PMC OFF B737
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PMC OFF B737
Thinking of a PMC OFF takeoff in a B737 classic, a few doubts came to my mind, and I hope you'll be able to clarify everything.
The PMC keeps engine RPM and thrust constant during takeoff, right?
With the PMC inop, the N1 value will increase during the takeoff roll by as much as 7%.
The initial setting is also higher without PMC.
In fact, the Vmcg also increases by 4-6 knots.
So why on earth there is a weight penalty with PMC OFF
since the engines are pushing harder??
The PMC keeps engine RPM and thrust constant during takeoff, right?
With the PMC inop, the N1 value will increase during the takeoff roll by as much as 7%.
The initial setting is also higher without PMC.
In fact, the Vmcg also increases by 4-6 knots.
So why on earth there is a weight penalty with PMC OFF
since the engines are pushing harder??
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PMC Off Performance
Hi LEM
thats a nice concern u got but if u look harder into yur QRH the answer is right there. i dont have -400 handy but i can give u an example of -800W. on the NGs there is some thing called as EEC (Electronic Eng Control) works similar to the PMC with more added features.
now as u rightly said that there MAYBE an increase in N1 as much as 7%...the question is 'do u really want it ???' when u know there could be an overboost !!! so if u do a full thrust TO u could end up overboosting your engine. maybe causing exceedances.
what the QRH does is that it gives u a reduced N1 in the first place, taking in consideratoin there could be an overboost. but since we are never sure that u would necessarily get a extra bit of thrust, u have to take the WT for that perticular reduced N1
Example -800W (26k)
EEC OFF @ 30deg Sea Lvl is 98.1
where as EEC ON @ 30deg Sea Lvl is 100.3
Hope this will ans yur Question
GLA
thats a nice concern u got but if u look harder into yur QRH the answer is right there. i dont have -400 handy but i can give u an example of -800W. on the NGs there is some thing called as EEC (Electronic Eng Control) works similar to the PMC with more added features.
now as u rightly said that there MAYBE an increase in N1 as much as 7%...the question is 'do u really want it ???' when u know there could be an overboost !!! so if u do a full thrust TO u could end up overboosting your engine. maybe causing exceedances.
what the QRH does is that it gives u a reduced N1 in the first place, taking in consideratoin there could be an overboost. but since we are never sure that u would necessarily get a extra bit of thrust, u have to take the WT for that perticular reduced N1
Example -800W (26k)
EEC OFF @ 30deg Sea Lvl is 98.1
where as EEC ON @ 30deg Sea Lvl is 100.3
Hope this will ans yur Question
GLA
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Hi Gold Leader.
Unfortunately, when it comes to the 737 classic, with PMC, your reply is not correct.
As I wrote in the first post,
I didn't specify how much higher, so here you go:
737 400 22k, takeoff N1 30 degrees Celsius sea level, is 96,1.
PMC off, same conditions: 96,6.
Thanks anyway for your reply.
LEM
Unfortunately, when it comes to the 737 classic, with PMC, your reply is not correct.
As I wrote in the first post,
The initial setting is also higher without PMC.
In fact, the Vmcg also increases by 4-6 knots.
In fact, the Vmcg also increases by 4-6 knots.
737 400 22k, takeoff N1 30 degrees Celsius sea level, is 96,1.
PMC off, same conditions: 96,6.
Thanks anyway for your reply.
LEM
Hi LEM,
With the PMC OFF, there is more variation of thrust with airspeed and altitude changes. In most cases, the thrust decreases to below what it would be with PMC ON (assuming you set the same takeoff %N1). So, for dispatching PMC OFF, two adjustments are made. First, a new (higher) Takeoff %N1 Target is set. This partially offsets the lower thrust as the airplane accelerates and climbs. Secondly, since there is still a thrust “shortfall”, decrements have to be made to field length, climb & obstacle limit weights. These adjustments vary with OAT and the pressure altitude of the departure airport.
With the PMC OFF, there is more variation of thrust with airspeed and altitude changes. In most cases, the thrust decreases to below what it would be with PMC ON (assuming you set the same takeoff %N1). So, for dispatching PMC OFF, two adjustments are made. First, a new (higher) Takeoff %N1 Target is set. This partially offsets the lower thrust as the airplane accelerates and climbs. Secondly, since there is still a thrust “shortfall”, decrements have to be made to field length, climb & obstacle limit weights. These adjustments vary with OAT and the pressure altitude of the departure airport.
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Hi Chris!
You say "This partially offsets the lower thrust as the airplane accelerates and climbs. "
Isn't the N1 increasing as the airplane accelerates?
You are right about the climb phase, thing is I didn't expect the N1 to decrease noticeably in the few hundreds of feet after liftoff.
Ps: Lulu has shipped your book, I'm looking forward to enjoy it!
LEM
You say "This partially offsets the lower thrust as the airplane accelerates and climbs. "
Isn't the N1 increasing as the airplane accelerates?
You are right about the climb phase, thing is I didn't expect the N1 to decrease noticeably in the few hundreds of feet after liftoff.
Ps: Lulu has shipped your book, I'm looking forward to enjoy it!
LEM
LEM,
There are several effects going on here. Without the PMC, the N1 will increase with speed but will also decrease with altitude. Just because N1 has increased with speed (ie windmilled) does not mean that it is producing more thrust. (Perhaps this is where EPR was a better measure?) I lay myself open to correction but surely you can’t get extra thrust from windmilling the engine faster by flying faster?
I know my limits and respectfully defer to the more knowledgeable contributors on such performance based subjects.
S&L
Hope the book doesn’t disappoint.
There are several effects going on here. Without the PMC, the N1 will increase with speed but will also decrease with altitude. Just because N1 has increased with speed (ie windmilled) does not mean that it is producing more thrust. (Perhaps this is where EPR was a better measure?) I lay myself open to correction but surely you can’t get extra thrust from windmilling the engine faster by flying faster?
I know my limits and respectfully defer to the more knowledgeable contributors on such performance based subjects.
S&L
Hope the book doesn’t disappoint.
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Yep, still puzzling me a little bit...
With PMC operating, N1 remains constant througout the acceleration, so thrust decreases with speed.
Then, during climb, thrust remains constant.
With PMC off, N1 increases during acceleration, so let's say thrust remains constant? (is that why Vmcg is 4-6 kts higher without PMC?)
Then thrust decreases with altitude.
Instinctively, I would have assumed thrust deterioration during the first few hundreds of feet of climb was less than the thrust gained (or kept constant) by the increasing N1 during acceleration.
But, if Boeing put a weight penalty with PMC off, actually this is not the case.
Now I'm seeing it more clearly, thanks for your help.
LEM
With PMC operating, N1 remains constant througout the acceleration, so thrust decreases with speed.
Then, during climb, thrust remains constant.
With PMC off, N1 increases during acceleration, so let's say thrust remains constant? (is that why Vmcg is 4-6 kts higher without PMC?)
Then thrust decreases with altitude.
Instinctively, I would have assumed thrust deterioration during the first few hundreds of feet of climb was less than the thrust gained (or kept constant) by the increasing N1 during acceleration.
But, if Boeing put a weight penalty with PMC off, actually this is not the case.
Now I'm seeing it more clearly, thanks for your help.
LEM
