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B737 nb electronic engine computer

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B737 nb electronic engine computer

Old 5th Jan 2018, 18:13
  #1 (permalink)  
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Join Date: Sep 2005
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B737 nb electronic engine computer

Hi folks
I do have a question about the functioning of the EEC on the 737-800
As far as i know the EEC when operated in the normal mode compares actual thrust to commanded thrust - either from the fmc or manually selected - and adjusts the fuel flow to achieve the commanded thrust based on ambient conditions and bleed air demand. Those ambient conditions include but are not limited to ambient pressure supplied by the T sensor while on the ground and by the ADIRU once airborne, and mach number derived by integrating static ambient pressure with total air pressure and TAT from the ADIRU.
Now my questions are: why does it need to know the Mach number and how can it derive the Mach number from the air pressure considering that the mach number is a function of the temperature?

Many thanks
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Old 6th Jan 2018, 00:37
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just google pitot-tube and read up on what it does.

afaik modern engines adjust fuel flow to match a certain EPR (Engine Pressure Ratio - measured as a difference of air flowing in and out of the engine). of course they also have limits regarding temperatures and rotational speeds.

and why would you integrate pressures? you mean subtract perhaps.
so you measure temperature and some form of airspeed and you get a mach-number? i'm quite sure some good reading material is out there including here on PPRuNe if you just use google with the keywords you used in your question.

anyhow why would it need mach-number, i don't know that it does, how did you get to that conclusion?
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Old 6th Jan 2018, 04:26
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B737ng eec

Hi
Well i am aware of the fact that the mach number is the ratio of tas to local speed of sound which in turn is a function of the ambient temperature.

As far as the ecc using the mach number to compute the required thrust it is mentioned in the maintenance manual for the b737ng apart from other publications in my possession.

From maintenance manual airframe page 189 of 418

Normal mode
In normal mode the EEC claculates the mach number with: two Pt values from the ADIRUís, P0 from the ADIRU or from the P0 ports on the EEC and the difference between the standard day temperature and the current ambient temperature. If Pt pressure is not valid or if the EEC switch is de-selected the EEC goes to one of the alternate modes.
T12 supplies current ambient temperature when the aircraft is on the ground and for the first 5 minutes after takeoff and then it uses T12 and TAT inputs from the ADIRU to compute current ambient temlerature - i stand corrected with regard to my previous T probe statement.

I do understand the implications that ambient pressure has in the functioning of the engine with regard to thrust limit and proper stoichiometric ratios and airflow patterns inside the engine, but the usage of the mach number escapes me, still.

Many thanks
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Old 6th Jan 2018, 04:49
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Join Date: Sep 2005
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B737 ng eec

Hi
I think i have figured out why the EEC needs static and ambient pressures to compute the mach number being the mach number TAS / LSS, TAS = EAS square root of the ratio of density at sea level in isa to ambient density and thus affected by temperature and pressure and LSS equal to 39 square root of temperature in degrees kelvin.

The question about why it needs the mach number still stands.

Many yhanks
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Old 6th Jan 2018, 09:17
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my guess is the equations to determine thrust are for some reason easier to formulate with the mach number instead of true airspeed for example.
with all the other data it doesn't really matter if the EEC gets the TAS or the mach number as the two can easily be calculated from the the other.

i found this 11.6 Performance of Jet Engines
and it seems the thermodynamic people like to calcuclate thrust based on mach number.

so as i said there are calculations to be done and which set of parameters is used to do these calculations does not matter as long as you have "enough" of the parameters as the others can be calculated from them. as the ADIRU calculates all parameters anyway i guess the EEC can choose which parameters to use.

what i could also imagine is that if the mach number gets too high in front of the engine there are aerodynamic problems so the engine needs to reduce power so the inlet stream remains laminar. but maybe someone who really understands these things will chime in.

may i recommend you the 120 page or so concorde thread there are quite some posts on engine operation there. there might me hints in there, but it's quite a long read
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Old 6th Jan 2018, 23:01
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Join Date: Jul 2013
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Mach number is not dependent on temperature - it's simply based on the ratio of static pressure to total (impact) pressure:


where:
qc is impact pressure (dynamic pressure) and
p is static pressure
is the ratio of specific heat of a gas at a constant pressure to heat at a constant volume (1.4 for air).

Temperature is needed for true airspeed, but not Mach. The Max ratings are a function of altitude and Mach number, (airspeed does get involved in some of the takeoff calculations, but is not used outside of the Takeoff envelope).
On the CFM, the EEC compares N1 Command to N1 Actual, and adjust fuel flow, etc. to match the actual N1 to the command. In Normal Mode, N1 command is based on thrust lever position (Throttle Resolver Angle or TRA), Altitude (pamb), Mach number, and Total air temp (TAT).
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