From Low pressure to High Pressure

[extricate]

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

[bubbers44]

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.

[MaxFL360]

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).

[The African Dude]

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

[bubbers44]

Or flying from higher pressure area to a lower pressure: From a high to a low, look out below.

[N1 Limit]

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?

[keith williams]

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.

[de facto]

Basically yes.