    “Why an aircraft stalls?
    An aircraft stalls when the streamlined/laminar airflow (or boundary layer) over the wing’s upper surface, which produces lift, breaks away from the surface when the critical angle of attack is exceeded, irrespective of airspeed, and becomes turbulent, causing a loss in lift (i.e., the turbulent air on the upper surface creates a higher air pressure than on the lower surface). The only way to recover is to decrease the angle of attack (i.e. relax the back pressure and/or move the control column forward.

    An aircraft will stall at a constant angle of attack (known as the critical angle of attack). Because most aircraft do not have an angle of attack indicator(s), the pilot has to rely on airspeed indications. However, the speed at which the aircraft stalls is variable, depending on the effects of the following properties. (This particular B737-800 had angle of attack indicator, that transmitted the signal to the software, which in turn was allegedly at fault.)

    Properties affecting an aircraft’s stall speed:
    1. Weight (actual weight, load factor, g, in a turn, effective weight/centre of gravity position
    2. Altitude
    3. Wing design/lift
    4. Configuration
    5. Engine power

    How does the stall speed vary with weight?
    The heavier the aircraft, the higher is the indicated speed at which the aircraft will stall. If an aircraft’s actual weight is increased, the wing must produce more lift (remember the lift force must equal the weight force), but because the stall occurs at a constant angle of attack, we can only increase lift by increasing speed. Therefore, the stall speed will increase with an increase in the aircraft’s actual or effective weight. The stall speed is proportional to the square root of the aircraft’s weight.

    What changes the aircraft’s angle of attack at the stall?
    The movement of the centre of pressure point at the stall causes a change in the aircraft’s angle of attack. Normally, a simple swept or tapered wing is designed so that the centre of pressure will move rearward at the stall. This is so because the stall normally is induced at the wing root first where the centre of pressure is at its furthest point across the wing span. The lift produced from the unstalled part of the wing towards the tips and therefore aft, is behind the root with an overall net result of the centre of pressure moving rearwards, resulting in a stable nose-down change in the aircraft’s angle of attack at the stall.

    What wing design areas delay the breakup of airflow (stall)?
    1. Wing slots are the main design feature that delays/suppresses stall speed. A slot is a form of a boundary-layer control that re-energises the airflow to delay it over the wing from separating at the normal stall speed. The wing therefore produces a higher coefficient of lift, and can achieve a lower speed at the stall angle of attack.

    2. Lower angle of incidence and a greater camber for a particular wing section, e.g., wing tips.

    What is a superstall?
    A superstall may be referred to as a deep stall or locked in stall condition which, as the name suggests, is a stall from which the aircraft is unable to recover. It is associated with rear-engined, high T-tail, swept wing aircraft, which because of their design trend, tend to suffer from an increasing nose-up pitch attitude at the stall with an ineffective recovery pitching capability.

    A superstall has two distinct characteristics:
    o A nose-up pitching tendency
    o An ineffective tail plane

    The nose-up pitching tendency at the stall is due to:
    Near the stall speed, the normal rooftop pressure distribution over the wing chord line changes to an increasing-leading edge peaky pattern because of the enormous suction developed by the nose profile. At the stall, this peak will collapse. A simple virgin swept or tapered wing will stall at the wing tip first (if the wing has not been designed with any inboard stall properties) mainly due to the greater loading experienced leading to a higher angle of incidence that causes the wing tip to stall. Because of the wing sweep, the centre of pressure moves inboard to a point where it is forward of the centre of gravity, therefore creating an increasing pitch-up tendency.

    The forward fuselage creates lift, which usually continues to increase with incidence until well past the stall. This destabilising effect has a significant contribution to the nose-up pitching tendency of the aircraft. However, these phenomena are not exclusive to high T-tail, rear engines aircraft and alone do not create superstall. For a supstall to occur, the aircraft will have to be capable of recovering from the pitch up tendency at the stall.

    The tail plane being ineffective because the wing wake, which has now become low-energy disturbed/turbulent air, passes aft and immerses the high-set tail when the aircraft stalls. This greatly reduces the tailplane’s effectiveness, and thus losing its pitching capability in the stall, which it requires to recover the aircraft. A control surface, especially the elevator, requires clean, stable, laminar airflow (high-energy airflow) to be aerodynamically effective.

    Stall warners are either an artificial audio warning and/or a stick shaker, which usually is activated at or just before the onset of the pre-stall buffet. Stick pushers are normally used only on aircraft with superstall qualities and usually activate after the stall warning but before the stall, giving an automatic nose down command. Both systems normally receive a signal from an incidence measuring probe.”

    S.M.Husain

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