From Low pressure to High Pressure
Thread Starter
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 234
Likes: 0
Received 0 Likes
on
0 Posts
From Low pressure to High Pressure
Hi,
Why flying from a low pressure airspace to high pressure airspace, the actual altitude is higher than the indicating altitude on the altimeter? How do I interpret this?
Thanks
Why flying from a low pressure airspace to high pressure airspace, the actual altitude is higher than the indicating altitude on the altimeter? How do I interpret this?
Thanks
Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes
on
0 Posts
Not sure what you mean but initially you set your altimeter to local pressure referenced to msl. As you fly you must update your altimeter to new local pressure settings to have an accurate msl indicated altitude read out.
Last edited by bubbers44; 7th Sep 2014 at 12:24. Reason: wording
Join Date: Aug 2014
Location: Australia
Age: 34
Posts: 40
Likes: 0
Received 0 Likes
on
0 Posts
For a standard altimeter as you enter a region of higher pressure so does the pressure within in the instrument itself. You can think of this as an increase in pressure relating to a decrease in altitude (lower alt = higher pressure). So for an instrument that is set at a specific hectopascal setting i.e. 1013/29.92 this will relate to an apparent decrease in altitude. To maintain the required altitude the aircraft will have to climb to return to an area of equal pressure (i.e. higher true altitude).
Join Date: Jul 2000
Location: Everywhere
Posts: 783
Likes: 0
Received 0 Likes
on
0 Posts
If you leave the Q setting on the altimeter the same, it measures the pressure height difference between that pressure level and where you actually are (i.e. what the altimeter SENSES).
If the Q setting does not refer to the actual ground Q, then you are not measuring (i.e. having displayed by the altimeter) the correct difference in height.
So as the actual Q value increases at the ground, if you leave it the same on the altimeter, the instrument is measuring an ever smaller and smaller difference, thereby displaying an ever decreasing altitude on the instrument.
As this appears to represent a descent on the dial, you will try to maintain your previous altitude by gradually climbing over a period of time. This is why over a cross-country flight with the wrong Q set, flying from an area of Low pressure to High pressure, you end up gradually climbing, i.e. higher than you think you are.
High to low, read high, be low
Low to high, read low, be high
If the Q setting does not refer to the actual ground Q, then you are not measuring (i.e. having displayed by the altimeter) the correct difference in height.
So as the actual Q value increases at the ground, if you leave it the same on the altimeter, the instrument is measuring an ever smaller and smaller difference, thereby displaying an ever decreasing altitude on the instrument.
As this appears to represent a descent on the dial, you will try to maintain your previous altitude by gradually climbing over a period of time. This is why over a cross-country flight with the wrong Q set, flying from an area of Low pressure to High pressure, you end up gradually climbing, i.e. higher than you think you are.
High to low, read high, be low
Low to high, read low, be high
Join Date: Nov 2009
Location: in the mix muff
Age: 44
Posts: 176
Likes: 0
Received 0 Likes
on
0 Posts
You are flying in the Alps at the same level as the summits on a hot day. What does the altimeter read?
A.)same altitude as the summit
B.)higher altitude as the summit
C.)lower altitude as the summit
D.)impossible to tell
Anyone to help me explain the correct answer of this question related to pressure systems?
A.)same altitude as the summit
B.)higher altitude as the summit
C.)lower altitude as the summit
D.)impossible to tell
Anyone to help me explain the correct answer of this question related to pressure systems?
The indicated altitude will depend upon what setting has been selected on the altimeter sub-scale.
But the OAT will also affect the indicated altitude. Altimeters tend to over indicate when the temperature is below ISA, and under indicate when the temperature is above ISA.
The combined effects of incorrect sub-scale setting and non-ISA OAT could cause the altimeter to over indicate, under indicate or (if the effects of the errors were equal and opposite) the indication could be correct.
The question does not give any information regarding the sub-scale setting or the magnitude of the temperature error, so it is not possible to predict what the indicated altitude will be.
But the OAT will also affect the indicated altitude. Altimeters tend to over indicate when the temperature is below ISA, and under indicate when the temperature is above ISA.
The combined effects of incorrect sub-scale setting and non-ISA OAT could cause the altimeter to over indicate, under indicate or (if the effects of the errors were equal and opposite) the indication could be correct.
The question does not give any information regarding the sub-scale setting or the magnitude of the temperature error, so it is not possible to predict what the indicated altitude will be.