All jetliners prior to about 1970 had flight controls that could, if need be, be manually operated without the use of any hydraulics:
- Some, such as the DC-9, had manual flight controls with hydraulic boosting available; the flight controls were manually operated even in normal flight, with the hydraulics providing additional force to assist in certain situations requiring very large control inputs.
- Others (707, 727, 737) had hydraulic flight controls with manual reversion available; the flight controls were normally operated hydraulically, but would revert to backup manual control if hydraulic pressure was lost.
After the first widebodies entered service in the early 1970s, which had no provisions for manual control, instead assuming that their triple-or-quadruple-string hydraulics would provide enough redundancy that a total hydraulic failure could never occur (an assumption which proved unwarranted), aircraft manufacturers mostly stopped including manual control capability (even for narrowbodies where it would have been easy to include); nowadays, the only manually-controllable jetliners still in commercial service in significant numbers are the DC-9 (produced from 1965 through 2006; hydraulically-boosted1 manual controls) and the 737 (produced since 1967;2 manual-reversion capable hydraulic controls).
The DC-9 holds no surprises for its pilots in full-manual mode, as this is its normal mode of operation (the hydraulics only kick in in a few extreme situations that would never be encountered in normal flight).
The 737, on the other hand, behaves considerably differently in manual-reversion flight than it does normally:
- First and foremost, manual reversion is available only for the ailerons and elevators; the rudder has no provision for direct manual control.3
- The control forces for the aircraft’s horizontal-stabiliser pitch-trim system are much higher when using the manual trim wheel than with the normal electrohydraulic trim (this is especially prominent for the 737 NG and MAX, which have a smaller manual-pitch-trim wheel than the 737 Original and Classic, reducing the mechanical advantage provided by the wheel).
- Control of the ailerons and elevators in manual-reversion flight is by servo tabs that, when deflected, produce aerodynamic forces that move their respective control surfaces; in contrast, when hydraulic pressure is available, the control tabs are mechanically locked to their respective surfaces and move with them. Consequently, the effective area of the control surfaces is slightly lower in manual-reversion flight (as the control tab, in order to move its respective elevator or aileron, is deflected in the opposite direction to the desired aileron or elevator deflection, rather than functioning as additional elevator or aileron area), somewhat increasing the amount of control-surface deflection required to produce a given aircraft response, and slightly decreasing the aircraft’s maximum pitch and roll control authority.
- Other, relatively-minor expected effects of the different methods of controlling the control surfaces in hydraulic and manual-reversion flight would be a change in the control surfaces’ damping characteristics and a slightly-different flight-control force-response curve (as the control forces experienced by the pilots in manual-reversion flight are a direct product of the aerodynamic forces on the aileron and elevator control tabs, which would be more sensitive to aerodynamic factors other than airspeed, such as a nonzero sideslip angle, than would the aircraft’s artificial-feels computer, which provides yoke and rudder-pedal force feedback in hydraulically-powered flight).
Given these differences in aircraft handling between normal and manual-reversion flight, are 737 pilots required to undergo training on flying the 737 in manual reversion (as is the case for other situations affecting aircraft handling, such as an asymmetric loss of engine thrust, a no-flap landing, a stall and recovery therefrom, a jammed pitch-trim system, etc.)?
1: Albeit only for the elevator, and only in certain stall-recovery situations where the required down-elevator force is greater than what the aerodynamic downforce from the elevator control tab can provide on its own.
3: A small amount of rudder deflection is attainable even without any hydraulic pressure, due to the design of the 737’s rudder system; however, this requires the application of an extreme amount of force to the rudder pedals (approximately 300 pounds4 per inch of rudder pedal deflection after the first inch, in addition to the additional force required to push the rudder pedals the initial inch required to take up slack in the rudder control system), and can only produce a very small amount of rudder deflection.
4: For context, the maximum force that an average pilot is physically capable of applying to an aircraft’s rudder pedals is generally around 500 pounds.