Hi,There is some technical issues regarding pneumatic/air conditioning systems I need to discuss with a 757/767 pilot. If there is anyone that would be interested please email me.Thank youTom Aguilo

Hi Tom,Why not ask the question right here so we all can learn ? Some knowledgable cats hanging out here and some of us have the real world manuals or at least the Thomas Gooch book.

Hi Jaaky- I was curious what the Thomas Gooch book is? Always looking for some more manuals to keep my 767 simming as real as possible. I have the Mike Ray book, and know about the Bulfer manual for the FMC. Anything else good out there??ThanksTravis

Ok guys here you have some tech issues regarding Penumatics and Bleed air logic functioning. I apologize for this being rather long and maybe boring to someone, but I would like to know how much accurate it is.These points refer to a 757-200 but may apply to a 767 as well. I would appreciate your opinions.1) ENGINE BLEED AIR VALVE Engine Bleed Air valve(s) open when Switch ON, respective engine is running and there is a pneumatic demand on duct(s) associated.2)BLEED AIR DEMAND Bleed air demand comes basically from these sources: -Left & Right Packs -Trim Air -Wing Anti-Ice -Engine Start -Hydaulic Reservoirs If all of these are "OFF" there is no pneumatic demand (Lack of Bleed Air demand, see below) 3) LACK OF BLEED AIR DEMANDEach Engine Bleed Air valve requieres air demand to operate, if there is a lack, valve closes and OFF light illuminates,even with engine ON and switch ON. Associate duct(s) remain(s) pressurized which is reflected in gauge's psi indication.4) ENG BLEED VALVE FROM OFF TO ON WITHOUT DUCT DEMAND (PACKS OFF,TRIM OFF, everything OFF)This happens when engine running with switch OFF, then switch turns to ON, or engine just started and switch ON. As associated duct(s) pressure is 0 psi or near, valve opens due to pressure differential. As soon as duct(s) fill(s) with air, pressure raises (50-55 psi) and valve closes again. Duct remain(s) pressurized. 5) ENG BLEED VALVE FROM OFF TO ON WITH DUCT DEMAND (PACKS ON case, take it as if it is the only air demand source)This happens a)when engine running with switch OFF, then switch turns to ON; or engine just started and switch ON AND Packs (ie) signaled AUTO/STDBY but PACKS OFF light illuminated due to lack of duct pressure. -Bleed valve opens, pressure raises form 0 to +- 50-55 psi then drops to +- 40 psi due to pack(s) demand and stabilizes. Bleed valve remains open. Packs valves open, PACKS OFF light blanks. b)engine running and Bleed switch ON AND Pack(s) switch OFF AND duct already pressurized -Bleed valve closed. Pack(s) switch to ON then: -Bleed valve opens, pressure drops to +- 40 psi due to pack(s) demand and stabilizes. Bleed valve remains open. 6)APU BLEED AIR -If APU is OFF, Switch to ON/OFF doesn't operate the valve,therefore VALVE light doesn't come on. -If APU is ON and Swich is ON/OFF, valve open/closes UNLESS there is Engine Bleed pressure on duct(s) WHile valve is in transit, VALVE light illuminates. -If APU is ON and Switch is ON (VALVE light off) and engine starts, valve closes and VALVE light turns ON to show valve disagreement with commanded switch. Starts the 60 secs cooling period. Switch to OFF (blank) turns the VALVE light off. -If APU is shut and valve is open, valve closes. Starts the 60 secs cooling period. WHile the valve is in transit, the VALVE light illuminates. - Valve open/closure as response to duct demand logic similar to Eng Bleed valve. However, VALVE light doesn`t illuminate while in transit as "disagreement" disabled when APU is running and Bleed switch is ON. (I doubt on this last) This is all for now. There are more aspects refered to proper PSI indications, relative to differential demand, a/c altitude, etcthat I could post later if you are interested.RegardsTom

what exactly is the quiestion or questions?

HI Tom,just read through that quickly and you amy have missed out something in the "demand" section:the demand hyd pump for the C Hyd Sys is driven by air (ADP). Thus it will have a pneumatic demand whenever it oerates (during flap/slat/gear movement)"There are more aspects refered to proper PSI indications, relative to differential demand, a/c altitude, etcthat I could post later if you are interested."Sure, go ahead!Regards,Mark

Hi Travis,http://ourworld.compuserve.com/homepages/T...ch/default.htmlUnlike the Mike Ray book, that focuses more on procedures, the Gooch book explains the different systems of the aircraft.

The point is, this is the way I think the logic works, but of course I might be wrong. So I was asking for anyone who can check it and help me know what is true and what is not.So far, so simple:-) Regards,Tom

Hi Mark,"...the demand hyd pump for the C Hyd Sys is driven by air (ADP). Thus it will have a pneumatic demand whenever it oerates (during flap/slat/gear movement)..."Actually, in the 757 only the hydraulic reservoirs are pressurized by bleed air, just to maintain positive fluid flow. Hydraulic pressure to the system is given by their respective pumps. I assume that, as long as reservoirs get pressurized, it shouldn't be any change in demand, unless reservoir quantities decrease or by leaks in the system.Regards,Tom

Hi Tom,I meant the 767.

The difference is that on the 757 the electric pumps run full timebut on the 767 the demand pumps cranck up when the associated system pressure drops to demand level or from an electrical signal that anticipates a pressure drop.Below the SYS PRESS lights there is in the 757 a RSVR light indicating either low quantity or low pressure. In the 767 there is a QTY light instead signaling only low fluid quantity.

"3) LACK OF BLEED AIR DEMANDEach Engine Bleed Air valve requires air demand to operate, if there is a lack, valve closes and OFF light illuminates,even with engine ON and switch ON. Associate duct(s) remain(s) pressurized which is reflected in gauge's psi indication."I don't believe this is correct, Tom (at least for the 767). The engine bleed valves should stay open even if there is no demand. Some of the bleed valves will modulate, but it's unlikely that they would close completely unless commanded closed (or if the engine was switched off). On the 767, the HPSOV will close when the engine rpms are high (otherwise you would overpressurize the system). However, the IP valve, PRV and PRSOV should remain open. AFAIK, the indication in the cockpit for the engine bleed valve being closed is only triggered by the position of the PRSOV (I assume you have a schematic of the engine bleed valves?). "-If APU is ON and Switch is ON (VALVE light off) and engine starts, valve closes and VALVE light turns ON to show valve disagreement with commanded switch."Are you talking about the engine bleed valve closing or the APU bleed valve? On the 767, the valves which close depend on whether the aircraft is in the air or not (and other things). On the ground during engine start, both engine bleed valves should be commanded closed if the APU is supplying air. In the air, only the engine bleed valve on the engine being started should close.Unfortunately, I wouldn't be able to prove my case for non-closure with no demand with photographs. A no demand situation is just too hard to simulate. There is always air being used somewhere in normal ops.... even if it's only air leaking from the aspirated TAT Probe.Hope this helps.Cheers.Ian.

Hi Ian,"3) LACK OF BLEED AIR DEMANDEach Engine Bleed Air valve requires air demand to operate, if there is a lack, valve closes and OFF light illuminates,even with engine ON and switch ON. Associate duct(s) remain(s) pressurized which is reflected in gauge's psi indication."I don't believe it either. But the 757 AOM says (refering to Eng Bleed Air) "...An OFF light is illuminated any time the associated valve is closed and will extinguish when bleed air pressure opens the valve. A Bleed Air OFF light may illuminate during periods of low bleed air demand, for example, with the air conditioning packs OFF." I find this rather confusing (meaning the word "may"), and is one of the reasons for what I was seeking a pilot's help, somebody who had the chance to verify it in the cockpit."-If APU is ON and Switch is ON (VALVE light off) and engine starts, valve closes and VALVE light turns ON to show valve disagreement with commanded switch."I was talking of the APU bleed valve."..On the ground during engine start, both engine bleed valves should be commanded closed if the APU is supplying air... "I guess this should be the same for the 757. However I can't find any reference of BOTH eng valves closing on APU eng start in my copy of the AOM.Regards,Tom