B737 questions

[Raptor71]

Hi guys,

Here are some 737 questions:

-Use of wing anti-ice above FL350 may cause bleed trip off and possible loss of cabin pressure……Why?

- Oil pressure is unregulated , therefore the yellow band (13-26psi) is only valid at take-off thrust whereas the lower red line (13psi) is valid at all times. Why?

-MEC will schedule FF to get the desired N1 using Thrust levers position.
How does the PMC work?
In the fuel circuit, why some fuel is diverted and reheated after the second stage pump? Is that for keeping enough fuel to the MEC to make sure priming is ensured?

Thank you

[c100driver]

Too higher bleed requirement from the high stage bleed valve resulting in high bleed/ duct temperature.

The oil pump is a simple spur gear driven by the main gear box so it's output is a direct function of engine speed. On the ground the low idle may not have enough speed to achieve the output, in flight the flight idle should be enough to always achieve 13 PSI, unless there is a failure in the system hence the shutdown below 13 PSI.

The PMC is simply a fuel trimming device, the MEC provides FF for thrust lever angle and the PMC trims within a small range (3%) for accuracy.

[too_much]

For more questions and answers have a look at Boeing 737 Systems Quiz (http://www.b737.org.uk/quiz.htm)

[tdracer]

-MEC will schedule FF to get the desired N1 using Thrust levers position.
How does the PMC work?
In the fuel circuit, why some fuel is diverted and reheated after the second stage pump? Is that for keeping enough fuel to the MEC to make sure priming is ensured?


The MEC is a hydromechanical device that will control to a fixed N2 (at a given throttle position). However thrust is N1, and the N1/N2 relationship will change with altitude, airspeed, and the thermal condition of the engine. On the 737, the PMC is an analog computer that provides a trim signal to the MEC to help maintain a constant N1 (on the 767, the PMC was a digital device, but the intent is similar).


The second (high pressure) state of the main engine fuel pump outputs considerably more flow than the engine needs - there is a valve that bypasses the excess fuel back to the intake side and maintain the desired pressure in the MEC.


BTW, Wing Anti-Ice is a real bleed hog - it takes a lot of air. Since 'conventional' icing (super-cooled droplets) doesn't exist above 30k (at least according to the regulations) the bleed system isn't sized for it. The icing threat above 30k is "Ice Crystal Icing" or ICI. ICI won't adhere to cold surfaces (it just bounces off) but it can melt and refreeze when it hits warm surfaces (which has been a big problem for the engines).

[Raptor71]

Hi guys,

Well thanks for your answers, it makes more sense.
Maybe you could help me with that too:


I have some questions on the ACP.

Coming from an airplane in which you have a PTT on the ACP:

( I/C: NORMAL POSITION, allowing you to hear and speak to your colleague (and the cabin crew when they call) without having to do anything with the transmitter switches when you use your boom set, however, if you use your microphone, then you need to select the INT transmitter switch; NEUTRAL POSITION: rarely used because You canon do anything with it; R/T: used when you do not want to use the PTT switch located on your Control wheel,when you're done transmitting that way, it will go back to the neutral position, if you don't select the I/C position, you colleague will not hear you talking to him)

and one on the Control wheel : two positions ( neutral and pushed (used to transmit when one transmitter switch is selected)

Pretty simple I must say.
I do not know if the B737 FCOM is unclear but the ACP use seems complicated

I mean, do I need to do anything to speak to my colleague and the cabin crew ( when they call the flight deck)? Can I let the PTT on the ACP in the I/C position and talk to them without using the PTT on my CW?
Is it equivalent to letting the ACP PTT in the neutral position and use the CW PTT in the INT position with the FLT INT transmitter sw selected?

Thank you

[RAT 5]

Intercom does not connect to the cabin crew telephone. You need to use SERV INT and a PTT switch. I/C is for comms between the pilots and the nose gear jack socket.

[Skyjob]

Hi guys,
( I/C: NORMAL POSITION, allowing you to hear and speak to your colleague (and the cabin crew when they call) without having to do anything with the transmitter switches when you use your boom set

You infer to its NORMAL POSITION, when this is actually not the normal case.
It is a Boeing modification to enable the I/C to "stick" in I/C, some older 737's have the spring loaded version still installed. This required crew who insisted in the use of the I/C for NORMAL communication to use elastic bands or similar to hold the button in the I/C position...

The NORMAL POSITION is the neutral middle position, not transmitting on either I/C or ACP selected channel. It also allows ground crew connected not to listen into your conversation in flight deck (even when your headset may be off it will still continue to transmit :O)

[Raptor71]

Ok ok it is still unclear….sorry guys…

If I want to speak to my colleague without using the PTT with on my CW, is it possible?
If yes, thank you to tell me how. It sounds crazy to me to have to use the CW to just speak to your colleague so I guess there is a way...

With the ACP PTT in I/C position, what will happen if I use the MIC position on the CW? ( same question if instead I use the INT position)?

IF ACP PTT is used in R/T ( which is not spring loaded), I understand It allows me to transmit on any frequency without using the PTT sw on the CW.

[Skyjob]

If I want to speak to my colleague without using the PTT with on my CW, is it possible?
Yes, with the ACP in I/C there is a open channel between the pair of you.
Problem is the fact that if you forget to turn it off after you finished using it, the channel remains open for all that plug in to listen to, including any ground staff plugged into the I/C outside the aircraft. Also, when it is selected, it mutes the speaker volume, thus on ground it makes sense to turn on speaker, as if a too large volume needs selecting on the ACP the most likely cause is an I/C switch on on of the installed ACP's being selected.

With the ACP PTT in I/C position, what will happen if I use the MIC position on the CW?
The MIC will override the ACP selected I/C, thus you can keep the channel open while responding to ATC.

( same question if instead I use the INT position)?
If I/C is selected, INT need not be used.
If I/C is deselected (e.g. during pushback) INT allows communication with external connected ground staff, inside flight deck you would need to speak up a bit louder to avoid having I/C selected and ground staff to hear all you checklist items and responses.

IF ACP PTT is used in R/T ( which is not spring loaded), I understand It allows me to transmit on any frequency without using the PTT sw on the CW.
The ACP PTT is identical to the function of the CW PTT. You transmit on the selected ACP function.

[de facto]

When you ll actually go in the SIM or aircraft,it will take you less than a min to figure it out...not rocket science:p