The727Flyer

Brake Pressure dropping too fast.

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Hello guys,

 

I have been enjoying the B747-400 a lot. It is a masterpiece.

However, I have a problem with the brake accumulator pressure. Since last update the pressure decreases too fast, it does not survive flight preparation. From around 3000psi to almost 0psi in less than an hour. I have parking brake on all the time until there is not enough pressure and it is released. With hydraulics on it recovers full pressure but as soon as there is no hydraulic it starts going down again slowly.

I have deleted my custom panel state and built again from your long stop panel state.

 

Best regards.

 

Miguelito de Lucas.

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Delete all the panel states, including the PMDG ones (or move them somewhere else if you don't feel comfortable doing that), then try creating one from default.

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What is a normal time frame for the pressure to drop? There must be a reason why chocks are being used at every airport and every stop.

depending on how a hydraulic supply is built it can hold its pressure for hours, if not days but it can also drop quite fast within minutes or seconds as soon as the pump is shut off.. 

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1 hour ago, Ephedrin said:

What is a normal time frame for the pressure to drop? There must be a reason why chocks are being used at every airport and every stop.

depending on how a hydraulic supply is built it can hold its pressure for hours, if not days but it can also drop quite fast within minutes or seconds as soon as the pump is shut off.. 

On most aircraft the parking brake hydraulic accumulator can hold the brakes applied for quite some time. On a real aircraft there are any number of things that could cause the pressure to drop too quickly. An external leak in the lines going to the brakes could do it of course, but that would be obvious to pilots or maintenance personnel when doing a walk around inspection. 

A more likely problem would be internal leakage in the check valve associated with the accumulator, or inside the accumulator itself. Every aircraft will have a specific time frame listed in the maintenance manual during which the accumulator must maintain a certain minimum pressure with parking brakes applied. I have no idea what the limit would be for a 747. Typically older aircraft would be more likely to have this problem than newer ones. 

Do you have system failures enabled? 

 

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2 hours ago, JRBarrett said:

Do you have system failures enabled? 

I don't think it's failure related as I see this too and it was evident on a pre-release video of the 747-8. As you say, the accumulator should be able to hold brake pressure for a long time. In the current 744 and 748 sim versions the accumulator pressure visibly reduces as if there is some leakage of fluid from the metering valve to return.

Edit: The leak is in the accumulator as pressure drops even with the parking brake not set.

Maybe PMDG wanted to introduce a little leakage to the simulation but at the moment it's too rapid.

4 hours ago, Ephedrin said:

There must be a reason why chocks are being used at every airport and every stop. 

The chocks allow you to release the brakes and properly cool them without risk of them seizing on.

Edited by kevinh
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The majority of times in real world ops you’d be asked by the engineer on the head set to release the brakes once the chocks are in place, and they report back that aircraft has settled on the chocks. When it’s particularly windy I’ll suggest to the engineer that I’d also like to leave the brakes set as aircraft can and do jump chocks if being battered by winds.

When the next crew get on the parking brake is then set on again as part of the initial preflight scan. The brakes are required to be on so that the brake wear pins protrude correctly as these are checked during the walk round .

 

Jon Bunting

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Miguelito

The rate of decrease in the brake pressure gauge within our simulation is based upon the weighted average of timed observation from 21 different in-service aircraft that varied in age from 28 years old to only 1 day in age.

While my training specifically told me to expect this pressure to remain, line observation and discussion with experienced Boeing engineers demonstrated that my expectation was not accurate.  So you aren't the only one who started at the gauge and thought:  "Now that cannot be right."

We put a huge amount of effort into researching such things- I hope that helps!

 

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Hi Robert and company,

 

I am back at my computer and I will do a test flight now.

According to the aircraft maintenance manual, the inspection/check task of the hydraulic brake accumulator states that the pressure indication on the P1 panel gauge should not decrease without hydraulic pressure althought it doesn´t says in which time range. My experience is with A320 family where the accumulator holds the pressure almost forever (plane arrives in the morning and the accumulator shows pressure on the top of the green at night) and although it is not the same plane, I think it is a bit extreme that the accumulator pressure is lost in less than an hour.

It was working ok when it was released one year and a half ago, even when it was broken by one update and was reading 300psi instead of 3000 psi it was holding pressure.

I will report back in around three hours.

 

Have a nice day guys.

 

Miguelito de Lucas.

 

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Hello again,

 

I got the parking brake released message (bottom left red message in Prepar3D). It went from aproximately 3100psi to 750-ish psi in 42 minutes with parking brake on and no hydraulic pressure. Then it remains there without decreasing for the remaining 18 minutes before departure.

Regards.

 

Miguelito de Lucas.

 

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3 hours ago, The727Flyer said:

Hi Robert and company,

 

I am back at my computer and I will do a test flight now.

According to the aircraft maintenance manual, the inspection/check task of the hydraulic brake accumulator states that the pressure indication on the P1 panel gauge should not decrease without hydraulic pressure althought it doesn´t says in which time range. My experience is with A320 family where the accumulator holds the pressure almost forever (plane arrives in the morning and the accumulator shows pressure on the top of the green at night) and although it is not the same plane, I think it is a bit extreme that the accumulator pressure is lost in less than an hour.

It was working ok when it was released one year and a half ago, even when it was broken by one update and was reading 300psi instead of 3000 psi it was holding pressure.

I will report back in around three hours.

 

Have a nice day guys.

 

Miguelito de Lucas.

 

Losing all accumulator pressure in an hour does seem extreme.

I work on Dassault Falcons, a much smaller aircraft, but its brake accumulator specifications are probably  similar to what one would find on most aircraft. The accumulator pressure gauge is only visible from outside the aircraft, through an inspection window in the right wing root.

The aircraft has a 3000 psi hydraulic system, and if the aircraft has just returned from a flight, and parking brakes were not used, the accumulator pressure will remain near 3000 psi for hours on end.

Even if the parking brake was applied and released once when parking (before engine shutdown), the residual accumulator pressure will still typically be above 2500 psi when the aircraft is brought into the hangar at the end of the day

As part of our post flight inspection after the last flight of the day, we pull the parking brake handle repeatedly to bleed off all residual hydraulic pressure - leaving only the nitrogen charge on the “gas” side of the accumulator piston. That should be between 1000 and 1200 psi, and should never drop any lower, no matter how much time passes.

The CRJ-200, which is another aircraft in our fleet, has a residual nitrogen charge of 750 psi when the brake hydraulics have been bled off by repeated brake applications with hydraulic pumps shut off.

I don’t know what the nitrogen precharge on a 747 brake accumulator would be, but the gauge in the cockpit should never read “zero”. With no hydraulic pressure from the engine or aux pumps the accumulator will lose pressure every time the parking brakes are applied and released, but the gauge should never show less than the nitrogen precharge pressure even if the hydraulics are completely depleted.

If I saw the brake accumulator lose more than a few hundred psi from its initial 3000 psi full charge, (without activating the parking brakes) I’d be looking at the accumulator check valve, or parking brake control valve, as this would be an indication that one of those components is leaking internally.

On the Falcon, a 24-month inspection item is a test of the parking brake accumulator. We attach a pressure gauge to the brake itself, fully charge the hydraulics, then set the brake and turn all hydraulics off. The brake pressure at the gauge will be initially 1500 psi. It must drop no lower than 850 psi after 24 hours with the brakes continuously applied, and the accumulator pressure must drop no lower than 1900 psi over the same time period.

I assume that the cockpit gauge in the 747 reads accumulator pressure.

 

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Hello Jim,

 

The nominal pressure of the brake accumulator is 750psi at 20ºC - 25ºC. The P1 press gauge in PMDG 747 goes down to 750psi and stays there. I am flying now without hyd sys #1, #2 and #4 at -26ºC and it doesn't goes below 750psi (should be around 650psi at that temperature).

Have a nice day.

 

Miguelito de Lucas.

 

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14 minutes ago, The727Flyer said:

Hello Jim,

 

The nominal pressure of the brake accumulator is 750psi at 20ºC - 25ºC. The P1 press gauge in PMDG 747 goes down to 750psi and stays there. I am flying now without hyd sys #1, #2 and #4 at -26ºC and it doesn't goes below 750psi (should be around 650psi at that temperature).

Have a nice day.

 

Miguelito de Lucas.

 

Thanks for the info. It appears that the gauge does indeed show the precharge. If that ever went to zero, it would definitely be a maintenance write-up.

I guess the only question is what would be a “typical” time frame for the accumulator to hold the full 3000 psi charge without leaving the parking brakes applied. PMDG has access to real 747 data and information from current and retired maintenance engineers, so perhaps the bleed down is average for what would be seen in the 747 fleet as a whole.

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