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  • Overspeed Question

    So, i was wondering, can a commercial airliner overspeed in level flight? say for example you took a 737 with no pax or cargo, leveled off at an altitude within the normal cruise range, and selected absolute full power (toga) and left it there, is it possible the aircraft could reach an unsafe speed for the airframe? if so, what would that speed be?

  • #2
    Originally posted by TeeVee View Post
    So, i was wondering, can a commercial airliner overspeed in level flight? say for example you took a 737 with no pax or cargo, leveled off at an altitude within the normal cruise range, and selected absolute full power (toga) and left it there, is it possible the aircraft could reach an unsafe speed for the airframe? if so, what would that speed be?
    Yes, it's possible even if you are full of passengers and cargo, as long as you are not too close to the ceiling (where the engines would lack the power to keep increasing the speed.

    The max speed that you can intentionally fly is Vmo/Mmo (mo stands for Maximum Operative). This is not an ultimate safe speed. There is margin between this speed and the "dangerous" speed. The manufacturer must show that, starting at Vmo/Mmo, you introduce certain prescribed upsets to the flight and a normal flight situation must be restored without exceeding the "dangerous" speed and with no exceptional alertness, strength or skills by the pilots.

    While the manufacturers internally use other "milestone" speeds during the certification process (like Vd/Md), they only publish Vmo/Mmo. Whenever you exceed those speeds you are officially "overspeed", and you don't know how close are to a really dangerous speed so recovery must be initiated immediately.

    As a side note, I can mention that sometimes cruise speed are really close to Vmo/Mmo.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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    • #3
      Indeed at normal cruise speed at altitude you can experience overspeed. Can happen due to high back winds.

      I have done overspeed inspections many a times.
      “The only time you have too much fuel is when you’re on fire.”

      Erwin

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      • #4
        Originally posted by TeeVee View Post
        ...an unsafe speed for the airframe...
        You mean such that something would bend or break?

        From the direct affect of the "straight and level speed"

        OR

        From the even higher speed that will occur when the plane goes into a dive after the super sonic lift shift / pitch down occurs.

        (By the way, I am parlour talking with a tiny shred of background)
        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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        • #5
          as in a week gets ripped off or some such other catastrophic failure. i know that these types of things theoretically could happen in a nose down overspeed. basically what i'm trying to find out is if the engines are powerful enough to push the plane passed it's "safe speed"

          perhaps a shitty analogy, but most cars have electronic speed limiters to prevent the driver from achieving a speed too fast for the car.

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          • #6
            Originally posted by TeeVee View Post
            perhaps a shitty analogy, but most cars have electronic speed limiters to prevent the driver from achieving a speed too fast for the car.
            I think it depends on how the auto-pilot was designed. Mechanically, it is certainly that a consistent TOGA power setting will allow the aircraft to go over the safe speed. But electronically whether a limit is imposed would be another question. Also, the safe speed would be different at different altitude under different weather, so it's not as "set-in-stone" as cars. Without access to the technical manuals and the design specification, I would think Airbus' automation philosophy would put such speed limit on the computer and Boeing would probably allow the pilots to manipulate the aircraft beyond its design limit. But that's pure speculation. Boeing would tell you that the incident of CI006 would show you the benefit of allowing such flexibility to the pilots and Airbus would tell you that the US Airways Hudson River incident would show you the benefit of a fixing a flight envelope, so there's no right-or-wrong answer here.

            Actually are you sure there are electronic speed limiters on cars? I know they generally put them on trucks and buses, but I am not sure about cars.
            Next:
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            • #7
              Originally posted by erwins View Post
              indeed at normal cruise speed at altitude you can experience overspeed. Can happen due to high back winds.
              WTF?????
              Are you Don (RIP)?

              --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
              --- Defend what you say with arguments, not by imposing your credentials ---

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              • #8
                Originally posted by TeeVee View Post
                as in a week gets ripped off or some such other catastrophic failure. i know that these types of things theoretically could happen in a nose down overspeed. basically what i'm trying to find out is if the engines are powerful enough to push the plane passed it's "safe speed"
                There are basically three failures modes due to speed itself.
                The "rip off" type that you mention, where the skin friction or the drag take thins apart.
                The sonic heat type. When you approach the speed of sound the temperatures of the leading edges and nose of the fuselage (where the air is compressed in a shock wave) increases a lot and parts not prepared to withstand those high temps can fail.
                Flutter, that is a vibration caused by interaction between aerodynamics, structural and inertia forces, that can be so divergent to the point of braking a wing in less than a full cycle.

                There is an indirect mode, which is what 3WE mentions: When you approach the speed of sound the center of pressure moves from about 25% of the chord to about 50% of the chord (in full supersonic regime). That means that the lift moves aft and generates a strong nose-down pitching moment. This can put the plane into a dive, which would increase the speed and would move the CP even further which worsens the dive and so on. The result is an uncommanded and sometimes uncontrollable and unrecoverable dive that, eventually, makes the plane fail due to one of the modes mentioned above, or when the elevator or wing fails due to excessive lift when the pilot applies a lot of nose-up command in an attempt to recover (of course this kind of thing can happen even at speeds well below overspeed if the pilot is too aggresive with the control inputs, ask AA A300). So even if the thrust by itself is not enough to put the plane in a "rip off" mood, the subsequent unavoidable dive can.

                --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                --- Defend what you say with arguments, not by imposing your credentials ---

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                • #9
                  Originally posted by CathayPacific View Post
                  Also, the safe speed would be different at different altitude under different weather
                  Not so much. It's more like "320 ktos calibrated airspeed or Mach 0.78, whatever comes first", regardless of the altitude, weight, etc.

                  Of course altitude (and temperature) will influence on which one comes first.

                  (the numbers above are just an invented example)

                  --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                  --- Defend what you say with arguments, not by imposing your credentials ---

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                  • #10
                    The answer TeeVee is yes, assuming no autothrottle/autothrust was engaged, if you simply set maximum power at a relatively low altitude and flew level, you would comfortably exceed the maximum speed.

                    Erwin's High Back Speed is an interesting thought - most VMO/MMO overspeeds (and yes, they do happen) are due to a relatively rapid change (read increase) in headwind.

                    On the aircraft I fly most overspeeds are flap overspeeds, where the aircraft goes faster than its flap limits. These are usually not a big deal - the amount you went over the limit determines the inspection to be carried out. A small overspeed only takes a few minutes (as the flaps are really designed to far higher speeds than our placarded limit).

                    Gabriel has thoroughly answered the question of what the speed would be - basically, it depends.

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                    • #11
                      got it. also, spoke to my 737 pilot friend who confirmed that the aircraft she flies can easily overspeed if throttles are at less than max but still too advanced, even in level flight. she also confirmed that other than a loud warning sound, the 737 will allow the pilot to fly it to pieces.

                      so now, the question is, does airbus have as part of its flight logic, an overspeed protection for level flight where say the auto-throttle backs the throttles off automatically?

                      any bus drivers around? evan?

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                      • #12
                        Originally posted by TeeVee View Post
                        so now, the question is, does airbus have as part of its flight logic, an overspeed protection for level flight where say the auto-throttle backs the throttles off automatically?

                        any bus drivers around? evan?
                        Yes, in normal law it has an overspeed protection as part of its envelope protection. What it does is to climb to prevent an overspeed.

                        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                        --- Defend what you say with arguments, not by imposing your credentials ---

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                        • #13
                          Originally posted by ErwinS View Post
                          Indeed at normal cruise speed at altitude you can experience overspeed. Can happen due to high back winds.

                          I have done overspeed inspections many a times.
                          Huh??? Tailwinds have nothing at all to do with exceeding any airspeed limitations, and there is no such thing as a groundspeed limitation on an airplane. If the airplane is indicating 100 knots on the airspeed in still air, it is going 100 knots over the ground). If the same airplane is indicating 100 knots with a 50 knot headwind, it is going through the air at 100 knots but over the ground at 50 knots (100-50 knot headwind=50 knots over the ground. If the pilot then turns around and flies the opposite direction, he now has a 50 knot tailwind. The speed through the air is still 100 knots, but now the ground speed is 150 knots...

                          Now, gusty winds can cause airspeed fluctuations, especially on approach, which can momentarily exceed airspeed limitations, usually the limit speeds for flap extension. Then, of course, there's the occasional bonehead who calls for flap extension too early or lets the airplane accelerate too fast on climb and accidentally overspeeds the flaps... Now for the big question...have I ever done that? No...nosir! Not me--it couldn't have been me! I would NEVER do such a thing!
                          The "keep my tail out of trouble" disclaimer: Though I work in the airline industry, anything I post on here is my own speculation or opinion. Nothing I post is to be construed as "official" information from any air carrier or any other entity.

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                          • #14
                            Now that we are left with no MDs in Argentina, I'll miss Miss Douglas' monotonic "flaaaps-over-speeeed - flaaaps-over-speeeed - flaaaps-over-speeeed"

                            http://www.youtube.com/watch?v=UxMDZ...eature=related

                            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                            --- Defend what you say with arguments, not by imposing your credentials ---

                            Comment


                            • #15
                              Originally posted by snydersnapshots View Post
                              Huh??? Tailwinds have nothing at all to do with exceeding any airspeed limitations, and there is no such thing as a groundspeed limitation on an airplane. If the airplane is indicating 100 knots on the airspeed in still air, it is going 100 knots over the ground). If the same airplane is indicating 100 knots with a 50 knot headwind, it is going through the air at 100 knots but over the ground at 50 knots (100-50 knot headwind=50 knots over the ground. If the pilot then turns around and flies the opposite direction, he now has a 50 knot tailwind. The speed through the air is still 100 knots, but now the ground speed is 150 knots...

                              Now, gusty winds can cause airspeed fluctuations, especially on approach, which can momentarily exceed airspeed limitations, usually the limit speeds for flap extension. Then, of course, there's the occasional bonehead who calls for flap extension too early or lets the airplane accelerate too fast on climb and accidentally overspeeds the flaps... Now for the big question...have I ever done that? No...nosir! Not me--it couldn't have been me! I would NEVER do such a thing!
                              I meant when you have a sudden loss of tailwind the aircraft can go in overspeed for a short moment......
                              “The only time you have too much fuel is when you’re on fire.”

                              Erwin

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