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Originally Posted by ArcticKnight
Hi again WOXOF
3 quick ones
1.Where I live the Northern Lights are awesome. I thought you noted that you did some flying in the north... have you been lucky enough to see any major displays of Northern Lights? How far north in Canada have you flown (if at all)?
I haven't and I keep hoping I will. My one view of the Northern Lights was kind of disappointing and I've heard from other pilots that it can be breathtakingly impressive. haven't flown that far north over Canada, though we now have an Amsterdam-Portland OR flight that overflies northern Canada, so maybe I'll get a chance to fly that one.
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2.Another question. There are overflights in the north where I live that are in the Vancouver to London (or other European destinations) route, and some flights from the east coast of the US to NRT. I was wondering, with the use of GPS and all, why do I still see overflights correcting over the VOR? I have been out of the field for a while but just assumed some of the need correct while ovr, by or abm VORs enroute would have been supplanted by routes using GPS, thus no need for even small corrections over VORs. Heck, do you even use VORs anymore as enroute nav aids?
It is kind of an anachronism, but the fact is that the airways are still defined by the location of the VORs. So even though we are using GPS or Inertial Navigation Systems (surprisingly, less than half of our 767s have GPS), we still fly the published legs of the airways, often including a turn over the VOR.
And although the pilots don't tune VORs or use the raw data for navigation, the plane
does tune them in automatically (transparent to the pilots) and uses azimuth and distance information from several VORs to provide position updates to the navigation system and reduce the error that can creep in over a long flight.
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3. Ever have any close calls in the transition between 18,000ft and FL190, due to going on to altimeter 29.92 or coming off of it?
For anyone else who wonders what ArcticKnight is talking about here, the altimeter has to be set to a local altimeter setting when we are flying below 18,000' to give an accurate readout of height above sea level for that area. Because the altimeter setting is just local barometric pressure, we must change it as we fly along (again, this is only below 18,000'). Flying Victor airways (those below 18,000'), we will get a new altimeter setting from every controller we talk to.
Because terrain avoidance isn't an issue above 18,000' (everywhere but Nepal, I suppose), and because we travel at speeds that mean the barometric pressure can be changing every few minutes, we use a common datum reference of 29.92" above 18,000' (1013 millibars in Europe).
We normally change the altimeter from local setting to 29.92 as we pass 17,000 in the climb. and now we refer to altitudes as Flight Levels rather than feet above Mean Sea Level (MSL). So if I check in with New York Center at 17,000', I say "Delta 123, level at one seven thousand", but if my altimeter says 18,000' I say, "Delta 123, Flight Level One Eight Zero" (not one eight thousand).
Because 1" of mercury is approximately equal to 1000' of altitude, we could have a problem if the local altimeter setting was, for example, 29.00" (a low pressure, but not unheard of). As we set our altimeter setting from 29.00" to 29.92", the readout on the altimeter will go up by about 920 feet. So we could have the situation of Aircraft A at 17,000 (using the local altimeter setting of 29.00") and Aircraft B at FL180 using 29.92, but Aircraft B would actually be at about 17,080' MSL, less than 100 feet of separation!
Because of this, when the local barometric pressure is low, ATC will not use FL180 at all.
Oh, and to answer your question, I've never had a close call due to this but I have forgotten to change altimeters before and then realized it later. Luckily, the change in the setting was small and so the discrepancy was small. Checking the altimeter setting is on our Takeoff, Climb, Descent, Approach and Landing Checklists (5 separate checklists, used at appropriate times during the flight), so this item gets checked and rechecked all the time.
Last edited by W0X0F; 06-19-2014 at 07:08 AM.