Altimeter Question

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.

Terry
PPL Downunder

terry 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.


Their answer confuses me too. It seems that if the altimeter setting wasn't
changed from day 1 to day 2, yet it reads a lower altitude, the barometric
pressure must have risen. The altimeter will translate lower pressure to higher
altitude and higher pressure to lower altitude. So I think the answer should be
"a". And if I accept "a", I have to accept "c" as well, as you did.

I'm probably going to regret posting this because I'm going off the top of my
head but it wouldn't be the first time a test guide had the wrong answer posted.

Alright guys.... pile on!




--
Mortimer Schnerd, RN
mschnerdatcarolina.

On Apr 17, 9:29 pm, "Mortimer Schnerd, RN" <mschnerdatcarolina.>
wrote:
> terry 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.
>
> Their answer confuses me too.  It seems that if the altimeter setting wasn't
> changed from day 1 to day 2, yet it reads a lower altitude, the barometric
> pressure must have risen.  The altimeter will translate lower pressure to higher
> altitude and higher pressure to lower altitude.  So I think the answer should be
> "a".  And if I accept "a", I have to accept "c" as well, as you did.
>
> I'm probably going to regret posting this because I'm going off the top of my
> head but it wouldn't be the first time a test guide had the wrong answer posted.
>
> Alright guys.... pile on!

No, its not the first time I have come across wrong answers either,
but it is the first time I have come across a wrong answer, pointed it
out to the author and was still told I was wrong. ( I dont want to
name the said author because I think he otherwise has published some
very good material and I am sure he will soon realise his mistake.. I
just wanted to be absolutely sure I was right before I responded to
his response!)
Thanks Mort

On Apr 17, 9:30 pm, "Barry" <a...@b.c> 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.

Yep, thats what I get
Thanks

On Apr 17, 11:38 pm, Stealth Pilot <notranspon...@aeroplanes.com.au>
wrote:
> On Thu, 17 Apr 2008 07:30:56 -0400, "Barry" <a...@b.c> 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

terry <tfmann@iprimus.com.au> wrote in news:234756e2-a583-4cbc-8f42-
9fc499a48ca3@c65g2000hsa..com:

> 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.

It's A. It could also be C if the airfield has a serious subsidance
problem.


Bertie

On Apr 18, 12:07 am, Bertie the Bunyip <A...@AA.AA> wrote:
> terry <tfm...@iprimus.com.au> wrote in news:234756e2-a583-4cbc-8f42-
> 9fc499a48...@c65g2000hsa..com:
>
> > 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.
>
> It's A. It could also be C if the airfield has a serious subsidance
> problem.
>
you are really confusing me now Bertie, I got learned that the
pressure ht was wot your altimeter reads when you have 1013.2 in the
window ( or 29.92 for you ...or maybe not cos everyone knows bunyips
are aussies ) 1000 is lower than 1390 therefore pressure altitude is
lower on day 2.. surely!.

terry <tfmann@iprimus.com.au> wrote in news:179c2f21-99ad-4415-a9b2-
b21d355211ea@2g2000hsn..com:

> On Apr 18, 12:07 am, Bertie the Bunyip <A...@AA.AA> wrote:
>> terry <tfm...@iprimus.com.au> wrote in news:234756e2-a583-4cbc-8f42-
>> 9fc499a48...@c65g2000hsa..com:
>>
>> > 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 f
> olks)
>> > 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.
>>
>> It's A. It could also be C if the airfield has a serious subsidance
>> problem.
>>
> you are really confusing me now Bertie, I got learned that the
> pressure ht was wot your altimeter reads when you have 1013.2 in the
> window ( or 29.92 for you ...or maybe not cos everyone knows bunyips
> are aussies ) 1000 is lower than 1390 therefore pressure altitude is
> lower on day 2.. surely!.
>

Yer right, of course, I just couldn't resist the joke! There's a name
for that, if I'm not mistaken, and it's QNE. But theyre going for A, I'm
pretty sure and made an error in what thye were trying to say in C. I'm
bi-barometric, BTW.

Bertie

On Apr 18, 1:53 am, Stefan <stefan@mus._INVALID_.ch> wrote:
> WingFlaps schrieb:
>
> > QNH means query Newquay Harbour which was the British reference for
> > sea level.
> > It should give the atmospheric pressure at that dtaum -but I doubt
>
> You certainly can cite a source for this urban legend?

Urban legend? O, it was not Newquay but Newlyn (both in cornwall). The
datum is sea level as set the British Admiralty back then so the
obvious Q code would have been QNH. Look up the history of the naval
tidal observatory if you don't believe it.

Cheers

On Apr 17, 9:40 am, Gig 601Xl Builder <wrgiac...@REMOVE>
wrote:
> terry wrote:
>
> > I wasnt aware you dont use the QNH term in the States.  So what do you
> > call the number you dial up to make the altimeter read airport
> > elevation?
>
> altimeter

The Department of Defense uses QNH for altimeter settings in its
forecasts; that replaced ALSTG when the old FT forecast code was
superseded by the TAF code in the late 1970's. Here's the latest TAF
for Offutt AFB:

KOFF 171410 36015G25KT 4800 -RA OVC040 QNH2996INS
TEMPO 1417 35015G35KT 3200 -TSRA OVC025CB
BECMG 1617 03015G25KT 4800 -RA BKN020 OVC040 QNH2997INS
BECMG 0203 36012G18KT 8000 -RA OVC008 QNH2985INS T10/21Z T06/14Z
1415