Is there Time Limitation For A340 & B747 During Windmill???
Thread Starter
Joined: Jun 2005
Posts: 182
Likes: 2
From: USA
Is there Time Limitation For A340 & B747 During Windmill???
Hi,
I read that certain aircraft like A340 have limitation of duration (7 or 8 hours) for windmilling engine (after engine shutoff in flight). Since quadjet/trijet are allowed to continue the flight in case of engine shutdown, is there such limitation in other aircraft like B747....? or even for twin during 180 or 207 minutes of diversion.
-Is this limitation due of the viscosity of the oil after being cold soaked during hours of windmill, or due to the lack of oil pressure because the oil pump does not provide the adequate lubrication during windmill?
-If the cause is the lack of adequate oil pressure, can this cause affect especially the N2 shaft bearings (assuming N2 is higher than N1)? OR
N1 shaft bearings? in certain flight conditions during windmill N1 is similar to N1rotation during flight idle (according to certain website)
-The questions mentioned above lead to another question:
What are the engine parameters (N1, N2, oil temperature, and oil pressure) for a stabilized windmilling engine after driftdown? may be it seems difficult to remember windmilling engine parameters during such rare event, but certain pilots are lucky (according to this forum one pilot performed about 30 inflight engine restart). If you cannot remember these parameters, it will be VERY HELPFUL AND NICE if someone can get these parameters from the sim and post these information here (if possible).
Feedback VERY appreciated. Thank you.
Best regards.
I read that certain aircraft like A340 have limitation of duration (7 or 8 hours) for windmilling engine (after engine shutoff in flight). Since quadjet/trijet are allowed to continue the flight in case of engine shutdown, is there such limitation in other aircraft like B747....? or even for twin during 180 or 207 minutes of diversion.
-Is this limitation due of the viscosity of the oil after being cold soaked during hours of windmill, or due to the lack of oil pressure because the oil pump does not provide the adequate lubrication during windmill?
-If the cause is the lack of adequate oil pressure, can this cause affect especially the N2 shaft bearings (assuming N2 is higher than N1)? OR
N1 shaft bearings? in certain flight conditions during windmill N1 is similar to N1rotation during flight idle (according to certain website)
-The questions mentioned above lead to another question:
What are the engine parameters (N1, N2, oil temperature, and oil pressure) for a stabilized windmilling engine after driftdown? may be it seems difficult to remember windmilling engine parameters during such rare event, but certain pilots are lucky (according to this forum one pilot performed about 30 inflight engine restart). If you cannot remember these parameters, it will be VERY HELPFUL AND NICE if someone can get these parameters from the sim and post these information here (if possible).
Feedback VERY appreciated. Thank you.
Best regards.
Joined: Feb 2006
Posts: 6
Likes: 0
From: Holland
A windmilling engine still produces enough rpm to produce sufficient oil pressure to supply oil to the breaings, and herein lies the problem, ALL 747/340 engines (infact most jet engines) use Labryinth seals at the bearing compartments, these seals utilise Internal Engine Bleed to pressurise the labryinth seals, this helps seal these bearing compartments tightly, thereby decreasing oil consumption radically.
A Jet Engine at windmill speeds cannot produce bleed pressure, this leads to the Labryinth seals being unpressurised, the oil consumption more than doubles because of this, the oil gets dumped overboard via the respective bearing compartment vents, (check the training or ground maintenance manual for precise figures), hence leaving an engine at idle power is far more sensible (if possible), than doing a precautionary shutdown. Doing a precautionary shutdown and then restarting in a few hours time might present you with a mandatory shutdown due to low oil quantity and Low oil pressure light illuminating.
This perhaps explains why there is a windmilling limitation on the 340, small oil tanks/massive endurance, on the classics with either the Pratt & whitney or GE motors there is no windmilling limitation.
If you dont find any info in your ground maintenance manuals let me know and I'll try and paste the Boeing info from their training manual in this thread.
A Jet Engine at windmill speeds cannot produce bleed pressure, this leads to the Labryinth seals being unpressurised, the oil consumption more than doubles because of this, the oil gets dumped overboard via the respective bearing compartment vents, (check the training or ground maintenance manual for precise figures), hence leaving an engine at idle power is far more sensible (if possible), than doing a precautionary shutdown. Doing a precautionary shutdown and then restarting in a few hours time might present you with a mandatory shutdown due to low oil quantity and Low oil pressure light illuminating.
This perhaps explains why there is a windmilling limitation on the 340, small oil tanks/massive endurance, on the classics with either the Pratt & whitney or GE motors there is no windmilling limitation.
If you dont find any info in your ground maintenance manuals let me know and I'll try and paste the Boeing info from their training manual in this thread.
Last edited by leftright; 2nd March 2006 at 12:28.
Joined: Jan 2002
Posts: 960
Likes: 0
From: Brisbane, Australia
It's also difficult to tell if you actually have any oil remaining in the oil tank on some engines at windmill rpm's. The B737 CFM56-7 has a habit of not scavenging the oil from the engine back to the tank at those sorts of low rpm's, so the oil tank quantity can show all the way to zero, even if the correct oil quantity is available once the engine is lit off and the scavenge pump picks up its game.
Just a thought !
Cheers, FD
Just a thought !
Cheers, FD
Thread Starter
Joined: Jun 2005
Posts: 182
Likes: 2
From: USA
Hi,
Thank you for your posts.
Leftright, I sent you a private message. it will be nice if you can post info from the manuals.
Originally posted by leftright:
I am not postulating, just a guess: a windmilling engine will provide very low oil pressure. Oil leakage via bearings will be not important (through labyrinth or carbon seals, I guess carbon seals are also used) since the oil pressure is very low and the compressor (windmilling engine) is still providing air (even tough not important) that may buffer the bearing seals.
I get a PM that states (during windmill):
Oil pressure almost zero, depends upon N2 ;and the N2 also depends upon airspeed. It will decay to zero below 250kts.
After I received this PM, my opinion is: N2 is turning initially at very low speed and will be driving the gearbox with all its accessories (more load for the N2). The oil pump will provide very low pressure if it is not nil. IF N2 is high the oil pressure will increase but also the N2 or HP compressor will increase the air pressure that will buffer the bearing seals (there is kind of certain proportion and balance between oil pressure and air in the bearing compartments). The oil leakage through bearing seals is existant but not very important: it may depend on the size of the oil tank (as you mentioned), the number of bearings, and the site of the bearings (on N1 or N2). The oil pump will provide the same pressure for all bearings (N1 & N2) but the bearing seals are not buffered by the same air pressure because the air pressure is not the same along N1 or N2: N2 bearing seals may be tightly sealed and N1 bearing seals may leak little bit (assuming N2 is higher than N1) and vice versa if N1 is higher than N2.
This is just assumption, I hope to get answers to be corrected.
In another thread I get this statement:
"FL150, 150kts which gives about N1=10% and N2=0% and FL240, 260kts which gives N1=18% and N2=15%, although these would be much lower if allowed to decay down to the steady state".
My question is: IN GENERAL after drift-down (twin, tri, or quad after engine shutdown) and in steady state and in stabilized flight, is N1 always > N2 ???(let's say especially in medium or high bypass engines, or even low bypass
if it is possible).
Feedback very appreciated. Thank you.
Best regards.
Thank you for your posts.
Leftright, I sent you a private message. it will be nice if you can post info from the manuals.
Originally posted by leftright:
A windmilling engine still produces enough rpm to produce sufficient oil pressure to supply oil to the breaings, and herein lies the problem, ALL 747/340 engines (infact most jet engines) use Labryinth seals at the bearing compartments, these seals utilise Internal Engine Bleed to pressurise the labryinth seals, this helps seal these bearing compartments tightly, thereby decreasing oil consumption radically.
A Jet Engine at windmill speeds cannot produce bleed pressure, this leads to the Labryinth seals being unpressurised, the oil consumption more than doubles because of this, the oil gets dumped overboard via the respective bearing compartment vents, (check the training or ground maintenance manual for precise figures), hence leaving an engine at idle power is far more sensible (if possible), than doing a precautionary shutdown. Doing a precautionary shutdown and then restarting in a few hours time might present you with a mandatory shutdown due to low oil quantity and Low oil pressure light illuminating.
This perhaps explains why there is a windmilling limitation on the 340, small oil tanks/massive endurance, on the classics with either the Pratt & whitney or GE motors there is no windmilling limitation.
If you dont find any info in your ground maintenance manuals let me know and I'll try and paste the Boeing info from their training manual in this thread
A Jet Engine at windmill speeds cannot produce bleed pressure, this leads to the Labryinth seals being unpressurised, the oil consumption more than doubles because of this, the oil gets dumped overboard via the respective bearing compartment vents, (check the training or ground maintenance manual for precise figures), hence leaving an engine at idle power is far more sensible (if possible), than doing a precautionary shutdown. Doing a precautionary shutdown and then restarting in a few hours time might present you with a mandatory shutdown due to low oil quantity and Low oil pressure light illuminating.
This perhaps explains why there is a windmilling limitation on the 340, small oil tanks/massive endurance, on the classics with either the Pratt & whitney or GE motors there is no windmilling limitation.
If you dont find any info in your ground maintenance manuals let me know and I'll try and paste the Boeing info from their training manual in this thread
I get a PM that states (during windmill):
Oil pressure almost zero, depends upon N2 ;and the N2 also depends upon airspeed. It will decay to zero below 250kts.
After I received this PM, my opinion is: N2 is turning initially at very low speed and will be driving the gearbox with all its accessories (more load for the N2). The oil pump will provide very low pressure if it is not nil. IF N2 is high the oil pressure will increase but also the N2 or HP compressor will increase the air pressure that will buffer the bearing seals (there is kind of certain proportion and balance between oil pressure and air in the bearing compartments). The oil leakage through bearing seals is existant but not very important: it may depend on the size of the oil tank (as you mentioned), the number of bearings, and the site of the bearings (on N1 or N2). The oil pump will provide the same pressure for all bearings (N1 & N2) but the bearing seals are not buffered by the same air pressure because the air pressure is not the same along N1 or N2: N2 bearing seals may be tightly sealed and N1 bearing seals may leak little bit (assuming N2 is higher than N1) and vice versa if N1 is higher than N2.
This is just assumption, I hope to get answers to be corrected.
In another thread I get this statement:
"FL150, 150kts which gives about N1=10% and N2=0% and FL240, 260kts which gives N1=18% and N2=15%, although these would be much lower if allowed to decay down to the steady state".
My question is: IN GENERAL after drift-down (twin, tri, or quad after engine shutdown) and in steady state and in stabilized flight, is N1 always > N2 ???(let's say especially in medium or high bypass engines, or even low bypass
if it is possible).Feedback very appreciated. Thank you.
Best regards.
Thread Starter
Joined: Jun 2005
Posts: 182
Likes: 2
From: USA
Hi,
Posted originally by Aerotech
The question posted above leads to me to wonder and think about the windmill itself:
-How the N1 & N2 are driven during windmill?
-Is it because of the airflow that drives the fan and the compressor? (in this case the fan and compressor drive the turbine which is the opposite when engine is operating)
OR
-The airflow drives the turbine which turns the fan and the compressor even though the airflow goes through the fan and the compressor first? which means there is no inversion of the role between the turbines and compressors during windmill or engine operation).
-What are the position of the VSV's (variable stator vanes) during windmill? Can this affect the N1 and N2?
Finally I am still wondering about the oil pressure (the lubrication problem) for a windmilling engine after drift-down, in steady state, and in stabilized flight since the trijet and quadjet are allowed to continue the flight after one engine shutdown. For example A340 engine can windmill up to 7 or 8 hours), is this regardless of the oil pressure?
Any toughts, comments, or feedback are very appreciated. Thank you.
Best regards
Posted originally by Aerotech
IN GENERAL after drift-down (twin, tri, or quad after engine shutdown) and in steady state and in stabilized flight, is N1 always > N2 ???(let's say especially in medium or high bypass engines, or even low bypass if it is possible).
-How the N1 & N2 are driven during windmill?
-Is it because of the airflow that drives the fan and the compressor? (in this case the fan and compressor drive the turbine which is the opposite when engine is operating)
OR
-The airflow drives the turbine which turns the fan and the compressor even though the airflow goes through the fan and the compressor first? which means there is no inversion of the role between the turbines and compressors during windmill or engine operation).
-What are the position of the VSV's (variable stator vanes) during windmill? Can this affect the N1 and N2?
Finally I am still wondering about the oil pressure (the lubrication problem) for a windmilling engine after drift-down, in steady state, and in stabilized flight since the trijet and quadjet are allowed to continue the flight after one engine shutdown. For example A340 engine can windmill up to 7 or 8 hours), is this regardless of the oil pressure?
Any toughts, comments, or feedback are very appreciated. Thank you.
Best regards
Joined: Feb 2005
Posts: 4,581
Likes: 0
From: flyover country USA
The front end of the N1 & N2 rotors (ie the fan & HP compressor) are removing most of the energy from the airstream to drive the windmilling process. Thus there is very little pressure energy back in the turbines - they contribute almost nothing to windmilling.
During shutdown or windmill, VSV's should be fully closed - their normal condition to be ready for the next start.
Since windmill is such a low rpm, there is little heat rejection from the bearings, so the requirement for oil cooling is almost nil. Maybe someone else can quote a case of bearing damage after prolonged windmill, but I've not heard of any.
During shutdown or windmill, VSV's should be fully closed - their normal condition to be ready for the next start.
Since windmill is such a low rpm, there is little heat rejection from the bearings, so the requirement for oil cooling is almost nil. Maybe someone else can quote a case of bearing damage after prolonged windmill, but I've not heard of any.
