Unusual
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Join Date: Sep 2004
Location: Ireland
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Unusual
With the unusually high pressure centred over the UK at present showing a QNH of up to 1044mb at some stations..what is the highest (or lowest for that matter) pressure setting people have experienced? where and when? and does it have any implications on operations..whats the max/min the altimeter subscale will show?
SC
SC
Join Date: Sep 2000
Location: Darraweit Guim, Victoria
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Haven't explored how high the subscale goes, but the FAA has a procedure for QNH above 31" (1049 in civilised speak) so it may be not unheard of:
g. When the barometric pressure is greater than 31.00 inches Hg., issue the altimeter setting and:
1. En Route/Arrivals. Advise pilots to remain set on altimeter 31.00 until reaching final approach segment.
2. Departures. Advise pilots to set altimeter 31.00 prior to reaching any mandatory/crossing altitude or 1,500 feet AGL, whichever is lower.
PHRASEOLOGY-
ALTIMETER, THREE ONE TWO FIVE, SET THREE ONE ZERO ZERO UNTIL REACHING THE FINAL APPROACH FIX.
or
ALTIMETER, THREE ONE ONE ZERO, SET THREE ONE ZERO ZERO PRIOR TO REACHING ONE THOUSAND THREE HUNDRED.
NOTE-
1. Aircraft with Mode C altitude reporting will be displayed on the controller's radar scope with a uniform altitude offset above the assigned altitude. With an actual altimeter of 31.28 inches Hg, the Mode C equipped aircraft will show 3,300 feet when assigned 3,000 feet. This will occur unless local directives authorize entering the altimeter setting 31.00 into the computer system regardless of the actual barometric pressure.
2. Flight Standards will implement high barometric pressure procedures by NOTAM defining the geographic area affected.
g. When the barometric pressure is greater than 31.00 inches Hg., issue the altimeter setting and:
1. En Route/Arrivals. Advise pilots to remain set on altimeter 31.00 until reaching final approach segment.
2. Departures. Advise pilots to set altimeter 31.00 prior to reaching any mandatory/crossing altitude or 1,500 feet AGL, whichever is lower.
PHRASEOLOGY-
ALTIMETER, THREE ONE TWO FIVE, SET THREE ONE ZERO ZERO UNTIL REACHING THE FINAL APPROACH FIX.
or
ALTIMETER, THREE ONE ONE ZERO, SET THREE ONE ZERO ZERO PRIOR TO REACHING ONE THOUSAND THREE HUNDRED.
NOTE-
1. Aircraft with Mode C altitude reporting will be displayed on the controller's radar scope with a uniform altitude offset above the assigned altitude. With an actual altimeter of 31.28 inches Hg, the Mode C equipped aircraft will show 3,300 feet when assigned 3,000 feet. This will occur unless local directives authorize entering the altimeter setting 31.00 into the computer system regardless of the actual barometric pressure.
2. Flight Standards will implement high barometric pressure procedures by NOTAM defining the geographic area affected.
Join Date: Oct 2002
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The 'great' storm of '87 had the QNH down to 925 millibars at Bournemouth,
and that was a few hours after the main event. Light aircraft used QNE as
their altimeters only went as far as 950.
and that was a few hours after the main event. Light aircraft used QNE as
their altimeters only went as far as 950.
Join Date: Jan 2003
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Years ago this same question was asked at the station where I was serving (How low does the altimeter go?). A squadron sent out a suitably briefed student to a cab parked on dispersal to investigate... some twenty minutes or so later, the Duty Instructor wandered out to check on his progress, only to find the stude going down through 700 and complaining of a stiff wrist.
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I remember when the FAA instituted that procedure, first by NOTAM and then in the ATC regs.. it was the winter of 89-90, I think, and a huge intensely cold high pressure system settled over Alaska for weeks. Pressures were running 31.20" I think.
