I want to understand altimetry - Part 1

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Simicro
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I want to understand altimetry - Part 1

Post by Simicro »

Hi all,

I've been flightsimming for 25 years and I confess that I've not quite understood altimetry.

I did some trials but I'm not good at maths and was probably lazy to persevere. So I ended up doing some fancy flying...
Of course I am aware that altimery is one of the most important things in aviation.
Recently I decided to re-start flightsimming from the ground up and re-learn the basics.
I've never talked about this on forums, but I feel in confidence on this A2A forum.
Probably because it is calm, friendly, not overcrowded however I can feel that there are a lot of knowledgeable people.

My goal is not to learn the full theory of altimetry. Instead I aim at understanding what I need to do, how, and why. What I plan to do is:
- Make some readings on the internet
- Take abstracted notes
- Take a few situations and say what I would do and share it with you, so that you can comment
- For clarity and legibility I'll create a thread for each situation (rest assured, there will no more than 5-6 situations I believe)

I would be most grateful if you could help me to achieve my goal.

Situation 1: Local flight with no pressure change

1. In all the following I will use QNH
2. I am the first guy to fly the plane on the day
3. When I board the plane, the altimeter reads 600ft (state of the plane left by a guy who flew the plane the day before)
4. I start the engine, listen to ATIS or contact the tower, and take note of the pressure, say 30 InHg
5. I set the altimeter to 30 InHg ==> Question: without reading the altimeter, is it possible to calculate/anticipate the altitude it will display?
6. Now I take a fictitious example because I do not know what the altimeter should read. Let's say 700 ft
7. ===> Question: if I read the QNH altitude of the airport on an aviation map, it would be 700 ft?
8. I take off and do a local flight
9. Before landing, I contact tower/listen to ATIS and the pressure is unchanged: 30 InHg
10. So I do nothing on the altimeter and land
11. ===> Question: after landing, the altimeter should read 700 ft?
P3Dv4.5 - V35B - C172 - Cherokee
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speedy70
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Re: I want to understand altimetry - Part 1

Post by speedy70 »

You will learn more from this site.

http://www.luizmonteiro.com/Altimetry.aspx

Cheers Chris

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Oracle427
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Re: I want to understand altimetry - Part 1

Post by Oracle427 »

Question: without reading the altimeter, is it possible to calculate/anticipate the altitude it will display?

No, as you would need to know the local air mass pressure in the area to determine the altitude. Temperature affects the reading as well, but you need not worry about that. The fact is that the altimeter should read within some calibrated range of the field elevation. In the US for IFR flight it had to read within 75 feet of the field elevation with the local pressure dialed in.

If you are operating out of an uncharted location, you will need to obtain some elevation data perhaps using a gps and then set the altimeter. You could also pick an altitude as your "zero" elevation for the field and then maintain visual separation until you are able to obtain an appropriate pressure reading for your location as you transit.

Question: if I read the QNH altitude of the airport on an aviation map, it would be 700 ft?

It would be approximately 700 feet, as the airport is not perfectly flat. As noted previously, the allowed error is 75 feet from the charted elevation.

Question: after landing, the altimeter should read 700 ft?
Same as before. Pressure is not static, so temperature variations and the change in the pressure of the airmass itself will alter the reading a little. It should be within the error range. Also note that most controlled airports don't publish the weather more than once an hour unless conditions are starting to get poor/dynamic. It isn't uncommon to get updated pressure with clearances that might be .3 off what the ATIS is broadcasting. That only amounts to 30 feet of change.
Last edited by Oracle427 on 24 Oct 2018, 07:25, edited 1 time in total.
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

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LZ-WIL
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Re: I want to understand altimetry - Part 1

Post by LZ-WIL »

Hi,

Check out this two videos in the given order - Pressure Altitude and then the Density Altitude, they helped me to understand better Chapter 11 from Pilot's Handbook of Aeronautical Knowledge. :)

Pressure Altitude

Density Altitude

Cheerz,
Vladimir
Bonanza, Skylane, Skyhawk, Cherokee, Cub, Texan, Mustang, Warhawk, Spitfire, Flying Fortress

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Simicro
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Re: I want to understand altimetry - Part 1

Post by Simicro »

Many thanks all for the links and responses.

Please do not post links anymore at this time please. I have enough material to read/watch. Sometimes "more" can be "less".

What helps me most is comments on my understanding and responses to my questions.

I rephrase/rewrite to memorize.

1. Boldly my understanding was correct
2. No need to calculate anything in this precise situation
3. I need to recall the error range (75 ft allowed):
- pressure is not static
- temperature plays a role
- airport is not perfectly flat
- controlled airports don't publish the weather more than once an hour
- there can be a difference between ATIS and airports published weather

One last question for this lesson-part 1: boldly, for all my learning can I do my rationale with QNH? Yeah I know QFE, etc. But I'd like to stick to one thing if possible. I cannot get it all at once if you understand.
P3Dv4.5 - V35B - C172 - Cherokee
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AKar
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Re: I want to understand altimetry - Part 1

Post by AKar »

Some additional points to what has been said above...

Temperature does not affect your altimeter reading at the QNH source. If your airfield from which you get your local QNH is at 700 ft, your altimeter would indicate approximately 700 ft at the airfield regardless of the temperature. However, if you get higher or lower, then your altimeter will show an erroneous altitude when the temperature deviates from the standard. If it was very cold, your altimeter set to QNH from airfield situated 700 ft above sea level would indicate negative altitude at sea level. Similarly, it would indicate too high when you fly some height above the airfield. In hot conditions, the effect is reversed. Farther away you are from the field elevation of your QNH source, larger the error.

Controlled airports may and do publish weather information more often than once in an hour if conditions require.

QNH and QFE are the most important concepts in altimeter settings, together with the 'standard setting' of 1013.25 hPa used when you don't fly at some distinct altitude but at some distinct atmospheric pressure (flight levels, used above local transition altitude).

-Esa

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Oracle427
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Re: I want to understand altimetry - Part 1

Post by Oracle427 »

To keep it very short and sweet, we aren't doing the mental math in the cockpit. Get the pressure, dial it in, keep flying at the correct altitude.

There is a saying, "From high to low, [pressure or temperature] look out below!".

It is a reminder that decreases temps and pressures will cause your indicate higher than you really are until you update the pressure. As Esa said, the ATIS is compensated for temp at the airport SURFACE.

In the end, there is no need to overthink it, and certainly no math required.
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

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Simicro
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Re: I want to understand altimetry - Part 1

Post by Simicro »

Esa and Oracle, thanks for those additional infos, attentation points and tips.
P3Dv4.5 - V35B - C172 - Cherokee
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Microsoft Force Feedback 2 modded with CH Fighterstick - VKB Sim T-Rudder Mk.IV Pedals

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AKar
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Re: I want to understand altimetry - Part 1

Post by AKar »

To add one further point to the discussion about temperature in what comes to altimeters, aerodromes that have instrument procedures that are adversely affected by temperature mention this in appropriate charts, so it is important to be aware of it. Sometimes temperature-corrected altitudes are provided, sometimes barometric-based VNAV is prohibited outside given temperature limit.

Note that in FSX/P3D it makes no difference, because the atmospheric model of the simulator does not correctly model the effects which temperature has to the altimeter behavior.

-Esa

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