About a month ago, the Surry sector up in Maine had an altimeter setting of 28.90, making the lowest usable flight level FL200. The rest of the center held steady at 28.92 or higher (as you can see from the posted weather report), so operations weren't affected much. However, there was a lot of discussion around the control room regarding the consequences of such a thing occurring. For example, LGA arrivals get handed off to a different sector, as does PHL arrivals. Alternatives include descending aircraft with a normal restriction of FL180 down to 16000, and allow the FL190 restrictions to descend to 17000. Yes, such a low altimeter setting would do crazy things, indeed.
But what about when the altimeter setting is too high? Too high? We're used to FL180 becoming unusable, so that's no big deal, but what could a high altimeter do to stir the pot? A month after the FL200 incident, the MSS altimeter got as high as 30.81. The weather is really getting out of control. So, my first question to every pilot out there is this: How high can you set your altimeter?
Can it go higher than 31.00?
Do you have any special considerations when the altimeter is that high? Lets whip out the AIM to 7-2-2a.
From an ATC standpoint, we simply state the altimeter and advise that you remain on 31.00 until final approach. Some aircraft cannot set above 31.00, so every aircraft remains on that setting (similar to Class A airspace). For those capable of setting the altimeter to the actual setting, pilots should do so when established on the final approach segment. For those who are unable to set above 31.00, they should remain on that setting, but take the difference into account when determining decision heights.
So, aside from an audible gasp, and asking the controller to repeat the altimeter, because you've never seen it this high in your life, do you do anything different? Do you know how to figure out your new DH if you can't set your altimeter higher than 31.00?