keight

Pneumatic thrust reverser operation

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Good day. 

Reverse thrust as well as nacelle anti-ice should not be available if PRV is closed. Today I got bleed air overheat, as a result the PRV closed, engine bleed valve closed,  NAI was unavailable but reverser was still working. It happened on RR version, later tested the same failure + PRV failure on GE it behaves the same.

pyKPQ.jpg

qSRrI.jpg

from fcom

6htb5.jpg

from qrh

ALBKq.jpg

 

Am I missing something or it is modeled inaccurately?

Thanks

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3 hours ago, keight said:

Good day. 

Reverse thrust as well as nacelle anti-ice should not be available if PRV is closed. Today I got bleed air overheat, as a result the PRV closed, engine bleed valve closed,  NAI was unavailable but reverser was still working. It happened on RR version, later tested the same failure + PRV failure on GE it behaves the same.

pyKPQ.jpg

qSRrI.jpg

from fcom

6htb5.jpg

from qrh

ALBKq.jpg

 

Am I missing something or it is modeled inaccurately?

Thanks

Submit a support ticket! 

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I do confirm, try the BLEED X OVERHEAT programmed failure and the engine X reverse keeps working...

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I suppose this happens with GE and RR engines, that have a pneumatic thrust reverser, but not with the PW, right?

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A bleed overheat should shut

On 14/10/2017 at 6:56 AM, keight said:

Reverse thrust as well as nacelle anti-ice should not be available if PRV is closed. Today I got bleed air overheat, as a result the PRV closed, engine bleed valve closed,  NAI was unavailable but reverser was still working. It happened on RR version, later tested the same failure + PRV failure on GE it behaves the same.

To be technically correct, the RR doesn't have PRV, but a FWSOV (Firewall Shutoff Valve). On the RR, an overheat automatically closes the HPSOV (High Pressure Shutoff Valve) and the FWSOV. The PRSOV (engine bleed valve) is not commanded closed by system logic, but closes anyway due to these other valves closing... i.e. there is no upstream pressure to keep the valve open. Having said this, I've always been warned not to touch the reverse levers on the ground even with no air available because there may be air trapped in the engine plumbing to activate the reversers. Whilst I'm not saying it's a certainty, I definitely wouldn't be climbing into the back of an engine to do maintenance without mechanically locking the reversers in position. However, for simulator purposes, I think it would be ok to ignore this possibility.


Cheers

 

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P.S. Also, I have a vague recollection that, on the real aircraft, the RR green NAI annunciation appears on the Upper EICAS when manual NAI is selected ON irrespective of bleed air availability. i.e. it's purely a switch position indicator. However, you will probably get a NAI valve message on the Upper EICAS and, on the Lower EICAS bleed synoptic, there will be no flow bar to the NAI branch.

(EDIT) I just looked at the 744 wiring schematics and this appears to agree with my real world recollections :cool:

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3 hours ago, Qavion2 said:

However, for simulator purposes, I think it would be ok to ignore this possibility.

So definately, "for simulator purposes", the thrust reverse logic needs to be revised and the reverse made unable to activate when there is an overheat... 

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14 hours ago, Qavion2 said:

A bleed overheat should shut

To be technically correct, the RR doesn't have PRV, but a FWSOV (Firewall Shutoff Valve). On the RR, an overheat automatically closes the HPSOV (High Pressure Shutoff Valve) and the FWSOV. The PRSOV (engine bleed valve) is not commanded closed by system logic, but closes anyway due to these other valves closing... i.e. there is no upstream pressure to keep the valve open. Having said this, I've always been warned not to touch the reverse levers on the ground even with no air available because there may be air trapped in the engine plumbing to activate the reversers. Whilst I'm not saying it's a certainty, I definitely wouldn't be climbing into the back of an engine to do maintenance without mechanically locking the reversers in position. However, for simulator purposes, I think it would be ok to ignore this possibility.


Cheers

 

Interesting to see SOV referred to as shut off valve, in industrial settings it is called a solenoid operated valve.  Generally found in shut off applications but there are many others.  They are typically a pneumatic actuated valve where the solenoid controls the pneumatics, but there are also solenoid actuated valves too in applications where you have the current available. The advantage to pneumatic is the valves typically fail close, so loss of air will shut the valve, and the air can be controlled with a much smaller solenoid with less current that the valve actuator would require.  Always enjoy your posts John.

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19 hours ago, jgoggi said:

So definately, "for simulator purposes", the thrust reverse logic needs to be revised and the reverse made unable to activate when there is an overheat... 

Sim designers option, I would say. However, without formal Boeing documentation on the subject, I would lean towards the FCOM descriptions. Other sims may not follow this rule. Note that DDGs/MELs applied to the reverser system usually involve both electronic (power) and mechanical lockout to prevent inadvertent operation.

 

8 hours ago, downscc said:

Interesting to see SOV referred to as shut off valve, in industrial settings it is called a solenoid operated valve.

Note that not all 744 pneumatic valves operate by direct control from the aircraft computers. Some of these engine valves may be activated by "controllers". i.e. a device which ports servo air pressure to the valve actuators. These controllers are located on the engine but are not always a part of the valve actuator assembly itself. These controllers have a number of devices (e.g. aneroids and solenoids) with multiple inputs (not just electric) which determine valve operation. Irrespective, Boeing documentation seems to use the title "SOV" to apply to both solenoid operated valves and controller-operated valves.


Cheers

John

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On 10/13/2017 at 8:56 PM, keight said:

Good day. 

Reverse thrust as well as nacelle anti-ice should not be available if PRV is closed. Today I got bleed air overheat, as a result the PRV closed, engine bleed valve closed,  NAI was unavailable but reverser was still working. It happened on RR version, later tested the same failure + PRV failure on GE it behaves the same.

pyKPQ.jpg

 

This is an interesting fault because on RR 744's the thrust reverser and NAI on the affected engine might actually still work.  On unmodified aircraft the EICAS warning message would simply be BLEED 4 plus a SYS FAULT light (however, there was no BLD 4 OVHT/PRV message on the early RR 744 aircraft).  The BLEED warning simply means that the Bleed air has exceeded temperature limits, or pressure limits, or the Firewall Shutoff Valve or HP Bleed Valve is open when commanded closed.  

The abnormal checklist for a BLEED 4 message on RR 744 aircraft initially calls for the Engine Bleed Air Switch to be selected OFF and the Nacelle Anti-ice Switch to be turned ON.  It also goes on to say that Reverser on the affected engine MAY not be available, because IP stage bleed air may not be sufficient to signal the affected thrust reverser to deploy. The landing distance is not affected with only one thrust reverser inoperative, so it is always safer to assume the Reverser will not work.  If it is found that the NAI is still available (i.e NAI VALVE message on the affected engine is not displayed with the switch in AUTO) the procedure calls for minimum N1 RPM's of 60% above 10,000ft and 55% below to ensure adequate IP stage air for anti-icing purposes. (NB:on modified aircraft the nacelle anti-ice will definitely be inoperative, so icing conditions should be avoided).

Bertie Goddard 

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2 hours ago, berts said:

On unmodified aircraft the EICAS warning message would simply be BLEED 4 plus a SYS FAULT light (however, there was no BLD 4 OVHT/PRV message on the early RR 744 aircraft).

So you're saying there was a modification to show a BLD X OVHT/PRV message on modified RR's, Bertie? In my manuals, I can't find any reference to it on the RR's, only on the CF6's. This makes sense to me because the RR doesn't have a PRV. However, my manuals mention a Status message "BLD X OVHT" which indicates the HPSOV and FWSOV has been commanded closed by the ASCTU (i.e. basically the same thing as a BLD X OVHT/PRV). 


Cheers

JHW

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So, summing up, the problem with the GE engines is: BLD X OVHT message --> PRV valve closes --> no more air from the engine (both IP and HP) for the thrust reverser. Therefore, unless it uses some trapped air, how can the thrust reverser work?

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3 hours ago, jgoggi said:

Therefore, unless it uses some trapped air, how can the thrust reverser work?

It shouldn't.... unless trapped air does cause reverser operation on the CF6.

The engineers who notified me of the trapped air issue did so prior to our airline acquiring CF6-powered aircraft. I don't know if it does or doesn't apply to the CF6. Even on the RR, I don't know to what degree the reversers operate. I don't know if they only unlock, drive a little way or drive to the fully deployed position. It may depend on how much "trapped air" there is. Since this information is only anecdotal, I don't know if there have been modifications which have stopped this happening.

General info: Note that the bleed air for the reversers is sourced from different locations on the RR and CF6. A closed PRV on a CF6 will remove the normal source of bleed air from the reversers. A closed FWSOV on an RR will not stop the reversers getting their normal supply of air. The RR reverser actually uses two sources of air for proper operation (one tapping point is prior to the HP, the other is after the HP (and IP) supply, but prior to the FWSOV)

 

 

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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.

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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.

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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. 

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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.

 

 

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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.

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

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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.

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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?

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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?

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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.

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