A : On Boeing 737, the FEEL DIFF PRESS indication on the flight control panel can illuminate in the following cases:

(The feel system simulates “actual feel forces” at the control column from the hydraulically supported elevator panels)

  1. The first one is related to a differential of A & B hydraulic pressures to the elevator feel system. When hydraulic system pressure drops > 25% related to the higher pressure, the FEEL DIFF PRESS light illuminates on the flight control panel with a 30 second delay. The 30 second delay prevents the light from “flickering” when pressure drops in either system by a high demand such as gear selection.
  2. The second is related to the dynamic air pressure supply to the Elevator Feel Computer. It receives dynamic pressure from the two pitot tubes mounted on either side of the vertical stabilizer. When the computer receives an erratic signal, it’d be the same as the pressure drop and the light illuminates. (failed probe heater and icing conditions)
  3. The third is related to the Stall Management and Yaw Damper (SMYD), and a so-called Elevator Feel Shift module (EFS), which creates a ±4 times higher forward control column force when approaching the stall region. This force uses a reduced system “A” pressure and when this reducer fails, opening prematurely providing a higher-than-normal A system pressure to the feel actuator, the FEEL DIFF PRESS also illuminates after 30 seconds.

Note on the last system, it’s inhibited <100 ft. RA and AP selected, and when the EFS is not operational.

The EFCTB will detect an obstruction or leak in the system’s pneumatic network (2nd failure case described above).
Then, the device will detect a leak in the hydraulic system (1st case of failure described above).
Finally, it will detect any inconsistency in the pressure build-up slopes of the two hydraulic circuits A and B (which may help identify the 3rd failure case described above).