The MRFCS is an emergency system for use when dual hy-draulic failure is impending or has occurred. The mode is ad-equate for executing moderate maneuvers. Landing should only be attempted under ideal conditions or when ejection is not pos-sible, since any degradation beyond normal reversion may make landing impossible.
Emergency transitions to manual reversion are automatic and instantaneous in pitch and yaw, with stick and pedal commands transmitted directly to the elevator and rudder surfaces through the actuators, which are in the hydraulic bypass mode. Transi-tions in roll must be initiated. When MAN REVERSION (Fig-ure 1-17) is selected, roll control is transferred from the ailerons to the aileron tabs. Selecting MAN REVERSION also closes hy-draulic shutoff valves preventing unexpected return to hyhy-draulic powered flight control. Manual reversion trim is provided only in pitch.
PITCH MRFCS.
Pitch transition to manual reversion occurs due to hydraulic pressure depletion. The same components are used for man-ual and hydraulic pitch control. As hydraulic pressure drops to 600 to 400 psi, elevator control automatically changes from hydraulic to mechanical. Electrical control of the two elevator trim tabs is automatically achieved when both hydraulic power sources have dropped below 1,000 to 800 psi. Artificial feel is retained.
Transition is reversible. Power control of the elevators is in-stantly restored as pressure at one (or both) of the actuators is increased above 700 to 900 psi. Both elevator trim tabs trim to neutral when either pressure switch senses 1,000 to 1,200 psi.
YAW MRFCS.
Yaw transition to manual reversion occurs due to hydraulic pres-sure depletion. The same mechanical elements are used for man-ual and hydraulic yaw control. As hydraulic pressure drops to 600 to 400 psi, rudder control automatically changes from hy-draulic to mechanical. Transition is reversible. Power control is instantly restored as pressure is increased to 700 to 900 psi.
ROLL MRFCS.
To achieve roll control when hydraulic pressure is not present, the flight control mode switch must be set to MAN REVER-SION. When MAN REVERSION is selected, stick commands are disconnected from the aileron actuators and connected to the aileron tabs. In this tab drive mode, the aileron tabs fly the aileron surface to the position commanded by the stick. Feel at the stick is proportional to air loads on the tabs.
Aileron Float-Up Transition.
After loss of hydraulic pressure, the trailing edges of the ailerons float up to a position that is higher than the powered neutral po-sition. Aileron float up normally induces an aircraft pitch change which can be nose up or down depending on aircraft cg, eleva-tor trim tab setting, power setting, and flap position. The pitch change intensity varies and is dependent on aileron float up rate, airspeed and altitude, and can range from minus 2.0 to plus 6.8 g’s during transition.
To soften pitch onset, the aileron float-up rate is limited by damping in the actuators. The time for the ailerons to float-up after hydraulic pressure loss/bleed off is approximately 4 sec-onds. If the flight control mode switch has not been placed in MAN REVERSION after a dual hydraulic failure, the ailerons will float up, the stick will act directly on the actuator linkage and almost no roll control will be available. The stick feel will be the same as experienced on the ground before engine start.
Flight Control Mode Switch.
The flight control mode switch (Figure 1-17), located on the emergency flight control panel, is placarded FLT CONTR, with positions NORM and MAN REVERSION. The switch is lever-locked in both positions.
In MAN REVERSION both hydraulic systems are shut off. The switch simultaneously drives the aileron/tab shifters to tab drive.
All other roll transfer logic is automatic. The switch controls two independent circuits, powered through the L & R AILERON TAB circuit breakers by the DC essential bus.
NOTE
All flight control mode switch functions are fully reversible at any time if hydraulic power is avail-able.
Aileron/Tab Shifting Transition.
Selecting MAN REVERSION initiates aileron/tab shifting immediately. The shift cycle takes approximately 4 seconds to complete (in either direction). Tab shift action is progressive, providing increasing roll control.
As the shifters move from the normal position switches:
• Deactivate both normal and emergency roll trim
• Drive the roll trim actuators to neutral
• Cause the corresponding L or R AIL TAB caution light to come on.
Driving roll trim to neutral during MRFCS operation assures that the ailerons will go to neutral when power is restored.
Aileron Tab Caution Lights.
The aileron tab caution lights (Figure 1-158), placarded L AIL TAB and R AIL TAB, come on if the corresponding aileron/tab shifter is not at the full normal position.
Aileron/Tab Shifter Malfunctions.
Failure to shift to tab drive after the flight control mode switch is placed to MAN REVERSION is indicated by:
• Respective AIL TAB caution light off
• Very high lateral stick force - approaching locked stick feel
• Aileron jam light(s) may be on depending upon stick forces applied
• Stick moves toward the side of the nonfunctioning shifter.
If failure to shift is experienced after switching to MAN RE-VERSION and hydraulic power is available, return to NORM for the remainder of the flight. Should hydraulic power not be available, some roll control may be achieved by disengaging the aileron for the side with the nonfunctioning aileron/tab shifter.
Flight in manual reversion with an aileron dis-connected has not been tested and may be
im-Failure of a shifter to return to aileron drive after selecting NORM is indicated by:
• Stick movement toward the side of the malfunctioning shifter
• High lateral stick force required to keep wings level
• Respective AIL TAB caution light remains on when oppo-site side AIL TAB caution light goes
• Aileron tab on side with nonfunctioning shifter responds to stick movement with aileron remaining in neutral position
• Aileron trim inoperative.
If failure to shift is experienced after shifting to NORM, roll con-trol can be increased, if necessary, by disengaging the aileron for the side with the nonfunctioning aileron/tab shifter as indicated by the AIL TAB caution light.
With one side disengaged, maximum roll capability will be re-duced approximately 50%, and stick input for a given roll will be twice normal.
Roll trim can be restored by pulling the AIL TAB circuit breaker for the side with the nonfunctioning shifter. The corresponding AIL TAB caution light will go off when this circuit breaker is pulled and both ailerons will respond to roll trim.
Failure to complete the shift to tab or aileron drive degrades roll control for the mode selected. The degree of control available is based upon the amount of shift accomplished prior to failure.
Hydraulic pressure and neutral aileron position will be normal for the mode selected. The aileron tab caution lights should pro-vide an indication of which shifter has malfunctioned, unless an AIL TAB circuit breaker has opened. If an AIL TAB circuit breaker is open, the associated caution light is inoperative. The aileron tab circuit breaker should be checked whenever a shifter failure is suspected.
MRFCS OPERATION.
Shifting to MRFCS Mode (Hydraulic Power Available).
Most conversions to MRFCS will be intentionally initiated. If intentional transition is planned, the MRFCS should be ground checked prior to flight. Reasons to transfer to MRFCS, while hydraulic power is still available, include the following:
• Training in the MRFCS mode
• Check-out of the MRFCS mode
• Precautionary transfer to MRFCS mode; e.g., one hydraulic system failed and failure of the second system is imminent.
When accomplishing an intentional shift into manual reversion, comply with operating limitations in Section V. Selecting MAN REVERSION simulates a dual hydraulic failure, while simulta-neously initiating roll transition to manual reversion. When the flight control mode switch is placed to MAN REVERSION, the following events occur:
• Hydraulic supply pressure is shut off and bleeds off to zero psi. This can take up to 10 seconds. Bleed off can be ob-served on the cockpit hydraulic pressure gauges and by not-ing the L/R HYD PRESS caution lights.
• The aileron tabs initiate (on switch actuation) shift to tab drive and this is indicated by the L/R AIL TAB caution lights coming on. Complete shift can take up to 4 seconds.
When hydraulic supply pressure bleed off is complete, aileron float up begins. Float up will take up to 4 seconds. Once the ailerons are floated up, the aileron actuators are in a bypass mode. Since supply pressure bleed off and aileron float up are se-quential, the total time to regain roll control after selecting MAN REVERSION can be up to 14 seconds. If the stick is moved lat-erally prior to completion of aileron float up, the ailerons may float up abruptly and asymmetrically.
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Failure of one or both hydraulic systems to drop below 250 psi after switching to MAN REVERSION may result in locked ailerons after shift to aileron tab drive commences.Under these circumstances, control stick feel will be near normal for manual reversion;
however, roll capability will be slight and in the opposite direction to stick displacement.
Therefore, should one or both hydraulic pres-sure gauges fail to drop below 250 psi within approximately 10 seconds, and, if roll is in opposite direction to stick displacement, re-turn the switch to NORM.
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If the stick is moved laterally prior to comple-tion of aileron float up, the ailerons may float up abruptly and asymmetrically.NOTE
The L/R AIL or L/R ELEV jam indicator lights may come on during manual reversion, due to airloads.
Shifting to MRFCS Mode (Hydraulic Power Not Available).
In a dual hydraulic failure the stick will essentially lock on roll.
Stick feel will be the same as experienced on the ground before engine start. Pitch and yaw control will be available immedi-ately, but MAN REVERSION must be selected to initiate tab shifting which will make it possible to regain roll control. Roll control will be available in approximately 4 seconds after select-ing MAN REVERSION.
If hydraulic failure occurs, when operating single engine or with an asymmetric loading, the aircraft will begin a slow roll into the dead engine or heavy wing. The throttle should be retarded on the operating engine (to IDLE if conditions permit) after select-ing MAN REVERSION. Coordinate rudder and aileron as thrust is increased after transition is complete. Asymmetric loadings can be corrected by selective jettison, as conditions permit.
Shifting Back to PFCS Mode.
Hydraulic power, if available, is immediately applied to flight control actuators when the flight control mode switch is returned to NORM. All logic functions are fully reversible and powered control of the elevators and rudders is immediate. Pitch trim change may be required. The ailerons drive down to neutral trim position, but roll control is not fully effective until aileron/tab shifting is complete. Roll trim control, both normal and emer-gency, is available at the completion of the shifting operation.
Yaw trim control is regained after YAW SAS is reengaged.