Brake accumulator oddities?

[badderjet 181]

Hey folks, just stumbled across this one a couple days ago... The brake accumulator gets depleted nicely with no hyd px and applying brakes a few times. Now one should know the accum is precharged with dry nitrogen at 1000 psi, which seems to be the case here. Now this depleting looks convincing when the accum was fully charged at 3000 psi. What the system seems to do though, is to just subtract a constant number on every brake application. After about six applications, the needle happily rests at a thousand psi. Good so far.However, this leads to the following behavior: Deplete the accum like said above. Charge with pressure but use only an EMDP this time. This will yield about 2850 psi on the accum, at least sightly less than 3000. Shut down hydraulics. Deplete accum and watch. On the last stroke, the system seems to think "ah, pressure still more than 1000 psi so once again subtract pressure". This time though it goes well below 1000 psi, eventually down to around 700, which to my understanding is an impossibility. As I said, the accum is 1000 psi precharged, so I would tend to think that you can't go below a thousand. That's the first thing, the other is that the accum would happily be charged by system A pressure, which is pretty much impossible as I think, because system A should in fact isolate the accum by closing it's isolation valve. I don't think there's any way in the lines that would allow system A pressure to reach the accum. Anything I have missed?

[tcas_climb 45]

intersting catch

[badderjet 181]

Well thanks... do you see the same at least? You never know, there might be config options that trigger different behavior on different systems and the like.

[ZachLW 37]

Under what scenario would this issue become apparent in the real aircraft??

[Houghton11 12]

Well, as a maintenance item you'd be testing things like that. In the mean time just pretend some lazy apprentice forgot to recharge the accumulator pressure. Stuff like this is a good find...it gives us some insight on how PMDG programmed all this stuff. In this example it's probably what the OP described - just arithmetic.

[ajacied551 1]

Pre-Charger of the Brake-accumulator is 1000 PSI as a nominal value, nitrogen is senstive for temp changes, so the brake pre-charger pressure could be lower due a lower ambient temp or higher due to a higher ambient temp. if it going lower than the 750 PSI than the brake accumulator needs to be serviced. And for the second point about Brake Accumulator pressure. It could never be charger with Hydraulic system A Because the Accumulator Isolation Valve prevents that. The Hydraulic system B is for the normal brake system and Hydraulic system A is for the Alternate Brake system. The Brake Accumlator is connected to the Hydraulic B system. When only Hydraulic system A is pressurized the accumulator Isolation valve closes. This is to prevent that Alternate brake system is going to supply hydraulic A power to the normal brake system.

[badderjet 181]

Under what scenario would this issue become apparent in the real aircraft??

I understand your concern, and the answer is probably "none come to mind". But that's not the point here, this is simply a system anomality that shouldn't be there.

Well, as a maintenance item you'd be testing things like that. In the mean time just pretend some lazy apprentice forgot to recharge the accumulator pressure. Stuff like this is a good find...it gives us some insight on how PMDG programmed all this stuff. In this example it's probably what the OP described - just arithmetic.

Not only for maintenance, I'm testing these things to see how the simulation goes. And yes in fact I do find it interesting how they did this stuff. Especially since they said numerous times they had somehow built the pumps, lines, valves, px sensors etc into the code. As I understand it they really got the structure of the systems into the code that way. Whatever that means. However, then I'd assume such thing would probably not be a hard thing to fix.

Pre-Charger of the Brake-accumulator is 1000 PSI as a nominal value, nitrogen is senstive for temp changes, so the brake pre-charger pressure could be lower due a lower ambient temp or higher due to a higher ambient temp. if it going lower than the 750 PSI than the brake accumulator needs to be serviced.

True, but even if N₂ temperature induced volume changes were modelled, that still doesn't justify the seen behavior. Because then it would still deplete to a constant value, be it more or less than 1000 psi, but it would be constant as long as the temperature doesn't change. Right now it depletes to different "final" pressures at constant temperature, and only depending on your "start" pressure. As I said, this seems to be a direct result of substracting a constant number but not checking for the last stroke if it would go below pre-charge pressure. I could be wrong of course, but this is only what I can observe. I agree with your second point. System A should isolate the accumulator via the isolation shuttle valve, not charge it in any way.

[Cristi_Neagu 134]

If they would have simulated all systems using accurate formulas for everything, you'd need Deep Blue to run FSX. You do have to remember FSX is "as real as it gets", not "real... period.". So compromises must be done. Even so, somebody should probably add an IF statement in there somewhere to fix this.

[badderjet 181]

If they would have simulated all systems using accurate formulas for everything, you'd need Deep Blue to run FSX. You do have to remember FSX is "as real as it gets", not "real... period.". So compromises must be done.

I tend to disagree. In fact they simulated lots of this plane with way more formulae than any other developer has ever dreamed of. Just imagine the thermodynamics going on behind the scenes to correctly simulate airflow through the cabin, or hydraulic fluids in the lines. So it's definately a way lesser task to fix one or two wrong valves one would think. And still, as you see you don't quite need anything close to Deep Blue to run this simulation which still produces convincing results for the most part.