Meteo Aspect of Altimetry

Question 1

What is the definition of an ‘Altimeter Setting’ (29.92 / 30.54 / etc) and how do meteorologists get it?

Answer 1

Altimeter settings can be defined as Station Pressure correct to ISA Sea Level Pressure using the ISA of 15oC and the ISA lapse rate of 1.98oC / 1000ft

They take the elevation of the aerodrome and the pressure read there and then add an imaginary column of air below it down to ISA sea level (using the average temperature over the last twelve hours at the aerodrome) then add them together.

They do this because they need a baseline to compare all the station pressures to, otherwise they don’t really mean anything?

Question 2

If station pressures were not corrected to ISA Sea Level, why would putting the station pressure into the kollsman windown while on the ground give you an indicated altitude of zero feet above sea level?

Answer 2

Think of pressure settings as the column of air going right through the station and down to sea level, instead of the column stopping at the station elevation.

Because the 29.92 pressure is sealed in the bellows, if they don’t add the pressure of the elevation of the airport to it, the altimeter would just be reading zero

Question 3

When meteorologists draw pressures on their maps, they don’t use Altimeter Settings, they use _____

Answer 3

The actual average temp over the last 12 hours of the stations/elevations/whatever the hell

Question 4

What is the difference between the Altimeter Setting and the MSL (Mean Sea Level) pressure reported?

Answer 4

MSL Pressures are calculated using the actual average temp of the station over the last 12 hours.

Altimeter Settings are calculated using ISA temps/lapse rates

Question 5

How is an altimeter setting found for a station?
How is a MSL (Mean Sea Level) Pressure found for a station?

Answer 5

Altimeter Setting is found by taking the station’s current pressure reading and correctig it to Sea Level using ISA temperature. In other words, the station pressure is for that altitude above sea level, they just ‘made up’ the column of air up through the ground to the elevation of the station using the ISA lapse rate.

MSL Pressure is found by taking the current pressure of the statio and reducing it to Sea Level using the actual average temperature over the last 12 hours.

Question 6

Decreasing pressure will cause an altimeter to indicate a ________ in altitude

Answer 6

Increase

Question 7

What is Pressure Altitude?

Answer 7

The altitude that is indicated when the altimeter is set to 29.92 OR it is the current pressure/altimeter setting once corrected for non-standard pressure conditions

This is the altitude we need to do all our charts (PA is used to find Density Alt for performance charts) and calculations

Question 8

When standard atmospheric conditions exist, what can we assume about all the other altitudes?

Answer 8

That pressure, density, indicated, calibrated, and true altitudes are all equal/the same

Question 9

What is Density altitude?

Answer 9

Density altitude is when you take the pressure altitude and correct it for non-standard temperatures

Question 10

Why do we calculate density altitude?

Answer 10

For use of our performance charts

Question 11

What is Density Altitude?

Answer 11

The altitude that the plane thinks its flying at.

A high density altitude means the plane thinks it is much higher in the sky than it actually is. This is because the air at a high density altitude has loosely packed particles, immitating an actual high altitude.

Question 12

For every _ degrees off standard temp, we chang density altitude by ____ft

Answer 12

8 degrees
1000 feet

Question 13

Density Altitude Practise Questions

Answer 13

Question 14

What are True and Absolute altitde?

Answer 14

True: Our exact height above sea lever
Absolute: Exact height above the terrain

Question 15

When it comes to True Altitude (exact height above sea level) calculations, what do you need to remember about colder temperatures?

Answer 15

Cold temperatures cause the pressure levels to compress (rise)

When the temperature is 0oC or less, our indicated altitude must be corrected. Particularly when we are depending on the altimeter for abstacle clearance during IFR flying.

Question 16

What two options for calculating true altitude (exact height above sea level) do you have?

Answer 16

1. Rule of Thumb: (See Picture Below)

• Multiply 4ft per 1000ft for each 1°C that the temperature varies from ISA

Example:

• OAT O°C
• ISA at 5000ft (PA) is 15°C

Question 17

IF THE EXAM DOESNT STATE OTHERWISE, YOU CAN ASSUME WHAT ALTITUDE?

Answer 17

SEA LEVEL

Question 18

How do you use the E6B to calculate your True Altitude (exact height above sea level)?

Answer 18

**Make sure you use the LEFT side of your E6B for True Altitude clculations.

There is an example around page 201 in FTGU

Question 19

Colder than ISA temperatures will place the aircraft _____ than the indicated altitude on the altimeter

Answer 19

Lower.

From hot to cold, you are being bold

Conversely, warmer than ISA temperatures will place the aircraft higher than what is indicated on your altimeter

Question 20

Anything stated as FLxxx or as being in the NDA, you will automatically assume is ________ altitude

Answer 20

Pressure altitude!

Question 21

What gives you your indicated altitude? The altitude you read off your altimeter?

Answer 21

Either the pressure setting of the airport (29.92)

OR

If you dont have the pressure altitude, then you put the elevation of the airport on your altimeter (which, will also then give you the pressure setting)

Question 22

What does the altimeter tell us and how does it calibrate the information/reading it gives us?

Answer 22

How high we are above sea level.

The altimeter is made in the factory with 29.92 inches of pressure sealed in the bellows/baloon inside the instrument. What the altimeter measures is the difference between the factory set 29.92 sea level pressure trapped in the bellows and the actual pressure coming in through the static port.

This then reads on the altimeter as our hight above sea level. The altimeter setting you get every time you listen to ATIS is the pressure reported on the ground at the elevation and area of the airport. This pressure setting is what is read on the ground but is then corrected to include the amount of pressure in an imaginary column of air the extends from the field elevation down to ISA sea level pressure of 29.92.

When you put this in to the kollsman window of your altimeter, it is then able to accurately read your height above sea level.

Question 23

How do you figure out how much clearance you have over a particular obstacle or ridge? This will be worded on the exam as something like “Your altimeter is set to X but the station altimeter is X. If you don’t change it, by how much will you clear the ridge/station/obstacle?”

Answer 23

Subtract the higher altimeter setting from the lower one and times it by 1000. The answer is by how many feet you are off your indicated altitude. We’ll say 250ft just to have a number.

Remember that if your setting in the plane is higher than that of whatever you are trying to clear, then you will be 250 too low. (From high to low, look out below).

So then take your 250ft and subtract it from whatever your indicated altitude is. This is your True Altitude.

Then take the height of the ridge/obstacle and subtrackt your true altitude. This will give you your clearance in feet.

Upon landing in the scenario in the picture, your altimeter would show you at an altitude of 150ft when you were sitting on the ground.

Convert everything to ASL first