Originally Posted by
jagema
FCeng84,
Given your familiarity with 737 systems I wanted to address a question to you:
As far as I understand, FCC-enabled MCAS receives single channel sensory data from the ADIRU, mainly AoA (so as to trigger MCAS in the first place) and speed (so as to regulate just how much trim input is required based on Mach number schedule).
My question is about the total control column forces with both SMYD stall protection features ANDhaving MCAS in its maximum setting. Allow me to explain further:
1. MCAS: assuming an AoA sensor failure where the data is relayed as equal or higher than the activation threshold of MCAS, then MCAS kicks in. Assuming a low speed readout by MCAS, it is then set to its maximum throughput (10 seconds of trim and 2.5 degrees nose down, since it works on a speed schedule based on Mach).
2. SMYD/EFS: As I understand, the SMYD computer enables the EFS (Elevator Feel Shift) and STS (Speed Trim System) to do the following during low airspeed conditions - EFS on one hand doubles Hyd A pressure to control column in its AFT axis, and STS trims the stab Nose Down (though respecting the column cutout switches, something MCAS ignores altogether).
The conclusion is the STS trimming you down (so as long as you don't pull aft), the EFS doubling aft forces, and MCAS also trimming at up to 2.5 degs nose down within 10 seconds (if uninterrupted). Essentially, you can end up in full Nose Down stab trim with doubled elevator control forces, thus severely affecting your authority.
Do you agree with my reasoning that SMYD/EFS interaction with MCAS (when low speed is relayed) can be additive in terms of struggling to keep the nose up and having limited authority?
Makes you wonder just how much attention Boeing engineering gave to both these systems' interaction at the design stage.
jagema,
In order to work out your scenario let's be specific about a few aspects of the proposed sequence:
1. Are we starting from takeoff with indicated AOA failed high?
2. What is the speed time history following liftoff and through this sequence?
3. Are there any thrust changes or is that left at takeoff setting throughout?
4. What flap setting are we starting with and when are the retractions including when do we go Flaps Up?
5. What is the pilot doing with respect to pitch trim?
- No trim inputs at all
- Trimming as needed to keep column forces less than 5 lbs
- Allowing column forces to build up to a higher level and then trimming off force (What level before applying trim?)
6. I suggest having the pilot add column as needed along the way to keep pitch rate at about zero after having complete the takeoff rotation.
I'd be glad to walk through the sequence once the scenario specifics per above are defined. If you want to define more than one variation go ahead.
FCeng84