How a Metro 'ground loops'

[VH-FTS]

The comment was made in another thread that they'd like to learn more about why a Metro would suddenly pull to one side of the runway. Leaving the rubbish and speculation about the Archerfield incident to one side, the following is some info for those interested. Others with more experience, like the Green Goblin I believe, are welcome to chip in or correct any errors or terms (it's been a few years since I flew the death pencil).


The Metro has an interesting nose-wheel steering set up. There is a switch to 'arm' the nose wheel steering (NWS), which is then 'activated' by a number of conditions. When 'activated' hydraulic fluid is applied to provide much more responsive and sensitive steering. When the system is not 'activated', but in a 'standby' kind of mode, it basically castors.


At low speed you want the system to be fully activated therefore allowing very responsive steering. At high speed i.e. above over 50 knots, the steering is very responsive, and combined with the aerodynamic effect of the rudder, can cause way too much yaw. Without knowing all of the facts, this could have occurred the other day, but moving on...


So, how does the NWS become 'armed' only? Simple, you turn on a switch to the left of the captain. Now, if you also want the full NWS working i.e. 'activated', either the speed levers (no condition levers in a Metro) need to be in the low position, or, the nose-wheel steering button on the power lever pressed in (using your thumb).


For taxi the speed levers are in the low position, so NWS is always activated and you don't have to press the thumb button. For take off, the speed levers are set to 'high', so the captain must hold the NWS button for full steering responsiveness during the early part of the roll, but lets go at about 30-50 KIAS (depending on your technique).


Now, lets say hypothetically you are landing in a very strong crosswind. You are applying a lot of rudder to stop the yaw etc., then you either a. press the NWS button, or b. pull the speed levers back to low, then you suddenly have a butt load of steering input which could take you off a runway. If the runway was wet, I could see how a 'ground loop' could occur, but I don't really think it's the correct use of the term.


There was a Metro that also went off the runway at Emerald a couple of months ago during the start of the take-off roll. They claim one of the props wasn't off the locks (now that's another story), but given a trainee was flying, I wouldn't be surprised if it was a NWS technique issue too.


At the end of the day, NWS can bite in the Metro, along with other stupid things like not turning the inverters on (which supplies AC power to the AI etc - guess what a failure or incorrect use could do on a dark night!). But then again, it was built by Mexicans.


Love to hear from others with 'fond' memories about the Metro (or corrections to the above). I don't want to hear from that YBAF plane spotter, who claims to fly Barons, that the plane was overloaded. Once he/she gets some time up in a Metro, or at least a better understanding, then I'm happy to hear from him/her.

[CharlieLimaX-Ray]

Won't be the first time a Metro has decided to head for the scrub after a nosewheel steering glitch-either pilot induced or machine generated.

[megle2]

FTS, excellent post. Never flown a Metro so your contribution makes interesting reading. Thanks

[601]

Never flown a Metro so your contribution makes interesting reading

Glad I never did!!!!!!!!!!!!!!!

[waren9]

a groundloop is when the cog tries to overtake the point that drag/braking can be said to act through.

tail draggers with mains forward of the cog, yes.

tricycle types with mains aft of the cog and betw 3 points of contact, not so much.

sufficient loss of traction in the pissing wet will turn a metro into a 5T weathervane and gravity will take it downslope but it aint what i was taught to be a groundloop

[Mail-man]

Thanks fts, appreciate it.

[Kharon]

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.

[The Green Goblin]

NWS is armed at decent.

It is activated with the PB on the power lever or by the right speed lever in the low position.

On the landing roll you centre the rudder and activate it though 40 kts with the PB. After which you call for speeds low.

If you are holding off rudder and call for speeds low without engaging it with the PB you scare yourself silly.

The metro is an honest ship. Anyone who knocks it never flew it. Metro drivers are well regarded in airline interviews and generally pole the sim check with ease. After all, a 737 classic sim feels like a metro once you get used to large pitch changes with thrust.

There has been plenty of metro drivers taught the NWS quirks the hard way. Myself with 4 years on type included.

[Mail-man]

Does anyone remember a hazelton metro that ended up off the side of 16r at yssy round 2000? Same nws issue/misapplication?

[Caboclo]

As VH said, the NWS becomes very over-sensitive at higher speeds. My boss got tired of pulling his planes out of the weeds, so he banned us from using it for either T/O or landing. He also told us not to bother with the test procedure, because it always failed.

Even without the NWS trying to kill you, you can still wind up in the weeds. Not sure why the Metro is so hard to rig properly, but we frequently had large splits between power levers. Talking about "parallel power" is all very well, but the reality is you had to remember which plane you were in, and which power lever to lead with on both T/O and landing. When I found myself using full reverse on one side with the other side forward of flight idle, I considered it was due for a write-up.

a 737 classic sim feels like a metroMy deepest sympathies to all those 737 pilots!

Can't believe I'm the first one to mention that the entire NWS is MEL'able for 10 days. Did Fairchild not manage to sell that detail to any agencies outside the FAA?

Referring back to the OP, the sudden lurches would be either engaging the NWS with the rudder pedals cocked after a crosswind landing, or fighting with an improperly rigged aircraft.

[dribbler]

If the RWY was contaminated with standing water, it is possible that the nose wheel threw up enough water to cause a NWS fault via a leaky connection box. Had it happen before.... The Metro is a honest stable single pilot machine.
Only the crew will know. Doubt the FDR/CVR is that sophisticated to catch all the parameters.

[The Green Goblin]

Why would you have at sympathy for 737 pilots?

The metro flies like and feels like a transport category aeroplane. If you want an agile nicely balanced spritely performer, join the airforce. Hopefully you won't end up flying a chopper :P

Having flown both the boeing and the bus, give me a metro (equipped with freon cycle aircon) anyday.

[Caboclo]

Wow. If that's what I had to look forward to, I'm glad I quit aviation. I credit the Metro with giving me the strong wrists I have today.

[18-Wheeler]

Love to hear from others with 'fond' memories about the Metro (or corrections to the above).

Here's something I wrote about Metros here a while ago. It's intended as satire but it's perhaps disturbingly accurate.

I'll explain, as it's also my most hated aeroplane.

The Metro.
The cockpit was designed by a midget and anyone of even normal size is very uncomfortable in them. Especially if you have long legs as the pedals are simply far too close to the seat so you have to bend your legs so far that it's practically impossible to get your feet onto the pedals at anything like a comfortable angle.
Fortunately the brakes, although fitted, don't actually work; they are there for show only and to amuse the ground engineers. So it's really not that important to get your feet where you would normally have them.
The control column was designed by a personal trainer, to improve your fitness by means of a continuous physical workout whenever any movement at all is required as they forces needed to move any control surface is in the tens of kilograms. If you can't benchpress two or three hundred kilograms, forget it.
Fortunately again, the engineers that designed the Metro had a sense of humour and made the ailerons so they don't really work much. They will roll the aeroplane roughly as fast as an oil tanker can turn, but no faster.
Whilst on the subject of controls, I must mention the rudder; like most small aeroplanes it also control the nosewheel steering and this is where the Metro engineers must still be peeing their pants with laughter, with the nosewheel system being the result of something like ten completely unrelated bits of machinery all incorporated to make the Metro perform the complex tasks of .... left ..... and ...... right. The nosewheel system requires approximately two weeks to do a complete systems test on the taxi out to the holding point and so many Metros must return to be refueled before flight. Fortunately it works randomly but the aeroplane can be steered on the ground by means of futilely stabbing at the brakes (I use the term loosely) and moving the power levers to vary the noise left to right.
Then there's the stall detection and Stall Avoidance System, which is designed to kick in just before landing; to do this, it senses a completely normal airspeed in the landing configuration just before landing, and then activates the stick pusher without warning, thus making the Metro land on the nosewheel and bounce down the runway.
Normally this would be uncomfortable for the passengers, if you are carrying any, but they are probably still unconscious from the lack of pressuristation due to the Metro being fitted with door seals made from molten ear wax and the bleed source a small 12 volt hair drier fitted to each engine. Even though the door seals may keep a tiny fraction of the air in the cabin and they whistle a loud revelry to keep the punters awake, they are often wearing foam ear plugs to block the noise and are likely to be blacked out from the cabin being something like 1,000' below the cruise altitude.
Fortunately, the Metro engineers had already considered the dual problems of trying to keep the passengers subdued and also making the ailerons a mere amusement, so they gave the machine a tiny little thin wing that only works when you are travelling along at many hundreds of knots. The wing is also very handy for smacking your head into, to remind you how lucky you are to be in aviation .... in case you had forgotten.
Speaking of engines, again the Metro engineers excelled themselves by choosing engines that have the unique blend of being complete and utter p***ks to start unless you have a nuclear power station plugged in (good forethough again - the ground power plug is often on the side of the nacelle near the prop, to help keep the ground crew cool as they unplug you), they also make more noise than said nuclear power station blowing up, and they also take 1.2 weeks to complete a start cycle. So at least when flying a Metro you develop the ability to think ahead. The propeller is also capable of reverse pitch, and this is used to make more noise on landing with the noise reflecting on the airframe to slow it to taxi speed.
Back to the cockpit. Whilst it may seem great fun to blast around the skies in a semi-pressurised aircraft that can barely be controlled - assuming it'll start - you can't actually see where you're going due to the windows being too small. To be fair there's a large-ish window on the side, but it often vibrates so much that nothing useful can be seen from it. This is assuming you're lucky enough to be flying in warm weather, for in colder conditions that require window heat only the window directly in front of each pilot has a tiny section of it heated, so if you thought you couldn't see much before you will see even less now.
The various controls & instruments in the cockpit were strategically positioned by means of a very large shotgun and many cases of beer I suspect. Fortunately, most of them don't read very accurately or indeed at all. A large number of the switches were labelled by means of picking suitable-sounding words from a brown paper bag. The radar is merely a Nintendo Gameboy converted to black & white, yet still has the batteries removed so it doesn't work. Every internal light is designed to illuminate dimly and also burn you if you foolishly fiddle with it to make it brighter.
If you do decide to go attempt to commit aviation in one, you first have to tackle the front door. It's hinged at the bottom and has a single handle to open & close/lock it. Sounds simple, but again the trusty Metro engineers decided that the door should be fitted with a totally reliable lift-assist device that detects the strength of the person trying to operate it, and if the person is weak then the door is scheduled to get no assistance at all, and vice-versa. The chains fitted to either side to stop the door from (mostly) plunging deep into the ground when opening are also designed to fall to the side when closing the door to make it jam, with the bonus of also damaging the door seal. But the Metro engineers weren't finished there, they decreed that even if you'd managed to lift the heavy door up and grow another arm or two to get the chains out of the way, then it has just enough twist in it so it simply won't shut. Well some doors are scheduled to only shut when slammed for the 3rd or 4th time with at least a grunt and swear word. It's then impossible to visually sight if the door is close as the inspection ports for the door pins are made from cataracts extracted from the eyes of old pilots. Fortunately it works nearly perfectly every single time you shut it from outside, though it must be a member of the flight crew that shuts it, not any old person on the ground. Engineer humour again I suspect ....

I like the water/meth on takeoff.

Does that answer your question?

[VH-FTS]

Thanks 18 Wheeler. I remember that post from a few years back and actually tried to look for it to include in the OP. Brings back some very fond memories!

[VH-XXX]

How much crosswind would it take to "ground loop" a Metro, particularly on a smooth runway and is that even possible?

I ask this because I've seen a video of a Caravan "ground loop" with a strong crosswind. I've even seen a Caravan spin around in a strong cross wind onto the tail and smack into a hangar on the spinner.

There was report of a crosswind in the mentioned incident, was it significant?

[JezYBBN]

Green goblin, good call on the Metro pilot accolade , keep up the good work, and yes totally agree regarding the Metro skill set one acquires with having flown them:ok:

[717tech]

Great post 18 Wheeler!

[glendalegoon]

I flew the Metro II, METRO III and METRO IIII. ( I put this down instead of IV for 4 as IV kind of thing makes me think of hospital)

Both as captain and as copilot. I learned the best way to handle the throttles as a copilot was to do it underhanded so my left thumb could engage the nws.

There are pluses and minuses to this plane. The older ones had a JATO bottle to assist in maintaing a safe airspeed if the engine quit.

We had one guy have the blades come off the prop, come through the fuselage, missing a passenger. He could only maintain control above 200 knots but lucked out as he was at cruise altitude lined up with the long runway at FRESNO (KFAT). My hat is off to him.

I hated the CLICK CLACKS. I watched when one of our planes couldn't get the front door open. YIKES


But it did have stabilizer trim and not elevator trim. A bit more jet like.

I flew two of the planes that airlines seemed to like to see in the log book. the original HP 135 Jetstream and the Metroliner.

But this was back in prehistoric times.

YOU ALL BE CAREFUL out there!

[j3pipercub]

It's been 3 years since I have flown one, but I have just over 1000hrs in them. They are with the exception of a cub, the most enjoyable aircraft I have flown, you just need to know the systems. NWS was a good one, the fact that the annunciator lights meant different things on the 3 and 23 was fun. Anyone remember what to do with an amber NWS light flashing once NWS was armed?

Great aeroplane. Hard to fly well.

[VH-FTS]

Wasn't there a Metro at Bankstowns with a flashing amber NWS light on approach? The pilot ended up on another runway? Or was that urban legend?

I believe the upcoming report about the Brisbane wheels up two years ago will also allegedly explain what happens when the undercarriage's emergency hydraulics are plumbed the wrong way.

[waren9]

well, that was a walk down memory lane thanks 18wheeler

[Kharon]

Cheers Uncle – as I said -the snags were burning,-http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/eek.gif -glad you knew what I meant though, good catch. - http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif - Error corrected. Geez, - ya shag one goat. :D

[The Green Goblin]

I laughed again at 18 wheelers post. Having read it the first time around 2008?

I remember at the time he was most upset after trading a 747 for the mighty metro. Who wouldn't be I suppose!

Secretly every metro driver enjoys all it's quirks which is what makes it quite a satisfying aeroplane to fly. Especially when you fly it well.

I'd say all in all she's a solid grounding for bigger things. You just have to relearn how to flare again!