B737 Pneumatics
 

For some reason the Engine Bleed switches of the B737 should be in the "ON" position before engine start. I see no specific reason for this. I know that the valves are only open if there is engine bleed pressure, and anyway the valves are closing when the starter switch is in "GROUND", going to the selected position after starter cut-out. But what is the reason why we may not select the bleed to "OFF" before engine start? :confused:

Could be as simple as having the bleeds setup for flight...nice to have pressurization .

Allerbeste,

May i reverse your question...why wouldn't you want the bleeds selected ON?

Having the Bleeds ON is their STANDARD position, switching them OFF would require you to go through the SUPPLEMENTARY procedure.

Good to know your systems,but make your life easier and follow standard ops:ok:

Whatever any technical reason, one word - Helios. Something as simple as a bleed switch, pressurisation mode selector etc can be the first card in a deck to cause a disaster. To this day there are NO warnings of an incorrectly configured bleed/press panel on the 737.
Our airline requires a safety report to be filed and subsequent investigation if we ever arrive at an airplane and find bleeds off or PMS set wrongly.

ALLERBESTE,

This is to prolong the life of the solenoid. It is actually de-energized on the ON position, not the other way around.

If this is the main thing your airline learned after the Helios accident, they did not understand what happened there.

Quote out of the final Report:
Direct Causes
1. Non-recognition that the cabin pressurization mode selector was in the MAN (manual) position during the performance of the:
a) Preflight procedure;
b) Before Start checklist; and
c) After Takeoff checklist.
...

Contributing Factors to the Accident
1. Omission of returning the pressurization mode selector to AUTO after unscheduled maintenance on the aircraft.
...

/Quote
At the time the accident happened the AMM did not demand to put the pressure mode selector to the position AUTO after the the leakage test.

capt solopsist correct
 

The captain S is correct. Very simple reason.
The fail safe for bleed air is open hence the solenoid powers the engine bleed off.
Same for many hyd systems.Fail safe to hyd on, powered to off position.

Having the bleed valve open removes some compressed air from the 5th and 9th stage compressor, before that air enters the combustion chamber. This enrichens the mixture, acts as a surge control and helps the start.

Do the engine bleed air switch positions control the bleed valves associated with surge control, starting, low/high thrust conditions? Or, are they purely connected to the pneumatic system in terms of supply on/off?

Maybe no aural warning like configuration or other bells and whistles. But there are two qualified Being 737 pilots on the flight deck. Plus two gauges that show cabin rate of climb and cabin differential and cabin altitude. Plus a checklist read as challenge and response. Plus airmanship. How many other warnings do you want if the problem cannot be seen by a competent crew. The operative word being "Competent".

Ifixplanes - We had this policy even before Helios. There are of course many other points to notice and rectify the problem, but I think there is no harm in having good processes in place to ensure things like this don't get left in those positions to start with i.e. the best method of detecting the problem is to remove it in the first place! Obviously other things have changed since thing including the takeoff config/cabin alt warning lights as well as a mentioning it in the brief on first flight of the day. Its amazing that the AMM didn't require the return to the auto position - didn't know that until now (not that any of it excuses the flight crew from checking it)

There is no surge control valve in the CFM56. I remember one fitted to the TFE731 (found on many business jets), I think. The bleed air valve on the CFM simply provides air to the packs for air conditioning, and for cross-bleed starting.

Now the AMM stated at the end of the check:

"Move the pressurization mode selector on the forward overhead panel to AUTO."

Lesson learned ...

checkboard
 

What you say is probabaly right from an operational point of view (not a driver here) but the bleed s/w's are on 99% of the time so they dont overheat/ wear or burn out while being powered off all the time.
When you climb into the front seat almost without fail you'd find the bleeds on whether your procedure requires them on or off for starting.

An engine start sense relay located in the M324 air conditioning accessory unit for the left
engine and in the M1455 air conditioning accessory unit for the right engine is energized when
the respective engine start switch is positioned to GRD. This relay sends a closing signal to the
latching solenoid on the bleed air regulator. This eliminates the control pressure to the PRSOV
pneumatic actuator and ensures the valve remains closed during starting.

Quote: From IFix Planes
Originally Posted by ballyboley
... Its amazing that the AMM didn't require the return to the auto position - didn't know that until now (not that any of it excuses the flight crew ...

Now the AMM stated at the end of the check:

"Move the pressurization mode selector on the forward overhead panel to AUTO."

Lesson learned ...

The AMM wasn't SPECIFIC. It did however have the statement "return the airplane to the usual condition" or words to that effect.

No, the wording was “Put the airplane back to its initial condition” and this was only the headline.
Then the AMM list 3 steps which have to be done. The selector was not mentioned.

Regarding Original Question
 

The start relay energizes when the engine start valve opens.
The energized relay contacts send a signal to close the bleed
air regulator solenoid valve.

i.e. Bleed is closed automatically during start, restores to selected position after start.

The open and close coils on the bleed air regulator solenoid
valve are constant duty coils. The solenoid valve also has a
mechanical latch. When there is a loss of electric power, the
solenoid valve will stay in the last electrically commanded
position.

The Engine Hyd Pumps are enegized OFF. The bleed system is not the same it's a twin coil latched type.

That last post has thrown me. To clarify,
When the bleed switch is in the OFF position, is it be energised to OFF or is it being held by a mechanical latch?

When the switch is selected OFF, the close coil of the solenoid is continuously powered. If power is lost the mechanical latch will keep it closed. The same applies when the switch is selected ON. As Terraplaneblues said, its a twin coil latch solenoid.