reply to post by nextguyinline
nextguy, your link may not work, at least not for me, but could be my system. But, I get your point....
I have to do this, it has been brought up, as relates to American Airlines 77, commonly and calously referred to as 'AA77' (yes, by me as well, I am
sorry to say).
Since we are discussing about the ability of an airplane to 'vaporize', I think it is important to first, remember the victims. Not only the ones
on the airplane, but those killed or severely injured in the building....
There have been many comments proffered about AA77, some can be shown to be fallacious. The one thing that sticks out, in my mind, is the question
about the 'Altimeter Setting' question.
Background: In the US Airspace (including Alaska and Hawai'i) we use a standard of 18,000 feet (or 19,000 feet, depending...when pressures are very
low, we use 19,000) as the basis for setting altimeters to 'Standard'...that is, 29.92 Hg. ('Hg' is the term for mercury...it is the standard for
measuring atmospheric pressure. Yes, there are metric standards, 29.92 inches is equal to 1013 millibars...This means that every airplane operating
above 18.000 feet is at the same 'true' altitude, or as we call it, 'Flight Level'.
Some countries define a 'Flight Level' as anything above, oh...4000 feet. I'm thinking of the UK here. In France, I think it's at 5 or 6
thousand...
Point is, it varies, the setting of the alitmeters to 29.92, or 1013Mb, depending on altitude, and this is what we learns and know, as we operate into
various countries. Heck, in Russian airspace, they still use metres, not feet...and we can set a button to display the atltitude in metres, instead
of feet....
The reason we have a barometric setting knob is because there must be a way to adjust the altimeter to the local atmospheric pressure, as it will
vary.
Back to above...a 'Flight Level' assumes that in the area where you are operating, there are no geographical obstacles to 'hit'...hence, in
western Europe, 4000 or 6000 feet work as the lower limits, and anything above is a 'Flight Level', which means every airplane has the same
altimeter setting (29.92 InHg or 1013 Mb, same thing).
Reason the US has 18,000 feet as a standard for changing altimeters from 'pressure altitude' to FL, is because there are very high mountains in
Alaska, and in the Rockies, and in the Northwest of the contiguous US too. When we fly in Europe, the FL changes, and this is denoted on our charts.
Varies by country, and FL.....
So....DID the altimeters get reset on AA77?
DID the DFDR record BOTH altimeters, the Captain's and the FO's, and also the standby altimeter, all three of them get reset?
The prevailing pressure, as has been presented on this thread, was 30.22. This means, it was a genearl higher pressure than normal, since we are
pretty close to Sea Level, and everyone knows that 'Standard' pressure at Sea Level is 29.92 InHg, or 1013.3 Millibars.
So, an altimeter is designed to 'adjust' by use of a knob, to correct for what the ground station reports as the prevailing pressure, which of
course varies...hence the ATIS must be recorded at least once each hour, or more often if conditions warrant.
BUT, all of this pre-supposes something that seems to be lost in the discussions.....setting the proper altimeter correction only matters if you CARE
ABOUT FOLLOWING regulations!
NO, it is not automatic, as it relates to the cockpit displays. DID the altimeter get reset? Did all three....notice ....there are THREE altimeters
in a B757....did they ALL get reset?
There are three altimeters, two are supplied info from the ADCs, one is considered 'Standby'...it gets 'raw' data...
So, the ADCs provide data to both the Captain and the F/O side....but there are two ADCs, they are designed to cross-check each other, and besides
that, there are other computers that will flag bad data....