reply to post by scrapple
scrapple, that is a very good question. I seriously doubt he used the ACARS to pull up a hard copy of the ATIS from DCA.
I've seen this argument debated back and forth, did they or did they NOT re-set the alitmeters below 18,000 feet? Did all THREE altimeters get
I am not taking any stand either way, just hoping that all info is accurate, and not tainted by biases either way....
For clarity, for non-pilots...various high and low pressure systems can cause an altimeter to read incorrectly, so there is something called a
barometric setting we dial in to correct for prevailing conditions, in order to reference MSL (mean sea level)
THis is a local correction, but above a certain altitude (18,000 feet) in the USA we switch to a 'Flight Level' system, by setting the altimeter to
29.92 in/hg. (1013 mm/hg in international airspace).
Other countries use a different 'transition level'...for instance, in the UK, it is at 4,000 feet. In France, 6,000 feet. This is primarily due to
the prevailing terrain within the Nation's airspace. England doesn't have any high terrain, so FL begins at lower than in the USA.
The above was just background for those who don't fly.
Again, scrapple, I haven't studied all of the opinions of naysayers re: AA77, perhaps you have? If so, let us know.
BTW, setting the altimeter to correct for local pressure variations to reference MSL is known as setting to 'QNH'. One airline...American
Airlines...used to use 'QFE' in their procedures...this would be for Instrument approaches, I think, since I'm not familiar with the practice...but
they were the only Major Airline that did that for years, not sure when they modified the procedure. Oh, QFE means that the altimeter is set to read
'zero' at the airport's field elevation. A proper atimeter setting is necessary, of course, for separation from othr traffic, and to ensure proper
clearance from terrain hazards, when operating in IFR conditions. Obviously, in clear weather, your eyes are your 'altimeter'....
[edit on 31-3-2008 by weedwhacker]