BLEED 1, 2, 3, 4

[emerydc8]

This is for the CF6 engine. Maybe someone can shed some light on this checklist for me and help correlate it to this diagram. https://drive.google.com/file/d/0B-WRh0Hf7VdZMWkzOFJhR1NKMUU/view?usp=sharing

There is no malfunction feature that I am aware of in PSX to duplicate the BLEED 1,2,3,4 EICAS. From what I can tell from the checklist, if you are able to get the anti-ice to operate with the bleed switch closed, then it means the Intermediate Pressure air was allowed to get past the PRV and into the engine nacelle for anti-ice protection. I'm presuming that's the reason for the checklist's warning about minimum N1 (55%/70%) for sufficient protection.

This is the scenario: Following a BLEED 1 EICAS, anti-ice was found to be available with the bleed switch closed. So, what caused the problem? From what I can tell, there are three choices: (1) Bleed overpressure; (2) HP bleed valve failed to close when commanded closed;, or (3) PRV valve failed to close when commanded closed.

I think we can rule out an overpressure because you won't get A/I for an overpressure; although if the overpressure was corrected by the bleed switch being turned off, then maybe you would. Also, the attached diagram says that with the bleed switch closed you will not get A/I if the PRV is closed due to an HP valve failed open. So, which valve is bad -- PRV or HP? It is really confusing and, honestly, there is a major gulf between systems knowledge and understanding the theory behind some of these checklists. Most of the time in the sim, you just see the EICAS and run the checklist. There's not much time to discuss what exactly caused the EICAS.

Also, is it safe to assume that in the above scenario, turning off the bleed switch managed to close a valve (either PRV or HP) that wasn't going closed when "commanded" by the automatic pressure regulating system? I'm not sure whether "commanded" means by the automatic pressure regulating system or by the bleed switch too, since the bleed switch will also close the HP, PRV and Bleed Valve (PRSOV).

Thanks. Jon D.

BLEED 1, 2, 3, 4

Condition: Engine bleed overpressure, or HP bleed valve or PRV
failed to close when commanded.

Light: SYS FAULT

ENGINE BLEED AIR SWITCH
(Affected engine). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .OFF

NACELLE ANTI-ICE SWITCH
(Affected engine). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ON

If NAI VALVE message for affected engine displayed:
Nacelle anti-ice for affected engine not available.
ANTI-ICE message may be displayed.

NACELLE ANTI-ICE SWITCH
(Affected engine). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .OFF

If NAI VALVE message for affected engine not displayed:
Operate nacelle anti-ice normally.
Sufficient bleed air may not be available for nacelle
anti-ice if N1 less than 70% at or above 10,000 feet
or less than 55% below 10,000 feet.

Do not accomplish the following checklist:
BLEED OFF

[John H Watson]

From what I can tell from the checklist, if you are able to get the anti-ice to operate with the bleed switch closed, then it means the Intermediate Pressure air was allowed to get past the PRV and into the engine nacelle for anti-ice protection.

Correct. There is logic in the ASCTU which allows the PRV to open (despite the PRV being initially commanded closed by the Bleed switch (in the OFF position)

I'm presuming that's the reason for the checklist's warning about minimum N1 (55%/70%) for sufficient protection.

The HP valve will be commanded closed with the Bleed switch OFF, so only IP pressure will be available for A/I. IP pressure will be quite low at low rpms.

This is the scenario: Following a BLEED 1 EICAS, anti-ice was found to be available with the bleed switch closed. So, what caused the problem? From what I can tell, there are three choices: (1) Bleed overpressure; (2) HP bleed valve failed to close when commanded closed;, or (3) PRV valve failed to close when commanded closed.

According to my books, a BLEED X Advisory can be caused by:
1) PRV open when electrically commanded closed
2) Bleed overpressure due to PRV failure
3) HP open when pressure >127psi
4) HP open when electrically commanded closed

Finding precise details/logic for these will involve a lot of in-depth research (which I'm not so keen to do at the moment).

Regarding 2)..... Overpressure will command the PRV and HP to close. On the GE engine, the PRV and HP valve controllers should regulate or even close the valves.  If the HP closes but the PRV is stuck open, you will get A/I but the pressure will be limited (because the HP is closed) and because the PRSOV is closed via crew action*, you won't get damaging pressures going into the bleed ducts. The AI valve modulates the pressure going into the nacelles, so you shouldn't get nacelle damage.

*The PRSOV will also close if the PRV closes completely (there will be no engine bleed pressure to open it)

Note that HP. PRV and PRSOV valve control is achieved via pneumatic feedback circuits. Each valve has a controller. The controller senses pressure downstream of each valve via a small air sensing tube. The sensed pressures are converted (in the controllers) into pneumatic servo pressures which control the valve itself. The ASCTU however, can send electrical signals to the valve controllers to either stop the servo pressures or enable the servo pressures. i.e. the ASCTU can only shut a valve or allow it to operate. It can't modulate servo pressures (or control the valve directly).

[Hardy Heinlin]

bleed switch closed

I think, to avoid confusion we should use the words "closed" and "open" for valves only, not for switches and their electrical contacts.

• A valve is open/closed -- the flow stops when it's closed.
• A contact is open/closed -- the flow stops when it's open.
• A switch is pressed/not pressed (or on/off).

Also, when comparing PSX to the real aircraft or to a text, and we say, for example, "it goes to 11", then we should also tell what we're referring to: "In PSX it goes to 11", "in the text it goes to 11".

Otherwise we'll just add more confusion ... :-)


Cheers,

|-|ardy

[emerydc8]

Sorry for the confusion, Hardy. I should have said "bleed switch off."

John,
Thanks for all the info. So, if you get anti-ice in the above scenario, then the likely culprit was the PRV stuck partially open? Turning the bleed switch off manually closes the HP valve and you get whatever comes from the PRV for anti-ice? Could there be any other failure that would allow you to get anti-ice in this case?

With regard to the use of the word "commanded," this would only apply to when the automatic system was commanding it closed? (not when you turn the bleed switches off?).

[John H Watson]

Entered in error

[John H Watson]

So, if you get anti-ice in the above scenario, then the likely culprit was the PRV stuck partially open?

As you say, this is the only possibility. If the HP was partially stuck open, the ASCTU won't allow the PRV to open with the Bleed switch OFF.

With regard to the use of the word "commanded," this would only apply to when the automatic system was commanding it closed? (not when you turn the bleed switches off?).

I can't find a definition for "commanded". The manual and automatic function is collated in the ASCTU. There is no separate wiring from the ASCTU to the valves for manual and automatic control (unlike some hydraulic systems which have separate wiring.. i.e. with manual control bypassing the computer/s)

(EDIT) Re: "The manual and automatic function is collated in the ASCTU." (See below). By automatic function, I mean ASCTU automatic intervention. e.g. Overtemperatures will cause the ASCTU to command the valves to close automatically. On the RR, overpressure (too) can cause ASCTU-commanded valve closure.

[John H Watson]

P.S. Note that Status Messages and CMC messages may provide additional information to the pilots and engineers, but, of course, your actions are dictated by the QRH

[John H Watson]

I've just noticed a difference between High Pressure (HP) valve control on the RR and the GE.

On the RR, overpressure will send a signal directly to the ASCTU and this will command the HP valve to latch closed (and generate a fault indication).
On the GE, overpressure will be sensed by the HP Controller. The HP Controller then sends the overpressure signal to the ASCTU, but on the GE, this signal simply generates a fault indication (if it sees the HP has not closed by normal HP Controller operation), but the ASCTU does not command the HP closed. This may complicate the diagnosis somewhat. We would have to start looking at the internals of the HP Controller to see if manual intervention overrides the fault in the HP controller (which caused it not to shut the HP as the pressures got higher (with higher rpms)).

And after further reading, it looks like duct overpressure (as sensed by a switch after the pre-cooler) on the GE also doesn't cause the ASCTU to command the PRV closed. It just puts on a fault light.

I'll update my earlier message to reflect this discovery

[emerydc8]

And after further reading, it looks like duct overpressure (as sensed by a switch after the pre-cooler) on the GE also doesn't cause the ASCTU to command the PRV closed. It just puts on a fault light.

Does an overpressure at least close the HP valve automatically? I was thinking both the HP and PRV would close for an overpressure, but after checking the Polar/Atlas FCOM maybe it's just for an overheat:

If a bleed air overheat is detected, the PRV and HP bleed valves close. Pushing an Engine Bleed Air switch from off to ON resets the engine bleed fault detection system.

Trying to look beyond the face of the QRH and understand what is really going on with this is confusing.

[John H Watson]

Does an overpressure at least close the HP valve automatically?

The HP should be closed just by normal pressure regulation. The HP valve closes (normally) well before overpressure is reached.

Let's take this step by step (simplifying the process a little). The HP produces more pressure than the IP.
At idle, the HP is open (and producing more pressure than the IP). Because of this, the check valve on the IP system is closed. i.e. only the HP is producing bleed air. The IP check valve is just a one way valve with a spring on it (shown in the diagram as an arrow in a box).
As the engine rpms rise from idle, the bleed pressure from the HP rises with rising rpms and has the potential to provide too much bleed pressure, but at a point called "switchover", the HP valve is closed by the HP Controller (operating normally).... and the IP check valve opens allowng the IP to provide all the pressure for the bleed system. The pressure sort of stabilises after this because the PRV and HPSOV are controlling the bleed pressure.

Except at low engine rpms, the IP system should provide sufficient pneumatic flow for all purposes. When the rpms reduce (e.g. top of descent), the HP should re-open.

If a bleed air overheat is detected, the PRV and HP bleed valves close. Pushing an Engine Bleed Air switch from off to ON resets the engine bleed fault detection system.

When an overheat is detected, signals are sent to the ASCTU (in the equipment centre). The ASCTU turns these into electrical signals which command the valves to close (The electrical signals go to the respective valve controllers which close the valves pneumatically). If the valves do close as commanded, the overheat should disappear (no hot air = no overheat). However, just because the overheat condition has disappeared, the ASCTU won't command the valves to re-open. There are fault latching circuits in the ASCTU which keep the valves closed. However, if you cycle the bleed switch, this unlatches the circuits and normal ops will resume. Of course, if the fault returns, the ASCTU will simply command the valves closed again (and the latches will be set again). (EDIT) I don't know if this reset is allowed by pilots?

[emerydc8]

Thanks, John. So, to summarize, if you can get anti-ice to operate with the bleed switch off, after getting a BLEED 1 EICAS, then it was the PRV that was stuck open? If you can't get anti-ice to work, then it was an HP valve failed open or an HP valve malfunction?

[John H Watson]

These are certainly plausible reasons.

However, I wouldn't rule out pressure sensor problems, valve position switch problems, wiring or ASTCU internal problems as potential reasons. Whilst EICAS provides you with cues, the CMC is pretty smart at isolating certain components. The EICAS leads the pilots to the QRH, the EICAS and CMC lead the engineers to the Fault Isolation Manual which tells them how to isolate the problems with various tests. The bleed system is one system which has a lot of clunky mechanical devices which can't be tested by computers, so often the tests include high power engine ground runs to test these parts properly.

[emerydc8]

It looks as if there is no way to identify what really failed here other than to check the CMC and status messages. That will teach me to try to make some sense out of any action taken via the QRH.

[John H Watson]

I don't know about you, but I certainly learned a lot thanks to your questions 

I even found in my RB211 training notes something I missed in the classroom.... i.e. that the HP valve, even when "closed", does let a certain quantity of air around the butterfly valve. On an engine working very efficiently, this leaking air can cause temporary increases in bleed pressure at high rpms which may reach the overpressure limit if the engine has low bleed demand (e.g. if an APU to pack takeoff is carried out). To help combat nuisance BLEED X messages, a one minute time delay is incorporated into the logic.

[emerydc8]

I don't know about you, but I certainly learned a lot thanks to your questions.

I learned that Boeing doesn't want the pilots to know what's going on with the system or to even think about it.

[John H Watson]

It may be a result of the missing Flight Engineer. There is simply too much complexity under the surface of the aircraft for two men to contemplate and fly the aircraft.

There is simply too much to remember. Engineers have access to full libraries of manuals and in many cases, have time to read them. Pilots shouldn't have to fly and spend long periods reading (after engine start). Fully understanding defects may go as far as knowing which relays are energised or whether there is a "1" or a "0" at logic gate XYZ.

I remember learning in high school maths how to derive the forumula for solving quadratic equations, however, in exam situations, I'd be using the memorised formula to solve quadratics, not working out the problem using basic principles. As an analogy, Boeing is probably just giving the pilots the formula. 

I'm just wondering where the QRH sits in the "aviate, navigate, communicate" mantra 

[emerydc8]

It is sometimes hard to figure out how far down the rabbit hole a pilot should go to understand the systems. Most of the flight engineers I flew with on the DC-8 and DC-10 were not pilots -- they were A & P mechanics and most of them had experience working on the same type of airplane. On the Douglas, it was very rare to do one leg without something breaking or malfunctioning. It was normal to troubleshoot during flight. Sometimes it involved pulling things from the radio rack or swapping gauges from the panels.

Fast forward to just a few years ago, a friend of mine was flying a boat-load of soldiers in a North American 767 from Constanta to Connecticut and when he put the gear down on the approach he didn't get three green lights. He told ATC about it and requested a hold while they determine if it was just a bulb. He called the flight mechanic up to the cockpit and the mechanic successfully changed the bulb in a few seconds. When he landed, the FAA was there to ask him why he was performing maintenance during flight. The only thing that saved him was a recent memo from the company that told the pilots not to do any maintenance during flight. They said to always leave the maintenance to the mechanics. There wasn't much they could say.

We both laughed about what the feds would have said about some of the things we used to do 25 years ago.

[emerydc8]

I found this under Bleed Air System Non-normal Operations. I guess I have my answer

The EICAS alert message BLEED is displayed for bleed overpressure, or PRV or HP bleed valve failed to close when commanded. If the respective NAI VALVE message is displayed after pushing the related Nacelle Anti-ice switch ON, the PRV is closed because the HP bleed valve failed open and nacelle anti-ice is not available.

[John H Watson]

If the respective NAI VALVE message is displayed after pushing the related Nacelle Anti-ice switch ON, the PRV is closed because the HP bleed valve failed open and nacelle anti-ice is not available.

That's a horrible way of explaining what is happening.

Anyway, I think you kinda guessed what was going on back in the first message 

[ASCTU744]

I learned that Boeing doesn't want the pilots to know what's going on with the system or to even think about it.

This kind of proves your point
https://www.pprune.org/tech-log/522962-777-pilots-guide.html