What is the purpose of the altimeter sub scale?

[fly12345678]

what is the purpose of the altimeter sub scale????

[VFE]

To set the relevant pressure setting (datum) so that altitude or height is displayed correctly on the altimeter.

VFE.

[bucket_and_spade]

The atmospheric pressure at any given time and on any given day varies and so, to make sure your altimeter gives you an accurate reading of height above sea or airfield level, you need to tell it what the current pressure is at sea or airfield level so it has a reference point.

Simply put, the current air pressure at sea level in the area you're operating in is called the local QNH. Plugging this into your altimeter subscale will cause your altimeter to show your height above sea level.

The current pressure at airfield elevation at the airfield you're at is called QFE and plugging this into your subscale will allow the altimeter to show your height above the airfield in question.

So, in both cases, you use the subscale to set a reference point and the altimeter will then display your height above this datum (be it sea or airfield level).

Altimetry is a very deep subject but that's the gist of the basics!

B&S :)

[Keygrip]

Bucket - I understand you are trying to help answer the original poster with simplicity - but, as I'm convinced you already know, it's an altitude when using QNH above sea level. I'll agree with the height for QFE and ground level.

"Vertical distance above......" is the all encompassing phrase that leads us neatly into the explanation of the other terms.

No need to teach "wrong" in an effort to make it simpler to understand.

[bucket_and_spade]

Bucket - I understand you are trying to help answer the original poster with simplicity - but, as I'm convinced you already know, it's an altitude when using QNH above sea level. I'll agree with the height for QFE and ground level.


Yep, I'm more than happy with altitude v. height. I did think about using the terms but thought it might muddy the waters so, rather than changing the word in each sentence, I just mentioned a different datum to make the point that this is the purpose of the subscale. In my defence, "altitude" is one and the same as "height above sea level", but I concede that if the original poster is in training it could lead to a little confusion when the books start talking about QFE v. QNH and height v. altitude.

"Vertical distance above......" is the all encompassing phrase that leads us neatly into the explanation of the other terms.

Good point, this would have been even better, if I'd thought of it! Semantics :bored:

Now, who's going to wade in with explanations of QNE, 1013 and ISA...? :O

B&S

[Neptunus Rex]

QNE is not an altimeter setting. It is the reading in feet (or metres) on an altimeter with 1013.2 set in the subscale.

The reason is that some older altimeters could not be set to QFE at high altitude airports, so you just leave 1013.2 set and request QNE. ATC would then give you the figure your altimeter would read with your aircraft on the ground there. I used it in Entebbe many moons ago.

Cheers,

Neppie :ok:

[Nearly There]

Are you training to be a pilot? if so then the answer will be in one of your books, if its purely a niggling question then the link below should help:ok:

http://en.wikipedia.org/wiki/Altimeter

[corsair]

fly12345678 is some kind of odd troll who specialises in asking damm silly questions. Check his other posts.

[bucket_and_spade]

Only looks like he's posted once...?

B&S

[Curtis E Carr]

He/she has posted before on several occasions but these posts appear to have disappeared.

[A Very Civil Pilot]

Only looks like he's posted once...?

Poor old fly 12.... has started several posts asking unusual questions (I should know, I've tied to answer half of them!).

It seems the threads have been removed.

Perhaps he is Franks Abagnale Jr.



Cutise E Carr beat me to it

[corsair]

It's like he's sitting in the briefing room with his Instructor who asks what is the purpose of the altimeter sub scale????

!23 says 'Let me think about it, esteemed Instructor sir!'
The Instructor nips out for a fag and a chat with the other Instructors about last nights drinking session and ole 123 fires up the laptop, clikety click click click. He posts the question on PPRuNe hoping one of us knows. Alas, no doubt his plan fails we don't know either but when the Instructor comes back he explains to 123 anyway because he likes being called sir by his students. This is in spite of being still mad at 123 for the disastrous solo navex he sent him on recently when he infringed LHR's airspace and was followed home by a Met helicopter ;)

[A Very Civil Pilot]

Corsair, so if you're the instructor, when you nip out for a fag after asking the question, do you go onto pprune as well, and give him a wrong answer to see if he really knows his stuff?

[DFC]

Instructors should never critise people for asking questions.

The purpose of the altimeter subscale is to set the pressure which will cause the altimeter to read zero.

No more, no less.

Regards,

DFC

[2close]

The purpose of the altimeter subscale is to set the pressure which will cause the altimeter to read zero.


......as you hit the water!! ;) ;) ;)


Sorry, DFC, I know what you mean but I couldn't resist that! :E

[charliegolf]

Surely the purpose of the subscale is simply to allow the user to reference the altimeter readout for any given circumstance? Zeroing isn't necessarily the aim when you set a QFE- it's rarely zero anyway! It's so that any given readout has the same meaning for all the users. Isn't it?

CG

[2close]

At some airfields, even when set to airfield QFE, the altimeter can vary significantly at different points on the airfield but it should read '0' at the airfield datum point.

My home base is like a roller coaster with at least 30' difference between some points so when flying circuits privately I '0' the altimeter at the threshold.

Students, however, are taught to use the stated QFE and apply normal accuracy tolerances.

Saying that, for flying circuits setting threshold QFE seems a more accurate and reliable method than using the QFE given by ATC, in the same way that I prefer to use visual reference to the windsock next to the runway rather than use a wind vector from a device 30' AGL, next to the tower, 1/4 - 1/2 mile away from my landing point. At my home base, the stated wind vector always appears to be about 30 degrees different to the direction of the windsock.

How often do these devices require calibration?

[corsair]

Corsair, so if you're the instructor, when you nip out for a fag after asking the question, do you go onto pprune as well, and give him a wrong answer to see if he really knows his stuff?

LOL Well he's not my student but I wonder if his Instructor is posting on this thread:confused:

[BigEndBob]

To give Medical Examiners an excuss to test your eyesight on that teeny weeny writing.