On Thu, 17 Apr 2008 06:51:24 -0700 (PDT), terry
wrote:

On Apr 17, 11:38*pm, Stealth Pilot
wrote:
On Thu, 17 Apr 2008 07:30:56 -0400, "Barry" wrote:
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
* * * * ( 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.

I agree with you - answers a and c are both correct. *Day 2 QNH is about 1026.

playing with an altimeter here in my lap.
simulating 1390 by setting 390. the qnh reads as 1032.

moving the needle lower sees the qnh values going lower.

so to correct for the reduced reading I would need to move the qnh the
other way.(increase it)
I agree a is correct from playing with an actual altimeter.
c is correct just from reading the data in the question.

fwiw
Stealth pilot- Hide quoted text -

gday Stealth,
I just think of an altimeter as being a inverse pressure guage cos
pressure decreases with altitude, so for altitude reading to have gone
down , atmospheric pressure has to have gone up.
I gotta ask, why have you got an altimeter in your lap?
Terry

that's like the goon show joke. turn the knob on your side.
but I dont have a knob on my side. ...there's one on the door though.

anyway it is the old altimeter out of my auster restoration. it lives
with a few other instruments in the cupboard behind me as I type.