Pneumatic thrust reverser operation

[jgoggi 65]

There must be something wrong with the PRV in the PMDG 744: if I activate or arm the failure ENG X PRV VALVE on the ground, nothing happens, no EICAS messages and no yellow indication on the ECS synoptic. Maybe that's also the reason why the reversers keep working.

[Qavion2 39]

Does the PMDG fault page specify the type of PRV failure? The PRV is designed to modulate the engine manifold pressure to 97psig. Did you advance the thrust levers to see if there were any overpressure/temperature problems? These problems should trigger messages.

[jgoggi 65]

No, it just says "ENG X PRV VALVE". Anyway yes, I get an EICAS message only when in flight or with high thrust settings, but I don't see any other effect. On the ground I tried spooling to max reverse for several minutes with this failure on but everything works normally, even if I am not sure what this failure does and if it causes the PRV not to open. In that case the reverser should not work. 

[Qavion2 39]

Unfortunately, I don't have a Fault Isolation Manual for the CF6, so it's difficult to figure out a possible failure which shows these symptoms. You would expect that if the PRV failed to open or close, there would be a cascade of secondary events. If the PRV failed open (i.e it was physically jammed open), you would expect an overheat and/or overpressure (and related message/s or lights) at higher thrust levels.

A CF6 engine bleed overpressure will be recognised by the bleed air computer (ASCTU), but it doesn't seem to have the same impact as bleed overtemperature. As far as I can see (quickly looking at my books), overpressure will not cause the ASCTU to command valve closure. If the PRV problem generates an overpressure, but not an overheat, you may still get reverser ops. You should get more than a single message though. Excessive pressures should cause BLEED X messages (Upper/Lower EICAS) and overhead lights.

Anyway, I'm far from an expert in this field. I think I'll have to do more reading on this and try to find the complete logic behind reverser and PRV operation.

 

 

[jgoggi 65]

Thanks, in this case it seems that the system works ok, because when I increase thrust with that failure active, I get a BLEED X eicas message and overhead light, so it should be an overpressure and the reverser keeps working. The problem remains why the reverse still works with a BLD X OVHT/PRV message.

[jgoggi 65]

Has the cause of the reverse working with bleed overheat been found?

[Jet Tech 31]

HI James,

As I told you in the support ticket that you submitted for this issue,  I have submitted it to the team to review the logic and operation.

[Copper. 78]

John, what do your GE manuals say about PRV and HPSOV behaviour AFTER being commanded closed by ASCTU as a result of a engine manifold overheat event?

[Qavion2 39]

 

On 13/11/2017 at 3:05 AM, Copper. said:

ohn, what do your GE manuals say about PRV and HPSOV behaviour AFTER being commanded closed by ASCTU as a result of a engine manifold overheat event?

My books say that the valves are latched closed until the overheat condition has gone AND the bleed switch has been cycled. However, my manuals also have the following statement:

Deployment of the thrust reverser will cause the HPSOV to be pulsed open provided a real time overheat conditon does not exist.

Looking at the logic circuits in the ASCTU, it seems that reverser activation is possible if the overheat goes away (even if the bleed switch hasn't been cycled).

The same statement is given for the PRV (and the logic diagrams seem to agree).

Interesting. I wonder if James' overheat was not real time?

[Copper. 78]

4 hours ago, Qavion2 said:

Interesting. I wonder if James' overheat was not real time?

I wonder the same. The original post says that this happens on the RR also, and maybe it shouldn't.

[jgoggi 65]

6 hours ago, Qavion2 said:

Interesting. I wonder if James' overheat was not real time?

Well, I just triggered the overheat failure through the FAILURES page of the PMDG 747 CDU. I think that how the logic of that failure is simulated in the PMDG 747 is known only to PMDG, anyway I tried several times and each time I could use the thrust reverse, with the failure still on, without cycling the bleed switch and with the EICAS message not extinguished. I hope at PMDG they have already looked inside the logic and found out where the problem is...

[Copper. 78]

5 hours ago, jgoggi said:

I hope at PMDG they have already looked inside the logic and found out where the problem is...

Problem?

RR - with HPSOV and FWSOV closed, it is possible for reversers to translate.

GE - with HPSOV and PRV closed, it is possible for reversers to translate.

 

[jgoggi 65]

11 minutes ago, Copper. said:

Problem?

RR - with HPSOV and FWSOV closed, it is possible for reversers to translate.

GE - with HPSOV and PRV closed, it is possible for reversers to translate.

 

"Deployment of the thrust reverser will cause the HPSOV to be pulsed open provided a real time overheat conditon does not exist". Our 747 opens the reverser even with a real time overheat condition...

[Copper. 78]

BLD X OVHT/PRV shows for 1 or 2 of 2 reasons. How are you certain the overheat exists?

[jgoggi 65]

I cannot be certain, but I tried at least 10 times, and all 10 times I had the reverse working... The failure to trigger is "bleed overheat", so why shouldn't the overheat exist?