Why Switch bleeds off on shutdown.
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Why Switch bleeds off on shutdown.
There is a is alot on bleeds but not to my specific question I was asked yesterday. I fly a lear 40. An nexpeperienced chap on type not experienced in general asked me why turn bleeds off on shutdown. I am a simple kinda chap myself and and said well if it aint being used turn it off and viser verser. Nope he was not satisfied with that answer. So he is trying to work out the mecanhics behind it. I have to say I have always done this and all the people I have flown wth do this. So why now re invent the wheel I thought....So could anyone tell me what else I could have said?
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would guess it maybe something to do with preventing combustion gases getting into the air con system?
As the engine winds down, tailwind may blow gasses forward and enter the manifold making a not very nice smell in cabin?
A lot of these valves are pneumatically operated; not knowing anything about the Lear, if you shut down the engine, you may possibly run the risk of not being able to shut them until you next get bleed air from a running engine.
Both of the above (as you can probably tell) are guesses.
As the engine winds down, tailwind may blow gasses forward and enter the manifold making a not very nice smell in cabin?
A lot of these valves are pneumatically operated; not knowing anything about the Lear, if you shut down the engine, you may possibly run the risk of not being able to shut them until you next get bleed air from a running engine.
Both of the above (as you can probably tell) are guesses.
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In most jet aircraft the bleed valves are designed to go to the open position with the loss of electrical power. So the primary reason to close the bleeds prior to shut down is basically to use make sure that they are exercised and do not become stuck in the open position.
Having a non-controllable bleed valve is a no-go item.
Having a non-controllable bleed valve is a no-go item.
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That Lear (Garrett) is electric start, so no reason there.
I like the idea of the gas backflow thru the cooling turbine, but suspect gases after shutdown, even without cooling flow shouldn't be a problem... but then air into cabin never mixes with bleed air anyway....
The Old/Bold rule and reason to do it like that is cuz it's in the checklist that way. I'm rated in the Lears, but do not remember the philosophy, it being 20 years since flying one.
Call FSI and ask. They will get you an answer no matter what.
I like the idea of the gas backflow thru the cooling turbine, but suspect gases after shutdown, even without cooling flow shouldn't be a problem... but then air into cabin never mixes with bleed air anyway....
The Old/Bold rule and reason to do it like that is cuz it's in the checklist that way. I'm rated in the Lears, but do not remember the philosophy, it being 20 years since flying one.
Call FSI and ask. They will get you an answer no matter what.
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In most jet aircraft the bleed valves are designed to go to the open position with the loss of electrical power.
Most are however spring-loaded closed and need pneumatic pressure to open.
So why some would close them before shutdown is a good question, as they would close anyway as the compressor pressure bleeds off.
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Some of the newer electronic bleed valves don't like repetitive cycles of low voltage (engine start up and shut down) and therefore you can get a much higher TBF by shutting them off during that phase.
The above is a pilot's interpretation of how the maintenance folks explained it. Besides, it's in the checklist.
The above is a pilot's interpretation of how the maintenance folks explained it. Besides, it's in the checklist.
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The primary reason is to exercise the valve, and verify it works. However, it also ensures more cooling air through the engine prior to shutdown and is good practice...and as others have stated, it's in the manufacturer's procedures.
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To clarify things a bit...
Are we discussing starting with the L/R BLEED AIR switches ON as the OP asked, or the PACK/CABIN AIR switch ON?
I have no Lear -40/-45 experience, but in the -30 series the L/R BLEED AIR switches are not normally ever closed by the crew, including during start or shutdown, but the CABIN AIR is turned on prior to takeoff and closed after landing. In the -30 series aircraft, the BLEED AIR SHUTOFF VALVES (SOV) are spring-loaded open and require DC power to close them.
A quick glance at the LR-40 checklist seems to confirm that the L/R BLEED AIR switches, which control the ECS SOVs (opened and closed by DC power) remain on/open during start, but the PACK switch is closed.
So, after all that, I'd say BLEED AIR remains ON, PACK is turned off. Beyond that, I'd guess the reason is to allow the engine to experience cooler ITTs prior to shutdown. Contradictorily, the Challenger 604 directs that the APU or Engines be shutdown with PACKS ON. I believe this is to allow for a more linear temperature decrease for the engine/APU.
Are we discussing starting with the L/R BLEED AIR switches ON as the OP asked, or the PACK/CABIN AIR switch ON?
I have no Lear -40/-45 experience, but in the -30 series the L/R BLEED AIR switches are not normally ever closed by the crew, including during start or shutdown, but the CABIN AIR is turned on prior to takeoff and closed after landing. In the -30 series aircraft, the BLEED AIR SHUTOFF VALVES (SOV) are spring-loaded open and require DC power to close them.
A quick glance at the LR-40 checklist seems to confirm that the L/R BLEED AIR switches, which control the ECS SOVs (opened and closed by DC power) remain on/open during start, but the PACK switch is closed.
So, after all that, I'd say BLEED AIR remains ON, PACK is turned off. Beyond that, I'd guess the reason is to allow the engine to experience cooler ITTs prior to shutdown. Contradictorily, the Challenger 604 directs that the APU or Engines be shutdown with PACKS ON. I believe this is to allow for a more linear temperature decrease for the engine/APU.
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Hello snorkboggler,
I am surprised that your checklist calls for the Left and Right Bleed switches to be turned off during the shutdown check.
I fly the Lear 45XR and to the best of my knowlege the systems are the same as the Lear 40. The FAA Approved Flight Manual (AFM) does not call for the L/R Bleed Switches to be turned off in the Shutdown Check (see Shutdown section on page 2-23 of the AFM). Nor does it call for them to be turned off prior to start.
Checking of the L/R Bleed switches is done in item 16 of the Starting Engines Check on page 2-15 of the AFM, and only on the first flight of the day.
What does your AFM say regarding the L/R Bleed Switches on the Lear 40?
best regards,
Bruce Waddington
I am surprised that your checklist calls for the Left and Right Bleed switches to be turned off during the shutdown check.
I fly the Lear 45XR and to the best of my knowlege the systems are the same as the Lear 40. The FAA Approved Flight Manual (AFM) does not call for the L/R Bleed Switches to be turned off in the Shutdown Check (see Shutdown section on page 2-23 of the AFM). Nor does it call for them to be turned off prior to start.
Checking of the L/R Bleed switches is done in item 16 of the Starting Engines Check on page 2-15 of the AFM, and only on the first flight of the day.
What does your AFM say regarding the L/R Bleed Switches on the Lear 40?
best regards,
Bruce Waddington
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I have no Lear -40/-45 experience, but in the -30 series the L/R BLEED AIR switches are not normally ever closed by the crew, including during start or shutdown, but the CABIN AIR is turned on prior to takeoff and closed after landing. In the -30 series aircraft, the BLEED AIR SHUTOFF VALVES (SOV) are spring-loaded open and require DC power to close them.
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Not knowing the lear 45 as well, it could be deenergization of the solonoids that port muscle pressure to open the valves. I disagree with the failsafe open, especially H/S bleed if stuck open would deliver damaging pressure to the bleed system at high engine settings. Bleed off in my oppinion just saves the lifespan of the component likely.
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http://www.rcoco.com/sdoc/data/pdfs/...ironmental.pdf
In the Lear 40, like the LR35, the bleed air switches control both the ability to shutoff the bleed from an individual engine, as well as emergency bleed from that engine. They bleed valves fail to the open position with loss of DC power.
Bleed from the powerplant is routed through the shutoff and regulator (mod) valves. Reverse flow is prevented by check valves. Shutting off a bleed isn't done to prevent pressure loss at altitude, though shutting off the bleed is a precaution that backs up the check valve. In normal operation, the bleed air check valves should prevent reverse flow and subsequent pressure and bleed loss into the dead engine.
In flight, one of the chief functions of a bleed valve is isolating an engine which may be contaminating the air supply, or which may be outputting too much pressure or too much heat due to a mod valve failure or other problem. Cycling these valves prior to shutdown does two things: it helps ensure their continued functioning by exercising them (they buildup debris like any other valve, and should be regularly cycled), and it ensures that excess bleed is shut off prior to engine shutdown (which aids in engine cooling).
In the Lear 40, shutting off the bleeds also shuts off the ECS control valves, thus exercising them. These valves also fail open, and require DC power to shut them off; intentionally exercising the valves helps keep them functional.
There are occasions when one may need to shut off the bleeds and shut off emergency pressurization. Regularly manually shutting off the bleed ensures the valves stand a better chance of functioning properly when needed, as well as providing a better engine condition prior to shutdown. After shutdown and DC power is off, the valves will open on their own, as a fail-safe function.
In the Lear 40, like the LR35, the bleed air switches control both the ability to shutoff the bleed from an individual engine, as well as emergency bleed from that engine. They bleed valves fail to the open position with loss of DC power.
Bleed from the powerplant is routed through the shutoff and regulator (mod) valves. Reverse flow is prevented by check valves. Shutting off a bleed isn't done to prevent pressure loss at altitude, though shutting off the bleed is a precaution that backs up the check valve. In normal operation, the bleed air check valves should prevent reverse flow and subsequent pressure and bleed loss into the dead engine.
In flight, one of the chief functions of a bleed valve is isolating an engine which may be contaminating the air supply, or which may be outputting too much pressure or too much heat due to a mod valve failure or other problem. Cycling these valves prior to shutdown does two things: it helps ensure their continued functioning by exercising them (they buildup debris like any other valve, and should be regularly cycled), and it ensures that excess bleed is shut off prior to engine shutdown (which aids in engine cooling).
In the Lear 40, shutting off the bleeds also shuts off the ECS control valves, thus exercising them. These valves also fail open, and require DC power to shut them off; intentionally exercising the valves helps keep them functional.
There are occasions when one may need to shut off the bleeds and shut off emergency pressurization. Regularly manually shutting off the bleed ensures the valves stand a better chance of functioning properly when needed, as well as providing a better engine condition prior to shutdown. After shutdown and DC power is off, the valves will open on their own, as a fail-safe function.
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I cannot stress enough the importance of having control of the bleed valves prior to flight. If you cannot control the bleed valves in flight you can end up causing a lot of damage to the aircraft.
Case in point. One winter afternoon we departed Aspen Co. (KASE) in a Sabre 65. As it was a cold day we did a bleeds on takeoff, the normal configuration for takeoff. In Aspen when the ambient temperature was above 10c we would do a bleeds off takeoff for performance, however that day it was -05c so we had more than adequate performance with the bleeds on.
Going through FL180 the automatic cabin temperature controller failed and called for full hot air. The first thing I noticed, before we realized that the controller failed, was a sudden increase in the noise of the air coming out of the ducts, then a rapid increase of the air temperature coming out of the ducts and then the smell of something getting too hot. I looked at the valve position indicator and it was nearly at the full open position. Then we got a "HOT CABIN AIR" warning on the annunciator panel. I turned the cabin thermostat down to full cold, no change. Then I switched the cabin temperature controller to manual and toggled the switch to full cold, no change.
The air in the cabin and now the cockpit was becoming extremely hot. By throttle movement we determined that it was the right engine bleed valve that had failed. At that point we decided that we were going to have to shut down the right engine or risk serious consequences. Before we shut the engine down I decided to try one more thing. According to valve position indicator the valve was not full open, just about 90% open. So I toggled the valve to hot, it moved and I immediately switch the direction of the valve to cold and it moved to the full cold position. The noise stopped, the cabin and cockpit air cooled down and the "HOT CABIN AIR" light went out. We then continued on the flight back to our home base.
Writing this story took longer than the actual incident. We were lucky as I was able to regain control of the bleed valve before any damage was done to the duct works. On tear-down of the valves we found all kinds of debris inside the valves that had caused the right valve to jam. I had the old valves replaced and told the director of maintenance to modify our inspections to remove and clean the valves more frequently. We notified Sabreliner of the incident and the change in our maintenance procedures. Shortly after this Sabreliner sent out a recommendation to inspect the valves more frequently.
We had most likely brought this problem on ourselves, as we only checked the valves once a day prior to the first flight and, this is my fault as I was the Chief Pilot, we did not close the valves on every shutdown as called for. Now on the Falcon 50 and 900 aircraft all takeoffs are bleeds off, but that is a automatic function, as N1 passes around 60% the bleeds close and open shorty after you are airborne.
Case in point. One winter afternoon we departed Aspen Co. (KASE) in a Sabre 65. As it was a cold day we did a bleeds on takeoff, the normal configuration for takeoff. In Aspen when the ambient temperature was above 10c we would do a bleeds off takeoff for performance, however that day it was -05c so we had more than adequate performance with the bleeds on.
Going through FL180 the automatic cabin temperature controller failed and called for full hot air. The first thing I noticed, before we realized that the controller failed, was a sudden increase in the noise of the air coming out of the ducts, then a rapid increase of the air temperature coming out of the ducts and then the smell of something getting too hot. I looked at the valve position indicator and it was nearly at the full open position. Then we got a "HOT CABIN AIR" warning on the annunciator panel. I turned the cabin thermostat down to full cold, no change. Then I switched the cabin temperature controller to manual and toggled the switch to full cold, no change.
The air in the cabin and now the cockpit was becoming extremely hot. By throttle movement we determined that it was the right engine bleed valve that had failed. At that point we decided that we were going to have to shut down the right engine or risk serious consequences. Before we shut the engine down I decided to try one more thing. According to valve position indicator the valve was not full open, just about 90% open. So I toggled the valve to hot, it moved and I immediately switch the direction of the valve to cold and it moved to the full cold position. The noise stopped, the cabin and cockpit air cooled down and the "HOT CABIN AIR" light went out. We then continued on the flight back to our home base.
Writing this story took longer than the actual incident. We were lucky as I was able to regain control of the bleed valve before any damage was done to the duct works. On tear-down of the valves we found all kinds of debris inside the valves that had caused the right valve to jam. I had the old valves replaced and told the director of maintenance to modify our inspections to remove and clean the valves more frequently. We notified Sabreliner of the incident and the change in our maintenance procedures. Shortly after this Sabreliner sent out a recommendation to inspect the valves more frequently.
We had most likely brought this problem on ourselves, as we only checked the valves once a day prior to the first flight and, this is my fault as I was the Chief Pilot, we did not close the valves on every shutdown as called for. Now on the Falcon 50 and 900 aircraft all takeoffs are bleeds off, but that is a automatic function, as N1 passes around 60% the bleeds close and open shorty after you are airborne.
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767 tech manager reply
I asked one of our tech Managers for our 767 fleet. He indicated that it’s very important to close the engine bleed switches before engine shut down and to close the APU shutoff valve before APU shutdown.
He said the engine bleed switches control the Pressure Regulating Shutoff Valve (PRSOV) which is regulated by balancing a differential between upstream and downstream air pressure. When the bleeds are shut off before the engines are shut-down there is enough duct pressure to ensure sufficient muscle pressure in the regulating lines to close the valve completely. When this is not done there is a chance, through normal wear of the piston spring and ring wear, that the valve may not fully close, resulting in a failure to illuminate the ENG BLEED OFF LIGHT. This results in log write ups for BLEED WILL NOT CLOSE WHEN SELECTED OFF on ENGINE SHUTDOWN. Many of these write ups result in No Trouble Found, with Ops Checked good Maintenance write-ups.
He said the engine bleed switches control the Pressure Regulating Shutoff Valve (PRSOV) which is regulated by balancing a differential between upstream and downstream air pressure. When the bleeds are shut off before the engines are shut-down there is enough duct pressure to ensure sufficient muscle pressure in the regulating lines to close the valve completely. When this is not done there is a chance, through normal wear of the piston spring and ring wear, that the valve may not fully close, resulting in a failure to illuminate the ENG BLEED OFF LIGHT. This results in log write ups for BLEED WILL NOT CLOSE WHEN SELECTED OFF on ENGINE SHUTDOWN. Many of these write ups result in No Trouble Found, with Ops Checked good Maintenance write-ups.
