On Thu, 17 Apr 2008 02:20:46 -0700 (PDT), terry
wrote in
:

I am confused by this practice commercial nav question. ( at least I
am confused by the answer in the book which was b. but I think both a
and c are correct), but I appreciate some other opinions.

Day 1 Altimeter reads elevation of 1390 feet with 1013 HPa set on
subscale

http://en.wikipedia.org/wiki/Mbar
Atmospheric air pressure is often given in millibars where
"standard" sea level pressure is defined as 1013.25 mbar (hPa),
equal to 1.01325 bar. Despite millibars not being an SI unit, they
are still used locally in meteorology in some countries to
describe atmospheric pressure. The SI unit is the pascal (Pa),
with 1 mbar = 100 Pa = 1 hPa = 0.1 kPa. Meteorologists worldwide
have long measured air pressure in millibars. After the
introduction of SI units, others use hectopascals (which are
equivalent to millibars) so they could stick to the same numeric
scale. Similar pressures are given in kilopascals in practically
all other fields, where the hecto prefix is hardly ever used. In
particular, Canadian weather reports use kilopascals (which could
also be called centibars).


( thats equivalent to 29.92 inches of Hg for the US folks)
Day 2 Altimeter reads elevation of 1000 feet
Assuming the altimeter subscale was not changed between day 1 and day
2 it could be said that

a. The QNH is higher on day 2
b. The QNH is lower on day 2
c. The pressure altitude at the airport is lower on day 2
d. The atmospheric pressure at the aerodrome has not changed.

Terry
PPL Downunder



http://www.acronymfinder.com/acronym.aspx?rec={8F1A7DDE-89E8-11D4-8351-00C04FC2C2BF}
What does QNH stand for?
Atmospheric Pressure (Q) at Nautical Height (aviation)