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810935 and 810937 - VA - fixed or variable?

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  • 810935 and 810937 - VA - fixed or variable?



    Q. Assuming no compressibility effects, the correct relationship between stall speed, limit load factor (n) and VA is:
    A. VA >= VS * SQRT (n)
    ...At lower masses, VS will be lower so published VA may be greater than VS times the square root of 2.5

    [this suggests VA is fixed regardless of mass].


    Q. How does VA (EAS) alter when the aeroplane's mass decreases by 19%?
    A. 10% reduction.
    (Wrong answer: No change)
    VA is the stalling speed of an aircraft at its maximum load factor (g). We know that ANY stalling speed changes with the square root of the change in mass.
    A 19% reduction in mass is a change of mass by a factor of 0.81. So the stalling speed will reduce by a factor of (√0.81), which = 0.90
    For example, if VA was 100kts, the new VA at the lower mass will be:
    100kts x 0.90 = 90kts. This is a reduction of 10%.

    [i.e. VA varies with mass, just like VS does. I put 'no change' and was wrong...]

    I am struggling to see these answers as consistent. Please will someone point me to what I'm missing?

    Many thanks.

    Edit, addition:

    Q 810939 adds to the mix:

    Q. The manoeuvring speed VA, expressed as indicated airspeed, of a transport aeroplane:
    A. Depends on aeroplane mass and pressure altitude.

    [I thought that VA was a fixed speed set by the manufacturer assuming MAUM (and maybe PA 1013mb?), then looking at the max speed where full back stick would give 2.5g, as per Q 810935. I didn't realise it changed as a function of the mass at a given time (or indeed altitude, PA or otherwise). Either way, I started thinking I understood VA, now I'm confused. Please help!]
    Last edited by TouchTheFaceOfGod; 31-03-2018, 10:32. Reason: Another question seen...

  • #2
    Q 810937: Va is the speed at which the aircraft stalls at its g-limit. So, in reality, the pilot will find that it varies with mass - just like any other stalling speed. So this question is all OK.

    Q 810935: This is an old question written at a time when they seemed to like offering cryptic mathematical answers! What the correct answer is actually trying to say is that :

    Published Va >= Vs*sqrt(n).

    The point is that, when only a single value of Va is published in the aircraft manual, it is for MAUM. Taking off at this weight, the published Va will be accurate. But, as soon as any fuel is burnt and mass reduces, the actual Va will decrease, and published Va will become greater than (>) the actual Va. (The formula to find the actual Va is Vs*sqrt(n))

    That's why we must pick the answer beginning Va > other words, published Va could be equal to or greater than the actual Va the pilot will find at the time.

    Large wide bodied aircraft usually publish several values of Va for different masses, as the actual mass reduces so much as fuel is burnt on during a long flight. A 747-400 flying from Singapore to London could be 40% (or more) lighter on landing than it was at take-off.

    I'll improve the explanation provided to this question. Although it's an old question, it has been reported in a recent exam.


    • #3
      QID 810939: This is testing two separate bits of knowledge in a single question.

      1) Does Va vary with mass? Yes it does.....its just another stalling IAS. That eliminates 2 of the answers.

      2) So does a stalling IAS vary with altitude? This is a bit more complicated.

      The value of lift created depends on the value of EAS. At lower altitudes EAS and IAS are pretty much equal, so as we climb at lower altiudes the stalling IAS remains constant. (Reduction in density is compensated by increased TAS).

      But increasing Mach number at higher altitudes brings compressibility and IAS becomes higher than EAS. The questions asks about stalling IAS, so it will eventually start to increase with altitude when when we reach higher levels. That eliminates a third answer.


      • #4
        Thanks Ben, those explanations really help, especially the differentiation of 'published' and 'actual' Va. I hadn't previously considered 'actual' Va...


        • #5
          Just encountered this question.

          Whereas (of course) the actual Va, i.e. the stall speed at 2.5g, varies with mass, the questions do not seem to differentiate between 'actual' and 'published' Va. The inequality:

          Va >= Vs * SQRT (n)

          Exists because whilst Va is fixed, Vs varies with mass. This again supports the notion that the questions are referring to a fixed, published Va. In fact, if Va were allowed to vary in accordance with mass, then we'd have:

          Va = Vs * SQRT (n)

          With no inequality sign. The fact that the first formula is correct really does seem to indicate to me that the syllabus refers to a published Va, not the actual Va.

          I honestly don't understand how, unless the question specifically states whether actual or published Va is to be used, one can infer the correct answer.


          • #6
            These 2 are amongst the worst in the PoF exam question bank. Its time to employ the old multiple-choice exam technique of studying the 4 answers to try to clarify what you are being asked to do.

            810935: All 4 answers have inequality signs. So you can't do a precise calculation using any of these and it isn't asking you to do one. It's just asking you to choose a relationship. There is only 1 correct answer, as explained in the post above dated March 2018.

            810937: The 4 answers suggest we are being asked to do a calculation. It's a bit ambiguous, but, if challenged, my guess is that the authorities would say that a question about the effect of a change of mass is obviously not about the published Va, which is for a fixed mass. Debatable?

            These questions have been around for many years, and we know that these are the answers to choose in the exam.

            Refer to QID 811389 and 811390 and their explanations for the more recent style of questions on Va.