Hi folks. Well I have gotten able to take off and fly straight. That's a beginning.

 

OK so I am looking at my altimeter (Cessna 152) on the ground and it says like something 2040 feet. When I am in the air I will be just over the houses and it will say like 4000 feet. I pressed 'b' to set the altimeter while on the ground but it still says 2040 feet while idling on the runway.

 

Can someone please explain what I am doing wrong and if I am not doing anything wrong, how am I misinterpreting the data?

 

UPDATE: OK I see that Las Vegas is actually roughly 2000' above sea level so obviously that's what the altimeter in the plane is showing me. But then how do I get a reading of how high above the ground I am rather than how high above sea level I am?

 

Thanks so much folks,

 

Rob

 

P.S. On a side note, I always check the "Notify me of replies" thing at the bottom of the post but I never get any kind of notification. How do I get set up so that I actually get notifications of some kind?

Edited December 10, 20205 yr by Vegas-Rob

If you want the altimeter to read the approximate height above ground, when sitting on the runway, adjust the altimeter setting (barometric setting) knob so the altimeter reads 0 feet.

In addition to the barometric altimeter, some aircraft also have a Radar (or Radio) Altimeter that reads height above ground by timing how long it takes for a signal from the aircraft to reflect off the ground back to the aircraft. Radar altimeters typically only work up to about a few thousand feet above the ground.

In aviation, QNH is the altimeter pressure setting so your altimeter reads your height above mean sea level. QFE is an altimeter setting such that your altimeter will read the approximate height above the ground (typically the airport you are landing at or taking off from).

So when sitting on the runway, if you have set the QNH value into your altimeter, it will read the airport's height above sea level. If you have set the local QFE value into your altimeter, it will read zero.

QNH is the setting typically used.

Al

Edited December 10, 20205 yr by ark

1 hour ago, Vegas-Rob said:

Hi folks. Well I have gotten able to take off and fly straight. That's a beginning.

 

OK so I am looking at my altimeter (Cessna 152) on the ground and it says like something 2040 feet. When I am in the air I will be just over the houses and it will say like 4000 feet. I pressed 'b' to set the altimeter while on the ground but it still says 2040 feet while idling on the runway.

 

Can someone please explain what I am doing wrong and if I am not doing anything wrong, how am I misinterpreting the data?

 

UPDATE: OK I see that Las Vegas is actually roughly 2000' above sea level so obviously that's what the altimeter in the plane is showing me. But then how do I get a reading of how high above the ground I am rather than how high above sea level I am?

 

Thanks so much folks,

 

Rob

 

P.S. On a side note, I always check the "Notify me of replies" thing at the bottom of the post but I never get any kind of notification. How do I get set up so that I actually get notifications of some kind?

The kind of altimeter you'r looking for isnt fitted on a cessna (radio altimeter). Best get used to the idea of a classic altimeter that read above sea level, as these are the kond you'll find the most. If for a personal reason you would rather have it reading above ground altitude then you could do as suggested above. 

Okay. So, you have a little rotary dial at the side of your altimeter, and when you turn it (put your mouse pointer over it and roll the mouse wheel), it calibrates the altimeter based on the air pressure. On a 'standard day' the air pressure measures 1013.25 Millibars (the system used in Europe) or 29.92 inches of mercury (the system used in the US). So if the air pressure is pretty average on a particular day, and you are parked on a beach in your aeroplane (i.e. it is at sea level) if you dialled in 29.92/1013 on your altimeter, the altitude needle would be reading zero. You will typically see the altimeter setting numbers go up on a warm sunny day, and drop on a rainy winter day.

This is why if you call up an airport on the radio, they will tell you the altimeter setting, so they will say something like 'QNH 29.98' or 'QFE 1027' or whatever. This tells you what setting to dial into your altimeter to get an accurate localised reading which will work for that airport. You might hear them say QNH, and you might hear QFE (not that often). Alternatively, if you happen to know exactly how high above sea level your airport is, you can alter the dial until it reads that height on your altimeter, and that will almost certainly be equivalent to the QNH setting or pretty close to it. Airports change this setting regularly (usually once every hour, but more frequently if there is a significant change and you can tune the Air Traffic Information Service - ATIS - frequency of an airfield to hear that plus other info about the airport).

The easy way to remember which is which, is to imagine that QFE is short for query field height (i.e. what is the pressure setting to put on my altimeter dial so that it reads zero feet when I'm on the runway), and imagine QNH is short for query nautical height, (i.e. what setting do I put into my altimeter so that it is reading my height above sea level.

All of this means that you could quite easily be sat on the ground at Mexico City Airport with your altimeter set to QNH, and your altimeter needle pointer would be reading that you were over 8,000 feet.

Since the pressure is never the same in different places, and it changes almost minute by minute, there has to be a system in place to ensure that all aeroplanes have the same altimeter settings when they are trying to avoid one another up at cruise altitudes. This is what something called 'transition height' is about. Aeroplanes will take off with their altimeters set to a local QNH setting, but when they get to the 'transition altitude' they all switch to the same standard setting (1013/29.92) to ensure that all aeroplane altimeters are reading the same, which will assist with vertical separation of traffic. The transition altitude can vary in different countries, some have it at 18,000 feet, some have it at 10,000 feet (these are the most common values) but some places have other weird values and some places (notably Eastern Europe) use meters instead of feet as a measurement, which further confuses things on occasion.

It seems confusing at first, but you'll get used to it.

Edited December 10, 20205 yr by Chock

Great explanation

Generally if you are flying VFR in a GA aircraft like a Cessna you generally adjust the altimeter to read the correct height ASL for the runway and work out height above the ground in your head or write them down.  Among other things knowing the height above the runway you left 20 minutes ago is not overly useful when flying through the hills 20 miles away so setting the altimeter to runway altitude is only useful in circuits.

Even then, you typically set your altimeter to ASL not AGL.  To fly a circuit in training you would typically takeoff and maintain the runway heading until you are both past the end of the runway and also at least at 500 feet AGL before turning crosswind and eventually level out at 1000 feet AGL (circuit height) and then, depending on the aircraft, as a general rule of thumb want to be turning from base onto final at around 600 feet AGL. Whilst you CAN set the altimeter to read 0 feet on the runway to make it easy to know when you are at 500 feet or 1000 feet, typically you would not do that, you would set the altimeter to ASL and work out your heights and headings beforehand and maybe even write them down.

EXAMPLE:  you are doing left hand (anti clockwise) circuits starting from runway 23 at 2050 feet ASL.  Before taking off you might make the following notes:

1. Runway altitude is 2050 feet
2. maintain runway heading of 230 degrees until at least 2550 feet
3. at 2550 feet turn to a crosswind heading of 140 degrees
4. level off at circuit height of 3050 feet
5. your downwind heading is 50 degrees
6. your base heading is 320 degrees
7. adjust descent rate on base so you are turning final (lined up with runway at heading 230 degrees) at roughly 2650 feet

Though it is important to eventually be able to do this mainly by feel and looking out the window and not fly with your face attached to the gauges.

Note that there is also a thing called density altitude used to calculate stuff like max take-off weight but you do not need to worry about that at this stage.

Edited December 11, 20205 yr by Glenn Fitzpatrick

11 minutes ago, Glenn Fitzpatrick said:

... depending on the aircraft, generally want to be turning base around 600 feet AGL.

Typo -- think you meant turning final around 600 feet AGL. 

Al

3 minutes ago, ark said:

Typo -- think you meant turning final around 600 feet AGL. 

Al

thanks - my bad

I have fixed it

Some good info there.

I have a question on Altimeters in MSFS2020.

Is there a way of recalibrating them to read/ show heights in metres rather than ft??

4 hours ago, merlin_66 said:

Some good info there.

I have a question on Altimeters in MSFS2020.

Is there a way of recalibrating them to read/ show heights in metres rather than ft??

Not the steam gauge ones.

Some glass cockpits will do it.

Another point not mentioned is once you get really high (as in tens of thousands of feet) all aircraft will reset their altimeter to standard rather than real local pressure to ensure that all aircraft are calibrated the same to maintain separation and avoid accidents.

It amazes me how many pilots don't understand how an altimeter works, and how to check it before takeoff.  This is one of the things I learned on my first pre solo  lesson. 

On 12/10/2020 at 5:06 PM, Vegas-Rob said:

Hi folks.....

When you press the B key, the altitude you read on the ALT is the height above the Sea.

Read more about altitudes here:

https://pilotinstitute.com/altitude-types/

18 hours ago, merlin_66 said:

Is there a way of recalibrating them to read/ show heights in metres rather than ft??

 It's just China, North Korea and few other smaller countries in that region that use metric altitudes. That being said, some aircraft with more modern glass cockpits have the ability to swap in their settings. 

Most of the aviation world uses feet for altitude, and knots (nautical miles per hour) for speeds, even in areas that use the metric system for non aviation purposes.  Although, having said that, there are a few aircraft in the sim whose gauges show speed in km/hr.