B737NG-Pre-pressurization on the ground

[B738HT]

Hi friends,
The captain I flew with tonight told me that B738 would be pre-pressurized when you press the TO/GA, and the cabin altitude could be approximately 200ft(0.1psi) lower than the ambient field altitude.

Question is why this pre-pressurization makes PAX and crewmembers feel more comfortable? Is this 200ft PA so helpful?

[Fullblast]

That's correct.

From FCOM:
The cabin begins to pressurize on the ground at higher power settings. The controller modulates the outflow valve toward close, slightly pressurizing the cabin. This ground pressurization of the cabin makes the transition to pressurized flight more gradual for the passengers and crew, and also gives the system better response to ground effect pressure changes during takeoff.

[de facto]

It is to prevent a pressure bump during lift off.
When you said your captain,you mean you were his passenger right?

[flyingchanges]

Pretty sure it is TLA dependent, not a TOGA function.

[STBYRUD]

AMM SDS 21-30-00
Takeoff
These changes cause the takeoff phase to start:
* N1 on both engines increases to more than 60% for at least
1.5 seconds
* N2 on both engines increases to more than 89% for at least
1.5 seconds.
In the takeoff phase, the system pressurizes the cabin to 0.1
psid below field elevation. This prevents the uncomfortable
pressure bump (momentary pressure increase) at airplane
rotation.
The cabin pressurization rate of change during the takeoff
phase is 350 slfpm.

[Amadis of Gaul]

Interesting. We've had 737NGPilot, 738HT, Sky738 (or whatever) all asking "interesting" questions. Hmmmm....

[ImbracableCrunk]

You guys crack me up.

It's like George Carlin on driving: 'Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?'

Anyone who knows less than you about the 737 is a Simmer. I'll give you some of these questions are pretty basic, but some are not taught in every ground school. Boeing has dumbed down their curricula so much that questions that were once required (useless) trivia are weeded out.

[STBYRUD]

I agree. It doesn't even matter what the background of somebody is, if its a valid question its worth asking and worth answering as well... Mostly ;)

[john_tullamarine]

If a question is excessively trivial, it will be moved to Questions in line with Corporate policy.

Otherwise, if it be answered and not need further thread development, it will come and go rapidly.

I have trouble seeing what the concern is with simple questions, regardless of the poster's background ? If you, as an individual Forum member, find the question uninteresting, it would seem appropriate to ignore it and move onto the next thread ?

A principal value of this Forum is its ability to educate the newer folk climbing up the totem pole. A principal value of the experience in the Forum is its ability to provide that education.

[Amadis of Gaul]

I was more thinking they're all the same person...

[Jwscud]

John, I do agree. I find the discussions that come from technical quirks not properly described to crews, and especially performance questions very illuminating

However, the answer to the OP's question is to be found prominently and in the appropriate section of the FCOM 2 discussing the pressurisation system on the 737 which I suspect is why some might be suspicious. Even in the "cheap" FCOM Boeing sells there is a very good description of the behaviour of the Pressurisation system in AUTO mode throughout a complete flight, and in this case gives the reasoning behind it, as quoted by several posters.

[john_tullamarine]

The dumbing down of training over recent decades is a major reason why resources such as PPRuNe have the potential to be so valuable. Unless one knows what the training exposure background of an individual may be it is very difficult to evaluate the background to, and reasonable validity of, the question.

It would help if folks were to put their heads into the books more but I guess that may be a consequence of attitude changes in many of the younger folk and we see that displayed in a variety of activities.

Be aware that there are many multiple identities on PPRuNe as there are on other sites. Indeed, on occasion where I know the person concerned and their IDs, I derive great amusement watching the poster stir up debate on a topic by using his/her multiple IDs. All part of the fun, I guess.

However, the main aim is to retain one's sense of humour and reasonable propriety.

And make no mistake about it, while there are a few Walter Mittys about, there are some very highly accredited, accomplished, and knowledgeable folk on PPRuNe ...