Aviation In Canada

Thursday, October 27, 2005

Altimeter, I say again, Altimeter

Have you heard ATC or FSS say the altimeter twice in your travels? Perhaps just listening to the scanner? Probably most who are reading this already know the rule for this little quirk.

The idea is to add emphasis to unusually high or unusually low altimeter settings. The vast majority of the time, we hear alimeters expressed in inches of Mercury (inHg) as "two niner" something something, or "three zero" something something. But what happens when a deep low or high high pressure system rolls through?

The bigger conern is the low pressure system. Given how often we set 29.XX in the little window, it's awfully easy to mistake 2898 for 2998 or 2989 or some other close-sounding setting. The big danger, of course, is having a subscale in the altimeter gauge set about an inch too high, meaning your alimeter will read higher than you actually are. This would lead a pilot to thinking he was 2,000 feet, when he's really at 1,000. He may be thinking he's 1,000 feet above the charted terrain, when really, he's about to get a close up look at it. If you're VMC, you can see it out the window, but if you're in cloud, that's a little tougher to do.

So when the barometric pressure is below 29.00, ATC and FSS personel are supposed to read it twice. The official phraseology is, "Altimeter two eight six seven. I say again, two eight six seven." That way if a pilot is dialing up 2967 in a bit of a rush, he might reply the transmission: "Did he just say that twice? That's unusual for 2967... Maybe I didn't catch that right." And then he can ask for it again to ensure he has set it right. It's a little safety mechanism in the system that evolved for a reason, and, while annoying for those who have to read it and listen to it, it is probably a good thing. The altimeter setting is supposed to be read twice when it's above 31.00 as well, though this side is less critical, since it results in an altimeter set too low, meaning it's showing lower than the aircraft really is. And hey, rarely has a pilot hit terrain or obstacles for being too low. This one is more of a concern with other traffic in the area.

Incidentally, they ran some stats in our local paper the other day. There was a record set by a hurricane this year for the lowest recorded barometric pressure in the eye of a North Atlantic storm. I forget which one scored it, but the pressure was 883 millibars, equivalent to 26.07 inHg. That's pretty low. The only storm lower was a typhoon in the Pacific, which measured 870mb, or 25.69.