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Eleven killed in Polish skydiving plane accident

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  • #31
    Some comments made on the PPrune website by people who actually fly or flew the PA31. It seems that the PA31, especially this pressurised version is not held in particularly high esteem. Bold red type is my highlighting.

    I have about 1000 hrs on the Pressurized Navajo, may be the worst of all.
    425 hp per engine but useless at heavy weights.
    I used to rotate the aircraft at 95 kts, because the manual says to forget to expect any climb below that speed on one engine. So in the briefing i was always telling to my copilot that as long as we are on the ground (below 95 kts) we abort in case of engine failure, it didn't matter how long the remaining runway was.
    I also used to take off with the cowl flaps closed (except in very high temperature) beacause they were real speedbrakes
    Gulfstreamaviator
    13th Oct 2011, 20:34
    fantastic aircraft, good load, and pax loved it.

    BUT it is a single......with two engines.
    2nd Nov 2011, 02:27
    I flew Navajo a couple of hours, an easy rule:

    Big guys behind Captain and First Officer seat

    All luggage inside nose compartment

    Girls at the back

    Place of CG it's really important in those type of aircraft
    cant speak for the PA31-325 Navajo, but over 1000hrs on its bigger sister PA31-350 Chieftain, what a lovely aircraft, flown for "old" Airlink, and when operated within W&B limitations would perform as per the AFM.

    One of my fellow PA31 Capt's had a magneto shaft failure (common shaft supplying both magneto's) after take-off at JNB, with a full load, and he climbed out and flew it all the way to a single engine landing, with a full load of pax and baggage...at MAUW

    critical was getting the "barn door" cowl flaps closed ASAP on the dead engine, and not flying even slightly overloaded, and as also mentioned the CofG must also be right.
    If it 'ain't broken........ Don't try to mend it !

    Comment


    • #32
      Originally posted by 3WE View Post
      ??????????????????
      Yes, an aft CG worsens the stall, and even more the spin. As I've said, you'll be in deep shit way before the stall. Just go below Vmc with one engine died and the other providing full power and stall and spin recovery won't matter.

      It's like what happens in any plane (typically singles) when they stall and spin when turning from base to final at 300ft. Spin recovery doesn't matter. There's no time to recover.

      The key thing here is keep the plane above Vmc (the red line), even if that means that the thing won't climb. Pulling up more will mean not only that it will not climb either, but that you'll lose control even if you don't stall and spin. The second key thing is to keep the speed at the single-engine Vy (the blue line). This is the speed that gives you the best climb rate. If it's zero or negative at that speed, it will be worse at any other speed (assuming that you already took all the drag reduction measures like gear up, flap up, cowl flaps closed and bad prop feathered).

      It's not easy. It's a lot of workload and the margins are very small. Figure that even with the weight within limits the single engine climb rate is 240 fpm at sea level 15°C!!! (ok, with this turbocharged engines it will be the same at a big range of altitudes and temperatures because the turbo will keep the manifold pressure up).

      Overweight is very bad for this but not for control issues but for performance issues. The overweight very quickly erodes the little climb performance left. The plane needs what is called "required power" to keep straight and level flight at one speed and weight, so it's what is needed to net the drag.

      If the plane has available more power than the required power, the excess power is called, well, excess power, and it WILL become a rate of increase of kinetic energy (speed) or a rate of increase of potential energy (altitude), or a combo of both. It is the pilot, with the elevator, who decides how to use this excess power, what to win, speed or altitude.

      Let's assume that you want to use it just to increase the altitude and that you are already flying at the speed of best climb.

      You need power to lift each unit of weight. And that's not an aviation thing. The same happens in an elevator (that's a lift, Brian ), in a car going up a slope, or anywhere. So, obviously, the excess power will mean a smaller climb speed the more units of weight you have.

      But in an airplane things are worse than in the elevator or in the car.

      The best-climb speed happens at the speed of lowest required power, which is close to the speed of lowest drag. Because the parasite drag increases with speed and the induced drag decreases with speed, there will be one speed of minimum drag and another of minimum required power, which are close to the point where parasite and induced drag are equal.

      More weight means more lift, more lift means more induced drag, more induced drag means that the speed at which the induced drag and the parasite drag are equal is faster, that meas that the best-climb speed will be faster, and that means that the required power will be higher due to the increase of drag (both induced and parasite) and because of the increase of speed itself (the required power is drag times speed).

      So an increase of weight does not only reduces the climb rate with any excess power. But will also strongly erode the excess power to the point that it's easy to make it negative, and there the "defect" power (technically called negative excess power) WILL become a rate of reduction of speed, a rate of descent, or both. Again, it's the pilot who decided what to lose, speed or altitude.

      The right choice is altitude, since losing speed will only make you lose even more altitude, not to mention of you go below Vmc and lose control.

      The problem here (and in many other places) is when the pilot try to squeeze from the plane more performacne that what Newton will give. You can never bear Newton, and trying to do so not only never helps, but too often it's lethal. The plane will not climb, the pilot is not happy with that because he will not reach a runway, perhaps he is overflying a forest and if he could stretch the flight a bit farther he will reach a clear. No boy, hit the trees the best you can and while in control. Trying to go farther than the plane can go will make you lose control and still hit the trees but in a much more nasty way.

      There is a point where the pilot needs to assume that he WILL NOT land in a runway, and he must reset to "survivor" mode, accept that he will crash (or crash-land) and try to do it in the most survivable way, and ALWAYS losing control plays against survivabililty. so "keep the thing in control" must be the highest priority.

      I'm not saying it's easy, and it's not. Again, lot of workload, small margins, and a strong psychological challenge of stop trying to save the flight, accept that you are going to crash and that you may die, and try to crash as benignly as possible to maximize the chances of survival.

      So an overweight is linked to the loss of performance, not linked with the loss of control that is what killed them here. Not, at least, until the pilot tries to squeeze performance that is not there to squeeze, and loses control.

      One could say that the pilot would have not done that if he had enough performance to sustain even a slight climb, so the overweight is a link in the chain of events. And I agree. But remember that, even with the overweight and with negative "best climb" performance, he could have not choose to do it and crash or crash-land in a different, more survivable way. Or maybe he actually attempted to do so, he really had all the intention to keep the speed up, but was too overloaded or not skilled enough and could not manage the situation. Then, again, the overweight would not be a strong factor. If the pilot wanted to, but was not up to the task, of keeping the speed up in this difficult situation, then he might very well have lost it with enough climb performance too.

      Two final things regarding overweight:
      - We still don't know of the plane was overweight.
      - If it was overloaded, and if you want to consider this overload a causal factor, it's still the pilot's fault. Not only the pilot's. Likely the culture of this operator is one that admits, maybe even encourage, this kind of operations.

      --- 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


      • #33
        Originally posted by brianw999 View Post
        Some comments made on the PPrune website by people who actually fly or flew the PA31. It seems that the PA31, especially this pressurised version is not held in particularly high esteem. Bold red type is my highlighting.
        Brian, maybe I'm missing something, but I don't understand how a slight aft CG will affect single-engine handling so badly.

        In the 4 comments below, only two mention CoG, only one of them seems to link it with single-engine handling (because it's in the same line with cowl flaps and feather of the bad engine), and none mentions how it affects.

        Of course the CG is very important, but it affects pitch, AoA, and speed stability and control (that is, longitudinal stability and control) much more than directional stability. MUCH more.

        I can't speak from own experience because I don't have, but from my theoretical knowledge it seems to me that, by when the CoG is so aft that it will become a significant detriment in directional stability and control, the longitudinal stability and control will have been long affected and spoiled.

        In other words, if the aft CG made the single-engine handling dangerous, then the longitudinal control will be already very dangerous with any number of engines operating or failed.

        Again, maybe I'm missing something.

        Of course, because speed control is so important in single-engine handling, there is an indirect link there. Just as there was one with the overload. However, keeping one target speed without touching the throttles (that is, keeping it with pitch) is equally easy or difficult with both, one, or no engine running. Only that a lousy speed control is not as critical with both or none engines running as it is with one.

        Again, if the CG out of range was a causal factor in this accident, that's still the pilot's fault.

        --- 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


        • #34
          From what I read on the PPrune site from the posts of the people that fly them, the Navajo is considered very susceptible to C of G changes and flight with only a small overload or slightly aft C of G should be avoided at all costs. I don't know why either, I've only got 18 hrs in a Cherokee 140 and that was 43 years ago but that's the message that comes across......and it really seems to me that on this occasion the pilot seriously screwed the pooch.
          Last edited by brianw999; 2014-07-13, 18:55.
          If it 'ain't broken........ Don't try to mend it !

          Comment


          • #35
            Originally posted by Gabriel View Post
            Yes, an aft CG worsens the stall...
            You quoted my question marks and not my bold stuff which was the point.

            It's all tightly linked and why folks are posting on PPRUNE and Brian is quoting that aft CG makes things bad.

            You can keep making Gabrielian posts or acknowledge that this plane (and actually lots of planes) act nasty with an aft CG.

            Are you Economy Class?
            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

            Comment


            • #36
              What I'm trying to say is that the Navajo is PARTICULARLY nasty if the CofG is even SLIGHTLY out of parameters compared to other aircraft, some of which may well have nasty traits about them but ARE NOT PART OF THIS DISCUSSION.

              Bloody hell...you've got me on caps and bold now.
              If it 'ain't broken........ Don't try to mend it !

              Comment


              • #37
                Originally posted by brianw999 View Post
                What I'm trying to say is that the Navajo is PARTICULARLY nasty if the CofG is even SLIGHTLY out of parameters compared to other aircraft, some of which may well have nasty traits about them but ARE NOT PART OF THIS DISCUSSION.

                Bloody hell...you've got me on caps and bold now.
                I acknowledge that (except the "bloody hell" part ), and, notwithstanding and not contradicting that, what I am saying is that I doubt that an aft CoG was a strong causal factor in this accident. I can be wrong.

                --- 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


                • #38
                  By the way, Brian,
                  Do you have an account in PPRuNe? I don't.
                  If you do, would you please ask those guys how does that nasty aft CoG manifest itself? I mean, take this sentence as an example:

                  "critical was getting the "barn door" cowl flaps closed ASAP on the dead engine, and not flying even slightly overloaded, and as also mentioned the CofG must also be right."

                  Remember: climb rate = excess power / weight = (power available - power required to fly straight and level) / weight = (power available - drag x speed) / weight.

                  So:
                  Barn door cowl flaps = more drag = more power required = less excess power = less climb rate.
                  Overload = more drag = less climb rate as above, and also overload = more weight = less climb rate.
                  CoG not right = ......?

                  It's a genuine question. Not saying that CoG doesn't matter at all. Just want to understand why it is critically more important in single-engine flight. I fail to see it and if this effect exists, then I want to understand it.

                  --- 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


                  • #39
                    I don't have an account at PPrune as I consider it a professional flyers website....and I ain't no professional flyer.
                    If it 'ain't broken........ Don't try to mend it !

                    Comment


                    • #40
                      Originally posted by brianw999 View Post
                      What I'm trying to say is that the Navajo is PARTICULARLY nasty if the CofG is even SLIGHTLY out of parameters compared to other aircraft...
                      I've got more flight hours than you...

                      However, I do not doubt this contention whether it's directly from you or if you are conveying it from PPRUNE.
                      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                      Comment


                      • #41
                        Originally posted by brianw999 View Post
                        I don't have an account at PPrune as I consider it a professional flyers website....and I ain't no professional flyer....
                        ... like the 99% of the people that does have an account at PPRuNe

                        --- 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


                        • #42
                          You're about the only one these days that does Brian.

                          99% of people on PPRuNe are nothing like professional aviators. Its quite sad for those of us who are - it used to be a brilliant forum, but these days the signal to noise ratio is disappointing.

                          As for aft C of G Gabriel, I'm thinking these pilots are talking more of controllability in general with the aft c of g, not specifically in yaw. I'm lead to believe they become very hard to handle in terms of pitch, which becomes a big problem if you're engine out and need accuracy in controls.

                          Comment


                          • #43
                            Originally posted by MCM View Post
                            As for aft C of G Gabriel, I'm thinking these pilots are talking more of controllability in general with the aft c of g, not specifically in yaw. I'm lead to believe they become very hard to handle in terms of pitch, which becomes a big problem if you're engine out and need accuracy in controls.
                            Yea, that's exactly what I was thinking (and saying):

                            Originally posted by Gabriel
                            Of course the CG is very important, but it affects pitch, AoA, and speed stability and control (that is, longitudinal stability and control) much more than directional stability. MUCH more.

                            Of course, because speed control is so important in single-engine handling, there is an indirect link there: Keeping one target speed without touching the throttles (that is, keeping it with pitch) is equally easy or difficult with both, one, or no engine running. Only that a lousy speed control is not as critical with both or none engines running as it is with one.

                            --- 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


                            • #44
                              Originally posted by MCM View Post
                              ?........ but these days the signal to noise ratio is disappointing.
                              Love it !!
                              If it 'ain't broken........ Don't try to mend it !

                              Comment


                              • #45
                                I do have a PPRUNE account- (I think I once asked a question)...but yeah- it's best to shut up and listen to those who know.

                                But yep- PPRUNE has become no better than the parlour fora.

                                (Moderated by admin. Removed uncalled for, unnecessary comments about this site.)
                                Last edited by brianw999; 2014-08-05, 19:39.
                                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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