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Budbud

Pneumatic failure rate on real aircraft

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Hi,

 

I set the service based failure rate to x10 and must say that I am not disappointed regarding the number of failures I get.

 

However I have noticed that almost every flight I get HPSOV valve failure on engines. Counting the fact that the rate is x10 and my average flight durations are approx 8h, it means that failures would occur every 80h or so.

 

Does that kind of failure occur so often on the real aircraft? Do the HPSOV valves so weak or is it a normal wear rate?

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I doubt that the MTBF tables are anywhere near actual numbers.  Such data would probably be costly and PMDG would be promoting it as a feature.  In fact, did a 15 sec Find in Introduction and they explain that the failure rates are adjusted to give you something to work with.  They do promote that failures are otherwise realistic and cascade into other systems realistically.  I remember one note from Robert that mentioned he had been spending hours making sure every light bulb in the cockpit drew power from the correct source LOL.

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According to the explanation of the service-based failures in the Introduction document:

 

"SERVICE BASED FAILURES: When using service based failures, you will experience a highly realistic statistical model for mechanical failures that uses the airplane’s age and operating experience in order to predict equipment failures in a fashion similar to the real world operating experience of 777 operators. This model was created using real world data defining the Mean Time Before Failure for nearly every operational part of the airplane. This being the case, equipment will wear out, and mechanical failures will happen, requiring you to use the Quick Reference Handbook to manage mechanical failures, and the Aircraft Maintenance menus in the FMS to service the airplane as you use it."

 

 

 


I doubt that the MTBF tables are anywhere near actual numbers.  Such data would probably be costly and PMDG would be promoting it as a feature.

I sometimes do so misreading as English is not my native language but I thought of the contrary when reading the text above.

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I have seen the HPSOV failure a few times as well even though I do not have service based failures activated.

Been a while though so maybe this has been fixed or maybe it was a panel state problem.

I suggest trying a default PMDG panel state for while and see what happens.

 

I just tried and activated the failure manually.

After that, I got the "Bleed Off Eng L" EICAS message and the HPSOV L status message.

(it takes a few seconds and a lot of engine N1).

So at that point you have no more bleed air from the left engine.

 

I checked the MEL of the real 777 and you are actually allowed to dispatch with this failure using bleed air from the other engine and/or APU to supply all pneumatic systems.

 

As for how often this happens on the real 777 .......I have never had this problem in 13 years!

So it more an FSX bug than anything else if you ask me.

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As for how often this happens on the real 777 .......I have never had this problem in 13 years!

So it more an FSX bug than anything else if you ask me.

 

Ok I was quite surprised of the recurrence of that kind of failure and what you say confirm my doubts.

 

I will check with panel state.

 

My last flight was a bit different, I had APU bleed valve and generator failure, nose tire balance and crossbleed valve forward. Only the bleed valve failure was indicated on the lower EICAS and visible on the air page of the upper.

For the gen failure I noticed it only when on blocks and shutting engines off and for the nose tire balance, it is quite easy to know it when a loud rattling noise during lift up at take off. Regarding the crossbleed valve I didn't even notice it for the whole flight as I didn't check them. I usually check test them only for ETOPS flights.

 

That makes the flight much more interesting with the possibility to have hidden failures. I'm still waiting for more serious failures like gear, hydraulics, or even engines.

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The HPSOV failure is one I've seen passing by quite a few times myself. Your doubts are certainly merited. If a single component on the aircraft was prone to such frequent failures, there would have been a design change without doubt. (In real life, of course)

 

 

 


nose tire balance

The nose tire balance "failure" is caused by speeding through the corners. The 777 is a big plane, so it will have an even more pronounced wearing rate than the 737NG when doing this.

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Yep I doesn't have the tire balance failure much but it's true that sometimes I'm a little fast in the turns when I'm late. :P

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Although the failure rate of the HPSOVs seems rather high in the PMDG 777 with service-based failures "on" in my experience in r/w aviation maintenance over the years, bleed valves are indeed prone to malfunctions. They operate under very high temperature conditions, controlling the flow of bleed air at temperatures of 450 degrees Celsius (or more) The actual valves themselves are often butterfly type - like the throttle plate in an old-fashioned carburetor. As the engine ages, it tends to start passing some oil in the bleed air, which forms a baked-on carbon deposit in the valve assemblies that can cause the moving parts to start sticking.

 

The other high-failure item on commercial aircraft in my experience are hydraulic actuators for control surfaces, landing gears, thrust reversers etc. These tend to develop leaks as they age, and the internal seals become worn.

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I sometimes do so misreading as English is not my native language but I thought of the contrary when reading the text above.

 

 

Your English is not a problem, I missed that additional information you found in the Introduction.  Taken together, both excerpts from the document, I conclude that PMDG used MTBF rates modified to provide a user experience.  For example, almost all pilots flying turbofans today have never had an engine failure but you can experience one with the PMDG 777 using accelerated failure rate feature.

 

My second career, after the USAF, was in petrochemical/refinery project engineering.  Critical SOV applications, such as direct fired heaters fuel supplies, were always designed with redundant valves and sensors. Valves have improved over the decades but they are still considered a likely failure node in fault analysis.

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Taken together, both excerpts from the document, I conclude that PMDG used MTBF rates modified to provide a user experience.  For example, almost all pilots flying turbofans today have never had an engine failure but you can experience one with the PMDG 777 using accelerated failure rate feature.

 

Hi Dan, 

 

Thanks for the explanations. Now compiling the info from the different posts here, remains the question whether the recurrence of failures I get is a technical issue due to corrupted panel states or normal service-based failures.

 

However I would rather go for the second option as everything works perfectly in the cockpit since the beginning and all these failures are handable through the service-based failures menu. And I have no weird behaviour of the plane or anything else related to corrupted panel states.

 

The only strange thing is that I sometimes get one of these failures on a flight following the servicing of the pneumatics. Which would defy the lows of probability.

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Which would defy the lows of probability.

 

I've had several real world problems immediately following maintenance.  I'm really diligent after an annual!  Material does have an increase rate of failure when new, called infant mortality rate, and extensive sampling of production is required if it is to be minimized.

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True, it is the same for merchant vessels (I'm a seaman) when new built or after technical calls... brand new or repaired and everything gets into trouble! :P

Hope it would be better in aviation.

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Hi everyone,

 

I'd like to share my FSX experience of that failure on a specific flight I've just done from OMDB to RJBB (8h30 of flight).

 

My service-based failure rate is set to 10x as well.

 

I've departed OMDB with no failures from my previous flights. The only think I've miss, and I notice it when at my cruise FL, is my oil quantity was a bit low. (16 qt on the left, and 8 qt on the right).

 

After about 6 hours of flight, I get a fail of the left HPSOV valve displayed on the EICAS. I monitor the DUCT pressure of my left engine, which was fine (around 50 psi on the pneumatic synoptic display), so I decided to continue the flight. No corrective action have been done at this stage of the flight (perhaps I should have done something ? I ask you the question..)

 

The real problems start at the descent after 8 hrs of flight. As the aircraft was pitching down, the oil quantity of the right engine drop to the low zone (3 qt), and while I was trying to solve that problem, many others appear :

 

- fail of the Right HPSOV valve,

- the oil temperature of the Right ENG rise until 270 °C,

- fail of the Left HPSOV valve,

- fail of the Left ENG PRSOV valve,

- fail of the Right ENG electrical GEN,

- fail of the Right ENG electrical backup GEN.

 

While descending, I've follow each abnormal checklist for the oil temp problem and the electrical problems.

 

So I had my A/T Left and Right off, Right ENG cut off, throttle idle (was at 270 kts of speed, FL280 descending to my destination RJBB), APU started (which supply me with electrical power on the Right ENG side).

 

But what was strange during my descent, at around FL120, I had a Master Warning of AP disconnected (was'nt intentional, no actions on the joystick, I was on LNAV and FL CHG at this time).

 

And the plane start banking weirdly left ! Yet, at this time, I had my throttle on idle and the Right ENG cutoff...

 

It was very difficult for me to make the aircraft banking on the other side. It was like I had to fight a weird "left banking force".

 

I thought it was an hydraulic problem that act on the flight controls, but on the corresponding synoptic page, the status was shown as NORMAL.

 

So I flew the aircraft manualy to the airport, compensating at every moment the left banking with more or less right joystick deflection.

 

Finally I manage to land the aircraft on the runway without damages or excessive V/S speed at touchdown (-150 ft/min).

 

At the gates, I service all the corresponding systems, except the engine oil temp one.

 

I left the aircraft for about 30-40 minutes to let the fuel temp warm a bit, and to search the web some informations on these failures. And surprisingly, when I come back to the sim, I had a new failure of Nose tire balance (while I was at the gates for at least 30 minutes !).

 

This last one is very surprising as I'm VERY VERY careful on my turns (not greater than 7 kts). The only way I think it could happens, it's when I was flaring the aircraft and trying to keep the nose of the aircraft centerlined (refer to my left banking problem).

 

 

 

Now, I'd like to ask you experts if you think all these failures are correctly linked, or is there a possible bug (AP disconnect, then having the aircraft banking weirdly left, Nose tire balance problem).

 

I'll be also please to understand if I've manage the HPSOV valve failure correctly (probably not, as I don't know what have to be done for handling such failure.).

 

Thanks for your comments.

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The HPSOV message is just a STATUS message.

It shows on the lower center display correct?

 

No action is required in flight (after engine start is the definition) for status messages.

Only maintenance has to be informed so they can repair things upon arrival.

In flight action is only required for YELLOW/RED EICAS caution and warning messages.

So if you get the "Bleed Off Eng L" EICAS caution message in addition to the HPSOV status message then you have to do the checklist for that.

 

 

I was thinking that maybe engine oil is also used to lubricate the Integrated Drive Generators but it is not.

(I checked a maintenance manual and the engine and the IDG have their own fill ports)

So low engine oil or overheated engine oil should not cause a problem with the electric generator.

(so either this not modelled correctly, or you simply had multiple failures due to your x10 failure rate settings)

 

Flight control problems should not occur just because an engine has been shut down.

If however you use the yoke while the AP is engaged, then the AP will disengage (you overruled it).

So maybe a hardware device on your end got out of trim?........maybe you were unintentionally trying to correct for assymetrical thrust? ......that input could disengage the AP.

In any case, use rudder to compensate for assymetric thrust (not aileron).

Rudder input will not disengage the AP.

Or maybe the TAC was doing something crazy (it should not, but who knows)........you could have turned the TAC off for a minute to see if that helps

 

Maybe x10 failure rate is just too high.

I have not tried but can one set x5?

If not then maybe it is possible to limit the ammount of failures that occur on one flight (I have not checked, but maybe there is a setting for this in the CDU?)

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I found why the aircraft was banking left !

The way it happen makes me think about a joystick deflection, and finaly it was ! Even my joystick was physically dead center, the calibration tool of the Thrustmaster Hotas X shows me it wasn't.

I had to unplug, then plug again the USB cable to solve the problem.

 

I have a last question. After a failure, when I'm on ground, do I have to "Clear it" or "Service" the concerned system ?

What's the difference between these two ?

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As Jim correctly said, the HPSOV valve does have a tendancy to start wearing during operation and i believe that it has had numerous modifications done to increase its reliabillity, in the RW aircraft the HPSOV valve message can appear if the butterfly valve sensing mechanism is worn and is giving erronious messages (could acount for why the data on its faliure is quite large, even though the valve hasnt completely failed but is simply giving a false position)

 

777Simmer, you are correct about the drive generator oil, Both the Backup Gen and IDG have their own oil systems, the IDG cooed by a Fuel heat exchanger and the BUG cooled with a combination of an OIL/OIL cooler (on fanframe) and a big AIR/OIL cooler on the left side of the engine core. Not sure why people get the excessive oil temp, ive never experienced it, but seems like a common occurence.

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