How a Metro 'ground loops'
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.