747 Boost problem

[YUHAO WANG]

Hello, dear sir. I once saw the boost control mode on the 737ng class d cae simulator in the manual or automatic position. When the outflow valve is in any position, the cabin height will change with the engine speed. Is this the same in the 747?
Sincerely ask the teachers.
Thank you for your answer.

[John H Watson]

I once saw the boost control mode on the 737ng

Can you tell us what "boost control mode" is (on the NG)? It's not mentioned in my 737NG engineering training manuals.

If the pressurisation changes with thrust changes, it must be very uncomfortable for the passengers (?) It's the purpose of the outflow valves to keep cabin at the required pressure altitude (as per the pressurisation schedule). The schedule is normally based on the cruise altitude (in the FMC on the 744 and as selected on the control panel on the 737NG)

There is a slight increase in cabin pressure during the takeoff roll, but this is not related to an increase in thrust. It's just the way the system is programmed.

[Dirk Schepmann]

Not an expert, but I thought that there is a series of valves in the bleed air system to keep the pressure of bleed air relatively constant before entering the packs, regardless of thrust setting?

[Hardy Heinlin]

"Boost control mode" might be an error in the Chinese autotranslator, actually meaning "pressurization" or "bleed air", i.e. outflow valve control mode?

When the outflow valve is in any position, the cabin height will change with the engine speed. Is this the same in the 747?

When the outflow valves control is in manual mode and the valves are in an open position, and the bleed air pressure is low, the cabin altitude will tend to be equal to the aircraft altitude. Same effect as when a window or door is open. The higher the aircraft altitude and the lower the (regulated) bleed air pressure output from the engines, the higher the cabin depressurization rate.

[Jeroen Hoppenbrouwers]

"Boost control mode" might be an error in the Chinese autotranslator, actually meaning "pressurization" or "bleed air", i.e. outflow valve control mode?

Very well spotted. I need to remind myself that in this age of possibly invisible autotranslators, we need to be even more vigilant for cultural and linguistic differences that are very difficult to spot if you are not consciously aware of them.


Hoppie

See also:
https://en.wikipedia.org/wiki/Darmok

[John H Watson]

Not an expert, but I thought that there is a series of valves in the bleed air system to keep the pressure of bleed air relatively constant before entering the packs, regardless of thrust setting?

My notes don't really go into much detail. They just state that the final engine bleed valve "controls the bleed pressure downstream of the valve to 45psi". The valve needs at least 10psi to operate and is fully open at <30psig.

Duct pressure is not always 45psi, so there will be some variation in psi going to the packs. The packs also operate at different flow rates, so it will be the pressurisation outflow valves which have the final say regarding cabin pressure. Under normal circumstances, the flight crew would not be looking at variations in cabin pressure, so it's not possible to know how fast the outflow valves react to pack output pressure changes. Their ears would be the best judge of this. 

[Hardy Heinlin]

Though the universal translator can translate their words, the Tamarians communicate only through allegory, which baffles the Enterprise crew because they do not understand this yet nor do they even know the stories to which the Tamarians are referring.

I remember this episode and found the story and idea very smart and unique. (In the German version the crew called it "metaphors" instead of "allegory".) Later I realized that this principle is not really unique; it also happens in our languages, I think. We too talk in metaphors. Always. We're often just not aware of it. Now I'm writing on a white background. "White" is just a symbol for a color. It's not the color itself. And "background" too is just a symbol for something we imagine. This symbol "background" is not the background itself, if there is such a thing like a background at all. Some centuries ago the symbol "white" might have stood for snow. The symbol "snow" may be a metaphor for frozen water, or the symbol "frozen water" may be a metaphor for "snow". You see, everything cross-references everything and vice versa. No symbol is the thing itself. All language is metaphor language.

[Jeroen Hoppenbrouwers]

The famous pipe. How people reproached me for it! And yet, could you stuff my pipe? No, it's just a representation, is it not? So if I had written on my picture 'This is a pipe', I'd have been lying!
    — René Magritte

[Hardy Heinlin]

Yes,  René, but by third thoughts we may return to the initial thoughts and conclude that all things in the world are not matter or energy but plain information. Matter itself is not solid, and energy itself is not hot; the smallest fractions of the world are just pieces of information. What is information? It's a reference. The world is a network of references. -- My "reply" on those initial and second thoughts 17 years ago:



:-)

[Will]

Another play on similar ideas is the Lord Chandos Letter ("Ein Brief") by Hugo von Hofmannsthal, from 1902, in which the fictional Lord Chandos writes to his friend Byron, decrying the insufficiency of language, saying that he (Chandos) is forswearing writing entirely after a successful career because he has realized the inability of language to accurately convey human experience. Chandos' crisis comes when he realizes that no writer, no matter how talented, could ever completely bridge the gap between language and experience, between an idea formed of having an experience and the experience of the experience itself. The meta point, of course, is that the letter itself is a magnificent piece of prose and very admirably communicates, through language, the experience of realizing that experience and language are forever separated.

The Lord Chandos Letter: http://depts.washington.edu/vienna/documents/Hofmannsthal/Hofmannsthal_Chandos.htm
Ein Brief: http://gutenberg.spiegel.de/buch/ein-brief-997/1

[Hardy Heinlin]

... when he realizes that no writer, no matter how talented, could ever completely bridge the gap between language and experience.

Luckily, there's an alternative to "completion": Approximation.

And I think that's why the world is alive and not dead -- thanks to approximation. Especially in quantum theory. That's the drive. When it is completed, it's done and over.


|-|

[Will]

"Approximation" ... I'm reminded of the equation of Arno Holz:

Art = Nature - x

In which the artist's job, supposedly, is to minimize the x. Or achieve as faithful an approximation as possible. But I think what links Hofmannsthal and Magritte together, along with Kurt Gödel and Pablo Picasso, is the idea that x can never be arbitrarily small.

The previous generation had tried to paint the landscape as it looked; writers had tried to wordsmith ideas perfectly, and Russell and Whitehead had tried to describe number theory on the assumption that formal logical systems could be both complete and consistent.

Along comes Hofmannsthal, who doesn't appreciate the implied imperative in Holz's equation and instead contemplates the irreducibility of x, saying like Magritte that we think and communicate in symbols only; Gödel also mused on the irreducibility of x, thinking in the space of unknowing, as it were, and asking what that means for us.

All of that being said, as Hofmannsthal pointed out by writing the Chandos Letter, approximations are good enough to get the job done. It's a fine letter, after all.

[Jeroen Hoppenbrouwers]

All of that being said, as Hofmannsthal pointed out by writing the Chandos Letter, approximations are good enough to get the job done. It's a fine letter, after all.

Most if not all aviation stuff is Newton... very little Einstein... let alone Feynman.

[cagarini]

At this moment the OP is saying... - "I swear I just wanted to know about that boost stuff..."  :-)

[Hardy Heinlin]

Thrust is Newton ... GPS is Einstein ... noise is Feynman ... airline operation manual is Picasso.

[cagarini]

I'll take Brigitte, the stewardess :-)

Joke apart, this has been one of the most interesting threads in this forum.