737m MCAS
 

This MCAS thingy is puzzling me.
Okay 737m has two AoA sensors. Both are used for onside EAS correction, right?
AoA data are also fed to the MCAS.

What about this alternate switching of AoA data and MCAS?
Has the 737m one stick shaker or two independent SS?
Is it that way one AoA drives the shaker, the other one MCAS? Hhmm.

Anyone in the know?
Thx

From the thread on Rumors - (so it must be true :8 )
Each AOA feeds the control position on its side of the aircraft the side with a bad AOA indicating stall will get stick shaker
Both AOAs feed the ADIRUs - so if the AOA's mismatch the effect is UAS
A single AOA feeds MCAS and this switches one from the other after the squat switch indicates weight on wheels.
MCAS only operates when the aircraft is being flown manually

I am sure people will correct me if that is wrong

Yes, I know.....

737m has two Flight Control Computers (FCC). Each FCC contains the control logic for the MCAS function. Each FCC's MCAS function uses the AOA vane signal from that side of the airplane to compute its MCAS command. When flying manually, only one FCC is active at a time (active channel switches each flight).

If one AOA vane signal as received by the FCCs is erroneously high, the stick shaker on that side of the airplane can activate erroneously. If the vane signal that is erroneously high happens to be the one feeding the active FCC, MCAS may activate erroneously. If the vane signal that is erroneously high happens to be the one feeding the non-active FCC, MCAS will be computing its commands based on the other, correct AOA signal and thus MCAS will behave as intended.

Great reply FCeng84, thx a lot.:ok:

What about EAS/IAS correction by AoA sensors?

Let's say LH AoA sensor is wrong and MCAS erroneous active. What about LH EAS/IAS values?

Thx

It is my understanding that static pressure port measurements are slightly affected by AOA. To account for this, processing of static pressure raw data includes a correction based on measured AOA. For 737 I believe that static pressure correction for each sensor is based on the AOA vane signal from the same side of the airplane. If one of the vane signals is incorrect, that side's static port pressure signal will be improperly "corrected" and thus read slightly different from the side that has the correct AOA reading. The first result is that altitude as computed based on static pressure will mistrack slightly between left and right.

Pitot pressure is not affected much by variation in AOA so there is no AOA correction applied to that signal. Left and right dynamic pressure readings would be expected to agree. Airspeed, however, is computed based on the difference between pitot and static pressures and thus will be slightly different when comparing the reading from the side of the airplane that has incorrect AOA and thus incorrect static pressure vs. the side that has proper AOA and thus correct static pressure.

The degree of altitude and airspeed disagree left to right will not be severe, but it is interesting to note that one errant AOA signal can yield all of the following:
1. Altitude mis-compare
2. Airspeed mis-compare
3. Stick shaker on one column but not the other
4. Erroneous MCAS if the FCC on the side with the errant AOA is the active one

Thanks a lot FCeng84:ok:

What is the B737 Max MCAS?

November 17 update to this article originally published on November 13.