How a Metro 'ground loops'

[Kharon]

I'll play; if you're serious.

It is an item common to all "metro types" in the AFM pre landing check: "ARM the Nose Wheel Steering". It is my personal opinion that this is incorrect, if the speed levers are not rigged or are slightly out, i.e. not disconnecting the 'thumb button' NWS through the speed (pitch) lever circuit the nose wheel is 'live' and any inadvertent input can spell trouble, particularly during the post touchdown period (Vref say 110 just for a number) @ 60 – 30 knots. After reverse, speed levers come aft, power reduces and the 'aerodynamic' energy is reducing, you need to pay attention during a real crosswind, and avoid as far as practical, 'relying' on the 'steering' to keep you straight. Good cross wind basics are essential. Any of the Metro types, with parallel power (torque) are a gentleman's aircraft above and below 30 knots KIAS, line up on the centre line, mind your wind, apply the early power carefully, you can with practice, even lead the upwind engine slightly (when needed); but if you keep the power essentially even, there is little need for any form of NWS to be active. If I had my way, the rotten system would be off for both take off and landing at any speed above 20 knots; but it's great for parking the dear old girls. However there is the AFM and etc. to contend with, so just ignore the thing until you have control of the aircraft at or near taxi speed. The following are years old, but may assist those interested in 'fine' machinery.

Start Locks.
1) Slowly retard the power levers, individually, toward reverse to remove the locks. Beta lamps should remain lit, should a lamp go out, hesitate at that position until the lamp is re lit. An increase in torque with power lever movement toward reverse indicates that the lock has been withdrawn. Verify lock withdrawal by advancing individual power levers during taxi, noting the turn away from the engine accelerated.

Beta Lights.
1) When the Power levers are aft of FLIGHT IDLE, full propeller governor oil pressure of 400 + 15 psi is provided to the propeller governor system. The Beta Pressure sensing system, when selected is set to operate at 350 psi, if this pressure is not available the system lights will not illuminate and REVERSE operations cannot be guaranteed.

N.B. Ensure both lights (one per engine) are on prior to selecting reverse.

2) It is acceptable to have the Beta lamps out during ground operations at engine speeds of between 65 – 85% RPM. The lamps must be ON when the engine is above 85%.

NWS.
1) Turn the steering system off and use differential braking if:-

• the NWS light flashes or,
• there is an unscheduled steering deflection or,
• the Park light is illuminated when the button is not activated.

2) It is normal for the park light to remain on for 4 – 7 seconds during transition from normal to park mode. The light may vary in intensity depending on how much 'authority' was scheduled e.g. 63° = very bright, 33° = half bright. The delay is due to the Computer/ amplifier taking time to process the signal and repositioning the Servo valve and solenoid.

3) Check Hydraulic system limitations 14 – 1700 psig minimum, 1700 – 2100 normal, 2100 – 2300 caution range, prior to operations.

NB. Corrective action when the Nose Steer Fail amber lamp in on depends on the operational status of the system. AFM procedures assume the NWS is armed. If the fault light is on, and the system is not armed, the system must be turned on (ARMED) prior to corrective actions being taken.

4) When the Speed levers are set to high, prior to take off, the NWS (rudder pedals) become non operational, if steering is required the 'thumb button' on the right power lever may be used for the take off roll,(electrically reconnects the rudder pedals to the steering) - the service will terminate once the squat switches open circuit. Parallel torque application precludes the need for activating the system after 40 / 60 knots during normal take off.

5) Thumb switch is not normally used for steering during landing unless there is a system malfunction, when the system may be used to ensure directional control.
6) The nose wheel cantering cam limits steering to approximately 65°. The limits are marked on the nose gear strut, if the gear is forced beyond these limits, physical damage to the system will occur.

7) Apart from the airmanship aspects of taxiing at reasonable speeds, recommended maximum 20 knots (GPS), taxi at sensible speeds allows the heavier components of the engine time to achieve proper heat soak, which helps relieve the stresses imposed on the engine during take - off. Brake temperature (Vmbe) limits should be considered

NWS AFM II – 33. Check List :

NWS ARM SWITCH ARMED.

This selection provides DC power to the nose steering relay contacts. A steady green annunciator lamp will illuminate indicating the electrical components of the system are operational. Check Hydraulic power (1700 – 2100 psi) is available prior to selection.

CHECK NOSE STEERING lamp ON.
RIGHT SPEED LEVER FORWARD.

Moving the RH speed lever forward of LOW, open circuits a quadrant micro switch, which acts to maintain DC power to the nose wheel steering (rudders) system, by passing the thumb switch. Once the speed lever is moved forward only the thumb button can supply electrical power for steering.

CHECK FAIL LAMP OFF and STEERING INOP
THUMB SWITCH OPERATE.
CHECK STEERING NORMAL.
RIGHT SPEED LEVER AFT.
TEST SWITCH TEST LEFT and RIGHT.

The test switch provides a fault check of the NWS electrical relay system. The switch is provided to verify proper operation of the fault protection system. There are two main faults, i.e. No steering when commanded, or Steering when not commanded. In either case, once there is more than 3° difference between the rudder pedal position (rudder pedal monitor potentiometer) and the Hydraulic actuator, the protective circuitry within the steering computer will isolate the system and the FAIL lamp will flash.

The test procedure is designed to provide a non commanded steering input and a failed steer command. Select LEFT test and note the turn to left is initiated, oppose the turn with rudder and note blinking lamp and automatic disengagement of the system. Repeat on a right turn. This check also confirms the hydraulic actuator is functioning correctly. At the end of the check, return the rudder pedals to the centre and note a steady ON lamp.

NWS ARM SWITCH VALVE TEST.
CHECK SYSTEM INOPERATIVE.

The switch is used to verify, during taxi, that the hydraulic arming valve has returned the actuator to caster mode. When released from the test (hold) position and reset to ARM the system should function normally and a steady lamp should be noted.

NWS ARM SWITCH ARMED.
PARK BUTTON OPERATE and CHECK.

A transition, over several seconds, of increasing authority and lamp brilliance should be noted, followed by a dimming and reduction of authority once the button is released. The time delay is electrically induced to prevent abrupt transition from Normal to Park mode.

Notes.
The amber NOSE STEER FAIL light must blink ON every time the NWS is deactivated either by the right speed lever, the power lever button, the test switch or, the arm switch.

Enjoy.....No idea what happened to the Archerfield crew; could be any thing or any number of things. Those who may have an idea will shut up and see what evolves; those without – well we did have a thread closed. Thanks Tail-wheel, for having more forbearance than I have. A lot more.