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Calculation of N1

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  • Aeromar
    As you say, the top of the amber band shows you the MINIMUM MANEUVER SPEED. In airliners we don't usually reference Va where the airplane will stall at any load factor before suffering structural stress. Maybe during certification they reference this speed but I don't know.

    The top of the amber bar shows the speed that provides manoeuvre capability to 1.3G. If you're flying exactly at that speed (top of the amber bar) and you pull 1.3g (40┬║bank in level flight), the minimum speed (red and black) bar will start rising and then this will happen:
    -Below 20000ft: Stick Shaker.
    -Above 20000ft: Stick Shaker and actual low speed buffet.

    I haven't tested this myself in MSFS. In the real sim this is also not simulated 100%. All sims break down a little bit on the edges of the envelop even though they still provide positive training on upset prevention and recovery techniques. Keep in mind that you're flying above the maximum ceiling (FL410) and certainly above any limiting level from the CRZ page. So it's is to be expected that the airplane is about to stop being an airplane at least temporarily.

    You can test things like this. Stay on autopilot and with heading select use the bank angle limiter to command turns at different angles to see what happens. You don't need to be so high to do this. Do it at a reasonable flight level and at slow speeds. You can also see how the PLIs behave when flaps aren't up or with flaps up at slow speeds.

    Last edited by Aeromar; 26Jun2022, 11:16.

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  • jrkob2
    Aeromar thanks a lot for the information, very interesting. Let me ask something else, I change the subject.

    This morning I was trying to get as close to the coffin corner as possible. It seems I was able to get to a difference of 10kt between Max Speed and Minimum Manoeuvre Speed. Now, my understanding is that below the latter, the buffet alert comes off but it is still possible to have - albeit limited - control over the airplane. But I tried, so let the plane going below the Minimum Manoeuvre Speed, but the airplane was not controllable, it sank fairly abruptly like in a stall.
    Is this realistic ?
    The reason why I'm asking is because in the C172/PA28 (these ones I fly as part of my PPL), there is Va (Manoeuvre Speed) under which the airplane is still largely controllable.
    Hence my confusion with the B737.

    * edit: I think Va in light aircraft is an upper limit speed, not lower limit. I think my comparison doesn't make sense.
    Attached Files
    Last edited by jrkob2; 25Jun2022, 09:58.

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  • Aeromar
    Originally posted by jrkob2 View Post
    Aeromar OMG yes this is exactly what I'm looking for, thanks a lot.
    Yes, that's pretty ATPLy stuff right there. So here's the thing in real life airline pilot training. You know the airplane's systems are broken down in 6 groups. So in every simulator recurrent we will train one group. Every 6 simulators you will have covered all the systems (simulators being about 6 months apart). The airline additionally adds a bunch of e-learning before each recurrent sim (CRM, Automation, UPRT and what not) and I know on one of the sims we do have a manual performance calculation exam. I think that e-learning module took me hours to finish (full paper load sheet, take off and landing performance, fuel consumptions). Now we do it all in literally 5 seconds with the ipad and we complain it's slow sometimes. Checkmate ATPL.

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  • jrkob2
    Aeromar OMG yes this is exactly what I'm looking for, thanks a lot. What I'm after specifically is on page 1.2.42. Just to let you know I completed an EASA ATPL (airplane) theory in 2019 and the performance charts included in your document raise a lot of memory (sadly I didn't get to do the CPL practical training because of coronavirus and I have now moved on to something else).
    But I think this table will do for my purpose. Using it won't add a meaningful amount of realism but will be fun.

    Kevin Hall yes I remember PMDG previous documentation (which was much much better than the current one IMO, and I'm quite positive it did include generic N1 tables), I had a look and unfortunately I no longer have the install files of the FSX version... I would have to ask them to PMDG and I think they would have removed those N1 tables anyway so no use to contact them.

    My question is answered, thanks fellas !
    Last edited by jrkob2; 25Jun2022, 01:17.

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  • Aeromar
    Before OPT (the app on the EFB to calculate performance), we used a set of printed papers called "runway or airport analysis". It had simplified tables for each individual runway to calculate take off performance and it allowed us to find the optimum thrust rating, Vspeeds and the MFRA in a few seconds. Using the actual generic tables that you're asking for would take you at least 5 minutes if you're really good at it. It would take me 30 minutes now to calculate performance using the generic tables. You would never find these tables in the FCOM (the document that PMDGs airplanes included in the past). These tables are found on the FPPM (Flight Planning and Performance Manual). PMDG never included the FPPM as far as I remember.

    MOD EDIT. Once more please do not share Copyrighted Material
    Last edited by Aeromar; 24Jun2022, 19:42.

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  • Kevin Hall
    PDF manuals containing the tables used to be provided by PMDG, but Boeing don't allow them to do this any more. When the EFB is added that should provide takeoff N1 calculations. In the meantime you can find copies of FCOM volume 1 online if you search the web.

    I've never used the FCOM tables as it's an involved process I'm not used to. Without an EFB, my "technique" is to pick an assumed temperature that feels right (typically in the 50-60 deg C range, less for a short runway) and let the FMC calculate the N1 and V speeds.
    Last edited by Kevin Hall; 24Jun2022, 11:11.

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  • jrkob2
    started a topic Calculation of N1

    Calculation of N1

    Guys, I vaguely remember there should be tables to refer to in order to calculate safe levels of N1 given the aircraft's weight, TORA/TODA etc.
    I had a look at PMDG's introduction and tutorial pdf documents but couldn't find anything on how to actually calculate this.

    Does anyone has this handy somewhere for this particular aircraft ? I'm not sure how it's called, performance tables may be.