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Old 5th Jun 2019, 08:08
  #441 (permalink)  
fdr
 
Join Date: Jun 2001
Location: 3rd Rock, #29B
Posts: 679
Originally Posted by tdracer View Post
Which time? The first bounce appears fairly benign, second is worse, third bounce all hell breaks loose.
The question remains - was the lightning (or other) induced systems damage so bad as to make the aircraft very difficult to fly, or did the crew botch what should have been a straight forward landing of a perfectly flyable aircraft?

thanks to longtimers post of some detail...


The aircraft is FBW, and has a similar control arm rest to Airbus designs. The aircraft has had an electrical systems failure and has reverted to direct law, and whatever gains that has in it for deflection of the SSC to control surface movement. Flying in windshear in such conditions is going to be a learning experience for the pilot. As I understand the SSJ's architecture, SCAC incorporated some changes to the airbus ideology, that may be a factor. The design had 3 modes, normal, degraded and minimum, with minimum still retaining damping in roll and pitch, which is a step up from AI direct law. That assumes that there is still some power in the system. The event suggests a significant loss of electrical power, and that raises a question as to what happens to the minimum law when unpowered? That this may have occurred, well, the video shows a full on PIO going on, which is more than would be expected from a system with damping.

SCAC increased the length of the SSC for the SSJ compared to the AI designs. That appears to have been retained through from the electric bird and initial sim, through design of the software by Liebherr, and on to the aircraft before and after RA-89098 MSN's. The design of the ATR was also changed from AI, SSJ elected to go part Boeing and incorporated servo control of the ATR to remove the no motion design towards more conventional designs as in the 777/787. The trim system also diverged from AI, AFAICS, SSJ has no trim pickle on the SSC, so follows AI design there rather than Boeing's, but does not have pitch trim wheels as the AI aircraft have had in their FBW design, it has a set of pickles on the rear of the pedestal. That is interesting in itself. It raises the question whether the FBW control stayed faithfully to the C* AI type design, (no speed trim input by the pilot in manual flight normal control law) or headed off towards, C*U, like the 777/787, where indexing of the speed is applied by the trim switches on the yoke. As the laws are enhanced from AI, while it looks a lot like C*, it may not be, it may have advanced towards a more advanced system, such as Dassault or Embraers later FPARC/FPAH type control law design. I digress; but only slightly. Cuz... the SSJ pilots are confronted with a major power loss, that appears to possibly have brought them into a new law, that is not part of the design, (I am way out in the breeze with this... but think it through before shooting...) The most degraded law still requires electronic input, which does not appear to have been available. Trim is by electrically actuated trim switches, not a wheel, and therefore is effectively FBW... the guys flying this probably have never before encountered a no trim available, out of trim, reduced capability below the minimum design trained degraded laws. Thats the worst case, and the guys would get a bit of slack for getting out of sorts. The slightly better case is that the aircraft is operating at minimum laws, but is in known windshear conditions, with manual flight for thrust and flight controls, necessitating input from both at the same time. The trim switches are.... way way way back out of the normal control handling, and that may result in the plane being out of trim purely by that cause. The guys are pitching and altering thrust, and those all result in moments that are being controlled by a system that is very dependent on minimising the random control input of the pilots arm, the reason the AI, SSJ, Dassault etc have configurable rests on the control side of the seat, to ensure that PIO's are avoided. Just add turbulence.

There is no question that the aircraft encountered a PIO (as defined by MIL-HDBK-1797A), it is not the first to do that, and won't be the last. Oddly, one major FBW system had a history of APIO... even electrons can get it wrong. The question is why did that happen, and in understanding that, perhaps until the TsAGI modelling of the actual architecture and conditions that the crew encountered is known, then I would back off throwing allegations of incompetence to the pilot. It potentially highlights an area of operation of electrojets for all driving those types, that if you have degraded laws, you may well consider having very simple conditions for your approach, to avoid finding out whether you end up on the wrong side of the Nyquist stability plot. O Caesar, thou art mortal, thou art mortal....

The indignation over this event appears to be railroading a quite competent design, that is a potentially valuable asset for the Russian civil economy. It would be a shame to bury the design through lack of understanding, where some comprehension may well easily remove the potential for such untidy outcomes. The loss of electrical systems may well be the bigger issue here to resolve to make the plane successful.

The SSJ in service appears to be disadvantaged by poor documentation, a poor MCD that is out of step with modern designs that the aircraft is, and lack of spares. These are symptoms of lack of funding. On the other hand, it is probable that it can be produced at a competitive price, be reliable as most Russian designs are. It needs additional support and understanding from customers in order to succeed, and possibly some fine tuning of the control law algorithm, additional training, and maybe a bit of extra beef in the gear-wing attachment area.

The SSJ is not the first aircraft to have some PIO potential, the MD11F at RJAA 34L is just one of many examples, as is the JAS Gripen, and the F22 in early days of test.




(Anyone with more information on the laws of the SSJ, please PM me)
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