Altimeter and flying in areas of changing pressures

[Banchod]

Hi, I'm a newbie here doing a PPL.

I'm reading up on the altimeter in the book "From The Ground Up" and have a question.

The books states that when an airplane is flying from an area of high pressure to low pressure, the altimeter (uncorrected) will read higher than what the aircraft actually is.

I'm having a hard time getting my head around this. My understanding is when you're flying from an area of high pressure to low pressure, air is exiting the altimeter case causing the aneroid capsule expand, as a result the altimeter will increase in altitude and the pilot descends until the plane has reached the indicated altitude. But the altimeter is reading higher than it actually is. Is that how it's interpreted??

Thanks,

- B

[old_man]

In this case they are assuming you are flying at the same indicated altitude. Thus, it has nothing to do with the inner workings of your altimeter. It is also assuming that you have not changed your altimeter setting in your Kollsman window. It is just saying that a same 'pressure' is at a different height in a high pressure system than a low pressure system.

In a high pressure system 29.80 may be at 1000 ASL because ground level altimeter setting is 30.80. In a low pressure system ground level altimeter maybe be 29.50 and a 1000 ASL would be 28.50.

Clear as mud?

viewtopic.php?f=3&t=67483

It is also discussed up there.

[Banchod]

Well assuming you're flying at the same indicated altitude and not changing the altimeter setting, flying from a high pressure to low pressure system means the inner workings of the altimeter are changing if you're ending up lower than what you actually are??

[Banchod]

Sorry, lol. I'm still confused.

I'm going to read that thread.

[old_man]

The Air Command weather manual might be a good read for you. Bonus if you can get it with the 2009 update.

[modi13]

In an area of low pressure the air is less dense, so your instruments will indicate the same readings as being at a higher altitude in an area of high pressure. As you fly from an area of high pressure to low pressure, if you maintain the same absolute altitude your altimeter will begin to show an increase; however, since most pilots fly at a constant altitude, as they see the increase in indicated altitude they will correct by descending. Therefore, if you maintain a constant indicated altitude, your actual height above the ground is decreasing, hence the term "High to low, look out below".

[C-GPFG]

If the plane maintains a constant true altitude while flying into an area of lower pressure, the pressure around the aircraft is decreasing, thus less air is filling the aneroid capsule in the altimeter, causing it to read a higher altitude. Opposite happens when you fly into and area of higher pressure...more air fills the aneroid capsule in the altimeter and it shows a lower altitude.

[lowapproach]

If the plane maintains a constant true altitude while flying into an area of lower pressure, the pressure around the aircraft is decreasing, thus less air is filling the aneroid capsule in the altimeter, causing it to read a higher altitude. Opposite happens when you fly into and area of higher pressure...more air fills the aneroid capsule in the altimeter and it shows a lower altitude.

The aneriod capsule is sealed in an altimeter

[modi13]

The aneriod capsule is sealed in an altimeter

Precisely. The density of the air outside the capsule decreases, decreasing the pressure, which produces a partial vacuum that the aneroid capsule expands to fill. It's like driving into the mountains with a pop bottle or a bag of chips: because the container is sealed it maintains its internal pressure, and as the external pressure decreases it tries to expand to compensate. Flying from an area of high to low pressure at a constant absolute altitude is just like leaving the plane on the ramp overnight as the pressure drops: the altimeter will show an increase in altitude.

[C-GPFG]

The aneriod capsule is sealed in an altimeter

Ah dammit! I knew that. Must have been thinking of the ASI. Good thing my instructor rating is invalid.

[modi13]

Ah dammit! I knew that. Must have been thinking of the ASI. Good thing my instructor rating is invalid.

Wrong again; the ASI uses a diaphragm, not an aneroid capsule. Perhaps you're thinking of the VSI, and perhaps you should refrain from teaching groundschool...