B737 NG / CFM56-7B Overspeed Protection
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B737 NG / CFM56-7B Overspeed Protection
I was recently asked a question regarding protection for the CFM56-7B if an incorrect OAT is entered in the FMC. Specifically, if an OAT is entered incorrectly as warmer than actual. I 100% understand the interaction of inserting an incorrect OAT colder than actual.
This would apply to an aircraft that doesn't have U12.0 and the required CDS Block Point software to support the OAT disagree functionality.
Here is the scenario:
Sea level airport
Actual OAT -30 C
FMC entered OAT +30 C in error
Assuming a 26K engine, the N1 setting in the FMC would likely be a little over 100% N1. With an actual OAT of -30, and this value correctly applied, the target N1 would be significantly lower say 90% N1.
My question is would the EEC limit the N1 to a lower value than 100% to avoid the FMC OAT error? And if so, how would this occur? Would the EEC somehow use a correct sensed temperature (not the FMC value) to determine a correct N1 setting or would it use other engine parameters to avoid an overpressure/overspeed situation.
I am told that the EEC will protect the engine, but really trying to find out what it does or uses to do this. I would appreciate any feedback.
This would apply to an aircraft that doesn't have U12.0 and the required CDS Block Point software to support the OAT disagree functionality.
Here is the scenario:
Sea level airport
Actual OAT -30 C
FMC entered OAT +30 C in error
Assuming a 26K engine, the N1 setting in the FMC would likely be a little over 100% N1. With an actual OAT of -30, and this value correctly applied, the target N1 would be significantly lower say 90% N1.
My question is would the EEC limit the N1 to a lower value than 100% to avoid the FMC OAT error? And if so, how would this occur? Would the EEC somehow use a correct sensed temperature (not the FMC value) to determine a correct N1 setting or would it use other engine parameters to avoid an overpressure/overspeed situation.
I am told that the EEC will protect the engine, but really trying to find out what it does or uses to do this. I would appreciate any feedback.
Yes, the EEC will protect the engine and limit N1 to the max rating for the temperature. It's very simple - the EEC has it's own inlet temperature probe (dual element - one for each channel). If the aircraft Air Data TAT agrees with EEC probe (within 2.5 degrees C) the EEC will use the Air Data temp to align things across the wing - but if the ADC differs by more than 5.0 deg C the EEC will use it's dedicated inlet temp probe (for a difference between 2.5 and 5 degrees in interpolates).
And the EEC will NEVER use the FMC input temperature to calculate ratings - only the ADC and inlet probe temps.
And the EEC will NEVER use the FMC input temperature to calculate ratings - only the ADC and inlet probe temps.
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Thank you very much for the reply tdracer.
That makes sense actually.
In the scenario I described, it would appear that the autothrottle system would still attempt to apply the incorrect thrust, but the EEC's would limit the N1 to maintain engine limits. At least the crew will still have adequate thrust which may not be the case if the inputted FMC temperature is lower than actual.
I talked to one of our engine guys and he is fairly certain that the EEC uses the T12 temperature on takeoff and then transitions to the ADC airborne (provided the comparison is good as you describe).
Cheers
That makes sense actually.
In the scenario I described, it would appear that the autothrottle system would still attempt to apply the incorrect thrust, but the EEC's would limit the N1 to maintain engine limits. At least the crew will still have adequate thrust which may not be the case if the inputted FMC temperature is lower than actual.
I talked to one of our engine guys and he is fairly certain that the EEC uses the T12 temperature on takeoff and then transitions to the ADC airborne (provided the comparison is good as you describe).
Cheers
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Yes, the EEC will protect the engine and limit N1 to the max rating for the temperature. It's very simple - the EEC has it's own inlet temperature probe (dual element - one for each channel). If the aircraft Air Data TAT agrees with EEC probe (within 2.5 degrees C) the EEC will use the Air Data temp to align things across the wing - but if the ADC differs by more than 5.0 deg C the EEC will use it's dedicated inlet temp probe (for a difference between 2.5 and 5 degrees in interpolates).
And the EEC will NEVER use the FMC input temperature to calculate ratings - only the ADC and inlet probe temps.
And the EEC will NEVER use the FMC input temperature to calculate ratings - only the ADC and inlet probe temps.
Would appreciate if you could direct me to any document that shows that.we are preparing a small performance class. Would like to corroborate that with some black and white.
